JP2007268241A - Computer assisted tomographic x-ray equipment, remodeling method thereof, and absorption filter therefor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide computer assisted tomographic X-ray equipment (hereinafter referred to as CT) of a simple configuration capable of easily producing DEM, a remodeling method thereof, and an absorption filter therefor. <P>SOLUTION: The CT 10 comprises an X-ray lamp 1 to emit an X-ray LX, a sensor 2 to detect an intensity of the X-ray LX, the absorption filter 3 to absorb a part of the X-ray LX, and a move-scanning mechanism 4 to move the X-ray lamp 1 and the sensor 2 in a move-scanning pattern that consists of a first state ST1 in which a first X-ray LX1 passes through a first beam path 11 to the sensor 2 and a second state ST2 in which a second X-ray LX2 passes through a second beam path 12 running counter to the first beam path 11 to the sensor 2. The absorption filter 3 is formed so that an energy spectral distribution of the first X-ray LX1 passing through a first absorption path 31 in the first state ST1 and that of the second X-ray LX2 passing through a second absorption path 32 in the second state ST2 could be different from a constant number times of the other's each other. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention aims to improve the basic performance of a computer-aided X-ray tomography apparatus (X-ray Computerized Tomography Scanner, hereinafter abbreviated as X-ray CT or CT) used for medical diagnosis using X-rays. The present invention relates to a computer-aided X-ray tomography apparatus, a method for remodeling a computer-aided X-ray tomography apparatus for obtaining such a CT, and an absorption filter used for such a CT.

  X-ray CT was developed in the 1970s and is now widely used as an indispensable device in medical diagnosis.

  In X-ray CT generally used in the field of medical diagnosis, a tube is used as an X-ray source, and the irradiated X-ray has a wide energy range within the energy range (10-150 [keV]) used in diagnosis. Has a spectral distribution. In addition, the X-ray absorption coefficient of a substance is not uniform with respect to the energy of X-rays, and the value of the X-ray absorption coefficient increases the energy of X-rays in X-rays in the energy region used for diagnosis. It tends to decrease with time. In addition, the tendency of this change varies depending on the material, so to speak, it indicates that each material has a unique color in X-rays. If color, that is, energy information can be used, it can be used for tissue identification and the like, which is considered to be extremely effective for diagnosis.

  However, in the CT that is currently popularized, the tomogram is reconstructed on the assumption that the irradiated X-ray has only a single energy, and the energy information cannot be completely extracted.

  Regarding the principle of extracting this energy information, efforts have been made for a long time. In Patent Document 1 and Non-Patent Document 1, the X-ray energy used in the medical diagnosis area has an absorption coefficient that can be expressed by two elements, that is, Compton scattering and photoelectric effect, and is different from the constant multiple of the other spectrum. It has been reported that energy information can be extracted by measuring with two types of X-rays having different spectra. This method is generally called a dual-energy method (hereinafter referred to as DEM).

  Hereinafter, the principle of DEM and the conventional realization method will be described. If the plane including the cross section of the object is specified by the x and y coordinates, and the position of this plane is represented by a vector x, the X-ray absorption coefficient at the position x at the energy ε is expressed as μ It can be expressed as (x, ε).

  A straight line connecting the X-ray tube and one sensor is called a beam line, and measurement is performed on the beam line. X-ray intensity measurement on the beam line necessary for tomographic reconstruction is called scanning. The beam line is basically specified by a two-dimensional quantity. In the first generation CT, which is the most basic form with the movement of linear scanning and rotation scanning by one thin rod-shaped X-ray (hereinafter referred to as pencil beam), the beam line is linearly scanned. The position and the rotational scanning position are specified in two dimensions. Further, in the third generation CT, which is the most popular form, using an X-ray tube that generates fan-shaped X-rays (hereinafter referred to as fan beam) and performing scanning only by rotational scanning, the rotation angle, A beam line is specified in two dimensions of the irradiation angle. The two-dimensional quantity that designates this beam line is defined here as a scan position i.

The X-ray intensity I _mi that reaches the sensor at a certain scan position i can be expressed by the following equation (S1).
Here, I _{e, i} (ε) is the irradiation X-ray intensity, s _i (ε) is the following equation (S2), more precisely, the absorption coefficient on the beam line as shown in equation (S3). This is integrated and called projection data.
Here, L _i is a set of coordinates through which the beam line at the measurement position i passes.

In DEM, it is assumed that the absorption coefficient μ (x, ε) can be expressed by a linear combination of the energy characteristic functions Γ (ε) and Ψ (ε) of the photoelectric effect element and the Compton effect element as in the following equation (S4). .

When this is applied to the equation (S2), the projection data can also be expressed by a linear combination of the energy characteristic functions of each element as in the following equation (S5).
Here, the coupling coefficients sγ _i and sψ _i represent projection data for each element.

Therefore, if the coupling coefficient for each element is obtained, projection data s _i (ε) for each energy can be obtained. For this purpose, two X-rays having spectra of ¹ I _{e, i} (ε), ² I _{e, i} (ε) on the same beam line, which are different from the other constant multiples (primarily independent from each other), are obtained. Generate in some way. Hereinafter, such X-rays are expressed as having different quality.

What is necessary is just to solve the nonlinear simultaneous equation of the equation (S6) including ^two measured values ¹ I _mi and ² I _mi by such two X-rays with respect to sγ _i and sψ _i .

  Numerical solutions such as Newton's method can be used to solve this equation. In recent years, DEM algorithms with improved noise tolerance have been developed in Non-Patent Documents 10 and 11 and so on, so that more accurate solutions can be obtained.

Now, in order to implement and realize the above-described DEM principle in CT, it is necessary to generate X-rays having different radiation qualities. To that end, it can be realized by switching the voltage applied to the X-ray tube and mechanically replacing the absorption filters with different thicknesses, so that various realization methods based on this principle have been proposed so far. . They are classified as follows.
(Prior Art 1) A method of switching and scanning the tube voltage.
(Prior Art 2) A method of mechanically switching and scanning an absorption filter.
(Prior Art 3) A method of using synchrotron radiation having an extremely sharp energy peak.
(Prior Art 4) A method in which an absorption filter is sandwiched between multiple sensors stacked in multiple stages.
(Prior Art 5) A method using a sensor having an energy analysis function.
(Prior Art 6) A method using a plurality of X-ray tubes and sensor pairs having different tube voltages.

Alvarez, R.E. et.al., `` X-ray spectral deconposition imaging system '', United States Patent, No. 710,359, July 1976. Alvarez, R. E. and Macovski, A., `` Energy-selective reconstruction in X-ray computerized tomography '', Phys. Med. Biol., Vol. 21, No. 5, pp. 733-744, 1976. Kelcz, F. and Joseph, P.M. and Hilal, S.K., `` Noise considerations in dual energy CT scanning '', Medical Physics, Vol. 6, pp. 418-425, 1979 Marshall, W. and Hall, E. and Doost-Hoseini, A. and Alvarez, R. and Macovski, A. and Cassel, D., `` An implementation of dual energy CT scanning '', J. Comput. Assist. Tomogr., VOl.8, No.4, pp.745-749, 1984. Makoto Sasaki, Masami Torikoshi, Masahiro Endo, Kazuyuki Hyodo, “Development of Mixed Two-Color X-ray CT System”, Proceedings of the 84th Annual Meeting of the Japan Society of Medical Physics Vol. 20, pp. 158-161, 2003. Torikoshi, M., Tsunoo, T.Sasaki, M., Endo, M., Noda, Y., Ohno, Y., Kohno, T., Hyodo, K., Uesugi, K. and Yagi, N., ` `Electron density measurement with dual-energy x-ray CT using synchrotron radiation '', Physics in Medicine and Biology, Vol.48, pp.673-685,2003. R. E. Alvarez, J. A. Seibert, and S. K. Thompson.``Comparison of dual energy detector system '' Med. Phys., Vol. 31, No. 3, pp. 556-565, Mar 2004. J. P. O. Evans. `` Stereoscopic imaging using folded linear dual-energy x-ray detectors '', Meas. Sci. Technol., Vol. 13, No. 9, pp. 1388-1397, Sep 2002. S. Gleason, H. Sari-Sarraf, MJ Paulus, DD Johnson, SJ Norton, and MA Abidi.``Reconstruc- tion of multi-energy X-ray computed tomography images of laboratory mice '', IEEE Trans. Nucl. Sci ., Vol. 46, No. 4, pp. 1081-1086, 1999. Bruder H, McCollough C, et al., `` Hot Topic: Design of a 64-Slice Dual-Source CT (DSCT) Scanner '', Scientific session, RSNA 2005. Predrag Sukovic and Neal H. Clinthorne `` Penalized weited least-squares image reconstruction for dual energy X-ray transmission tomography '' IEEE Trans. Med.Imag., Vol. 19, No. 11, pp. 1075-1081, Nov 2000 . JA Fessler, IA Elbakri, P. Sukovic, and NH Clinthorne.``Maximum-likelihood dual-energy tomographic image reconstruction '' In Proc.SPIE Vol. 4684, p. 38-49, Medical Imaging 2002: Image Processing, Milan Sonka ; J. Michael Fitzpatrick; Eds., Pp. 38-49, May 2002.

  As methods included in (Prior Art 1), there are Patent Literature 1, Non-Patent Literature 1, and 2. Further, there are Patent Literature 1 and Non-Patent Literature 3 as methods included in (Prior Art 2). These methods of switching the tube voltage and the method of switching the filter basically require a plurality of times of scanning and take time, so that the exposure dose of the non-analyte increases and the movement in the non-analyte is vulnerable. Further, in order to switch these at high speed, it is necessary to newly add an accompanying advanced mechanism.

  With regard to (Prior Art 3), efforts have been made in Non-Patent Documents 4 and 5, and this method can obtain a precise reconstructed image, but an extremely large apparatus such as a synchrotron is required to generate synchrotron radiation. Many technical issues remain to make it necessary and popular.

  (Prior Art 4) is a method of performing measurement with a plurality of sensors simultaneously on an X-ray beam line, and is reported in Non-Patent Documents 6 and 7. In this method, data necessary for DEM can be acquired basically by one scanning. However, since X-rays are filtered after passing through the subject, the exposure dose of the subject increases.

  In addition, (Conventional Technology 5) can realize DEM without increasing the exposure dose by one scanning, and is introduced in Non-Patent Document 8, but it is necessary to perform wave height analysis for each sensor, which is very large. This makes adjustment difficult.

  In recent years (Non-Patent Document 9), the method of (Prior Art 6) has been put to practical use, is a special device, so it has to be newly introduced, and the assets of the popular device cannot be utilized.

  As described above, as a method for realizing DEM in X-ray CT, a method of switching a tube voltage or an absorption filter of a bulb during scanning, a method of using radiated light, a method of using a multistage sensor, or a sensor having an energy analysis function is used. A method and a method using a plurality of tubes have been proposed. However, these realization methods have problems such as requiring a large-scale device, taking a long time for scanning, not being able to utilize the assets of conventional devices, and increasing the exposure dose of the subject. It was difficult to convert.

  The present invention has been made in view of such problems, and an object thereof is to provide a computer-assisted X-ray tomography apparatus having a simple configuration and capable of easily realizing DEM. It is another object of the present invention to provide a computer-assisted X-ray tomography apparatus remodeling method capable of realizing asset utilization so that DEM can be realized with an existing computer-assisted X-ray tomography apparatus, and an absorption filter used for these.

  The solving means is arranged to face the X-ray source via an X-ray source that generates X-rays having an energy spectrum distribution in a predetermined energy region, and an imaging target space in which the subject is arranged. One or a plurality of sensors that detect the intensity of the X-ray that reaches from the X-ray source via the imaging target space, and the X-ray source and the sensor are arranged, and the X-ray reaches the sensor. An absorption filter that previously absorbs a portion of the X-ray and a first state in which the first X-ray radiated from the X-ray source passes through the first beam path and reaches one of the one or more sensors. On the other hand, the second X-ray emitted from the X-ray source passes through the second beam path that travels in the same place as the first beam path in the imaging target space, and passes through the second beam path. Or a second that reaches one of a plurality of sensors A moving scanning mechanism that moves the X-ray source and the sensor in a moving scanning pattern in which a state exists, and scans the imaging target space with the X-ray, about a virtual central axis in the imaging target space A computer-aided X-ray tomography apparatus including a rotational scanning mechanism including a rotational movement mechanism for rotationally moving the X-ray source and the sensor, wherein the absorption filter has a relative position with respect to the rotational movement mechanism. The first and second beam paths are fixed, and the first beam path and the second beam path are positioned in the absorption filter of the first beam path, and the second beam path, except when both the first beam path and the second beam path pass through the virtual central axis. Among the paths, the second absorption path located in the absorption filter is arranged so as not to overlap, and in the first state, the first or the plurality of sensors pass through the first absorption path. Energy spectrum distribution of the first X-ray reaching one of the above, and energy spectrum of the second X-ray reaching one of the one or more sensors through the second absorption path in the second state. This is a computer-aided X-ray tomography apparatus which is an absorption filter having a configuration in which the distribution is different from the other constant multiple.

The computer-aided X-ray tomography apparatus according to the present invention moves and scans an imaging target space with X-rays by moving an X-ray source and a sensor in a pattern in which a first state and a second state corresponding to the first state exist. It has a mechanism. The first state is a state in which the first X-ray reaches the sensor through the first beam path in the imaging target space, and the second state is the reverse of the same location as the first beam path in the second X-ray. It reaches the sensor through the second beam path traveling in the direction.
On the other hand, the absorption filter is arranged so that the first absorption path and the second absorption path do not overlap, and the energy spectrum distribution of the first X-rays that reach the sensor through the first absorption path and the second absorption path. Thus, the energy spectrum distribution of the second X-rays reaching the sensor is different from the other constant multiple.
Moreover, in the CT of the present invention, since the relative position of the absorption filter is fixed with respect to the rotational movement mechanism that rotates and moves the X-ray source and the sensor, The relative position of the source, sensor and absorption filter will not change.
In the CT according to the present invention as described above, the first X-ray and the second X-ray are travel directions opposite to each other even when the subject is placed and operated in the imaging target space, but the first X-ray and the second X-ray overlap each other. It travels on the first beam path and the second beam path, and passes through the same place in the imaging target space including the subject. Moreover, the distribution shape of the energy spectrum distribution of the first X-ray and the second X-ray reaching the sensor is different from the other constant multiple by the absorption filter. That is, the quality of the first X-ray and the second X-ray reaching the sensor are different.

