JP4649150B2 - Radiation imaging apparatus and imaging method - Google Patents

Description

  The present invention eliminates the influence of fluctuations in a subject in an X-ray CT (Computer Tomography) apparatus or the like that performs imaging using radiation such as X-rays, for example. The present invention relates to a radiation imaging apparatus and an imaging method for imaging a characteristic distribution.

  Conventionally, X-rays are exposed to a subject, X-rays transmitted through the subject or scattered by the subject are detected by an X-ray detector, and this X-ray detection output (number of X-ray photons) is used. An X-ray CT apparatus that captures a fluoroscopic image, a tomographic image, or a three-dimensional image of a subject is known.

  As such an X-ray CT apparatus, a cone beam CT (CBCT) has been developed. In a normal X-ray CT apparatus, the X-ray beam is cut out thinly in the Z direction, which is called a fan beam. In the cone beam CT, an X-ray beam that spreads in a cone shape (cone shape) in the Z direction is used. This X-ray beam is called a cone beam.

  As a CBCT, a format corresponding to a so-called third generation type or R / R type method has been examined with respect to a conventional type CT having only one row. In the third generation CT, a pair of an X-ray source and a detector scans around the subject while scanning to collect projection data.

  FIG. 6 shows an example of a CBCT apparatus, which belongs to the third generation CT apparatus. The X-ray detector 1 and the X-ray detector 2 rotate around the subject P around the Z axis as a rotation axis. The scanning of the region of interest is completed in one rotation.

  In a normal X-ray CT apparatus, in order to sample in the channel (CH) direction, detection elements are arranged in one line in the CH direction, and each element is identified by a channel number. On the other hand, in the CBCT apparatus, the detection elements are further arranged in the Z direction (ROW direction). That is, the X-ray detector 2 in the CBCT apparatus is configured by two-dimensionally arranging detection elements in an orthogonal lattice shape.

  According to such a CBCT apparatus, the X-ray detector 2 is configured by arranging detection elements in a lattice shape in two directions of the Z direction (ROW direction) and the CH direction, and the X-rays are also thick in the Z direction. By providing and exposing in a cone shape, projection data for a plurality of rows can be obtained collectively.

  The cone angle is a problem when photographing a plurality of slices simultaneously. In regions where the cone angle is large, reconstruction errors occur due to the lack of an X-ray beam transmitted through the cross section. In order to avoid this, the distance (FDD) between the X-ray focal point and the FPD (flat panel detector) may be increased and the cone angle may be decreased.

  However, if the distance is increased, the photographing system becomes larger, and it becomes difficult to rotate the X-tube and the FPD at a high speed, and it also hinders installation in the examination room. In view of this, it has been considered that the subject is gradually rotated to take an image instead of rotating the X tube and the FPD.

  When rotating the human body, 3 to 5 seconds per rotation seems to be appropriate. However, in such a long-time shooting, in addition to the body movement of the subject, the movement of the internal organs due to the heartbeat is a problem. It becomes. In particular, in pulmonary field imaging, pulmonary blood vessel movement due to the pulsation of the heart is significant. As shown in FIG. 7, during the period from the R wave to the T wave of the electrocardiogram, the size of the heart changes abruptly, and a large pressure is also applied to the artery so that the position of the blood vessel is extremely displaced. This extreme displacement of the blood vessel position not only lowers the resolution of the reconstructed image, but also generates artifacts and becomes an obstacle to disease diagnosis.

  Patent Document 1 discloses an X-ray CT apparatus that can achieve both reduction in X-ray exposure and suppression of image quality deterioration in electrocardiographic synchronous scanning. A high voltage generator that applies a high voltage to the X-ray tube to generate X-rays from the X-ray tube, an X-ray detector that detects X-rays coming from the X-ray tube through the subject, and this detection A tomographic reconstruction processor for reconstructing a tomographic image based on projection data detected by the instrument, an electrocardiograph for measuring an electrocardiogram relating to the subject, and an X-ray for a specific period in the heartbeat cycle of the subject And a system controller that controls the high-voltage generator based on the electrocardiogram in order to stop generation and generate X-rays in a period other than the specific period.

