JP4006451B2 - X-ray CT apparatus and misalignment correction method thereof - Google Patents

Description

  The present invention relates to an X-ray CT apparatus used for imaging a tomographic plane of a subject or a misalignment correction method thereof, and in particular, a direction around a rotation axis of a system (channel direction) and a direction of a rotation axis perpendicular thereto. The present invention relates to a multi-slice CT device or a cone beam CT device in which X-ray detector elements are arranged two-dimensionally (in the slice direction) and a misalignment correction method thereof.

  Conventionally, a fan beam X-ray CT apparatus is known. This is a CT that irradiates a fan-shaped X-ray beam from an X-ray source and collects data of X-rays that have passed through a subject with an X-ray detector of about 1000 channels arranged in a line around the rotation axis. Device. Normally, data is collected about 1000 times in one rotation while rotating around the subject (one data collection is called one view), and an image is reconstructed based on the data. As a reconstruction method in such a fan beam X-ray CT apparatus, fan beam projection data is rearranged and interpolated to convert fan beam projection data into parallel beam projection data, and this conversion-created parallel beam projection A fan-para conversion method that backprojects data to obtain a desired tomographic image, and a centering method that backprojects fan beam projection data once to a reference axis (centering axis) and backprojects each pixel row of the reconstructed image again. Two types of methods are devised and adopted.

  Both methods perform high-speed processing using dedicated hardware, but these reconstruction methods naturally do not have parameters in the slice thickness direction (the body axis direction of the subject), The configuration is completed by processing in each plane.

  A helical scan method is known as a method for reducing the data collection time. The basic form of the fan beam X-ray CT apparatus is based on the principle that the X-ray focal point rotates on the same cross section of the subject, and the subject is sent in the body axis direction by the slice thickness every time it rotates. In response to a request to scan a wider range in the body axis direction of the subject in a short time, scanning is performed while moving the subject relatively in the body axis direction (the X-ray focal point for the subject is The helical scan method (which draws a spiral trajectory instead of a circular trajectory) has been devised and put into practical use.

  In the helical scan method, since the position of the focal point in the body axis direction is shifted for each focal angle, a strong artifact (false image) is generated if the fan beam reconstruction is performed as it is. As a countermeasure, a fan beam reconstruction method is adopted by interpolating from two data of the equivalent rotational phase across the reconstruction plane, converting and creating the projection data of the reconstruction plane of the corresponding rotational phase. ing. As this interpolation method, a 360 ° interpolation method using projection data for two rotations with the reconstruction plane sandwiched, and a projection data for one rotation with the reconstruction plane sandwiched, the reconstruction is performed using the opposite beam. Two types of counter beam interpolation methods (180 ° interpolation methods) for converting and creating planar projection data have been devised, and they are properly used according to the purpose.

  The reconstruction method in the helical scan uses parameters in the body axis direction in the projection beam interpolation, but in the back projection, it is the same as the basic form of the fan beam X-ray CT apparatus, and in the slice thickness direction (the body of the subject). It does not have parameters in the (axial direction), and is in the same plane.

  In general, the X-ray tube alignment adjustment in the channel direction in the conventional single slice CT apparatus and the fan beam CT apparatus measures the position of the focus in the channel direction, and moves the tube so that it matches the designed position. A fine adjustment mechanism is used. However, a position error (misalignment) of about several tens of microns finally remains due to the limit of mechanically moving the tube and the limit of processing and assembly accuracy of the tube. In such a state, the design X-ray path and the actual X-ray path are misaligned, and as a result, there is a problem that artifacts occur in reconstruction (particularly, high resolution mode). In view of this, the misalignment amount finally remaining due to the limit at the time of alignment adjustment is used to correct the reconstruction parameter and perform correction to match the actual X-ray path.

  Next, focal point movement and countermeasures in a conventional fan beam X-ray CT apparatus will be described.

The principle of the fan beam X-ray CT apparatus is based on the premise that the focal point rotates in the same plane, and the position of the fan beam incident on the X-ray detector needs to be stable. Even in the helical scan method, although a spiral trajectory is formed relative to the subject, in reality, the rotation of the focal point and the linear movement of the subject in the body axis direction are combined.

  Actually, however, the focal position shifts from the center plane of the X-ray detector and optical system (such as a pre-collimator) due to the limit of the position adjustment (alignment) of the X-ray tube and the X-ray detector (focal misalignment). ) Cannot be reduced to 0, or the focal position moves in the slice thickness direction depending on the tube structure, such as thermal expansion due to temperature rise of the target when X-rays are generated.

  The deviation of the focal position can be suppressed to 0.1 mm or less. In the current X-ray CT apparatus, the influence is within an allowable range and does not cause a big problem.

  On the other hand, the focus movement is a problem that cannot be ignored. The focus movement of a rotating anode type X-ray tube as shown in FIG. 16 will be described as an example.

  In general, an X-ray tube accelerates and focuses thermoelectrons emitted from a filament (cathode) 101 by a high voltage applied between the cathode and anode, and collides the high-velocity electrons with a target (anode) 103. X-rays are generated.