Therefore, according to the CT of the present invention, for each part of the subject, information obtained by using the first X-ray and the corresponding second X-ray is used, and the tomographic image is regenerated by the dual energy method (DEM). If configured, the difference in the absorption coefficient of each part in the tomography of the subject, and hence the difference in the material of each part (for example, brain, bone, muscle, etc.) can be detected appropriately.
Moreover, in the CT of the present invention, it is not necessary to switch the tube voltage during the scanning as described in the prior art 1, and the absorption filter is mechanically operated during the scanning as described in the prior art 2. There is no need to switch to. Further, as described in the prior arts 3, 4, 5, and 6, it is not necessary to provide a large-scale device or a complicated mechanism. In addition, one time (360 degrees) of rotational scanning is sufficient, and the X-ray exposure dose to the subject is not increased. Thus, DEM can be easily realized with CT having a simple mechanism. In addition, the amount of information for diagnosis by CT examination is greatly increased by DEM, and the medical effect can be improved.

As a computer-aided X-ray tomography apparatus, any scanning method may be used as long as the CT satisfies the above-described requirements. Therefore, in addition to the so-called first generation CT, the third generation Also included is CT.
That is, a pencil beam X-ray source that generates thin rod-shaped X-rays (pencil beam) and a single sensor, called the first generation, are maintained in a state of being opposed to each other on linear scanning line segments parallel to each other. Applied to CT with a mechanism that scans linearly, and further rotates an X-ray source, a sensor, and a linear rail, linear movement mechanism, etc. for linear scanning to rotate around the virtual central axis of the imaging target space can do.
In addition, a fan beam X-ray source that generates fan-shaped X-rays (fan beams) and a plurality of sensors (sensor groups) arranged in an arc shape are rotated around a virtual central axis in the imaging target space. The present invention can also be applied to a CT having a scanning mechanism.
Further, the fan beam X-ray source and a plurality of sensors (sensor groups) arranged in an arc shape are rotated at high speed around the virtual central axis, and the subject is moved along the virtual central axis to perform spiral scanning (virtual central axis). In a CT capable of scanning by a so-called fan beam helical scan method (scanning by a combination of linear movement in the direction along the axis and rotational movement around the virtual center axis), the subject is moved in the direction along the virtual center axis. Without rotating the fan beam X-ray source, sensor, etc. (If necessary, the subject is moved in the direction along the virtual central axis, and then rotational scanning is performed again). When used, the present invention can be applied in the same manner as the third generation CT described above.
Also, an X-ray source called cone beam CT that generates cone-shaped X-rays (cone beams) and a plurality of sensors (sensor groups) arranged in an arc shape and a band shape around a virtual central axis in the imaging target space This also occurred when using a scanning method in which a cone beam X-ray source, a sensor, and the like are rotated without moving the subject in the direction along the virtual central axis of the CT. Of the X-rays, the present invention is applied to fan beam X-rays (hereinafter referred to as orthogonal fan beam X-rays) that travel along a plane orthogonal to the virtual central axis, as in the third generation CT described above. Can do. In the case of using a cone beam X-ray source, the cone beam X-ray source is substantially made by using a diaphragm (collimator) that suppresses radiation of X-rays other than the orthogonal fan beam X-ray among the generated X-rays. It may be used as a fan beam X-ray source. Alternatively, although cone beam X-rays are generated, it is also possible to perform processing in which a reconstructed image or the like by DEM is obtained only by data using orthogonal fan beam X-rays.

As the absorption filter, the energy spectrum distribution of the first X-ray that reaches the sensor through the first absorption path and the energy spectrum distribution of the second X-ray that reaches the sensor through the second absorption path are the other constants. It has a form with a distribution shape different from the double.
Specifically, for example, in the absorption filter, at least the material through which X-rays pass is constant (homogeneous), but the shape is different between the first absorption path and the second absorption path. Absorption filter. Moreover, although the length of the 1st absorption path and the 2nd absorption path is the same, an absorption filter is comprised with the material from which 2 or more types of absorption characteristics differ, and the part used as a 1st absorption path, and a 2nd absorption path Since the ratio of the length occupied by each material is made different in each portion, the energy spectrum distribution of the first X-ray and the second X-ray reaching the sensor is made different from the other constant multiple. There are also filters. Furthermore, since the above-mentioned combination, that is, the shape and each material of the absorption filter are made different between the portion serving as the first absorption path and the portion serving as the second absorption path, the first X-ray and the second X-ray reaching the sensor are different. There is also an absorption filter in which the distribution of energy spectrum is different from the other constant multiple.

In many CTs, a so-called bow-tie filter having a symmetrical shape is provided at the X-ray irradiation port between the X-ray source and the imaging target space. On the other hand, in the CT of the present invention, as long as the above-described characteristics are obtained as an absorption filter, the configuration other than that of the existing CT is sufficient. Therefore, it is preferable to install an absorption filter having a configuration in which the energy spectrum distribution of the first and second X-rays is the above-described distribution shape instead of the bow tie filter. Alternatively, an additional absorption filter may be additionally stacked on the bow tie filter, and these may be combined to form an absorption filter having a configuration in which the energy spectrum distribution of the first and second X-rays passing through them becomes the above-described distribution shape. .
Accordingly, the CT of the present invention is also newly designed or manufactured as a CT that satisfies the above requirements. However, the CT of the present invention can also be configured by replacing the bow tie filter used in this CT with an absorption filter that satisfies the above-mentioned conditions in the already manufactured CT. Alternatively, the CT of the present invention can also be configured by adding an additional absorption filter to an existing bow tie filter and forming an absorption filter that satisfies the above-described conditions as a whole.
Some CTs are provided with a plurality of absorption filters (bow tie filters), and the bow tie filters can be switched appropriately according to the measurement situation. In such a CT, an absorption filter that satisfies the requirements of the present invention is attached as one of a plurality of absorption filters that are connected, and this filter is selected when imaging by a DEM is required. You may make it do.

  Furthermore, the absorption filter should just be arrange | positioned between the X-ray source and the said sensor. Therefore, as described above, the absorption filter can be disposed between the X-ray source and the imaging target space (subject) and the imaging target space (subject) and the sensor. . Alternatively, absorption filters can be arranged between the X-ray source and the imaging target space (subject) and in both the imaging target space (subject) and the sensor.

  In the CT of the present invention, a cross-sectional image or the like that has been processed according to the material of each part in the subject is obtained by reconstructing a tomogram from the obtained data using DEM software. Obtainable. As software for realizing such a DEM and obtaining a reconstructed image or the like, known software or software obtained by appropriately changing the software may be used in accordance with the configuration and characteristics of each CT.

  Furthermore, in the computer-assisted X-ray tomography apparatus according to claim 1 or 2, wherein the first beam path and the second beam path of the absorption filter pass through the virtual central axis. When the first absorption path and the second absorption path that pass through the absorption filter coincide with each other and the portion corresponding to the first absorption path is a coincident portion, the absorption filter is at least in the vicinity of the coincident portion, The first X-ray energy spectrum distribution and the second X-ray energy spectrum distribution have an approximate distribution shape as the first absorption path and the second absorption path are brought closer to the coincident portion. A computer-assisted X-ray tomography apparatus is preferable.

First, let us consider a case where the first beam path and the second beam path both pass through the virtual central axis, and the first absorption path and the second absorption path coincide with each other in the absorption filter and pass through the coincident part. Then, in an ideal state in which the absorption filter is attached without positional deviation, an absorption filter having a form in which the X-ray absorption characteristic of the absorption filter changes sharply with the coincidence portion as a boundary is considered. For example, consider a case in which the absorption characteristic also changes in a step shape around the coincidence portion because the thickness changes in a step shape (in the form of a snake side function).
In this case, a problem arises in an ideal state where the absorption filter is accurately attached to the CT attachment portion, and the position where the absorption characteristics change stepwise and the coincident portion exactly match. Absent. For example, in the case of using an absorption filter whose absorption characteristic changes stepwise with respect to the position coordinate x = 0 of the absorption filter, the absorption filter accurately matches the CT so that the coincident portion coincides with the position coordinate x = 0. For example, it is attached.

  However, in reality, the ideal state may not be achieved, and there may be a case where a position shift occurs in the attachment position of the absorption filter. In this case, the coincident part in the absorption filter is also shifted from the original position. For this reason, even when the first beam path and the second beam path do not coincide with each other and do not pass through the virtual central axis, that is, when the DEM should be realized, the absorption characteristics of the absorption filter are the same. Thus, there occurs a portion where the energy spectrum distribution of the first X-ray and the energy spectrum distribution of the second X-ray have the same distribution shape. Then, in this part, DEM cannot be realized, and problems such as noise in the reconstructed image occur.

On the other hand, in the CT of the present invention, the form of the absorption filter is as described above. That is, the absorption characteristic of the absorption filter is changed gently at least in the region near the coincidence portion. Accordingly, even when the absorption filter is attached to the CT while being displaced or when the absorption filter is displaced during use, the energy spectrum distribution of the first X-ray and the energy of the second X-ray are obtained. DEM can be appropriately realized without having a distribution shape that matches the spectral distribution.
As a specific example, when the absorption characteristics of the absorption filter are changed depending on the X-ray transmission distance (the thickness of the portion through which X passes) in the absorption filter, the first beam path and the second beam path. Of the absorption filter through which X-rays pass in a region sandwiching a portion (matching portion) corresponding to the first absorption path and the second absorption path that coincide with each other when passing through the virtual central axis An absorption filter having a mode in which the thickness is gradually changed can be mentioned.

  In the absorption filter, as the first absorption path and the second absorption path are brought closer to the coincidence portion, the energy spectrum distribution of the first X-ray and the energy spectrum distribution of the second X-ray have an approximate distribution shape. As described above, the range may be at least in the vicinity of the coincidence portion as described above. Thereby, when the magnitude | size of the actual position shift which arose in the absorption filter is a range smaller than the said vicinity range, DEM can be implement | achieved appropriately. More preferably, when the attachment position of the absorption filter is displaced, a range where a portion that is supposed to be a coincident portion can be disposed, that is, the largest range in which the attachment position can be displaced (for example, a tolerance range). good. In this way, it is possible to eliminate a problem that a portion where DEM cannot be realized due to the displacement of the absorption filter. Furthermore, it is preferable to adopt the above-described form within a range including this range. Furthermore, the above-described configuration is preferable over the entire range of the absorption filter.

  Alternatively, in the computer-assisted X-ray tomography apparatus according to claim 1, the X-ray source is a pencil beam X-ray source that generates the X-ray in a thin rod shape, and the moving scanning mechanism is the pencil A pencil moving X-ray source and a sensor when the pencil beam X-ray source and the sensor are rotated by the rotary moving mechanism. An arbitrary X-ray emitted from the pencil beam X-ray source linearly moved by the linear movement mechanism and perpendicular to the linear movement trajectory of the pencil beam X-ray source in a virtual rotation plane including As a position X-ray, a center perpendicular line emitted from the pencil beam X-ray source, parallel to the reference position X-ray, and taken from the virtual central axis to the linear movement locus is When an X-ray passing through a position symmetric with respect to the reference position X-ray is a symmetric position X-ray, the absorption filter has the energy spectrum distribution of the reference position X-ray and the symmetric position X-ray after passing through the absorption filter. It is preferable to use a computer-aided X-ray tomography apparatus having a distribution shape different from the other constant multiple.

The CT of the present invention is an invention relating to a CT having a so-called first generation configuration for an X-ray source, a sensor, a moving scanning mechanism, and the like. In the CT of the present invention, an absorption filter is used in which the energy spectrum distribution of the reference position X-ray and the symmetric position X-ray is different from the other constant multiple. That is, the reference position X-ray and the symmetric position X-ray transmitted through the absorption filter are absorption filters having a form in which the quality of the radiation is different from each other.
Therefore, according to the CT of the present invention using such an absorption filter, it is possible to appropriately realize the DEM in the first generation CT in which the pencil beam X-ray source and the sensor are linearly moved and rotationally moved.

  Further, in the above-described computer-aided X-ray tomography apparatus, the absorption filter absorbs the reference position X-ray and the symmetric position X-ray closer to the central perpendicular at least in the vicinity of the central perpendicular. It is preferable that the computer-assisted X-ray tomography apparatus has a form in which the energy spectrum distribution of the reference position X-ray after passing through the filter and the energy spectrum distribution of the symmetric position X-ray are approximated.

  By configuring the absorption filter as described above, in the first generation CT, when the absorption filter is attached to the CT, the absorption filter is attached in a misaligned state, or the absorption filter is misaligned during use. Even in such a case, the DEM can be appropriately realized.

  In the absorption filter, as described above, the reference position X-ray and the symmetric position X-ray are closer to the central perpendicular, and the range in which these energy spectrum distributions approximate to the distribution shape is at least the center as described above. It may be in the vicinity of the perpendicular. Thereby, when the magnitude | size of the actual position shift which arose in the absorption filter is a range smaller than the said vicinity range, DEM can be implement | achieved appropriately. More preferably, when the mounting position of the absorption filter is displaced, the range in which the central perpendicular can be arranged, that is, the largest range (for example, a tolerance range) in which the mounting position can be displaced is good. If it does in this way, the malfunction which the site | part which cannot implement | achieve DEM by the position shift of attachment of an absorption filter can be eliminated. Furthermore, it is preferable to adopt the above-described form within a range including this range. Furthermore, the above-described configuration is preferable over the entire range of the absorption filter.

Alternatively, in the computer-assisted X-ray tomography apparatus according to claim 1, the moving scanning mechanism includes a linear moving mechanism that linearly moves the X-ray source and the sensor, and the X-ray source has a thin rod shape. A pencil beam X-ray source that generates the X-ray, and when the X-ray source and the sensor are rotated by the rotational movement mechanism, in a virtual rotation plane including the X-ray source and the sensor, The coordinate in the direction along the linear movement locus of the pencil beam X-ray source moved linearly by the linear movement mechanism is t, and the foot coordinates of the central perpendicular line taken from the virtual central axis to the linear movement locus are reference points ( When t = 0), the absorption filter has a form satisfying the following formula (C1) with respect to the thickness h (t) in the direction perpendicular to the linear movement locus at the coordinate t: h (t) ≠ h ( -T) (t 0) ... (C1)
A computer-assisted X-ray tomography apparatus is preferable.

In the CT of the present invention, the thickness in the direction perpendicular to the linear movement locus of the absorption filter, that is, the X-ray transmission thickness satisfies the formula (C1): h (t) ≠ h (−t). . Specifically, it has an asymmetric form at any location where t ≠ 0.
Therefore, in the first state, the energy spectrum distribution of the first X-ray that reaches the sensor through the first absorption path, and in the second state, the energy spectrum distribution of the second X-ray that reaches the sensor through the second absorption path Can have a distribution shape different from the other constant multiple. Thereby, DEM is realizable using the 1st X ray and the 2nd X ray.
Note that the absorption filter is preferably made of a uniform material and density at least in a portion through which X-rays pass.

  5. The computer-aided X-ray tomography apparatus according to claim 4, wherein the absorption filter is configured such that the thickness h (t) gradually changes before and after the coordinate t = 0. A assisted X-ray tomography apparatus is preferred.