JP 2000-51208 A

  In this patent document, one cycle of heartbeat is morphologically classified into a systole, a relaxation period, and an equivalent relaxation period, and X-rays are emitted during an equivalent relaxation period with a small morphological variation. To collect. Here, the scanning speed is assumed to be 0.75 seconds per rotation, and the data collection is assumed to be half scanning. Here, if the fan angle is 50 degrees, 0.75 · (180 + 50) /360=0.48 seconds. That is, if the equivalent relaxation period is 0.48 seconds or less, the collection of half-scan data in the equivalent relaxation period is completed during one rotation.

  Therefore, considering the length of the equivalent relaxation period, according to Patent Document 1, it is assumed that the pulsation period is about 1 second, and that the equivalent relaxation period can be secured about 60% or more in one cycle of the heartbeat. 7 and the equivalent relaxation period will last approximately 0.6 seconds. In short, the invention according to Patent Document 1 has been completed based on the above premise, and in order to set the heart rate to 60 times / second, the patient is allowed to rest for about 1 hour by administering β blocker.

  However, as described above, in consideration of standing type CBCT using a large FPD, it takes 3 to 5 seconds or more per rotation to rotate the subject. The invention cannot be applied.

  An object of the present invention is to solve the above-described problems and to provide a radiation imaging apparatus and an imaging method for obtaining a CT image with reduced artifacts by removing a characteristic fluctuation period of a subject.

In order to achieve the above object, a radiation imaging apparatus according to the present invention includes an X-ray generation means, a two-dimensional detector that detects X-rays transmitted through a subject as a two-dimensional distribution, the X-ray generation means, and the two In a radiation imaging apparatus having a rotating means for rotating a subject in an imaging region formed by a dimension detector, a fluctuation detecting means for detecting inherent fluctuations of the subject, and a fluctuation period p from the fluctuation detecting means A period calculating means for calculating, a stationary period determining means for determining a ratio q of the stationary period to the period p from the fluctuation detecting means, and t = (n−q) · p (n is selected according to the age of the subject ) natural number) and so that that, and a rotation time calculating means for calculating a rotation time t of one rotation of said rotating means, said rotating means, said the rotating the subject by the rotation time t To do.

Further, the radiation imaging method according to the present invention is generated from the X-ray generation means and transmitted through the subject while relatively rotating the subject in an imaging region formed by the X-ray generation means and the two-dimensional detector. X-rays are detected as a two-dimensional distribution by the two-dimensional detector, the inherent variation of the subject is detected, the period p is calculated, the ratio q of the stationary period to the period p is determined, and t = ( The rotation time t of one rotation of the subject is calculated and rotated so as to be nq) · p (n is a natural number selected according to the age of the subject ).

  According to the present invention, it is possible to reduce artifacts due to the heart beat of the subject, and it is possible to cope with not only full scanning but also half scanning. If the static ratio cannot be limited, the image quality can be improved by shifting the static ratio when rearranging the data after completion of imaging, and a suitable table rotation time is determined based on the subject's pulsation period It becomes possible.

  When the stationary period in the fluctuation period can be specified before imaging, it is possible to reduce the exposure of the subject by performing X-ray exposure only during the stationary period. Even when the ratio q exceeds 0.5, the duplicate X-ray data can be systematically eliminated, so that the exposure of the subject can be reduced. Even when a pulse oximeter or the like is used, it is possible to obtain a reconstructed image free from artifacts similar to the case where an electrocardiograph is used.

  FIG. 1 is a plan view of the embodiment, and FIG. 2 is a side view. A subject P is located between the X-ray generation means 11 and the two-dimensional detector 12. The subject P rides on the rotary table 13 and touches the chest with a breastplate 14 attached to the table 13. The two-dimensional detector 12 is composed of a semiconductor sensor having one pixel of 250 × 250 μm, a total number of pixels of 1720 × 1720, and an outer shape of 43 × 43 cm, and its output is connected to the reconstruction means 15 via a BUS described later. Yes.

  X-rays emitted from the X-ray generation means 11 pass through the subject P, pass through the breastplate 14 and the scattered radiation removal grid (not shown), and reach the two-dimensional detector 12. The data acquired by the two-dimensional detector 12 is transferred to the reconstruction unit 15 and reconstructed to calculate a tomogram.