  The conversion efficiency from electric energy to X-ray energy is proportional to the product of the acceleration voltage and the atomic number (Z) of the target material. For example, if the tube voltage is 100 kV and the target material is tungsten (Z = 74), the X-ray The conversion efficiency is only 0.74%, and the remaining 99% or more is converted into heat.

  For this reason, a material having a high melting point and a high atomic number is used for the target, but a rotating anode structure is employed to effectively reduce the amount of heat generated per unit area of the target. That is, the target 103 is formed in a disk shape, and a rotor 111 is fixed to a rotating shaft 105 connected to the center of the target 103, and the rotor 111 is rotationally driven from the outside of the glass bulb 113 by a stator (not shown) that generates a rotating magnetic field. .

  In such a rotating anode structure, since it is difficult to directly cool the anode, most of the heat generated in the target 103 is emitted from the target 103 as radiant energy, but a part of the generated heat is generated. Is released through the rotating shaft 105 by heat conduction. When X-ray irradiation is repeated from such a rotating anode X-ray tube, the amount of heat accumulated in the target 103 in the X-ray tube increases, and the temperature of the rotating shaft 105 and the like increases. As the temperature rises, the rotating shaft 105 and the like undergo thermal expansion, and the X-ray focal point moves in the slice direction as indicated by a position 123 (white circle) moved from the designed focal point 121 (black circle) (FIG. 16, the thermal expansion is exaggerated.)

  Since the X-ray CT apparatus adjusts the X-ray beam shape by an optical system such as the slit 115, the beam position incident on an X-ray detector (not shown) changes from a solid line arrow to a broken line arrow when focal movement occurs. Causes fluctuations as shown. When the beam position changes, the X-ray detector output data varies depending on the sensitivity distribution (slice thickness direction) on the X-ray detector side, and the sensitivity distribution varies for each element of the X-ray detector. Cause artifacts on the image.

  Countermeasures for artifacts due to this focal movement include reducing the focal movement, a method of preventing the X-ray detector incident position of the beam from changing even if the focal spot is moved, and reducing variations in sensitivity distribution of the X-ray detector. In addition to the above, a method for correcting the sensitivity change of the X-ray detector has been devised.

  Details of these correction methods are described in JP-A-6-169914 (publication date: June 21, 1994, Japanese Patent Application No. 4-325159, application date: December 4, 1992) and JP-A-6-269443. Publication (publication date: September 27, 1994, Japanese Patent Application No. 5-57645, application date: March 18, 1993). In this correction method, the focus movement range is divided into several parts, X-ray detector sensitivity correction data is collected and stored in advance for each small range, and the X position corresponding to the focus position at the time of data collection of the subject is obtained. This is a method of correcting the X-ray detector sensitivity using the line detector sensitivity correction data.

  In addition, an X-ray source that emits a conical X-ray beam and a plurality of fan beam X-ray detector rows are stacked on the cylindrical surface such that N rows are stacked in the Z-axis (rotation axis, body axis of the subject) direction. A cone beam (multi-slice) X-ray CT apparatus is known which is composed of a two-dimensional X-ray detector in which X-ray detector elements (M channels × N rows) are arrayed.

  A typical reconstruction (Feldkamp reconstruction) method in this cone beam X-ray CT apparatus is disclosed in Non-Patent Document 1.

  This reconstruction method is obtained by extending a fan beam (in a two-dimensional plane) reconstruction algorithm [Filtered-Backprojection], which is a mathematically exact reconstruction method, in the Z-axis direction. This approximate three-dimensional reconstruction algorithm is intended for conventional scanning (static scanning) using a cone beam, and includes the following steps (a) to (c).

(A) Projection data weighting step
The projection data is multiplied by a term depending on the Z coordinate and a cos term.
(B) Convolution calculation step
A convolution operation between the data of step (a) and the same reconstruction function as the fan beam is performed.
(C) BackProjection step
The data of step (b) is back projected onto the path (from the focal point to the channel of the X-ray detector) through which the X-ray has passed. That is, the point where the straight line passing through the voxel to be back projected intersects the X-ray detector plane is calculated from the focal point, and the data to be back projected from the data of step (b) around that point is created by interpolation or the like. Back projection by weighting with FCD / FVD2. Backprojection takes place over 360 °.

  Next, the fan beam reconstruction method and the cone beam reconstruction (Feldkamp reconstruction) method are very similar, but the back projection method is greatly different. In two-dimensional fan beam reconstruction, all the pixels (pixels) in the reconstruction plane are back-projected from X-ray detector data arranged one-dimensionally, whereas cone beam (Feldkamp) reconstruction is performed. In, the point where the straight line connecting the voxel to be reconstructed intersects the two-dimensional X-ray detector plane is obtained, and the data of the X-ray detector located at this intersection is back-projected onto the voxel on the straight line.

  Therefore, when a certain surface is reconstructed, the data of a specific X-ray detector channel of a specific X-ray detector array is back-projected only to a part of the voxels of the reconstructed surface, so that each voxel Since it is necessary to select data (X-ray detector row and X-ray detector channel) to be back-projected, the three-dimensional positional relationship between the straight line connecting the reconstructed voxel and the focus and the X-ray detector surface Become important. In addition, when considering an X-ray detector array having the same Z coordinate and a straight line connecting the X-ray detector element and the focal point, the voxel is similar to the X-ray detector surface centered on the focal point (the cylindrical detector). In this case, the calculation of the positional relationship becomes very complicated.