In this CT, the thickness of the absorption filter gradually changes before and after the coordinate t = 0. Accordingly, even when the absorption filter is attached to the CT while being displaced or when the absorption filter is displaced during use, the energy spectrum distribution of the first X-ray and the energy of the second X-ray are obtained. DEM can be appropriately realized without having a distribution shape that matches the spectral distribution.
The range before and after the coordinate t = 0 in the absorption filter can be selected as appropriate, but the attachment position of the absorption filter can be displaced in the direction along the linear movement locus (that is, the direction of the coordinate t). The largest range (for example, a tolerance range) is preferable. In this way, it is possible to eliminate a problem that a portion where DEM cannot be realized due to the displacement of the absorption filter. Moreover, it is preferable to use the above-mentioned form within a range including this range. Furthermore, it is preferable to adopt the above-described form over the entire range of the absorption filter.

  Furthermore, in the computer-assisted X-ray tomography apparatus according to claim 4, the absorption filter has a form in which the thickness h (t) increases toward an edge side at both ends in a direction along the linear movement locus. It is preferable that the computer-assisted X-ray tomography apparatus be configured.

  In the CT of the present invention, the amount of X-ray irradiation tends to increase at the outer periphery of the subject. Therefore, in the CT of the present invention, the thicknesses h (t) and h (−t) are increased toward the end edge at both ends in the direction along the linear movement locus while maintaining the relationship of h (t) ≠ h (−t). ) Becomes larger. Thereby, it is possible to suppress the X-ray irradiation amount on the outer periphery of the subject while realizing DEM.

  Alternatively, in the computer-assisted X-ray tomography apparatus according to claim 1, the X-ray source is a fan beam X-ray source that radiates the X-ray in a fan shape, and the fan beam X-ray In the sector formed by the X-rays radiated from the source, two X-rays having arbitrary central angles symmetrical to each other with respect to the source-center axis connecting the X-ray source and the virtual central axis are expressed as a reference angle. When the X-ray and the symmetric angle X-ray are used, the absorption filter has a distribution shape different from the other constant multiple of the energy spectrum distribution of the reference angle X-ray and the symmetric angle X-ray after passing through the absorption filter. A computer-assisted X-ray tomography apparatus having a configuration may be used.

The CT of the present invention is an invention of CT using a fan beam X-ray source. In the CT of the present invention, an absorption filter is used in which the energy spectrum distribution of the reference angle X-ray and the symmetry angle X-ray is different from the other constant multiple. That is, the absorption filter has a form in which the reference angle X-ray and the symmetric angle X-ray transmitted through the absorption filter have different quality.
Therefore, according to the CT of the present invention using a fan beam X-ray source and such an absorption filter, DEM can be appropriately realized.

  The computer-assisted X-ray tomography apparatus according to claim 6, wherein the absorption filter generates the reference angle X-ray and the symmetry angle X-ray at least in the vicinity of a radiation source-center axis line. -Computer-aided X-ray tomography having a form in which the energy spectrum distribution of the reference angle X-ray after passing through the absorption filter approximates the energy spectrum distribution of the symmetric angle X-ray as it approaches the central axis. An apparatus is preferred.

  Even when the absorption filter is attached to the CT in the CT as described above, the absorption filter is attached in a misaligned state, or even if the absorption filter is misaligned during use. DEM can be realized appropriately.

  As described above, the range of the absorption filter in which the reference angle X-ray and the symmetry angle X-ray are close to the radiation source-center axis and the energy spectrum distribution becomes a distribution shape is approximated as described above. , At least near the source-center axis. Thereby, when the magnitude | size of the actual position shift of an absorption filter is a range smaller than the range of the said vicinity, DEM is appropriately realizable. More preferably, when the attachment position of the absorption filter is displaced, the range in which the radiation source-center axis line can be arranged, that is, the largest range (for example, a tolerance range) in which the attachment position can be displaced is set. In this way, it is possible to eliminate a problem that a portion where DEM cannot be realized due to the displacement of the absorption filter occurs. Furthermore, it is preferable to adopt the above-described form within a range including this range. Furthermore, it is good to use the above-mentioned form in the whole range of the absorption filter.

3. The computer-assisted X-ray tomography apparatus according to claim 1, wherein the X-ray source is a fan beam X-ray source that radiates the X-ray in a fan shape, and the absorption filter is the fan shape. In the fan shape formed by the X-ray, the fan beam X-ray source passes through the fan beam X-ray source and the reference angle straight line that forms the central angle φ with the source-center axis line connecting the fan beam X-ray source and the central axis is along this line. When the thickness in the direction is h (φ), the following formula (C2) is satisfied. H (φ) ≠ h (−φ) (φ> 0) (C2)
A computer-assisted X-ray tomography apparatus is preferable.

In the CT of the present invention, the thickness of the absorption filter in the direction along the reference angle straight line, that is, the transmission thickness of X-rays, satisfies the formula (C2): h (φ) ≠ h (−φ). Specifically, it has a form that is asymmetric with respect to the angles φ and −φ at any place where φ ≠ 0.
Therefore, in the first state, the energy spectrum distribution of the first X-ray that reaches the sensor through the first absorption path, and in the second state, the energy spectrum distribution of the second X-ray that reaches the sensor through the second absorption path Can have a distribution shape different from the other constant multiple. Thereby, DEM is realizable using the 1st X ray and the 2nd X ray.
Note that the absorption filter is preferably made of a uniform material and density at least in a portion through which X-rays pass.

The computer-assisted X-ray tomography apparatus according to claim 7,
The absorption filter is
A computer-aided X-ray tomography apparatus in which the thickness h (φ) gradually changes before and after the central angle φ = 0.

In this CT, the thickness of the absorption filter gradually changes before and after the central angle φ = 0. It is a form that changes slowly. Accordingly, even when the absorption filter is attached to the CT while being displaced or when the absorption filter is displaced during use, the energy spectrum distribution of the first X-ray and the energy of the second X-ray are obtained. DEM can be appropriately realized without having a distribution shape that matches the spectral distribution.
Of the absorption filters, the range before and after the central angle φ = 0 can be selected as appropriate, but the mounting position of the absorption filter is displaced in the direction in which the central angle changes (that is, the direction of the central angle φ). The largest possible range (for example, a tolerance range) is preferable. In this way, it is possible to eliminate a problem that a portion where DEM cannot be realized due to the displacement of the absorption filter. Moreover, it is preferable to make it the above-mentioned form in the range including this range. Furthermore, the above-described configuration is preferable over the entire range of the absorption filter.

  Furthermore, in the computer-assisted X-ray tomography apparatus according to claim 7, the absorption filter has the thickness closer to the edge side at both ends of the fan-shaped circumferential direction in which the central angles φ and −φ have large values. A computer-assisted X-ray tomography apparatus in which h (φ) and h (−φ) are increased may be used.

  In the CT of the present invention, the amount of X-ray irradiation tends to increase at the outer periphery of the subject. Therefore, while maintaining the above-described relationship of h (φ) ≠ h (−φ), the thicknesses h (φ) and h (−φ) are increased toward the edge side at both ends in the circumferential direction of the sector. Thus, the X-ray irradiation amount on the outer periphery of the subject can be suppressed while making DEM realizable.

Furthermore, an X-ray source that generates X-rays having a wide energy spectrum distribution, and an absorption filter that simultaneously satisfies the following conditions (a) and (b) are provided. Further, the spatial distribution of the X-ray absorption coefficient in the subject for each energy CT provided with software for the purpose of obtaining a reconstructed image is a means for solving the problem.
(a) X-rays incident on the sensor when there is no X-ray absorption of the subject with respect to X-rays along a certain straight line in the imaging target space and X-rays along the same straight line but having the opposite emission direction The spectrum of the lines is different from the other constant multiple.
(b) The absorption filter is a component that performs only a rotational motion having a center and an angular velocity common to the rotational scanning among the components of the CT (for example, a linear scanning line segment of an X-ray source, an X-ray source that performs only rotational scanning, The relative position is fixed with respect to a sensor group that performs only rotational scanning, and a rotary pulley that is a base for fixing them.

Specifically, an X-ray source that generates X-rays having an energy spectrum distribution within a predetermined energy region, and an X-ray source that is disposed through an imaging target space in which the subject is disposed, One or a plurality of sensors that detect the intensity of the X-ray that reaches from the X-ray source via the imaging target space, and the X-ray source and the sensor are arranged, and the X-ray reaches the sensor. A computer-aided X-ray tomography apparatus comprising: an absorption filter that previously absorbs part of the X-rays and simultaneously satisfies the following conditions (a) and (b):
(a) The absorption filter includes the forward X-ray that travels in the imaging target space and the reverse X-ray that is along the same straight line as the straight line along which the forward X-ray is directed but whose emission direction is reversed. Except for the case where a straight line passes through the virtual center axis below, when there is no X-ray absorption in the subject, the energy spectrum distributions of the forward X-ray and the retrograde X-ray incident on the sensor The distribution shape is different from a constant multiple of.
(b) The absorption filter has a virtual center axis common to rotational scanning for rotating the X-ray source and the sensor among the components constituting the computer-assisted X-ray tomography apparatus, and performs only rotational motion. The relative position with respect to the component is fixed.

According to the CT of the present invention, since the relative position of the absorption filter is fixed with respect to the component that performs only the rotational motion, the relative position is maintained even if these are rotated with respect to the X-ray source, the sensor, and the like. Does not change. Further, the forward X-ray and the reverse X-ray that have passed through the absorption filter have different energy spectrum distributions from the other constant multiple, that is, the forward X-ray and the reverse X-ray have a quality of radiation. Is different.
Therefore, according to the CT of the present invention, if the tomographic image is reconstructed by DEM using information obtained by using the forward X-ray and the corresponding retrograde X-ray for each part of the subject, It is possible to appropriately detect the difference in the absorption coefficient of each part in the tomographic specimen, and hence the difference in the material (for example, brain, bone, muscle, etc.) of each part.
Moreover, in the CT of the present invention, there is no need to switch the tube voltage during the scanning as described in the prior art 1, and as described in the prior art 2, the absorption filter is mechanically operated during the scanning. There is no need to switch to. Further, as described in the prior arts 3, 4, 5, and 6, it is not necessary to provide a large-scale device or a complicated mechanism. In addition, one time (360 degrees) of rotational scanning is sufficient, and the X-ray exposure dose to the subject is not increased. Thus, DEM can be easily realized with CT having a simple mechanism. In addition, the amount of information for diagnosis by CT examination is greatly increased by DEM, and the medical effect can be improved.

  Alternatively, with respect to many existing CTs, a symmetrical bow-tie type absorption filter called a bow tie filter that is installed at the X-ray irradiation port and already satisfies the above condition (b) is replaced with the above condition (a). The CT of the present invention can also be realized by replacing with a satisfying one or by overlapping an absorption filter that satisfies the condition (a) on the bow tie filter and installing both filters so that X-rays pass through.

As a component that performs only rotational movement, for example, a first generation CT, that is, a thin rod-shaped X-ray (pencil) is used to obtain a tomographic image of a plane orthogonal to the virtual central axis in the imaging target space. A pencil beam X-ray source and a single sensor that emit a beam) are linearly scanned in a state of being opposed to each other on linear scanning lines parallel to each other, and further, a pencil beam X-ray source, a sensor, and these In CT having a mechanism for rotating and scanning a linear movement mechanism such as a linear rail for linear scanning with respect to the center on the virtual center axis of the imaging target space, a pencil beam X-ray source, a sensor, Examples include a linear rail (linear scanning line segment) for linear scanning with these mounted, and a rotary scanning mechanism such as a rotating pulley that rotates while rotating these around a virtual central axis. .
On the other hand, a fan beam X-ray source that emits fan-shaped X-rays (fan beams) and an arc are arranged to obtain a tomographic image of a plane orthogonal to the virtual central axis in the imaging target space, such as third generation CT. In a CT having a mechanism for rotating and scanning a plurality of sensors (sensor groups) with respect to the center on the virtual central axis in the imaging target space, a fan beam X-ray source, sensors (sensor groups), and Examples include a rotary scanning mechanism such as a rotating pulley that rotates while being fixed around a virtual central axis.
A cone beam CT having a mechanism for rotating and scanning a cone beam X-ray source that emits a cone beam and a sensor group arranged in an arc shape and a band shape with respect to the center on the virtual central axis in the imaging target space is also used in the cone beam CT. Examples include a beam X-ray source, a sensor (sensor group), and a rotary scanning mechanism such as a rotary pulley that rotates while fixing these around a virtual central axis.

Further, in the above-described CT, it is preferable to use a computer-aided X-ray tomography apparatus provided with software for obtaining a reconstructed image of the spatial distribution for each energy of the X-ray absorption coefficient in the subject.
Alternatively, software for performing DEM can be added to the computer software used for the existing CT. In this way, while using the existing CT, the tomographic image for each energy and the tomographic image appropriately representing the difference in the material of each part (for example, brain, bone, muscle, etc.), which was not found in the popular CT. The obtained functions can be realized.

  One of the features of the present invention is that forward X-rays traveling along a certain beam line in order to obtain two X-rays having different radiation qualities while being X-rays traveling on the same beam line, The point is to use the retrograde X-ray that travels in the beam line with the opposite X-ray traveling direction and the geometric positional relationship of the absorption filter. This makes it possible to perform a single scan with only a very simple mechanical change. Can realize DEM. Hereinafter, beam lines having opposite X-ray emission directions and passing through the same portion are referred to as opposite beam lines.

  Further, the DEM can be realized even if the absorption filter is installed between the subject and the sensor. However, in this case, X-rays are attenuated by absorption of the absorption filter after passing through the subject. For this reason, in order to obtain the same image quality, the X-ray dose has to be relatively increased as compared with the case where it is placed between the subject and the X-ray source (when placed at the same position as the bow tie filter), The exposure dose of the subject increases. Therefore, if the absorption filter is installed between the subject and the X-ray source, it is possible to relatively suppress the increase in the X-ray exposure dose of the subject and realize various DEMs shown in the prior art. Can solve the problems of law.

  Further, if the CT is the above-described CT and has linear scanning and rotational scanning, and the X-ray source belongs to the first generation that generates a pencil beam, any X perpendicular to the linear scanning line in the plane of rotation can be obtained. An absorption filter having a property that the spectrum after passing is different from the other constant multiple of the X-ray passing through a position parallel to the X-ray and symmetric with respect to the perpendicular at the midpoint of the linear scan Is also a means to solve the problem.