  FIG. 3 is a block diagram of the system, and the entire system is configured by a computer system. BUS21 indicates an internal bus of the computer. The BUS21 includes an X-ray generation means 11, a two-dimensional detector 12, a rotary table 13, a reconstruction means 15, a pulsation detection means 22, for example, a control means 23 comprising a CPU, an image display means. 24, a period calculation means 25, an interface means 26, a rotation time calculation means 27, and a stationary period determination means 28 are connected to transmit and receive control signals and data.

  FIG. 4 shows a flowchart of the photographing procedure. First, an instruction to start photographing is issued via the interface means 26 (step S101). When an imaging instruction is given, the pulsation detecting means 22 detects the pulsation of the subject P (step S102). As a means for detecting pulsation, an electrocardiograph, a pulse oximeter for detecting oxygen forward, or an X-ray generation means 11 that continuously emits X-rays and transmits a transmitted X-ray distribution to a two-dimensional detector. A morphological detection means can be used that images at 12 and detects the size of the heart in the image.

  By attaching an electrocardiograph and a pulse oximeter to the subject P, a periodic signal is detected. FIG. 7 described above shows a waveform detected by the electrocardiograph, and an R wave is obtained as a characteristic wave of the electrocardiograph waveform by the period calculation means 25. By measuring the interval of the R wave, the pulsation period is obtained. p can be measured (step S103). Specifically, calculation is possible by counting reference pulses counted between a certain R wave and the next R wave.

  Next, the static period ratio q in the pulsation period p is determined by the static period determining means 28. The rest period is a period in which the morphological variation is small due to the influence of the heart pulsation. The morphological change of the heart is divided into a systole, a relaxation period, and an equivalent relaxation period. The relaxation period is a period during which the so-called heart volume expands, and the equivalent relaxation period is a period during which the expansion contracts. The equal relaxation period is determined empirically based on the characteristic R wave as a stationary period, and can be roughly determined as the latter half of the pulsation period p starting from the R wave as shown in FIG. 7 (step S104). ).

  The rest ratio (= equal relaxation period) q determined by the rest period determining means 28 in the pulsation cycle p is limited to q ≧ 0.5 in this embodiment. This is because if the value is less than 0.5, it is complicated to extract still data from scanning. Since the ratio q of the still period is determined in relation to the image quality, even if the actual ratio q is 0.5 or less, the reconstructed image is extremely reduced by setting q = 0.5. It doesn't get worse. That is, limiting to q ≧ 0.5 does not depart from the image quality improvement intended by the present invention.

  Even when a pulse oximeter is used, it is possible to detect the R wave shown in FIG. 7, and therefore, the pulsation period p can be detected in the same manner as in the case of an electrocardiograph. Therefore, in determining the ratio q of the rest period, the pulse oximeter measures the oxygen forward with the fingertip, so that there is a delay from the actual pulsation timing. In addition, the delay depends on the human body, and the fluidity of blood in the blood vessels varies from individual to individual, so it cannot be determined uniformly.

  Therefore, when a pulse oximeter is used, an initial value is set as to which phase in the pulsation cycle p sets the rest period. This initial value may be set by determining a high frequency value from a statistical database based on information such as the age, height, weight, blood pressure, etc. of the subject P to be examined.

Next, the rotation time t of one rotation of the rotary table 13 is calculated by the following equation by the rotation time calculation means 27 from the pulsation period p and the ratio q.
t = (n−q) · p (n is a natural number) 0.5 ≦ q ≦ 1.0

  The rotation time t is empirically found to be 3 seconds ≦ t ≦ 10 seconds. If the rotation time t is too short, the subject P feels dizzy and moves, and if the rotation time t is too long, the patient P does not endure and moves and a body movement artifact occurs. There may be a plurality of n satisfying 3 seconds ≦ (n−q) · p ≦ 10 seconds, but they are selected according to the age of the subject P (step S105).