Next, a helical scan reconstruction method (Feldkamp-Helical) in a cone beam X-ray CT apparatus will be described.
Non-Patent Document 2 and Patent Document 1 disclose a method of helically scanning the periphery of a subject using a two-dimensional surface detector and performing reconstruction using a three-dimensional reconstruction method using the Feldkamp reconstruction method. .

Next, fan beam deemed multi-slice reconstruction in a cone beam X-ray CT apparatus will be described.
Various methods have been proposed in which cone beam data is collected, but cone beam data is regarded as fan beam data without performing cone beam reconstruction, and fan beam reconstruction is performed.

  In particular, interpolation methods in helical scanning are disclosed in Patent Documents 2, 3 and the like.

  The method described in Patent Document 3 is called a filter interpolation method, and procedure 1 for obtaining a plurality of interpolation data obtained by slightly shifting slice positions for an arbitrary phase, and weighting and adding the interpolation data to obtain a target phase. And procedure 2 for obtaining data. The slice thickness is shifted and the interpolated data is weighted and added, so the execution slice thickness increases slightly, but the effect of switching the X-ray detector array decreases in inverse proportion to the number of interpolation data, resulting in fewer artifacts and higher image quality. Improve.

Next, a dose change accompanying a focal shift in a cone beam (multi-slice) X-ray CT apparatus and countermeasures will be described.
Also in the cone beam (multi-slice) X-ray CT apparatus, it is expected that the focal position shift and the focal movement phenomenon occur as in the fan beam X-ray CT apparatus.

  The measures described in the section of the fan beam X-ray CT apparatus, including the correction method, can be considered for the focus movement. This correction method is aimed at correcting “focus movement → beam position variation → occurrence of artifacts due to variation in sensitivity distribution of X-ray detector”, but in a cone beam (multi-slice) X-ray CT apparatus, If the beam position fluctuates, there is a problem that the output of the detection element in each column changes even if the slice direction sensitivity distribution of the X-ray detector does not vary, and countermeasures for this are required. As a countermeasure against this, details of the correction method are disclosed in Patent Document 4. This is a method of providing a profile detector for monitoring the dose for each X-ray detector row and correcting the dose change for each main detector in each row.

  As described above, with regard to the accuracy of the focal position, in the fan beam X-ray CT apparatus, there are focal misalignment (adjustment deviation, not change over time) and movement over time due to heat, and the former allows for deviation. It is not a problem because it can be suppressed to a level, and the latter is solved by taking several measures including correction.

  On the other hand, it has been explained that a cone beam (multi-slice) X-ray CT apparatus already has a method for correcting the output fluctuation of the X-ray detector with respect to the focal point movement with time.

  In the multi-slice CT system, a data processing method called a segment bundling method is known. This method is a method in which data collected by a plurality of detection elements arranged in the slice thickness direction is added or bundled by interpolation or the like to obtain data corresponding to a thick slice and then processed. 6 etc.

  In addition, the detection elements arranged in the slice thickness direction are arranged not only in the same size but also in the arrangement of elements having different sizes as in Patent Document 7, for example. Has been.

  However, the misalignment correction in the channel direction in the conventional single-slice CT apparatus and multi-slice CT apparatus is intended to correct the misalignment remaining at the time of tube alignment, and changes with time, such as focus movement due to thermal expansion, etc. This is not intended for misalignment or misalignment due to an assembly error of each detection element.

  In addition, the cone beam (multi-slice) X-ray CT apparatus has the following problems related to the focal position.

  First, in an X-ray CT apparatus that performs cone beam reconstruction, projection data to be back-projected onto one pixel (voxel) in a certain reconstruction plane has a straight line connecting the focal point and the voxel as a two-dimensional X-ray detector. It is necessary that the projection data is an intersection point. By design, a plane that passes through the center of the slice direction of the two-dimensional X-ray detector and is orthogonal to the rotation axis is defined as a central cross section (Mid-Plane). When the focal position is deviated from the central cross section for some reason (the X-ray detector is displaced relative to the design position, it is relatively the same in terms of reconstruction. (Describe the case where the focus is shifted with respect to the line detector.)

  Output data at a point on the two-dimensional X-ray detector assumed from the designed X-ray path (obtained directly or obtained by interpolating the output data of detection elements around that point The output data at the point) is actually projection data of a completely different X-ray path. In other words, when considering the target reconstruction plane, data to be back-projected onto another voxel may be back-projected onto a certain voxel. If the projection data is back-projected to a completely different position, it can be easily estimated that it will cause an artifact.

  This problem occurs not only with focus movement but also with a constant misalignment in the channel direction, such as misalignment (misalignment parallel to the pre-collimator) when an X-ray detector is attached to the system. In addition, this problem is different from the output variation due to the variation in sensitivity distribution in the slice direction of the X-ray detector and the fluctuation of the beam on the X-ray detector, and thus there is no effect of the correction method shown in the prior art.