  Specifically, in the computer-aided X-ray tomography apparatus according to claim 9, a linear scanning mechanism that linearly moves the X-ray source and the sensor, the X-ray source, the sensor, and the linear scanning. A rotary scanning mechanism that rotates at least a part of the mechanism around the virtual central axis, and the X-ray source is a pencil beam X-ray source that generates the X-ray in the form of a thin rod, and the rotational scanning When the pencil beam X-ray source and the sensor are rotated by a mechanism, the pencil is moved on the linear scanning line by the linear scanning mechanism in a virtual rotation plane including the pencil beam X-ray source and sensor. An arbitrary X-ray generated from the beam X-ray source and perpendicular to the linear scanning line is defined as a reference position X-ray, parallel to the reference position X-ray, and within the linear scanning range of the pencil beam X-ray source. When the X-ray passing through a position symmetric with respect to the reference position X-ray is a symmetric position X-ray with respect to the midpoint perpendicular line at the point, the absorption filter is symmetrical with the reference position X-ray after passing through the absorption filter. A computer-aided X-ray tomography apparatus having a configuration in which the energy spectrum distribution of the lines is different from the other constant multiple is preferably used.

The CT of the present invention is an invention relating to a so-called first generation CT having a pencil beam X-ray source, a sensor, a linear scanning mechanism, a rotational scanning mechanism, and the like. In the CT of the present invention, an absorption filter is used in which the energy spectrum distribution of the reference position X-ray and the symmetric position X-ray is different from the other constant multiple. That is, the reference position X-ray and the symmetric position X-ray transmitted through the absorption filter are absorption filters having a form in which the quality of the radiation is different from each other.
Therefore, according to the present invention, it is possible to appropriately realize the DEM for the first generation CT in which the pencil beam X-ray source and the sensor are linearly moved and rotated.

Still further, in the above-described CT, it is preferable that the midpoint of the linear scanning range of the pencil beam X-ray source coincides with the leg of the central perpendicular line drawn from the virtual central axis to the linear scanning line. .
This is because the balance of the linear scanning range is most appropriate.

In addition, an absorption filter satisfying such a property has some kind of asymmetry. For example, if this is made of a uniform material (for example, metal or resin such as homogeneous aluminum or copper), specifically, if at least a portion through which X-rays are transmitted is created, it is within the range of the linear scanning line range. The absorption filter has different thicknesses at two symmetrical positions with respect to the point.
Among the first generation CTs, the CT of the present invention can be realized not only by newly manufactured ones but also by existing absorption CTs and by installing an absorption filter satisfying such properties. In addition, when data processing is performed in an existing CT, the above-described tomographic image for each energy can be obtained only by adding DEM software.

  Alternatively, if the above-mentioned CT has only rotational scanning and the X-ray source belongs to the third generation that generates a fan beam, on any two X-rays that are symmetric with respect to the center line of the X-ray fan shape A problem-solving means is also provided with an absorption filter whose spectrum after passing is different from the other constant multiple.

  Specifically, in the computer-aided X-ray tomography apparatus according to claim 9, comprising a rotary scanning mechanism that performs scanning by rotating the X-ray source and the sensor around the virtual central axis, The X-ray source is a fan beam X-ray source that generates the X-rays in a fan shape, and any two X-rays having mutually symmetrical central angles with respect to the fan-shaped center line formed by the X-rays, When the reference angle X-ray and the symmetric angle X-ray are used, the absorption filter distributes the energy spectrum distribution of the reference angle X-ray and the symmetric angle X-ray after passing through the absorption filter different from the other constant multiple. A computer-assisted X-ray tomography apparatus having a shape is preferable.

The CT of the present invention is an invention relating to a CT having a so-called third generation configuration for an X-ray source, a sensor, a moving scanning mechanism, and the like. In the CT of the present invention, an absorption filter is used in which the energy spectrum distribution of the reference angle X-ray and the symmetry angle X-ray is different from the other constant multiple. That is, the absorption filter has a form in which the reference angle X-ray and the symmetric angle X-ray transmitted through the absorption filter have different quality.
Therefore, according to the present invention, it is possible to appropriately realize the DEM for the third generation CT that rotationally moves the fan beam X-ray source and the sensor.

  The third generation CT is the most prevalent CT and is used in numerous medical facilities. If such a facility can easily obtain a tomographic image of each energy, it is considered to have an unmeasurable medical effect. The present invention has such a function even in the third generation CT only by replacing an existing bow tie filter with an absorption filter that satisfies the above conditions, or by simply installing an absorption filter that satisfies the above conditions on the bow tie filter. CT can be provided.

Still further, in the above-described CT, it is preferable that the sector-shaped center line is configured to pass through the virtual central axis.
This is because the balance of the radiation range of X-rays emitted from the fan beam X-ray source is most appropriate.

  The computer-assisted X-ray tomography apparatus according to any one of claims 1 to 11, wherein the absorption filter is disposed between the X-ray source and the imaging target space. A computer-assisted X-ray tomography apparatus is preferable.

  In the CT of the present invention, the absorption filter is arranged on the X-ray source side of the subject, so that the sensor detects X-rays having the same intensity as compared with the case where the absorption filter is arranged on the sensor side of the subject. However, the X-ray exposure dose in the subject can be reduced.

The absorption filter may be disposed at an appropriate portion between the X-ray source and the imaging target space. Specifically, for example, for example, an X provided between the X-ray source and the imaging target space is used. The form which arrange | positions an absorption filter so that a line | wire irradiation opening may be obstruct | occluded is mentioned.
Furthermore, the absorption filter is composed of one filter, and the two filters can be overlapped or spaced apart to form one absorption filter as a whole. Specifically, a combination of a bow-tie-shaped bow-tie filter having a symmetrical shape with respect to the center line and an asymmetric filter having an asymmetric thickness with respect to the center line or the like can be given.

  Still another solution is an X-ray source that generates X-rays having an energy spectrum distribution within a predetermined energy region, and an X-ray source that is disposed through an imaging target space in which the subject is arranged. One or a plurality of sensors for detecting the intensity of the X-ray that reaches from the X-ray source via the imaging target space, and the first X-ray emitted from the X-ray source passes through the first beam path and For the first state that reaches one of the one or more sensors, the second X-ray emitted from the X-ray source is located in the same space as the first beam path in the imaging target space. The X-ray source and the sensor are moved in a moving scanning pattern in which a second state exists that reaches one of the one or more sensors through the second beam path that travels in the opposite direction, and the imaging is performed. Scan the target space with the X-ray A computer-aided X-ray tomography apparatus comprising: a moving scanning mechanism including a rotational scanning mechanism that rotationally moves the X-ray source and the sensor around a virtual central axis in the imaging target space The absorption filter that absorbs a part of the X-ray is disposed between the X-ray source and the sensor, the relative position with respect to the rotational movement mechanism is fixed, and the first Except for the case where both the first beam path and the second beam path pass through the virtual center axis, the first absorption path located in the absorption filter in the first beam path and the absorption in the second beam path. A computer-aided X-ray tomography apparatus remodeling method that is arranged so as not to overlap a second absorption path located in a filter, wherein the absorption filter has the first X-ray and the second X-ray in the first state, Up The energy spectrum distribution of the first X-ray that reaches one of the one or more sensors through the first absorption path, and the one or more through the second absorption path in the second state. The computer-aided X-ray tomography apparatus is remodeled in such a manner that the energy spectrum distribution of the second X-ray reaching one of the sensors is different from the other constant multiple. .

As described above, in the existing CT, the first X-ray passing through the first beam path and the second X-ray passing through the second beam path have the same energy spectrum distribution. Therefore, a tomographic image is reconstructed by DEM. Thus, energy information, that is, material of each part (for example, differences in brain, bone, muscle, etc.) could not be detected.
On the other hand, according to the CT remodeling method of the present invention, the relationship between the first X-ray and the second X-ray can be set as described above by installing an absorption filter that satisfies the above-described conditions in the existing CT. it can.
Accordingly, if such first X-ray and second X-ray are detected by a sensor and a tomographic image is reconstructed by DEM using the obtained data, energy information, that is, the material of each part (for example, brain, bone, etc.) , Differences in muscles, etc.) can be detected.

In addition, when a bow tie filter is already attached to the CT in the modification of the computer-aided X-ray tomography apparatus, in addition to installing an absorption filter instead of this, an additional absorption filter is installed together with this bow tie filter, In total, the modification method of the present invention can be realized by using the above-described absorption filter. In CT having a plurality of absorption filters (bow tie filters) and having a function of selecting an appropriate bow tie filter according to the measurement, at least one of the plurality of bow tie filters absorbs the above-mentioned requirements. Remodeling can also be realized by replacing the filter.
As the computer-assisted X-ray tomography apparatus to be modified, any CT of fan beam helical scan CT and cone beam CT in addition to the first and third generation CTs can be applied.
Further, the arrangement position of the absorption filter may be between the X-ray source and the sensor, but for example, an arrangement in which the absorption filter is arranged between the X-ray source and the imaging target space and between the imaging target space and the sensor can be mentioned. . Further, the absorption filter can be arranged in these two places.

  Still another solution is an X-ray source that generates X-rays having an energy spectrum distribution within a predetermined energy region, and an X-ray source that is disposed through an imaging target space in which the subject is arranged. One or a plurality of sensors for detecting the intensity of the X-ray that reaches from the X-ray source via the imaging target space, and the first X-ray emitted from the X-ray source passes through the first beam path and For the first state that reaches one of the one or more sensors, the second X-ray emitted from the X-ray source is located in the same space as the first beam path in the imaging target space. The X-ray source and the sensor are moved in a moving scanning pattern in which a second state exists that reaches one of the one or more sensors through the second beam path that travels in the opposite direction, and the imaging is performed. Scan the target space with the X-ray A computer-aided X-ray tomography apparatus comprising: a moving scanning mechanism including a rotational scanning mechanism that rotationally moves the X-ray source and the sensor around a virtual central axis in the imaging target space The absorption filter is disposed between the X-ray source and the sensor, and absorbs a part of the X-ray before the X-ray reaches the sensor. When the first beam path and the second beam path both pass through the virtual central axis when arranged in a predetermined position at a position relative to the rotational movement mechanism, the first beam is excluded. The first absorption path located in the absorption filter in the path and the second absorption path located in the absorption filter in the second beam path are arranged so as not to overlap, and in the first state, The energy spectrum distribution of the first X-ray that reaches one of the one or the plurality of sensors through the first absorption path, and in the second state, the first or It is an absorption filter used in a computer-aided X-ray tomography apparatus having a form in which the energy spectrum distribution of the second X-ray reaching one of a plurality of sensors is different from the other constant multiple.

Since the relative position of the absorption filter of the present invention is fixed to a rotational movement mechanism that rotates and moves the X-ray source and the sensor and the X-ray source and sensor are rotated and moved in CT, The relative positions of the radiation source and sensor and the absorption filter do not change.
In addition, the absorption filter has an arrangement in which the first absorption path and the second absorption path do not overlap each other, and the distribution of the energy spectrum of the first X-ray that reaches the sensor through the first absorption path and the second absorption path The distribution of the energy spectrum of the second X-ray that passes through the sensor and has a different distribution shape from the other constant multiple.
Specifically, for example, the shape of the absorption filter has a different length between the first absorption path and the second absorption path, and thereby the energy spectrum of the first X-ray that reaches the sensor through the first absorption path. And the distribution of the energy spectrum of the second X-ray that reaches the sensor through the second absorption path have different distribution shapes from the other constant multiples.

For this reason, when the absorption filter of the present invention is used for CT and the subject is placed in the imaging target space and the CT is operated, the traveling directions of the first X-ray and the second X-ray are opposite to each other, It travels on the overlapping first beam path and second beam path and passes through the same part in the imaging target space. However, the distribution shape of the energy spectrum distribution of the first X-rays and the second X-rays reaching the sensor is different from the other constant multiple by the absorption filter. That is, the quality of the first X-ray and the second X-ray reaching the sensor are different.
Therefore, this absorption filter is used for the manufacture of a new CT or an existing (existing) CT, and the obtained data is used for each part of the subject by DEM, specifically using DEM software. By reconstructing a tomographic image, it is possible to detect energy information, that is, a difference in material (for example, brain, bone, muscle, etc.) of each part in the tomography of the subject.

  Still another solution is a computer-aided X-ray tomography apparatus that is arranged between a fan beam X-ray source that generates X-rays in a fan shape to an imaging target space, and a plurality of sensors from the fan beam X-ray source. An absorption filter that absorbs part of the X-rays emitted toward the computer, and in the state where the filter is disposed in the computer-aided X-ray tomography apparatus, the fan beam X-ray source and the virtual central axis of the imaging target space The reference angle X after passing through the absorption filter when two X-rays having an arbitrary central angle symmetrical with respect to the source-center axis connecting the two are defined as a reference angle X-ray and a symmetry angle X-ray It is an absorption filter having a form in which the energy spectrum distribution of the line and the symmetrical angle X-ray is different from the other constant multiple.

When the absorption filter of the present invention is arranged in the CT, the energy spectrum distribution of the reference angle X-ray and the symmetry angle X-ray after passing through the absorption filter for the reference angle X-ray and the symmetry angle X-ray is mutually multiplied by the other constant. And has a different distribution shape. That is, the quality of the reference angle X-ray after passing through the absorption filter and the symmetrical angle X-ray are different.
Therefore, this absorption filter is used for the manufacture of a new CT or for an existing (existing) CT, and the obtained data is analyzed by DEM, specifically using DEM software. By reconstructing a tomographic image for each part, it is possible to detect energy information, that is, a difference in material (for example, brain, bone, muscle, etc.) of each part in the tomogram of the subject.

Still another solution is a computer-aided X-ray tomography apparatus that is arranged between a fan beam X-ray source that generates X-rays in a fan shape to an imaging target space, and a plurality of sensors from the fan beam X-ray source. An absorption filter that absorbs part of the X-rays emitted toward the computer, and in the state where the filter is disposed in the computer-aided X-ray tomography apparatus, the fan beam X-ray source and the virtual central axis of the imaging target space With respect to a reference angle line that forms a central axis and a central angle φ with respect to the source line connecting the central axis and the central angle φ, when the thickness in the direction along this is h (φ), the following formula (C2) is satisfied: h (φ) ≠ h (−φ) (φ> 0) (C2)
Absorption filter.

When the absorption filter of the present invention is arranged in CT, the thickness h (φ) in the direction along the reference angle line is asymmetric with respect to the central angle φ with respect to the source-center axis. Has been. If this absorption filter is used for CT, the energy spectrum distribution of the two X-rays having the central angles φ and −φ after passing through the absorption filter can be made to have a distribution shape different from the other constant multiple. That is, after passing through the absorption filter, the two X-rays have different quality.
Therefore, this absorption filter is used for the manufacture of a new CT or for an existing (existing) CT, and the obtained data is analyzed by DEM, specifically using DEM software. By reconstructing a tomographic image for each part, it is possible to detect energy information, that is, a difference in material (for example, brain, bone, muscle, etc.) of each part in the tomogram of the subject.

Further, an absorption filter design method provided for the above-described CT, which includes all of the following procedures, provides a higher quality solution.
(Procedure 1) The filter shape and material are expressed by a finite number of parameters.
(Procedure 2) An evaluation function having both or one of tomographic reconstruction accuracy and exposure dose as elements is set.
(Procedure 3) The filter parameters are determined using a computer simulation and a numerical optimization method in consideration of the mechanical accuracy of the apparatus and the absorption filter mounting accuracy so that the evaluation function is minimized.

