JP2015019991A - Detector module position measuring method, radiation tomography apparatus and program - Google Patents

Abstract

PROBLEM TO BE SOLVED: To relatively easily measure the accurate position of each detector module constituting a radiation detector in a radiation tomography apparatus.SOLUTION: A detector module position measuring method executes: a step of collecting projection data of a plurality of views by scanning a columnar pin which is arranged at a position displaced from a rotation shaft and extends in a direction vertical to an imaging field plane by using a radiation tomography apparatus which includes a radiation source and a radiation detector in which a plurality of detector modules are arranged in a channel direction and performs scanning by rotating the radiation source and the radiation detector with the rotation shaft held therebetween; and a step of obtaining an actual measurement position based on collected projection data and a calculation position based on the calculation as a projection position of a pin in the projection data profile of the view to obtain the difference between the actual measurement position and the calculation position. Thereby, an amount of positional displacement of each detector module can be measured from the difference.

Description

  The present invention relates to a technique for measuring the position of each detector module constituting a radiation detector in a radiation tomography apparatus with high accuracy.

  Conventionally, a radiation tomography apparatus includes a radiation detector in which a plurality of detection elements are arranged in at least a channel direction. As a radiation detector provided in the radiation tomography apparatus, one configured by arranging a plurality of detector modules in the channel direction is known (see Patent Document 1, Abstract, etc.). In such a radiation tomography apparatus, if the detector module has an arrangement error in the channel direction, that is, a positional deviation from an ideal arrangement as designed, the projection data (data) includes an error component. Thus, an artifact is generated in the reconstructed image.

  On the other hand, the arrangement error of the detector module has been reduced by improving the assembly technique, but it is difficult to completely lose it.

  Therefore, a method for suppressing artifacts by measuring the accurate position of each detector module in advance and reconstructing an image in consideration of the measurement result can be considered.

JP 2011-245209 A

  However, the arrangement error of the detector module is generally as small as an order of several μm to several tens of μm. Therefore, it is not easy to measure the exact position of the detector module, and there are few established measurement methods that can be performed in the manufacturing process and maintenance process.

  Under such circumstances, there is a demand for a technique that can relatively easily measure the accurate position of each detector module constituting the radiation detector in the radiation tomography apparatus.

The invention of the first aspect
Radiation tomography comprising a radiation source and a radiation detector in which a plurality of detector modules are arranged in a channel direction, wherein the radiation source and the radiation detector are scanned with a rotation axis interposed therebetween. A first step of collecting projection data of a plurality of views by scanning a columnar body that is arranged at a position off the rotation axis and extends in a direction perpendicular to an imaging field plane using an apparatus. (Step)
For each of the plurality of views, an actual measurement position based on the collected projection data and a calculation position based on the calculation are obtained as the projection position of the columnar body in the projection data profile of the view, and the actual measurement position and the calculation position are obtained. A detector module position measuring method comprising:

The invention of the second aspect is
A radiation tomography apparatus comprising a radiation source and a radiation detector in which a plurality of detector modules are arranged in a channel direction, wherein the radiation source and the radiation detector are rotated with a rotation axis interposed therebetween and scanned. And
Using projection data of a plurality of views (views) collected by scanning a columnar body that is arranged at a position off the rotation axis and extends in a direction perpendicular to the imaging field plane, each of the plurality of views As for the projection position of the columnar body in the projection data profile of the view, an actual measurement position based on the collected projection data and a calculation position based on the calculation are obtained, and the actual measurement position and the calculation Provided is a radiation tomography apparatus including a calculation unit that obtains a difference from a position and generates correction data used for image reconstruction based on the difference.

The invention of the third aspect is
A radiation tomography apparatus according to the second aspect, further comprising reconstruction means for performing image reconstruction based on projection data obtained by subject scanning and the correction data.

The invention of the fourth aspect is
The radiation according to the third aspect, wherein the calculation means extracts a high-frequency component in a curve representing a relationship between the calculation position and the difference in each of the plurality of views, and generates the correction data based on the high-frequency component. A tomography apparatus is provided.

The invention of the fifth aspect is
The radiation tomography apparatus according to the fourth aspect, wherein the correction means extracts the high-frequency component by removing the low-frequency component of the curve from the curve.