  When the rotation time t is determined, a completion of photographing preparation is displayed on the interface means 26. When an instruction to start photographing is issued, the rotation table 13 starts rotating according to an instruction from the control means 23 (step S106). The control means 23 monitors an encoder signal (not shown) generated from the rotary table 13 and confirms whether a predetermined constant speed and angle are reached. When a predetermined constant speed and angle are reached, a signal is sent to the X-ray generation means 11 to start X-ray exposure to the subject P (step S107). This encoder signal is also used for determining the data integration timing.

When using an encoder for generating one rotation per 25 000 pulses of the rotary table 13, 1 if acquire projection data of 1000 views for rotation data for each 25 pulses from the two-dimensional detector 12 of the encoder signal Will be collected. The control means 23 counts the encode pulses, generates an integrated signal every 25 pulses, and counts the X-ray dose reaching the two-dimensional detector 12.

  Data from the two-dimensional detector 12 is sequentially transferred to the reconstruction means 15 via the BUS 21. The data transfer continues until the turntable 13 rotates a predetermined rotation angle and a predetermined number of views are collected. When the rotary table 13 rotates a predetermined rotation angle and reaches a predetermined number of views, the control unit 23 instructs the X-ray generation unit 11 to stop the X-ray exposure. Thereafter, the rotary table 13 is controlled to stop while being decelerated.

  Immediately after the X-ray exposure is completed, the last projection data is transferred to the reconstruction unit 15. When the projection data transfer is completed, a data rearrangement process is performed (step S108). The rearrangement process will be described with reference to FIG. 5. In t = (n−q) · p, n = 4 and q = 0.5. The rest period is assumed to be the latter half of the pulsation cycle p. In the pulsation row, a section indicated as “equal relaxation” is a stationary period, and this period corresponds to a data section.

  Assuming a full scan in which data is collected from the 360-degree direction, the data in the sections A0-A1, A2-A3, A4-A5, and A6-A7 are variable periods and cannot be used at the first rotation. However, the section A7-A8 in the second rotation corresponds to the section A0-A1 that was the fluctuation section in the first rotation. That is, if the data of the A7-A8 section is copied to the A0-A1 section, the still section data of the A0-A2 section is completed. Similarly, if the data of the A9-A10 section is copied to the A2-A3 section, the data of the A11-A12 section is copied to the A4-A5 section, and the data of the A13-A14 section is copied to the A6-A7 section, the stationary period for 360 degrees Data can be obtained.

  In the case of half scanning with a fan angle added to 180 degrees, it is not necessary to make two complete rotations. If the fan angle is 10 degrees, it is only necessary to collect data for 190 degrees. In the case shown in FIG. 5, the angle of each section such as A0-A1 is 360 degrees / 7≈51 degrees. If still period data can be collected in the A0-A4 section, reconstruction is possible. Therefore, if data in the sections corresponding to A7-A8 and A9-A10 corresponding to the sections A0-A1 and A2-A3 are collected, the scan may be completed. In other words, the rotation may be stopped when the rotation is collected from A0 to A10.

  In the embodiment, q = 0.5. However, when q ≧ 0.5, the first period stationary period data overlaps with the second period stationary period data. Since the overlapping part is in principle the same data, either one may be adopted.

  In order to reduce the exposure of the subject P, the X-ray exposure of the X-ray generation means 11 may be limited only to the stationary period. Specifically, only the equivalent relaxation period shown in FIG. Just shoot. In addition, when q ≧ 0.5, a portion where data overlap is generated as described above, but it is not necessary to expose the X-ray to the overlapping portion.

  However, the X-ray exposure can be limited only to the stationary period when the phase of the stationary period in the pulsation cycle p can be limited by an electrocardiograph, and as in the case where a pulse oximeter is used. When the phase of the period cannot be accurately defined, X-ray exposure cannot be stopped because it is necessary to evaluate the reconstructed image while shifting the stationary period as described above.

  The control unit 23 instructs the reconstruction unit 15 to perform reconstruction based on the rearranged projection data (step S109). The reconstruction includes preprocessing, filter processing, and back projection processing, and the preprocessing includes offset processing, LOG conversion, gain correction, and defect correction. In the filtering process, a Ramachandran function or a Shepp Logan function is generally used, and these are also used in this embodiment, and the filtered data is backprojected.

  The algorithm from the filtering process to the back projection uses the Feldkamp algorithm, but is not limited thereto. The following non-patent document 1 is known as a reference document.