  By designing the absorption filter according to this, it is possible to make the most of the effect. The present invention can be applied not only to an absorption filter to be mounted on a newly manufactured CT, but also to an absorption filter for replacement of an existing CT installed in each medical facility. The mechanical accuracy of existing CT and the mounting accuracy of the absorption filter vary depending on aging and the skill of the surgeon. With this design method, it is possible to immediately adapt to each existing CT for such individual constraints. Therefore, it is possible to design an absorption filter for replacement and enhance the medical effect.

  According to the CT of the present invention, a DEM can be realized by installing an absorption filter having a simple configuration in the CT. In addition, in a widely used existing CT that does not allow DEM, the tube voltage and the filter are switched during scanning with a simple modification that replaces or adds an already installed bowtie filter with an absorption filter that satisfies the conditions of the present invention. Therefore, it is possible to obtain a CT that can solve the problems of the prior art by performing a single scanning operation without increasing the exposure dose of the subject without requiring a large-scale apparatus. Moreover, the absorption filter which can be installed in such CT can be provided. In such CT, in order to obtain a reconstructed image or the like by DEM from the acquired data, the tomographic image reconstruction software for DEM may be used or replaced.

  Furthermore, a filter shape that maximizes the above effects can be provided by using a design method that takes into account the processing accuracy in the manufacturing process and the exposure dose of the subject.

  As a result, the amount of information for diagnosis is greatly increased by CT examination, and the medical effect can be improved.

Example 1
A first embodiment of the present invention will be described with reference to FIGS. FIG. 1 is an explanatory diagram of the CT 10 according to the first embodiment. The X-ray CT10 of the first embodiment scans a plane (designated by x, y coordinates) including the cross section of the subject HK arranged in the imaging target space SS with X-rays, and crosses the subject HK. It is a device to obtain the image inside. This CT 10 is opposed to the X-ray tube 1 via an X-ray tube (pencil beam X-ray source) 1 that generates and emits a thin rod-shaped pencil beam X-ray LX as a component, and the imaging target space SS. And a single sensor 2 for detecting the intensity of the X-ray LX reaching from the X-ray tube 1 via the imaging target space SS.
Further, the CT 10 is arranged between the X-ray tube 1 and the sensor 2, specifically, between the X-ray tube 1 and the subject HK, and more specifically, an X-ray formed on the holder 39. An absorption filter 3 having a predetermined shape that absorbs a part of the X-ray LX before the X-ray LX emitted to the sensor 4 is fixed to the irradiation port XOP is provided.

Further, the CT 10 includes a moving scanning mechanism 4 that moves the X-ray tube 1, the sensor 2, the absorption filter 3, and the holding body 39 that holds the X-ray tube 1, and scans the imaging target space SS with the X-ray LX. Yes.
The moving scanning mechanism 4 has a linear moving mechanism 45 that linearly moves the X-ray tube 1 and the single sensor 2. Specifically, the linear moving mechanism 45 includes a linear moving body 47 that can travel on the linear rail 46, and the X-ray tube 1 is fixed to the linear moving body 47. For this reason, the X-ray tube 1 can move within the range of the linear scanning range 45A by drawing a linear movement locus 1L in the direction in which the linear rail 46 extends. The X-ray tube 1 is attached so that the X-ray LX is emitted in a direction orthogonal to the linear movement locus 1L. In the first embodiment, the position in the direction D1L along the linear movement trajectory 1L of the X-ray tube 1 is expressed using the coordinate t, and the position of the midpoint 45M of this linear scanning range 45A is the origin t0 (t0 = 0).
It should be noted that the coordinates of a leg 1LO of the central vertical line CP taken along the linear movement locus 1L from a virtual center axis SO described later are also t0 (t0 = 0). Therefore, the central perpendicular CP also passes through the midpoint 45M. Therefore, the center perpendicular line CP is parallel to the X-ray LX and is also a midpoint perpendicular line at the midpoint 45M of the linear scanning range 45A.

  The linear moving mechanism 45 includes a linear rail 48 disposed in parallel to the linear rail 46 described above and a linear moving body 49 that can travel on the linear rail 48. The sensor 2 is fixed to the linear moving body 49. ing. For this reason, the sensor 2 can move linearly along the linear movement locus 2L, and thus in the direction D1L along the linear movement locus 1L, that is, in the direction of the coordinate t. Moreover, the linear movement mechanism 45 moves the sensor 2 in synchronism with the X-ray tube 1 and the coordinate t so that the sensor 2 is always disposed facing the X-ray tube 1. Thereby, the sensor 2 can always detect the pencil beam X-ray LX emitted from the X-ray tube 1.

  Furthermore, the moving scanning mechanism 4 includes the X-ray tube 1, the sensor 2, the absorption filter 3, the holding body 39, and the linear moving mechanism 45 (the linear rails 46 and 48, the linear moving bodies 47 and 49) among the components of the CT 10. Are rotated and moved around the virtual center axis SO in the imaging target space SS while maintaining their relative positions. A virtual rotation surface SSR including the X-ray tube 1 and the sensor 2 when the X-ray tube 1 and the sensor 2 and the like are rotated by the rotational movement mechanism 41 is represented by the above-described FIG. It is a plane. In the first embodiment, the origin of the coordinates (x, y) of the virtual rotation surface SSR is set on the virtual center axis SO of the rotation of the rotational movement mechanism 41. Then, the rotation angle θ (deg) of the rotational movement mechanism 41 is a value measured counterclockwise from the y-axis in FIG. In this way, the state (arrangement) of the X-ray tube 1 and the sensor 2 of the CT 10, the X-ray LX emitted from the X-ray tube 1 toward the sensor 2, and the beam line along the X-ray tube 1 are coordinate t And the rotation angle θ.

  The absorption filter 3 has a configuration in which the thickness h in the direction perpendicular to the linear movement locus 1L (direction perpendicular to the direction D1L) D1P, that is, the direction parallel to the central perpendicular CP is changed with respect to the coordinate t. Therefore, the thickness of the absorption filter 3 in the direction D1P at the coordinate t is represented as h (t). Then, the absorption filter 3 of the first embodiment shown in FIG. 1 is first asymmetric (h (t) ≠ h (−t)) with respect to the origin t0 (t0 = 0), that is, the foot 1LO of the central perpendicular CP. It has a form. Moreover, the thickness h (t) changes gently in the vicinity of the central perpendicular CP. That is, the thickness h (t) gradually changes before and after the coordinate t = 0. Further, in the absorption filter 3, at both end portions 36A and 36B in the vicinity of both end edges 35A and 35B in the direction D1L along the linear movement locus 1L, the thickness h (t) increases toward the end edges 35A and 35B. It is said that. In the first embodiment, specifically, the thickness h (t) has a form of a sum function (h (t) = at2 + 1 / (1 + exp (−αt))) of a parabolic function and a sigmoid function described later. ing.

  Thus, in this CT 10, after X-ray LX is irradiated from the X-ray tube 1 that generates X-rays having a wide energy spectrum distribution in the diagnostic region (about 10 to 150 KeV) and passes through the subject HK, this X The sensor 2 that receives the line LX is reached. The spectrum of this X-ray LX is assumed to be known (see FIG. 9). The X-ray tube 1 moves (straight line scanning) on the straight line (coordinate axis) indicated by the linear movement locus 1L. In addition, the sensor 2 moves with the X-ray tube 1 in synchronization with the X-ray LX so as to always receive the X-ray LX on a straight line of the linear movement locus 2L parallel thereto. In this CT 10, the X-ray tube 1 and the sensor 2 are covered over the entire linear scanning range 45, that is, the region where the subject HK exists at a certain rotation angle θ (deg) with respect to the y axis. The X-ray intensity at each coordinate t is measured. Further, this measurement is performed over all angles (0 to 360 °) with respect to the rotation angle θ by rotational scanning by the rotational movement mechanism 41.

Further, as described above, in this CT 10, the absorption filter 3 is installed with the relative position fixed with respect to the coordinate axis (linear movement locus 1L) for linear scanning (linear movement) of the X-ray tube 1. . The absorption filter 3 is made of a uniform material and density. As described above, the absorption filter 3 has two arbitrary points at symmetrical positions with respect to the central axis, that is, the central perpendicular CP (located at the origin t = 0 in the coordinates of the linear movement locus 1L), that is, The absorption filter thicknesses h (t) and h (-t) at the point of the coordinate t and the point of the coordinate -t are different from each other.
For this reason, the spectra of the two X-rays LXB and LXS (see FIG. 2) or the X-rays LX1 and LX2 (see FIGS. 3 and 4) passing through these two points (t, −t) are mutually multiplied by the other constant. It will not be.

In the CT 10 of the first embodiment, the relationship between the positions (coordinates t) of the absorption filter 3 and the X-ray tube 1 and the X-ray LX that has passed through the absorption filter 3 will be described with reference to FIG.
First, the X-ray LX radiated from the X-ray tube 1 located at the coordinate t on the linear movement locus 1L is set as a reference position X-ray LXB. On the other hand, an X-ray LX radiated from the X-ray tube 1 located at the coordinate −t, parallel to the reference position X-ray LXB, and passing through a position symmetric with respect to the reference position X-line LXB with respect to the central perpendicular line CP X-ray LXS.
At this time, the absorption filter 3 has a shape in which the thicknesses h (t) and h (−t) in the direction D1P perpendicular to the linear movement locus 1L at the coordinates t and −t are different from each other. As a result, the energy spectrum distribution of the reference position X-ray LXB and the symmetric position X-ray LXS after passing through the absorption filter 3 has a distribution shape different from the other constant multiple. That is, the absorption filter 3 has a form in which the energy spectrum distributions of the reference position X-ray LXB and the symmetric position X-ray LXS after passing through the absorption filter 3 are different from each other by a constant multiple.

  In order to realize DEM, it was necessary to acquire data by using two or more X-rays LX having different spectrum shapes with the same X-ray beam line. The fact that this condition is satisfied by installing the absorption filter 3 will be described below with reference to FIGS.

  FIG. 3 shows a first state ST1 in which the rotation angle by the rotational scanning mechanism 41 is θ ° (θdeg), and the linear scanning position of the X-ray tube 1 (coordinates of the linear movement locus 1L) is specified by t. On the other hand, FIG. 4 shows a second state ST2 in which the rotation angle is θ + 180 ° (θ + 180 deg) and the coordinates of the X-ray tube 1 are specified by −t. In the first state ST1 shown in FIG. 3, the X-ray LK (first X-ray LX1) emitted from the X-ray tube 1 is the first in the imaging target space SS (virtual rotation plane SSR) including the subject HK. It passes through one beam path 12. On the other hand, in the second state ST2 shown in FIG. 4, the second beam path 12 through which the second X-ray LX2 passes can be easily understood as compared with the case of FIG. It passes through the same part as the path 12 (beam line). The second beam path 12 (beam line and counter beam line) appears due to rotational scanning, as shown in FIG. 4, with a rotational angle of θ + 180 °, and the linear scanning position (coordinates) of the X-ray tube 1 Is the time point when −t is reached (hereinafter referred to as the “opposing position”).

  Therefore, if the first X-ray LX1 and the second X-ray LX2 are detected by the sensor 2 and have different X-ray spectra, the conditions for realizing DEM are satisfied. In the first state ST1 of the rotation angle θ ° and the linear scanning position t of the X-ray tube 1 shown in FIG. 3, the thickness (first absorption path 31) of the absorption filter 3 through which the first X-ray LX passes is h (t ). On the other hand, in the second state ST2 at the rotation angle θ + 180 ° and the linear scanning position −t of the X-ray tube 1 shown in FIG. 4, the thickness (second absorption path 32) of the filter 7 through which the second X-ray LX2 passes is h ( -T). In CT10 of the first embodiment, since it is assumed that these thicknesses (absorption path lengths) of the absorption filter 3 are different from each other, the following equation (S7) (same as (C1)) is established. This is a condition that specifies a special function among asymmetric functions.

  In the CT 10 according to the first embodiment, the above description passes through the first X-ray LX1 passing through an arbitrary first beam path (beam line) while performing a rotational scan of 360 ° with respect to the rotation angle θ. This means that there is a counter beam line by the second X-ray LX2 in which the thickness of the absorption filter 3 is different. Even if X-rays LX1 and LX2 having the same spectral shape are emitted from the X-ray tube 1, if the thickness of the absorption filter 3 (the lengths of the first and second absorption paths 31, 32 in the absorption filter 3) is different, It has already been mentioned that the different spectral shapes required by DEM. Therefore, it is possible to acquire data for realizing the DEM only by installing the absorption filter 3 that satisfies the formula (S7, C1).

  Note that the first and second X-rays LX1 and LX2 (first and second beam paths 11 and 12) are the origin of the imaging target region, that is, (x, y) = (0,0), that is, the virtual rotation plane. When passing through the virtual central axis SO that is the center of SST, the absorption filter 3 cannot generate two types of X-rays having different quality. However, since such a point is only the origin, that is, the virtual center axis SO, in the spatial discretization of the reconstructed image, if the setting is made so that the origin (virtual center axis SO) is not included in the calculation point, the problem is Does not occur.

  The first embodiment will be described by computer simulation. There are an infinite number of absorption filter thickness functions h (t) that satisfy the equation (S7, C1). The simplest shape is the snake side function shown in the following equation.

Here, a (> 0) is the maximum thickness of the absorption filter 30 (see FIGS. 6 and 7). The absorption filter 3 having the shape of the snake side function and made of a uniform material and density is called a snake side filter. FIG. 5 shows the shape of the snake side function (where a = 1). In the figure, the variable r _{* on the} horizontal axis is a normalized variable when the measurement interval (linear scanning range 45A) in linear scanning is 1.

  In the first embodiment, it will be shown below that DEM can be realized when the absorption filter 3 is made of aluminum and a snake side filter 30 of a = 1 [cm] is used (see FIGS. 6 and 7).

  As a simulation model of the subject phantom, muscle (Muscle), aluminum (Aluminum), polyvinyl (Polyvinil), lung (Lung), brain (in a circular water with a diameter of 19 [cm] as shown in FIG. A brain, bone, blood, and soft-tissue were prepared.

  The X-ray tube 1 emits a pencil beam X-ray LX having the spectrum shown in FIG. On the other hand, the sensor 2 is assumed to have a uniform sensitivity to the energy spectrum of the X-ray LX. The CT 10 described with reference to FIG. 1 follows the scanning method, and the linear scanning is performed at each point (each coordinate t) obtained by dividing the 30 [cm] linear scanning range 45A by 1024. Projection data is acquired using the X-ray tube 1 and the sensor 2 at each point (each rotation angle θ) obtained by dividing 360 ° into 1536. The number of pixels of the reconstructed image was 256 × 256.