  Further, even in a multi-slice X-ray CT apparatus that performs fan beam-reconstructed reconstruction, if the focal point moves in the slice thickness direction, the Z coordinate of the collected data deviates from the design value. There arises a problem that an error occurs in the filter interpolation in the helical mode described above.

  The above are problems associated with the relative misalignment between the focal point and the X-ray detector due to misalignment when the entire X-ray detector is attached, misalignment of the position, and movement over time. Apart from these, problems also exist inside the X-ray detector. Due to the structure of the X-ray detector, assembling all the elements integrally has a big problem in cost and the like, and it is general to divide and assemble several blocks having a plurality of elements.

  When the X-ray detectors are finally arranged, it is unavoidable that the alignment in the slice direction is out of order for each block or channel. Therefore, the deviation of the Z coordinate of the collected data described above is different for each block or channel. This leads to the problem of being different. Even if there is no focus shift or X-ray detector mounting misalignment, the Z coordinate of the collected data deviates from the design value. This problem occurs not only in the X-ray detector but also in the misalignment in the slice direction that changes in the channel direction, such as torsion with the pre-collimator when the X-ray detector is attached to the system.

  In a multi-slice CT system, in an actual system, not only the misalignment in the slice direction but also the misalignment in the channel direction may remain due to the limit of the assembly accuracy of the detector.

For this reason, when performing segment bundling data processing, it is expected that the misalignment amount in the channel direction of each of the detection elements arranged in the slice thickness direction supplying the original data to be bundled will be different. There arises a new problem that the amount of misalignment differs depending on the position of the original data.
Japanese Patent Laid-Open No. 9-19425 JP-A-8-19532 JP-A-9-234195 JP-A-8-154926 Japanese Patent No. 1513867 JP-A-4-224736 JP-A-6-169912 “Practical cone-beam algorithm”. A. Feldkamp, L.M. C. Davis, and J.M. W. Kress, J.M. Opt. Soc. Am. A / Vol. 1, no. 6, pp. 612-619 / June 1984. “Three-dimensional helical scan CT using conical beam projection”, Hiroyuki Kudo, Tohoku University, Tsuneo Saito, University of Tsukuba, IEICE Transactions DII Vol. J74-D-II, no. 8, pp. 1108-1114, August 1991.

An object of the present invention is to reduce an artifact caused by a deviation between a design X-ray path and an actual X-ray path.

The present invention includes an X-ray source that generates an X-ray cone beam, and a two-dimensional X-ray detector in which detection elements that detect X-rays transmitted through the subject are two-dimensionally arranged. X-ray CT for generating an image related to the reconstruction plane by cone beam reconstruction based on data obtained by each detection element obtained by rotating and scanning at least the X-ray source around the space to be arranged In the apparatus, focus position detection means for detecting a relative position of the focus of the X-ray source with respect to the two-dimensional X-ray detector with respect to the slice direction , and back projection to each of the voxels constituting the reconstruction plane the combination of detector elements for the data to be used to interpolate the data, changed according to the detected focus position, and back projection data generated by the interpolation to said voxel each re ; And a forming means.

  According to the present invention, misalignment (misalignment parallel to the pre-collimator) or focus misalignment (positional change over time (focal movement) and limit of accuracy of position adjustment when the entire X-ray detector is attached to the system. Artifacts that occur in principle during image reconstruction in accordance with (misalignment) due to (1) can be removed, and a good reconstructed image can be obtained.

  Next, an embodiment of an X-ray CT apparatus according to the present invention will be described in detail with reference to the drawings.

  FIG. 1 is a system configuration diagram showing an X-ray CT apparatus 1 according to the present invention. As shown in the figure, an X-ray CT apparatus 1 includes an X-ray tube 3 that is an X-ray beam generator, a slit 5, a main X-ray detector 7 in which detection elements are two-dimensionally arranged, and a profile. The detector 9, the main X-ray detector data acquisition device 11, the profile detector data acquisition device 13, the data processing device 15, and a plurality of re-reads respectively corresponding to a plurality of focal positions (a, b,...) In advance. , And a plurality of beam position information 33a, 33b,... Corresponding to a plurality of focal positions, a high voltage generator 19, a host controller 21, and a display 23, respectively. The reconfiguration device 25, the auxiliary storage device 27, and a bus 29 for connecting these devices are provided.

  In FIG. 1, a host controller 21 is a central control device that controls the operation of the entire X-ray CT apparatus 1. The high voltage generator 19 supplies a high voltage to the X-ray tube 3 according to a control signal from the host controller 21 to generate X-rays. The generated X-rays pass through the subject P, and the energy is converted into current by the main X-ray detector 7 arranged to face the X-ray tube 3.

  At this time, the X-ray tube 3, the slit 5, the main X-ray detector 7, and the profile detector 9 are integrally rotated around the subject P by a gantry rotating device (not shown). The gantry rotating device is controlled by a control signal from the host controller 21. The signals detected as currents by the main X-ray detector 7 and the profile detector 9 are amplified by the main detector data collecting device 11 and the profile detector data collecting device 13, respectively, converted into voltage signals, and A / D converted. After that, it is transferred to the data processor 15 as projection data of a digital signal.

  In the data processing device 15, dark current correction for each element of the X-ray detector is performed on each projection data, and then logarithmic conversion is performed and stored in the storage device 17.