Feldkamp and the method described by Davis and Kress ("Practical Cone-Beam Algorithm"), J.Opt.Soc.Am.A1,612-619, 1984

  When the back projection is completed and the CT cross-sectional image is reconstructed, the cross-section is displayed on the image display means 24. As described above, since the reconstructed image that is created first is rearranged based on the initial value of the stationary period, the reconstructed image may include artifacts due to heart rate variability. (Step S110). This evaluation may be performed by a human but can also be automated.

  In the automatic case, the evaluation of the image is determined by designating a region around the heart, calculating the variance of the region image, and comparing the variance value with a predetermined value. An image may be cut out by an operator based on a tomographic image or may be performed by a heart portion determination process. The heart portion determination process may be a predetermined region predicted from the body shape of the subject P, or pattern recognition may be used. When performed by a human, it is confirmed whether or not artifacts have occurred in the reconstructed image centering around the heart.

  If the artifact is confirmed, retry is supported, and the stationary period is changed according to the instruction (step S111). The change of the stationary period is performed by sequentially shifting the phase of the stationary period in the pulsation cycle p. The step width of the shift can be arbitrarily selected, but may be about 1/10 of the pulsation period p.

  If the evaluation does not satisfy the criterion, the rearrangement period ratio q is shifted as described above to rearrange the data. This loop is repeated until the reconstructed image is satisfied with respect to the artifacts. Finally, the reconstructed image is displayed and shooting is completed (step S112).

  In the embodiment, it is assumed that X-rays are generated continuously, but the present invention is not limited to this, and pulse-shaped X-rays are matched with the integration interval of the two-dimensional detector 12 based on the encoder signal. Lines can also be generated.

It is a top view of an Example. It is a side view. It is a block block diagram of a system. It is a flowchart figure. It is explanatory drawing of the data collection which specified the static ratio. It is explanatory drawing of the conventional cone beam CT apparatus. It is explanatory drawing of an electrocardiogram waveform and heart contraction.

Explanation of symbols

DESCRIPTION OF SYMBOLS 11 X-ray generation means 12 Two-dimensional detector 13 Rotary table 14 Chest pad 15 Reconstruction means 22 Beat detection means 23 Control means 24 Image display means 25 Period calculation means 26 Interface means 27 Rotation time calculation means 28 Rest period determination means 28

Claims (6)

X-ray generation means, a two-dimensional detector for detecting X-rays transmitted through the subject as a two-dimensional distribution, and rotation for rotating the subject in an imaging region formed by the X-ray generation means and the two-dimensional detector A radiation imaging apparatus comprising:
A variation detecting means for detecting inherent variations of the subject;
A period calculating means for calculating a fluctuation period p from the fluctuation detecting means;
The stationary period determining means for determining the ratio q of the stationary period to the period p from the fluctuation detecting means, and t = (n−q) · p (n is a natural number selected according to the age of the subject ) And a rotation time calculation means for calculating a rotation time t of one rotation of the rotation means, wherein the rotation means rotates the subject at the rotation time t.   The radiation imaging apparatus according to claim 1, wherein the rotation time t is 3 seconds ≦ t ≦ 10 seconds.   The radiation imaging apparatus according to claim 1, wherein the fluctuation detecting unit detects the fluctuation based on a fluoroscopic image obtained by an electrocardiograph, a pulse oximeter, or the two-dimensional detector.   The radiation imaging apparatus according to claim 1, wherein the X-ray generation unit irradiates the subject with X-rays only during the stationary period.   The radiation imaging apparatus according to claim 1, wherein the phase of the stationary period in the period p is changed based on a reconstructed image. X-rays generated from the X-ray generation means and transmitted through the subject are two-dimensionally detected by the two-dimensional detector while relatively rotating the subject in an imaging region formed by the X-ray generation means and the two-dimensional detector. Detecting as a distribution, detecting the inherent variation of the subject, calculating the period p, determining the ratio q of the stationary period to the period p, t = (n−q) · p (n is the subject's A radiation imaging method comprising: calculating and rotating a rotation time t of one rotation of a subject so that the natural number is selected according to age ).

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