  Assuming that there is no error in the attachment position of the aluminum snake side filter (absorption filter) 30 and no mechanical shift in the rotational scanning, a projection simulation is performed to acquire data.

As a DEM reconstruction algorithm, the nonlinear equation of equation (S6) is solved using Newton's method to obtain coupling coefficients sγ _i and sψ _i . In order to obtain a tomographic image of an absorption coefficient for each energy, projection data for each energy was obtained by equation (S5), and a filtered back projection (FBP) algorithm was applied to each.

FIG. 10A shows a tomographic image of an absorption coefficient μ at an energy ε = 70 KeV, that is, a true image, among the absorption coefficients μ (x, ε) of the subject phantom model. On the other hand, FIG. 10B is a tomographic image of an absorption coefficient μ at an energy ε = 70 KeV when a snake side filter is used as the absorption filter 3 and DEM is performed. For comparison, FIG. 10 (c) shows a reconstructed image based on the FBP algorithm that does not take into account X-ray energy, which is currently used in popular CT.
It can be seen that by using the CT 10 according to the present invention, it is possible to obtain an image (see FIG. 10B) that reproduces the true absorption coefficient μ well. Further, as the absorption filter 3, the asymmetric absorption filter 3 represented by the formula (S7) (for example, see the absorption filter 3 described in FIGS. 1 and 2) without using the snake side filter 30 shown in FIGS. ), It can also be understood that DEM can be realized.

(Example 2)
Next, the CT 110 according to the second embodiment will be described. FIG. 11 shows components of the CT 110 according to the second embodiment, which includes the X-ray tube 101 that generates the fan beam X-ray FX. The X-ray tube 101 used in the second embodiment is similar to the X-ray tube 1 used in the first embodiment. The X-ray FX having a wide spectrum distribution shape (see FIG. 9) is centered on the radiation center RO. It occurs in the form of a fan-shaped FS. Each X-ray FX passes through the imaging target space SS including the subject HK and then reaches one of the sensors 102 in the sensor group 120 arranged in an arc centered on the radiation center RO. Then, the intensity of each X-ray FX is measured. In this CT 110, the X-ray tube 101 can be rotated around the virtual central axis SO by a rotational movement mechanism 141 via a fixture 142. The sensor group 120 is also fixed to the rotational movement mechanism 141, and can be rotated by the rotational movement mechanism 141 while maintaining a relative positional relationship with the X-ray tube 101. Therefore, in the CT 110 of the second embodiment, scanning is performed only by rotational scanning of 360 ° for the entire circumference with respect to the rotation angle θ.

In the CT 110 according to the second embodiment, the relationship between the central angle φ formed by the X-ray FX radiated from the absorption filter 103 and the X-ray tube 101 and the X-ray FX transmitted through the absorption filter 3 is described with reference to FIG. I will explain.
First, in the fan-shaped FS formed by the X-ray FX radiated from the X-ray tube 1010, a central angle φ which is symmetrical with respect to the source-center axis CR connecting the radiation center RO of the X-ray tube 101 and the virtual center axis SO. And the two X-rays FX forming −φ are referred to as a reference angle X-ray FXB and a symmetric angle X-ray FXS.
At this time, the absorption filter 103 has a thickness h in a direction DBA along the reference angle straight line LFXB that forms the central angle φ with respect to the source-center axis CR along the reference angle X-line FXB in the sector FS. (φ) has a shape such that asymmetric h (φ) ≠ h (−φ) (where φ> 0). As a result, the energy spectrum distribution of the reference angle X-ray FXB and the symmetry angle line FXS after passing through the absorption filter 103 has a distribution shape different from the other constant multiple. That is, the absorption filter 3 has a form in which the energy spectrum distribution of the reference angle X-ray FXB and the symmetric angle X-ray FXS after passing through the absorption filter 3 is different from the other constant multiple.

FIG. 13 shows that in the CT 110 of the second embodiment, the rotational angle θ ° of the rotational movement mechanism 141 and the central angle of the X-ray FX (first X-ray FX1) with respect to the radiation source-central axis CR (fan beam central axis) are positive. The first state ST11 specified by φ ° in the direction of is shown. In the first state ST11, the first X-ray FX1 travels along the first beam path (beam line) 111.
In the second embodiment, the central angle φ of the X-ray FX is, as shown in FIG. 13, centered on the source-center axis CR (origin) and the clockwise direction is + (positive direction) and counterclockwise. The surrounding direction is expressed as-(negative direction).
On the other hand, an X-ray FX (second X-ray FX2) that passes through the opposite beam line to the first beam path 111, that is, the second beam path 112 having the opposite traveling direction but passing through the same part of the subject HK, It exists in the fan beam when the rotational angle of the rotational movement mechanism 141 for rotationally moving the tube 101 or the like is (θ + 180−2φ) °. Specifically, as in the second state ST12 shown in FIG. 14, the rotational angle of the rotational movement mechanism 141 is (θ + 180-2φ), and the central angle (injection angle) is −φ with respect to the source-center axis CR. The second X-ray FX2 when it is set to ° corresponds to that. As can be easily understood by comparing FIG. 13 and FIG. 14, in FIG. 14, the second beam path 112 is the opposite beam line of the first beam path 111, and these are the same as those of the subject HK. Has passed the part.

  Further, in the CT 110 shown in FIG. 11, the X-ray irradiation port XOP of the holding body 139 fixed to the rotational movement mechanism 141 is positive or negative with respect to the radiation source-center axis CR passing through the radiation center RO (emergence angle center). The thickness h of the direction DBA along two straight lines (first beam path 111 and second beam path 112) having symmetrical angles (center angles φ and −φ) is expressed by the following equation (S9) ((C2 The absorption filter 103 made of a uniform material and density satisfying the same)) is installed.

As a result, the X-ray FX is caused to travel along the first beam path 111 (beam line) and the second beam path 112 (opposing beam line), thereby having spectral shapes that are not constant multiples of each other, that is, the line quality. First and second X-rays FX1 and FX2 can be obtained.
In addition, the thickness h (φ) of the absorption filter 103 changes gently in the vicinity of the radiation source-center axis CR. That is, the thickness h (φ) gradually changes around the central angle φ = 0. Further, in the absorption filter 103, at both ends 136A and 136B in the circumferential direction of the sector FS having large values of the central angles φ and −φ, the thickness h (φ) increases toward the end edges 135A and 135B. It is in the form.

  Thus, if projection data is obtained by each sensor 102 of such a CT 110 and a DEM reconstruction algorithm having a fan beam correction function is used, a reconstructed image for each energy can be obtained as in the first embodiment. In the spatial discretization in the reconstructed image, the setting so that the origin, that is, the virtual central axis SO is not used as the calculation point is the same as in the first embodiment.

(Example 3)
Next, in the third embodiment, in the CT 10 of the first embodiment (see FIGS. 1 to 4, 6, and 7), there are fluctuations in the mounting accuracy of the absorption filters 3 and 30, and the rotational scanning mechanism 41 and angle measurement. The problem that occurs in the case is clarified, and the corresponding absorption filter design method and the absorption filter shape that takes this into account will function effectively.

In the snake side filter 30 (see formula S8, FIG. 5) used in the CT 10 of FIGS. 6 and 7, the spectrum pattern (spectral structure) of the X-ray LX after passing through the snake side filter 30 increases as the maximum thickness a increases. Since it can be changed, the conditions for realizing the DEM are good. However, the snake side filter 30 has a discontinuity in the thickness h (t) at the origin (t = 0).
For this reason, when the snake side filter 30 is attached without misalignment in the direction D1L along the linear movement locus 1L, it becomes as shown in FIG. That is, even in the vicinity of the central vertical line CP, one becomes the first X-ray LX1 transmitted through the first absorption path 31 having the thickness h (t) = a through the snake side filter 30, and the other does not pass through the snake side filter 30. (That is, the thickness h (t) = 0 of the second absorption path 32)) The second X-ray LX2. Therefore,
However, as shown in FIG. 16, the snake side filter 30 is mounted with a displacement in the direction D1L (the displacement ε in FIG. 16), and the discontinuity of this thickness is not on the central perpendicular CP. In this state, a region having the same filter thickness exists in the vicinity of the central perpendicular CP.

As shown in FIG. 16, the discontinuous edge of the snake side filter 30 given by the equation (S8) moves in the direction D1L (on the coordinates of the linear scanning axis) along the linear movement locus 1L from the central perpendicular CP to the negative side ε Consider a case where the installation is shifted by (> 0). In the first and second beam paths 11 and 12 (counter beam line pair) when the linear scanning position of the X-ray tube 1 is t (| t | <ε) at a certain rotation angle θ, the snake side filter 30 The thickness (passing thickness) h is equal to a and does not satisfy the conditions for realizing DEM.
That is, in the case of FIG. 16, in the vicinity of the central perpendicular CP, both the first X-ray LX1 and the second X-ray LX2 are transmitted through the snake side filter 30, and the sizes of the first and second absorption paths 31, 32 are the same. Both are equal to a. Therefore, DEM cannot be realized for the X-ray LX (first and second X-rays LX1, LX2) passing through the vicinity of the central perpendicular CP.
For example, FIG. 17A shows a reconstructed image at an energy of 70 [keV] when the deviation ε is 5% of the length of the linear scanning range 45A (linear scanning line segment). As can be seen from the comparison between the reconstructed image of FIG. 17A and the reconstructed image shown in FIG. 10B when the same snake side filter 30 is used but there is no positional shift, the snake side filter 30 was used. In this case, it can be seen that the positional shift of the snake side filter 30 greatly affects the accuracy of the obtained reconstructed image.

  Depending on the manufacturing process of the CT 10 or the installation situation when the snake side filter 30 is replaced, it may be difficult to precisely align the discontinuous edge of the snake side filter 30 and the central perpendicular CP. Considering that accuracy is not so necessary for the installation of the bow tie filter that is often used in general CT, this point is considered to be a particular problem when the snake side filter 30 is used. is required.

This problem is caused by the shape of the snake side filter 30, that is, the rise of the snake side function used for the absorption filter 3 at the origin is too steep, and can be solved by using a stepped function that rises gently. it is conceivable that.
As a step-like function that rises smoothly around the origin, there is a sigmoid function expressed by the following equation (S10).

Here, r _* is a normalized variable when the measurement interval (linear scanning range 45A) in linear scanning is set to 1, as in the case of the snake side function (see FIG. 5). This sigmoid function has one parameter α related to the shape. As shown in FIG. 18, as α increases, the slope near r * becomes steeper, and becomes a snake side function when α → ∞. Hereinafter, the absorption filter 3 having the shape of the sigmoid function is referred to as a sigmoid filter.

  FIG. 17B shows a reconstructed image at an energy of 70 [keV] when only the filter shape to be used is changed from the snake side filter 30 to the sigmoid filter of α = 3 under the conditions obtained in FIG. It was. According to this figure, when the snake side filter 30 is used (FIG. 17 (a)) and the sigmoid filter is used in spite of the same deviation ε (5%) in the filter, It can be seen that reconfiguration is taking place.

  From the above results, when there is a possible lateral shift in the installation of the absorption filter 3, it is considered that the shape of the absorption filter 3 having a smooth rise is good. For this reason, FIG. 19 shows the result of obtaining the error E from the true image by the following equation (S11) by changing the shape parameter α of the sigmoid filter from 1 to 600 for the lateral shift in this case.

Here, M _E is the energy element set, M _P is a pixel set, μ _j (ε) is legitimate true image, ∧μ _j (ε) is reconstructed image, I ^* _e (ε) is the X-ray The energy distribution is shown.

  As can be seen from FIG. 19, as the steepness of the sigmoid filter increases (the parameter α increases), the error E increases, and when there is a lateral shift in the installation of the sigmoid filter, it is as smooth as possible ( This shows that it is better to select a shape with a small α.

On the other hand, in CT10, there is also rotational scanning, and the rotational movement mechanism 41 for performing this rotational scanning also has mechanical play and errors in measuring the rotational angle θ. From these factors, it is conceivable that the rotation angle θ slightly shifts every time data is acquired. The shift of the rotation angle θ corresponds to the fact that the first X-ray LX1 and the second X-ray LX2 (first beam path 11 and second beam path 12, opposite beam) are shifted from each other. give.
Therefore, when obtaining each data, assuming that an error in accordance with a normal distribution having a standard deviation of 0.0025 [rad] (about 1.41 °) occurs in the rotation angle θ, the parameter α of the sigmoid function is in the range of 1 to 200. FIG. 20 shows the relationship between the parameter α and the error E given by the above equation (S11).

  According to the graph of FIG. 20, it can be seen that the absorption filter 3 should have a steep rise when it comes to the rotational displacement, as opposed to the lateral displacement of the mounting position (see FIG. 19). That is, it can be said that both have a trade-off relationship with respect to the steepness of the absorption filter.

  There are other elements that have such a trade-off relationship with respect to the shape of the absorption filters 3 and 103. For example, if the entire thickness of the absorption filters 3 and 103 is increased, the exposure dose of the subject HK can be reduced, but the image quality deteriorates because the X-ray intensity after transmission decreases. Further, if the difference in thickness, that is, the difference in path length between the first absorption paths 31 and 131 and the second absorption paths 32 and 132, is taken, a line between the first X-ray and the second X-ray (a pair of opposed beam lines). Although it is considered that the difference in quality can be increased, the intensity of transmitted X-rays is also reduced in a thick portion (portion having a large path), and it is considered that a good image cannot be obtained.

These mean that there is a filter shape that provides the best image quality and exposure dose, given the processing accuracy in the manufacturing process of the CTs 10 and 110 and the installation accuracy of the absorption filters 3 and 103.
This is an optimization problem with the filter shape as a parameter.

Although various factors can be considered as described above, in the third embodiment, CT10 is taken as an example, and a case where a lateral shift in the installation of the absorption filter 3 and a shift in the rotation direction occur simultaneously follows the filter shape design method. Attempts to determine the shape of the absorption filter 3.
The design method is as follows.
(Procedure 1) The filter shape and material are expressed by a finite number of parameters.
(Procedure 2) An evaluation function having both or one of tomographic reconstruction accuracy and exposure dose as elements is set.
(Procedure 3) The filter parameters are determined using a computer simulation and a numerical optimization method in consideration of the mechanical accuracy of the apparatus and the absorption filter mounting accuracy so that the evaluation function is minimized.

  First, according to (Procedure 1), it is necessary to determine the function system of the filter shape to be used. Here, the sigmoid filter given by the previously used equation (S10) is used.

  The subject phantom (see FIG. 8), the X-ray tube 1 and the like, and the scanning method were the same as those in Example 1 (see FIG. 1). In the third embodiment, the scattering effect is not considered. Similarly to the above description, when the absorption filter 3 is attached, the coordinate t is shifted by 5% in the positive direction with respect to the entire length of the absorption filter 3 (linear scanning range 45A), and the rotation is rotated. In scanning, it is assumed that an error in accordance with a normal distribution having a standard deviation of 0.0025 [rad] (about 1.41 °) occurs at the rotation angle θ when acquiring each data.