  The projection data stored in the storage device 17 is transferred to the image reconstruction device 25. The tomographic image imaged by the image reconstruction device 25 is transferred to the display device 23 and displayed. Further, the collected projection data or the imaged tomographic image is transferred to the auxiliary storage device 27 and stored therein.

  In order to detect the focal movement of the X-ray tube 3, the slit 5 is provided with a gap through which the X-ray beam XP for the profile detector 9 passes. The focus position calculation function incorporated in the profile detector 9 that detects the X-ray beam XP, the data detector 13 for the profile detector, and the data processor 15 is capable of relative to the X-ray tube 3 relative to the main X-ray detector 7. A focal position means for detecting a focal position is configured.

  An example of the profile detector 9 and the profile detector data collection device 13 will be described. As shown in FIG. 2, the profile detector 9 is composed of two detection elements A and B of the X-ray detector, and each element is arranged in the scintillator 41 and the lower part thereof as shown in FIG. The photodiode 43 is configured. The scintillator 41 emits light according to the incident X-ray intensity, and the light emitted from the scintillator 41 is converted into a current by the photodiode 43. The current is amplified by the profile detector data collecting device 13, converted into a voltage signal, and converted into a digital signal after A / D conversion.

  These digital voltage signals are corrected so as to be proportional to the incident X-ray energy, and then the ratio is calculated by the data processor 15. The calculated digital voltage signal indicates the ratio of the X-ray irradiation area on the elements A and B of the profile detector 9. As shown in FIG. 4A, the amount of movement of the X-ray focal point in the slice direction can be detected by combining the slit 5 and the profile detector 9.

FIG. 5 shows the flow of the correction process. There are two modes using a two-dimensional X-ray detector, a static mode with a bed fixed and a helical mode as acquisition methods, and a cone beam reconstruction method and a fan beam-reconstruction method as reconstruction methods, totaling 4 types. Suppose you have different modes. (Not all may be provided, for example, if there is only one mode, the mode is fixed.)
First, a scan mode is selected from the above four modes in accordance with an instruction from the operator (step S10). After the mode is selected, scanning is started in accordance with an operator instruction (step S20). When scanning is started, X-ray exposure from the X-ray tube 3 starts, and projection data collection of the subject P by the main X-ray detector 7 and the main X-ray detector data acquisition device 11 (step S30). ), The focus position is measured by the profile detector 9 and the profile detector data collection device (step S40), and the focus position is calculated by the data processing device 15 (step S50).

  Based on the obtained focal position, the reconstruction parameter closest to the actual focal position is selected from the reconstruction parameters 31a, 31b,... Respectively corresponding to the plurality of focal positions stored in the storage device 17. Similarly, the beam position information closest to the actual focus position is selected from the beam position information 33a, 33b,... Corresponding respectively to the plurality of focus positions stored in the storage device 17, and the reconstruction device 25 is selected. To correct misalignment (step S60).

  Next, the process proceeds to image reconstruction (step S70) and image display (step S80) using the collected projection data and the corrected reconstruction parameters. In step S60, the correction of the reconstruction parameter may be considered as a part of the reconstruction, and in the block diagram of FIG. 1, the reconstruction device 25 may have the function.

Next, a correction method will be described for each scan mode.
First, correction in cone beam reconstruction will be described. The static mode and the helical mode will be described without distinction.
FIG. 6 shows the geometry of the cone beam CT apparatus design. The axis of rotation is taken on the vertical axis, and the horizontal axis is set on a plane parallel to the slice plane. A cross section in a horizontal direction at a phase with a focal point on the left side and an X-ray detector on the right side. The X-ray detector will be described as 16 elements. As already described, in cone beam reconstruction, a point where a straight line connecting a focal point and a voxel to be reconstructed intersects a two-dimensional X-ray detector surface is obtained, and this is back-projected onto the straight line. Considering voxel a and voxel b in the reconstruction plane shown in FIG. 12, no. The data of the beam A incident on almost the center of the X-ray detector No. 13 (for example, obtained by interpolation from the data of the No. 12 and No. 13 X-ray detectors) is back projected onto the voxel a. The data of the beam B incident on almost the middle (near No. 10) of the X-ray detector of No. 11 (for example, obtained by interpolation from the data of the X-ray detectors of No. 10 and No. 11) is projected back to the voxel b. It will be.

  Here, when the relative position of the focus with respect to the X-ray detector changes for some reason such as thermal focus shift (hereinafter referred to as focus shift), the state shown in FIG. 7 is obtained. As data of the beam A (shown by a dotted line in FIG. 7) in FIG. 12. No. The data obtained by interpolation from the data of 13 X-ray detectors is actually incident on the X-ray detector through the position of the beam A shown in FIG. (The same applies to beam B). In other words, data that has passed through a completely different place is back-projected on voxel a. (Voxel b is the same). Back-projecting onto voxels on the path through which the beam has passed is the basis of reconstruction, and it is easily analogized that artifacts occur in the state of FIG.