  According to (Procedure 2), here, an error E from the true image in the image reconstruction given by the equation (S11) is used as an evaluation function.

In the sigmoid filter, the parameter relating to the filter shape is only α, and the optimization in (Procedure 3) is a one-dimensional search. FIG. 21 shows an error E that is a value of the evaluation function with respect to a change in the parameter α. In the figure, at the position of the arrow (α = 50), the error E shows the minimum value, and the shape having α as a parameter is optimal. FIG. 22 (a) shows a reconstructed image (70 [keV]) in that case (when the sigmoid filter parameter α = 50).
On the other hand, FIG. 22 (b) shows the result when deviating from the optimum shape (when parameter α = 220), and many streak-like artifacts appear compared to the image of FIG. 22 (a). ing. As can be seen from the comparison of FIGS. 22 (a) and 22 (b), even if there is a shift in the attachment of the absorption filter 3 and a fluctuation in the rotation angle θ of the rotational movement mechanism 41, according to the present design method. It can be seen that the resulting artifacts can be greatly reduced.

(Absorption filter configuration)
Next, the shape of the absorption filter 3 in Example 1 (see FIGS. 1 to 4) will be further examined. As described above, the absorption filter 3 shown in FIG. 1 of the first embodiment is first asymmetric (h (t) ≠ h (−) with respect to the origin t0 (t0 = 0), that is, the foot 1LO of the central perpendicular CP. t)).

Further details will be described. First, as shown in FIGS. 3 and 4, the first beam path 11 and the second beam path 12 of the absorption filter 3 pass through the virtual central axis SO, so that the first absorption path 31 and the first absorption path 31 passing through the absorption filter 3 are obtained. A part corresponding to the first absorption path 31 when the two absorption paths 32 match each other is defined as a matching part 33. At this time, the absorption filter 3 of the first embodiment is such that the first absorption path 31 and the second absorption path 32 are closer to the matching portion 33 (central perpendicular CP) in the vicinity of the matching portion 33 (that is, the coordinates t and −t). The energy spectrum distribution of the first X-ray LX1 and the energy spectrum distribution of the second X-ray LX2 have an approximate distribution shape.
Specifically, like the sigmoid filter in the above-described third embodiment, the thickness h (t) gradually changes in the vicinity of the coincidence portion 33 (central perpendicular CP). That is, the thickness h (t) gradually changes before and after the coordinate t = 0.
For this reason, when this absorption filter 3 is used, as in the case of the sigmoid filter shown in the third embodiment, even if the attachment position of the absorption filter 3 is displaced in the direction D1L, DEM can also be realized in the vicinity of the normal CP).
In addition, the absorption filter 3 shown in Example 1 has a form in which the thickness h (t) of the absorption filter 3 gradually changes in the entire range of the coordinate t. Therefore, even if the absorption filter 3 is mounted with a large positional deviation within the mounting tolerance range, the DEM is removed in any case except when the first X-ray LX1 and the second X-ray LX2 pass through the virtual central axis SO. Can be realized.

Further, as described above (see FIG. 1), in the absorption filter 3 of the first embodiment, in the both end portions 36A and 36B in the vicinity of the both end edges 35A and 35B in the direction D1L along the linear movement locus 1L, the end edges 35A, The thickness h (t) is increased toward the 35B side. In the first embodiment, specifically, the thickness h (t) has a form of a sum function (h (t) = at2 + 1 / (1 + exp (−αt))) of a parabolic function and a sigmoid function. .
In general, if the CT does not include a filter, the amount of X-ray irradiation (exposure dose) increases at the outer periphery of the subject HK. Therefore, in general CT, X-rays at the outer peripheral portion of the subject are used by using a so-called bow tie filter, which is a bilaterally symmetric and thick parabolic filter with a thickness increased toward both ends. This prevents an increase in radiation exposure.
On the other hand, in the absorption filter 3 used for the CT 10 of the first embodiment, the relationship of h (t) ≠ h (−t) described above is maintained, and the absorption filter 3 in the direction D1L along the linear movement locus 1L is maintained. In both end portions 36A and 36B, the thicknesses h (t) and h (−t) are increased toward the end edges 35A and 35B. By using such an absorption filter 3, it is possible to suppress the X-ray irradiation amount on the outer periphery of the subject HK while realizing DEM.

  The same applies to the shape of the absorption filter 103 in Example 2 (see FIGS. 11 to 14). That is, as described above, the absorption filter 103 shown in FIG. 11 and the like of the second embodiment first has the first and second beam paths with respect to the central angle φ with respect to the source-center axis CR passing through the radiation center RO. The thickness h (φ) of the direction DBA along the lines 111 and 112 is asymmetric (h (φ) ≠ h (−φ)).

Similarly to the absorption filter 3 of the first embodiment, as shown in FIGS. 13 and 14, the first beam path 111 and the second beam path 112 of the absorption filter 103 pass through the virtual center axis SO, thereby absorbing the filter. A part corresponding to the first absorption path 131 when the first absorption path 131 and the second absorption path 132 passing through 103 match is referred to as a matching part 133. At this time, in the absorption filter 103 of the second embodiment, the first absorption path 131 and the second absorption path 132 are brought closer to the matching portion 133 (radiation source-center axis CR) in the vicinity of the matching portion 133 (that is, the central angle). The smaller the values of φ and −φ), the energy spectrum distribution of the first X-ray FX1 and the energy spectrum distribution of the second X-ray FX2 have an approximate distribution shape.
Specifically, the thickness h (φ) changes gently in the vicinity of the coincidence portion 133 (radiation source—center axis CR). That is, the thickness h (φ) gradually changes around the central angle φ = 0.
For this reason, when this absorption filter 103 is used, the mounting angle of the absorption filter 103 is shifted in the circumferential direction DFS of the sector FS, considering the same as the adoption of the sigmoid filter for the positional shift shown in the third embodiment. Even if attached, the DEM can be realized even in the vicinity of the coincidence portion 133 (in the vicinity of the radiation source—the central axis CR).
Note that the absorption filter 103 shown in the second embodiment also has a form in which the thickness h (φ) gradually changes over the entire range of the central angle φ of the absorption filter 103. Therefore, even if the absorption filter 103 is mounted with a large angular deviation within the mounting tolerance range, the DEM is not measured in any case except when the first X-ray FX1 and the second X-ray FX2 pass through the virtual central axis SO. Can be realized.

Further, as shown in FIG. 11, in the absorption filter 103 of the second embodiment, at both end portions 136A and 136B in the vicinity of both end edges 135A and 135B in the circumferential direction DFS of the fan-shaped FS, the thickness is increased toward the end edges 135A and 135B. The h (φ) is increased.
As described above, generally, if the CT does not include a filter, the X-ray irradiation dose (exposure dose) increases in the outer peripheral portion of the subject HK. Therefore, the so-called bow tie filter is used in the outer peripheral portion of the subject. Prevents increase in X-ray exposure dose.
On the other hand, in the absorption filter 103 used in the CT 110 of the second embodiment, both ends 136A of the circumferential direction DFS of the absorption filter 103 are maintained while maintaining the relationship of h (φ) ≠ h (−φ). In 136B, the thicknesses h (φ) and h (−φ) are increased toward the end edges 135A and 135B. By using such an absorption filter 103, the X-ray irradiation amount on the outer periphery of the subject HK can be suppressed while realizing DEM.

In the above, the present invention has been described with reference to the first to third embodiments. However, the present invention is not limited to the above-described embodiments and the like, and it can be applied as appropriate without departing from the gist thereof. Not too long.
For example, in the first embodiment, the CT 10 using the pencil beam X-ray source is described, and in the second embodiment, the CT 110 using the fan beam X-ray source is described. However, the first beam path is changed by transmitting through different parts of the absorption filter. Two X-rays having different radiation qualities may be obtained using the first X-ray passing through and the second X-ray passing through the second beam path traveling in the opposite direction to the same part. Therefore, the present invention can also be applied to a CT having a cone beam X-ray source by making the scanning method appropriate as described above.

  Further, as the absorption filters 3, 30, and 103 in Examples 1 and 2, an absorption filter made of a homogeneous material and changing the spectrum distribution of transmitted X-rays by changing the thickness thereof is exemplified. However, it is also possible to use an absorption filter that changes the absorption characteristics by changing the material in the absorption filter. Further, in the first and second embodiments, the example in which the absorption filters 3, 30, and 103 are used without using the bow tie filter that is already widely used in the CT is shown. However, in addition to the existing bow tie filter, It can also be used as an absorption filter of the present invention in combination with an absorption filter.

  In the first to third embodiments, the CTs 10 and 110 using the absorption filters 3, 30, and 103 having a predetermined shape have been described. However, in realizing these CTs 10 and 110, the existing CTs are described above. Modification of attaching the absorption filter 3 or the like to the X-ray irradiation port XOP or the like can also be performed.

It is explanatory drawing which concerns on Example 1 and shows a mode that the asymmetrical absorption filter of this invention was integrated in X-ray CT which uses a pencil beam X-ray source and performs a linear scan and a rotational scan. In CT shown in FIG. 1, it is explanatory drawing explaining the relationship between the position (coordinates) of an absorption filter and an X-ray tube, and the X-ray which permeate | transmitted the absorption filter. In CT shown in FIG. 1, it is explanatory drawing which shows the mode of a measurement in the 1st state of a linear scanning position (coordinate t) and a rotational scanning position (rotation angle (theta)). In the CT shown in FIG. 1, it is explanatory drawing which shows the mode of a measurement in the 2nd state of a linear scanning position (coordinate -t) and a rotational scanning position (rotation angle (theta) +180 degrees), and a rotation angle differs 180 degrees from FIG. State. It is a graph which shows the shape of the snake side function (however, coefficient a = 1) shown to Formula (S8). In the CT shown in FIG. 1, when an absorption filter having a snake side function shape (snake side filter) is used, the linear scanning position (coordinate t) and the rotational scanning position (rotation angle θ) are measured in the first state. It is explanatory drawing which shows a mode. In CT shown in FIG. 1 using a snake side filter, it is explanatory drawing which shows the mode of a measurement in the 2nd state of a linear scanning position (coordinate -t) and a rotational scanning position (rotation angle (theta) +180 degrees), and is rotating. The angle is 180 ° different. It is explanatory drawing which shows the model for computer simulations of the subject phantom used in each Example. It is a graph which shows the irradiation X-ray spectrum of the X-ray tube model used in each Example. (A) is an image obtained by imaging the absorption coefficient when the X-ray energy is 70 [keV] for the subject phantom shown in FIG. (B) is an image obtained by reconstructing a cross-sectional image of an absorption coefficient of energy 70 keV by DEM using data obtained by CT in which an asymmetric absorption filter having a snake side function shape is mounted without positional displacement. (C) is an image reconstructed by a popular CT algorithm without using an asymmetric filter. It is explanatory drawing which shows a mode that it concerns on Example 2 and incorporated the asymmetrical absorption filter of this invention in X-ray CT which uses a fan beam X-ray source and performs rotational scanning. In CT shown in FIG. 11, it is explanatory drawing explaining the relationship between central angle (phi) which the X-ray radiated | emitted from an absorption filter and an X-ray tube, and the X-ray which permeate | transmitted the absorption filter. FIG. 9 is an explanatory diagram showing a state of measurement in a state of a rotation angle θ and an irradiation angle (center angle) φ in the CT shown in FIG. 8. FIG. 9 is an explanatory diagram showing a state of measurement in a state of a rotation angle θ + 180 ° + 2φ and an irradiation angle (center angle) −φ in the CT shown in FIG. 8. In CT of FIG. 1, it is explanatory drawing which shows the state which attached the asymmetrical absorption filter by the snake side function form without position shift. In the CT of FIG. 1, unlike FIG. 11, the attachment position of the absorptive absorption filter having a snake side function is shifted by ε. (A) is the image which reconfigure | reconstructed the measurement data when the attachment position of an absorption filter was 5% of the linear scanning range in the case of FIG. 12 by DEM. (B) uses a sigmoid filter of α = 3 (asymmetric absorption filter of sigmoid function type), and is reconstructed using measurement data when there is a 5% misalignment as in (a). It is an image. It is a graph which shows the shape of the sigmoid function shown to Formula (S10), and shows a mode that steepness changes with parameter (alpha). When the sigmoid filter is attached to the CT of FIG. 1, assuming that the attachment position is laterally shifted by 5%, the magnitude E of the image reconstruction error when the steepness (parameter α) of the sigmoid filter is changed is changed. It is a graph which shows a change. When a sigmoid filter is attached to the CT in Fig. 1, the error that follows the normal distribution with a standard deviation of 0.0025 [rad] (about 1.41 °) occurs at each data acquisition angle for the rotation angle that occurs in the rotational scan. 7 is a graph showing a change in the magnitude E of the reconstructed image error when the steepness (parameter α) of the sigmoid filter is changed. It is a graph which shows the change of the value of the evaluation function with respect to the change of the parameter (alpha) of a sigmoid filter, and takes the minimum value by the arrow part. (A) is obtained by using an asymmetric sigmoid filter designed according to the filter shape design method of the present invention and using the parameter α (α = 50) with which the evaluation function obtained in FIG. 20 is the minimum value. It is the image which reconstructed data. (B) is an image obtained by reconstructing the obtained data using an asymmetric sigmoid filter having a shape (α = 220) not conforming to the present design method.

Explanation of symbols

1 X-ray tube (X-ray source, pencil beam X-ray source, X-ray tube generating pencil beam)
1L (X-ray tube: pencil beam X-ray source) linear movement trajectory (X-ray tube linear scanning axis)
XOP X-ray irradiation port LX Pencil beam X-ray LX1 First X-ray (forward X-ray)
LX2 2nd X-ray (Reverse X-ray)
LXB Reference position X-ray LXS Symmetric position X-ray 11, 111 First beam path (beam line, straight line along forward X-ray)
12, 112 Second beam path (beam line, straight line along retrograde X-ray)
DBA directions ST1 and ST11 along the reference angle line, first state ST2 and ST12 second state 101 X-ray tube (fan beam X-ray source, X-ray tube generating fan beam)
FX (radiated from the fan beam X-ray source toward each angle) X-ray FS (formed by X-rays emitted from the fan beam X-ray source) Fan-shaped DFS (fan-shaped) circumferential direction FX1 First X-ray (forward) X-ray)
FX2 2nd X-ray (backward X-ray)
FXB Reference angle X-ray FXS Symmetric angle X-ray LFXB Reference angle straight line HK Subject SS Imaging target space SO Virtual central axis SSR Virtual rotation plane CP Central perpendicular (middle vertical line, straight line connecting the origin of the linear scanning axis and the rotation center)
1 LO Center perpendicular foot RO (X-ray tube) Radiation center CR Source-center axis (fan-shaped center line)
φ (radiation source-fan-shaped center axis and X-ray) center angle 2,102 Sensor 120 Sensor group (multiple sensors)
2L (sensor) linear trajectory (sensor linear scan axis)
3,30,103 Absorption filter (asymmetrical absorption filter)
30 snake side filter (snake side function type asymmetrical absorption filter)
31, 131 First absorption path 32, 132 Second absorption path 33, 133 Matching part 35A, 35B Edge 36A, 36B (in the direction along the linear movement locus of the absorption filter) Edge 36A, 36B (in the direction along the linear movement locus of the absorption filter) ) End h (t) Thickness h (φ) of absorption filter (in the direction perpendicular to the linear movement trajectory) Thickness 135A, 135B (in the direction along the straight line of the reference angle) of absorption filter ) Edges 136A, 136B End portions 39, 139 (holding the absorption filter in the circumferential direction of the absorption filter) Holding body 4 Moving scanning mechanism 41 Rotating moving mechanism 45 Linear moving mechanism 46 (for X-ray tube) ) Linear rail 47 Linear moving body 48 (for X-ray tube) Linear rail 49 (for sensor) Linear moving body D1L Direction t along the linear movement locus Direction of the coordinate t0 along line movement track (coordinates t) of origin D1P linear movement perpendicular 45A linear scan range trajectory 45M linear scanning range of the midpoint 141 rotational movement mechanism (component performs a rotational movement only)
142 (fixing the X-ray tube to the rotational movement mechanism) fixtures 10, 110 Computer-aided X-ray tomography apparatus (CT)

Claims (16)

An X-ray source that generates X-rays having an energy spectral distribution within a predetermined energy region;
One or more sensors that are arranged to face the X-ray source via the imaging target space in which the subject is arranged and detect the intensity of the X-ray that reaches from the X-ray source via the imaging target space When,
An absorption filter disposed between the X-ray source and the sensor and absorbing a portion of the X-ray before the X-ray reaches the sensor;
Second X-rays emitted from the X-ray source with respect to a first state where the first X-rays emitted from the X-ray source reach one of the one or more sensors through a first beam path. However, the second state of reaching the any one of the one or the plurality of sensors through the second beam path that travels in the same place as the first beam path in the imaging target space. A moving scanning mechanism that moves the X-ray source and the sensor with an existing moving scanning pattern and scans the imaging target space with the X-ray;
A computer-aided X-ray tomography apparatus comprising: a moving scanning mechanism including a rotational movement mechanism that rotationally moves the X-ray source and the sensor around a virtual central axis in the imaging target space;
The absorption filter is
The relative position is fixed with respect to the rotational movement mechanism,
Except when both the first beam path and the second beam path pass through the virtual central axis,
The first absorption path located in the absorption filter in the first beam path and the second absorption path located in the absorption filter in the second beam path are arranged so as not to overlap each other.
In the first state, through the first absorption path, the energy spectrum distribution of the first X-ray that reaches one of the one or more sensors, and in the second state, the second absorption path A computer-aided X-ray that is an absorption filter having a configuration in which the energy spectrum distribution of the second X-ray that reaches one of the one or more sensors is different from the other constant multiple. Tomography equipment. The computer-assisted X-ray tomography apparatus according to claim 1,
Among the absorption filters, the first beam path and the second beam path pass through the virtual central axis, so that the first absorption path and the second absorption path through which the absorption filter passes coincide with each other. When the site corresponding to one absorption pathway is the matching site,
The absorption filter is
At least in the vicinity of the matching site,
The closer the first absorption path and the second absorption path are to the matching site,
A computer-aided X-ray tomography apparatus having a form in which the energy spectrum distribution of the first X-ray and the energy spectrum distribution of the second X-ray have an approximate distribution shape. The computer-assisted X-ray tomography apparatus according to claim 1 or 2,
The X-ray source is a pencil beam X-ray source that generates the thin rod-shaped X-ray,
The moving scanning mechanism includes:
A linear movement mechanism for linearly moving the pencil beam X-ray source and the single sensor;
In the virtual rotation plane including the pencil beam X-ray source and the sensor when the pencil beam X-ray source and the sensor are rotated by the rotational movement mechanism,
An arbitrary X-ray radiated from the pencil beam X-ray source linearly moved by the linear movement mechanism and perpendicular to the linear movement locus of the pencil beam X-ray source is defined as a reference position X-ray,
X-rays radiated from the pencil beam X-ray source, parallel to the reference position X-ray, and passing through a position symmetrical to the reference position X-ray with respect to a central perpendicular line taken from the virtual central axis to the linear movement locus. When the X-ray is symmetrical,
The absorption filter is
A computer-aided X-ray tomography apparatus having a configuration in which the energy spectrum distribution of the reference position X-ray and the symmetric position X-ray after passing through the absorption filter is different from the other constant multiple. The computer-assisted X-ray tomography apparatus according to claim 1,
The moving scanning mechanism includes:
A linear movement mechanism for linearly moving the X-ray source and the sensor;
The X-ray source is a pencil beam X-ray source that generates the thin bar-shaped X-ray,
In the virtual rotation plane including the X-ray source and the sensor when the X-ray source and the sensor are rotated by the rotational movement mechanism,
The coordinate in the direction along the linear movement locus of the pencil beam X-ray source moved linearly by the linear movement mechanism is t, and the foot coordinates of the central perpendicular line taken from the virtual central axis to the linear movement locus are reference points ( t = 0),
The absorption filter is
The thickness h (t) in the direction perpendicular to the linear movement locus at the coordinate t has a form satisfying the following formula (C1): h (t) ≠ h (−t) (t> 0) (C1)
Computer-assisted X-ray tomography system. The computer-assisted X-ray tomography apparatus according to claim 4,
The absorption filter is
At both ends in the direction along the linear movement locus,
A computer-aided X-ray tomography apparatus in which the thickness h (t) increases toward the edge side. The computer-assisted X-ray tomography apparatus according to claim 1 or 2,
The X-ray source is a fan beam X-ray source that radiates the X-ray in a fan shape,
In the fan shape formed by the X-rays radiated from the fan beam X-ray source, two central angles that are symmetric with respect to the source-center axis line connecting the X-ray source and the virtual central axis are formed. When the X-ray is a reference angle X-ray and a symmetric angle X-ray,
The absorption filter is
A computer-aided X-ray tomography apparatus having a configuration in which the energy spectrum distribution of the reference angle X-ray and the symmetric angle X-ray after passing through the absorption filter is different from the other constant multiple. The computer-assisted X-ray tomography apparatus according to claim 1 or 2,
The X-ray source is a fan beam X-ray source that radiates the X-ray in a fan shape,
The absorption filter is
Within a fan-shaped X-ray sector, the fan beam X-ray source passes through the fan beam X-ray source and connects the fan beam X-ray source and the central axis. H (φ) ≠ h (−φ) (φ> 0) (C2) having a form that satisfies the following formula (C2) where
Computer-assisted X-ray tomography system. The computer-assisted X-ray tomography apparatus according to claim 7,
The absorption filter is
At both ends in the circumferential direction of the sector where the central angles φ and −φ are large values,
A computer-aided X-ray tomography apparatus in which the thicknesses h (φ) and h (−φ) are increased toward the edge side. An X-ray source that generates X-rays having an energy spectral distribution within a predetermined energy region;
One or more sensors that are arranged to face the X-ray source via the imaging target space in which the subject is arranged and detect the intensity of the X-ray that reaches from the X-ray source via the imaging target space When,
An absorption filter that is disposed between the X-ray source and the sensor, absorbs part of the X-ray before the X-ray reaches the sensor, and simultaneously satisfies the following conditions (a) and (b): A computer-aided X-ray tomography apparatus.
(a) The absorption filter is a forward X-ray that travels in the imaging target space, and a retrograde X-ray that is along the same straight line as the straight line along which the forward X-ray is directed but whose emission direction is reversed.
Except when the straight line passes through the virtual center axis below,
When there is no X-ray absorption in the subject, the energy spectrum distribution of the forward X-ray and the retrograde X-ray incident on the sensor is different from the other constant multiple. .
(b) The absorption filter has a virtual center axis common to rotational scanning for rotating the X-ray source and the sensor among the components constituting the computer-assisted X-ray tomography apparatus, and performs only rotational motion. The relative position is fixed with respect to the component. A computer-assisted X-ray tomography apparatus according to claim 9,
A linear scanning mechanism for linearly moving the X-ray source and the sensor;
A rotational scanning mechanism for rotating at least a part of the X-ray source, the sensor, and the linear scanning mechanism around the virtual central axis;
The X-ray source is a pencil beam X-ray source that generates the thin bar-shaped X-ray,
When the pencil beam X-ray source and the sensor are rotated by the rotational scanning mechanism, in a virtual rotation plane including the pencil beam X-ray source and sensor,
An arbitrary X-ray generated from the pencil beam X-ray source moved on the linear scanning line by the linear scanning mechanism and perpendicular to the linear scanning line is set as a reference position X-ray,
When the X-ray that is parallel to the reference position X-ray and passes through a position symmetrical to the reference position X-ray with respect to the midpoint perpendicular line at the midpoint of the linear scanning range of the pencil beam X-ray source is set as the symmetrical position X-ray ,
The absorption filter is
A computer-aided X-ray tomography apparatus having a configuration in which the energy spectrum distribution of the reference position X-ray and the symmetric position X-ray after passing through the absorption filter is different from the other constant multiple. A computer-assisted X-ray tomography apparatus according to claim 9,
The X-ray source is a fan beam X-ray source that generates the X-ray in a fan shape,
A rotation scanning mechanism that performs scanning by rotating the fan beam X-ray source and the plurality of sensors around the virtual central axis;
When any two X-rays having a symmetrical central angle with respect to the fan-shaped center line formed by the X-rays are set as a reference angle X-ray and a symmetrical angle X-ray,
The absorption filter is
A computer-aided X-ray tomography apparatus having a configuration in which the energy spectrum distribution of the reference angle X-ray and the symmetric angle X-ray after passing through the absorption filter is different from the other constant multiple. A computer-aided X-ray tomography apparatus according to any one of claims 1 to 11,
The absorption filter is
A computer-aided X-ray tomography apparatus arranged between the X-ray source and the imaging target space. An X-ray source that generates X-rays having an energy spectral distribution within a predetermined energy region;
One or more sensors that are arranged to face the X-ray source via the imaging target space in which the subject is arranged and detect the intensity of the X-ray that reaches from the X-ray source via the imaging target space When,
Second X-rays emitted from the X-ray source with respect to a first state where the first X-rays emitted from the X-ray source reach one of the one or more sensors through a first beam path. However, the second state of reaching the any one of the one or the plurality of sensors through the second beam path that travels in the same place as the first beam path in the imaging target space. A moving scanning mechanism that moves the X-ray source and the sensor with an existing moving scanning pattern and scans the imaging target space with the X-ray;
A computer-aided X-ray tomography apparatus remodeling method comprising: a moving scanning mechanism including a rotational movement mechanism that rotationally moves the X-ray source and the sensor around a virtual central axis in the imaging target space,
An absorption filter that absorbs part of the X-rays;
Arranged between the X-ray source and the sensor;
Fix the relative position with respect to the rotational movement mechanism,
Except when both the first beam path and the second beam path pass through the virtual central axis,
A computer-aided X-ray tomography arranged such that a first absorption path located in the absorption filter in the first beam path and a second absorption path located in the absorption filter in the second beam path do not overlap. It is a remodeling method of the photographing device,
The absorption filter is
About the first X-ray and the second X-ray
In the first state, through the first absorption path, the energy spectrum distribution of the first X-ray that reaches one of the one or more sensors, and in the second state, the second absorption path A computer-aided X-ray tomography having a configuration in which the energy spectrum distribution of the second X-ray reaching one of the one or more sensors is different from the other constant multiple. How to remodel the photographic equipment. An X-ray source that generates X-rays having an energy spectral distribution within a predetermined energy region;
One or more sensors that are arranged to face the X-ray source via the imaging target space in which the subject is arranged and detect the intensity of the X-ray that reaches from the X-ray source via the imaging target space When,
Second X-rays emitted from the X-ray source with respect to a first state where the first X-rays emitted from the X-ray source reach one of the one or more sensors through a first beam path. However, the second state of reaching the any one of the one or the plurality of sensors through the second beam path that travels in the same place as the first beam path in the imaging target space. A moving scanning mechanism that moves the X-ray source and the sensor with an existing moving scanning pattern and scans the imaging target space with the X-ray;
A moving scanning mechanism including a rotational movement mechanism for rotating the X-ray source and the sensor around a virtual central axis in the imaging target space;
In a computer-aided X-ray tomography apparatus comprising:
An absorption filter that is disposed between the X-ray source and the sensor and absorbs a portion of the X-ray before the X-ray reaches the sensor;
When the relative position with respect to the rotational movement mechanism is fixed at a predetermined position of the computer-assisted X-ray tomography apparatus,
Except when both the first beam path and the second beam path pass through the virtual central axis,
The first absorption path located in the absorption filter in the first beam path and the second absorption path located in the absorption filter in the second beam path are arranged so as not to overlap each other.
In the first state, through the first absorption path, the energy spectrum distribution of the first X-ray that reaches one of the one or more sensors, and in the second state, the second absorption path A computer-aided X-ray tomography apparatus having a configuration in which the energy spectrum distribution of the second X-ray reaching one of the one or more sensors is different from the other constant multiples. Absorption filter used. Of the computer-aided X-ray tomography apparatus, an X-ray is arranged between a fan beam X-ray source that generates X-rays in a fan shape and a space to be imaged, and is emitted from the fan beam X-ray source toward a plurality of sensors. An absorption filter that absorbs part of the line,
In the state of being arranged in the computer-assisted X-ray tomography apparatus,
Two X-rays having arbitrary central angles symmetrical with respect to a source-center axis line connecting the fan beam X-ray source and the virtual center axis of the imaging target space are converted into a reference angle X-ray and a symmetry angle X When a line
An absorption filter having a form in which the energy spectrum distribution of the reference angle X-ray and the symmetric angle X-ray after passing through the absorption filter is different from the other constant multiple. Of the computer-aided X-ray tomography apparatus, an X-ray is arranged between a fan beam X-ray source that generates X-rays in a fan shape and a space to be imaged, and is emitted from the fan beam X-ray source toward a plurality of sensors. An absorption filter that absorbs part of the line,
In the state of being arranged in the computer-assisted X-ray tomography apparatus,
When the thickness in the direction along the reference angle line that forms the central angle φ with the source-center axis line connecting the fan beam X-ray source and the virtual central axis of the imaging target space is h (φ),
It has a form that satisfies the following formula (C2) h (φ) ≠ h (−φ) (φ> 0) (C2)
Absorption filter.