Therefore, in the present invention, this deviation is corrected by the method shown in FIGS.
In FIG. 8, the beam A is assumed as data to be back-projected on the voxel a in accordance with the actual focal position. 12, no. Assuming beam A ′ instead of data obtained by interpolation from the data of X-ray detectors 13, 14, no. Data obtained by interpolation is selected from the data of 15 X-ray detectors. The same applies to voxel b. 10, no. Data of the beam B ′ (for example, data obtained by interpolation from the data of the X-ray detectors of No. 10 and No. 11) instead of the data obtained by interpolation from the data of the X-ray detector of No. 11 The method of backprojecting is taken. As a result, the data actually passing through the corresponding voxel is back-projected onto the voxel, and the original performance of cone beam reconstruction can be exhibited.

  Here, a simple implementation method of FIG. 8 will be described. The basis of the method of FIG. 8 is to change the combination of interpolation (the combination of detection elements and the weight) for obtaining data to be back-projected to the corresponding voxel in accordance with the actual focal position. In order to change the combination of interpolations for continuously changing focal positions, it is necessary to calculate and obtain the combination of interpolations each time. Therefore, first, the range of focus movement that is predicted in advance is divided by a width that allows artifacts due to misalignment, and one combination of interpolations is set as a representative of the range. Then, at the time of actual data collection, a method of selecting and interpolating a combination of interpolations at positions closest to the measured focal position is adopted. By setting and storing the interpolation combination in advance, extra time for calculating the interpolation combination becomes unnecessary.

  FIG. 9 shows a method of back-projecting the data of the beam A at the actual focal position onto the voxel a ′ that has actually passed. Assuming a voxel a ′ where the straight line connecting the actual focal position and the position on the X-ray detector on which the corresponding beam is incident intersects the reconstruction plane, the design should be backprojected to voxel a This data is back-projected onto the voxel a ′. As a result, the data actually passing through the corresponding voxel is back-projected onto the voxel, and the original performance of cone beam reconstruction can be exhibited.

  The method of FIG. 8 changes the data to be back-projected on the voxel based on the voxel position, while the method of FIG. 9 uses the data detected by the detector as a reference. Thus, the voxel position where this data or data interpolated from this data is to be backprojected is changed according to the focal point movement. As a result, the method of FIG. 8 is the same as the method of FIG. 9, and the same effect can be obtained regardless of which method is selected according to the configuration of the reconstruction device.

  Second, the mode using the fan beam deemed reconstruction method will be described. FIG. 10 shows the design geometry. The axis of rotation is taken as the vertical axis, and the horizontal axis is set parallel to the slice plane. A cross section in a horizontal direction at a phase with a focal point on the left side and an X-ray detector on the right side. Here, the X-ray detector is described as eight elements. As already described, the cone beam data is regarded as fan beam data, and the fan beam reconstruction is performed. This is a method of reconstructing the data obtained by the X-ray detector No. 6 as fan beam data in the shaded plane. For example, no. The position where the straight beam A connecting the center of the detection element 6 and the focal point position intersects with the rotation center axis is the center of the reconstruction plane, and this is the coordinates in the slice thickness direction (Z axis) of the image. Become.

nview: view number
Zview: Z coordinate of the plane including the design focal point in an arbitrary view (constant in static mode, changes according to helical pitch in helical mode)
nseg: X-ray detector seg number
Zseg: any seg. Of the X-ray detector. From the plane including the design focus on the X-ray detection surface
FCD: Distance from the focal point to the rotation axis (Z-axis)
FDD: If the distance from the focal point to the X-ray detection surface, the Z coordinate of the beam at (nview, nseg): Z (nview, nseg) is
Z (nview, nseg) = Zview + Zseg × FCD / FDD (1)
Equation (1) is obtained. In the static mode, this coordinate is the Z coordinate of the image as it is, and in the helical mode, the Z coordinate of each collected data is a parameter for determining the weight of interpolation.

  Here, when the relative position of the focal point with respect to the X-ray detector changes due to some cause such as thermal focal movement (hereinafter referred to as focal movement), the state shown in FIG. 11 is obtained. For example, no. The straight beam A connecting the center of the detection element 6 and the actual focal position changes as shown in FIG. 11, and the position intersecting the rotation center axis (the center of the reconstruction plane) is the design position (thin mesh). Is hung).

Here, if Δf is the amount of movement of the focal point, the Z coordinate of the beam in the focal point moving state at (nview, nseg): Z ′ (nview, nseg) is
Z (nview, nseg)
= Zview + (Zseg- [Delta] f) * FCD / FDD + [Delta] f = Zview + Zseg * FCD / FDD + [Delta] f * (1-FCD / FDD)
= Z (nview, nseg) + Δf × (1−FCD / FDD) (2)
Equation (2) is obtained. By using Z ′ (nview, nseg) based on the actually measured focus position instead of the design value Z (nview, nseg) as the Z coordinate of the image or the collected data, the relative position of the focus position The effect of misalignment due to various fluctuations can be suppressed.

  In cone beam reconstruction, complex correction processing is required to adjust the combination of interpolations, but in the case of the fan beam deemed reconstruction method, it is only necessary to add correction to the Z-coordinate of the collected data, so that simpler processing is possible. This makes it possible to correct misalignment.

  Next, a method of correcting misalignment in the slice direction that changes in the channel direction will be described.

  In this case, first, it is necessary to measure the displacement of the arrangement position of each detection element with respect to the system. The element arrangement inside the X-ray detector can be measured in the manufacturing process. For example, there is a method in which an X-ray film is disposed behind the X-ray detector, X-rays are irradiated from the front, and the array position shift of each detection element is measured from the shadow of the X-ray detector. The X-ray film may be replaced with another X-ray detector for measurement. The detection element data measured here is managed together with the X-ray detector main body, and when the X-ray detector is attached to the system, the X-ray detector element arrangement data is input to the system and used for correction.

  Next, when the X-ray detector is attached to the system, it is measured whether the pre-collimator is twisted. Measure the profile of the pre-collimator at the elements at both ends in the channel direction of the X-ray detector, and detect the difference between the profiles at both ends and the corresponding ends of the detection element array data measured and input to the system in advance. The twist between the pre-collimator and the X-ray detector center line (for example, the average value of all channels) is obtained from the element arrangement data.

Next, from these data, each seg. Each time, the array position (Z coordinate) for the system is calculated. The Z coordinate (Zseg) obtained here
(Nch, nseg)) can be used in place of Zseg described above to define the beam position of the data obtained by each element.

  The specific correction after that can be basically the same as the correction of misalignment in the slice direction, which is basically described above and constant in the channel direction. In the case of cone beam reconstruction, adjustment is performed by a combination that interpolates in accordance with the beam position. In the case of a multi-slice helical with no fan beam, the weight of filter interpolation is calculated for each detection element. If the variation of each element is small due to the structure of the X-ray detector and varies in units of blocks, a sufficient effect can be obtained by calculating in units of blocks instead of calculating for each element.

  However, a new method is also proposed in the case of the fan-beam deemed multi-slice static mode. As shown in FIG. 12A, consider a case where an X-ray detector is configured by using 6 detector blocks each having 4 channels and a total of 32 elements from 1 to 8 in the segment direction. When each detector block is incorporated into the system, it is assumed that a shift in the Z direction occurs for each block. In this system, a plurality of seg. Consider a case where two beams A and B are irradiated and data is bundled to form an image of two thick slices. If the data is added as it is without considering this misalignment in the Z direction, it is processed as an uneven beam as shown in FIG.

  Therefore, in this embodiment, a method is employed in which the data of the element irradiated with the beam is weighted and added based on the array data. Block No. with alignment 1, no. 4, no. 6 seg. Corresponding to the beam A and the beam B, respectively. No. 3, no. 4 and seg. No. 5, no. Each of the 6 data is added. On the other hand, in the other blocks, each beam is received (whether or not it is received can be determined from the detection element array data). The data is obtained by weighted addition. As a result, not the beam of FIG. 12B but the beam of FIG. 12A can be reproduced.

  Thus, even when the misalignment in the slice direction changes in the channel direction, the influence can be suppressed as in the case of the misalignment constant in the channel direction described above.

  In the above embodiments, misalignment in the slice direction has been described, but the present invention can also be applied to misalignment in the channel direction.

  That is, in the profile detector shown in FIG. 4A, the array of detector elements is expanded two-dimensionally, and a profile detection X-ray beam having an intensity distribution in the channel direction is used, thereby focusing in the slice direction. It is possible to detect not only the movement but also the focal movement in the channel direction.

  As shown in FIG. 4B, for example, a profile detection X-ray beam formed into a square is irradiated toward a profile detector in which four detection elements A, B, C, and D are squarely arranged. The fact that the focus movement in the slice direction is detected by the signal ratio of the detection elements A and B is the same as that in FIG. 4A, but in addition, the focus movement in the channel direction is detected by the detection elements A and C. Detection is possible based on the signal ratio (or B and D signal ratios).

  As for the data collection method for the detection error of the detector elements, a method using an X-ray film or the like has been shown in the manufacturing process, but this also involves collecting not only the slice direction but also the position information in the channel direction. Information necessary for correcting misalignment of directions can be obtained.

  Next, an embodiment of misalignment correction in the case of bundling data of a plurality of X-ray detection elements in a multi-slice system will be described.

  First, as shown in FIG. 13, when the detection elements in the slice direction are equal in size, an average value of the misalignment amounts of the respective elements to be bundled is obtained, and the misalignment amount of the data obtained by bundling the average values is obtained. As a result, the reconstruction parameter corresponding to the data is changed.

  In the case of FIG. 13, data from segment 1 to segment 4 having the same detection element size is bundled. That is, if the misalignment amounts of the elements belonging to the segments 1, 2, 3, 4 are δ1, δ2, δ3, and δ4, the misalignment amount of the bundled data is (δ1 + δ2 + δ3 + δ4) / 4.

  Next, as shown in FIG. 14, when the sizes of the detection elements in the slice direction are not equal, the weighted average value (weighted average value) of the misalignment amount is obtained according to the size of each element to be bundled, The weighted average value is regarded as a misalignment amount of the bundled data, and the reconstruction parameter corresponding to the data is changed.

  That is, if the sizes of the elements belonging to the respective segments 1, 2, 3, and 4 are L1, L2, L3, and L4, and the misalignment amounts are δ1, δ2, δ3, and δ4, errors in the bundled data are caused. The alignment amount is (δ1L1 + δ2L2 + δ3L3 + δ4L4) / (L1 + L2 + L3 + L4).

  As shown in FIG. 15, as a simple method, when the detection elements in the slice direction are not equal in size, the misalignment amount δ1 of the largest element (L1) among the elements to be bundled is bundled. The reconstruction parameter corresponding to the data may be changed by regarding the amount of data misalignment.

  The preferred embodiments have been described above, but these do not limit the present invention. The essence of the idea of the present invention is that the actual X-ray detector for the focal position detected by the focal position detecting means does not depend on the generation of the X-ray CT scanner, the scanning method, the type of scanning mode, or the difference in the interpolation method. According to the relative position, reconstruction parameters such as a combination of interpolation, beam position information, and the like are changed, and data for reconstruction is processed.

1 is a block diagram showing an embodiment of a cone beam X-ray CT apparatus according to the present invention. It is a focus position detection principle explanatory drawing of a focus position detection means. It is a block diagram which shows the structure of a focus position detection means. It is operation | movement explanatory drawing of a focus position detection means. It is a flowchart figure which shows the procedure of the focus movement correction | amendment in the cone beam X-ray CT apparatus concerning this invention. It is principal part sectional drawing explaining the geometry on the design of a cone beam X-ray CT apparatus. It is a fragmentary sectional view explaining the state before correction | amendment when misalignment, such as a focus movement, arises in a cone beam X-ray CT apparatus. It is a figure explaining the 1st correction method in the state where misalignment, such as a focal shift, occurred in the X-ray CT apparatus (cone beam) concerning the present invention. It is a figure explaining the 2nd correction method in the state where misalignment, such as a focus movement, occurred in the X-ray CT apparatus (cone beam) concerning the present invention. It is principal part sectional drawing explaining the geometry on the design of the multi-slice X-ray CT apparatus which performs fan beam deemed reconstruction. It is principal part sectional drawing explaining the position shift of the beam at the time of the focus movement of a fan beam deemed multi-slice X-ray CT apparatus. It is an X-ray detector development view explaining the positional deviation of the beam when there is variation between blocks. It is a figure explaining the amount of misalignment in the case of bundling X-ray detection data of a plurality of segments of the same size. It is a figure explaining the amount of misalignment in the case of bundling X-ray detection data of a plurality of segments of different sizes. It is a figure explaining the amount of misalignment (simple definition) in the case of bundling and using X-ray detection data of a plurality of segments of different sizes. It is a focus movement explanatory view of a rotating anode X-ray tube.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 ... X-ray CT apparatus, 3 ... X-ray tube, 5 ... Slit, 7 ... Main X-ray detector, 9 ... Profile detector, 11 ... Main X-ray detector data acquisition device, 13 ... Profile detector data acquisition Device ... 15 Data processing device 17 ... Storage device 19 ... High voltage generator 21 ... Host controller 23 ... Display device 25 ... Reconstruction device 27 ... Auxiliary storage device 29 ... Bus 31 ... Reconfiguration Parameter, 33: Beam position information.

Claims (4)

A space provided with an X-ray source that generates an X-ray cone beam and a two-dimensional X-ray detector in which detection elements that detect X-rays transmitted through the subject are two-dimensionally arranged, and in which the subject is arranged In an X-ray CT apparatus that generates an image related to a reconstruction plane by cone beam reconstruction based on data obtained by each of the detection elements obtained by rotating and scanning at least the X-ray source around
A focus position detecting means for detecting a relative position of the focus of the X-ray source with respect to the slice direction of the X-ray source with respect to the two-dimensional X-ray detector;
A combination of detection elements for data used for interpolating data to be back-projected on each voxel constituting the reconstruction plane is changed according to the detected focal position, and the data generated by the interpolation is changed. An X-ray CT apparatus comprising: a reconstructing unit that back-projects each voxel . Storage means for storing data relating to combinations of detection elements respectively corresponding to a plurality of positions of the focal point, and a combination of detection elements corresponding to the focal position closest to the detected focal position is specified. The X-ray CT apparatus according to claim 1. A space provided with an X-ray source that generates an X-ray cone beam and a two-dimensional X-ray detector in which detection elements that detect X-rays transmitted through the subject are two-dimensionally arranged, and in which the subject is arranged Misalignment of an X-ray CT apparatus that generates an image relating to a reconstruction plane by cone beam reconstruction based on data obtained by each of the detection elements obtained by scanning at least the X-ray source around In the correction method,
Detecting the relative position of the focus of the X-ray source with respect to the X-ray detector with respect to the slice direction during the acquisition of the projection data;
A combination of detection elements for data used for interpolating data to be back-projected to each voxel constituting the reconstruction plane is changed according to the detected focal position ,
A method of correcting misalignment of an X-ray CT apparatus, wherein the data generated by the interpolation is back projected onto each of the voxels . In order to specify the combination of the detection elements, the combination of the detection elements corresponding to the focal position closest to the detected focal position from the storage unit that stores data relating to the combination of the detection elements respectively corresponding to the plurality of focal positions. The method of correcting misalignment of an X-ray CT apparatus according to claim 3, wherein:

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