The invention of the sixth aspect is
The computing means obtains the position of the columnar body based on the image of the columnar body reconstructed based on the projection data of the plurality of views, and obtains the calculated position based on the position of the columnar body; A radiation tomography apparatus according to any one of the second to fifth aspects is provided.

The invention of the seventh aspect
The calculation means performs a curve fitting on a graph representing a relationship between a view and a projection position of the columnar body in the projection data profile of the view, and the position on the fitted curve is determined as the position on the curve. A radiation tomography apparatus according to the second aspect or the third aspect obtained as a calculation position is provided.

The invention of the eighth aspect
The radiation tomography apparatus according to the sixth aspect, wherein the fitting includes processing by any one of a least square method, a smoothing method, and a boot strap method.

The invention of the ninth aspect is
The calculation means performs a process of obtaining a representative value of a difference between the calculated position and the actually measured position for each detector module, and obtaining a displacement amount of the detector module based on the representative value. A radiation tomography apparatus according to any one of the eighth aspect to the eighth aspect is provided.

The invention of the tenth aspect is
The radiation tomography apparatus according to the ninth aspect, wherein the representative value is an average value.

The invention of the eleventh aspect is
The radiation tomography apparatus according to any one of the second to tenth aspects, wherein the columnar body is disposed in the vicinity of an outer edge of the imaging visual field.

The invention of the twelfth aspect is
There is provided a program for causing a computer to function as computing means in the radiation tomography apparatus according to any one of the second to eleventh aspects.

  According to the above aspect, the columnar body is installed at a position deviated from the rotation center of the radiation source and the radiation detector in the radiation tomography apparatus, and the columnar body is scanned to collect projection data of a plurality of views. Since the measured position based on the collected projection data and the calculated position based on the calculation are obtained as the projection position of the columnar body in the projection data profile of the view, the position of each detector module is calculated from the difference between the measured position and the calculated position. The amount of deviation can be detected. In other words, no special tools other than the columnar body and the means for installing the columnar body are required, and the projection position of the columnar body is specified with a resolution finer than the width of the detection element from the signal intensity distribution of the mountain shape in the projection data profile. can do. Thereby, the exact position of each detector module which comprises the radiation detector in a radiation tomography apparatus can be measured comparatively easily.

It is a block diagram which shows the principal part structure of the X-ray CT apparatus which concerns on 1st Embodiment. It is a figure which shows the structure of a X-ray detector. It is a flowchart (flowchart) which shows the flow of a process of the X-ray CT apparatus which concerns on 1st Embodiment. It is a figure for demonstrating the installation place of a pin (pin). It is a figure for demonstrating the identification method of the measurement projection position of a pin. It is a figure which shows an example of the measurement projection position graph G1 of a pin. It is a figure which shows an example of the ideal projection position graph G2 of a pin. It is a figure which shows an example of the channel unit projection position shift graph G3. It is a figure which shows a mode that the position shift of a detector module is reflected in the projection position of the pin in a projection data profile. It is a figure which shows an example of the module unit projection position shift graph G4. It is a figure which shows an example of the graph G5 showing the high frequency component of the module unit projection position shift graph G4. It is a flowchart which shows the flow of a process of the X-ray CT apparatus which concerns on 2nd Embodiment. It is a figure which shows an example of the theoretical projection position graph G6 of a pin. It is a figure which shows an example of the 2nd channel unit projection position shift graph G7. It is a figure which shows an example of the 2nd module unit projection position shift graph G8.

  Hereinafter, embodiments of the present invention will be described. Note that the present invention is not limited thereby.

(First embodiment)
FIG. 1 is a block diagram showing the main configuration of the X-ray CT apparatus according to the first embodiment.

  The X-ray CT apparatus 1 includes an X-ray tube 2, an aperture 3, and an X-ray detector 4.

  The X-ray tube 2 irradiates the subject 20 with X-rays 21 from the X-ray focal point f.

  The aperture 3 is provided between the X-ray tube 2 and the subject 20. The aperture 3 shapes the X-ray 21 irradiated from the X-ray tube 2 into a fan-shaped fan-beam or cone-beam that spreads at a predetermined fan angle.

  The X-ray detector 4 is arranged to face the X-ray tube 2 so as to sandwich the subject 20. The X-ray detector 4 detects X-rays irradiated from the X-ray tube 2 and transmitted through the subject 20.

  FIG. 2 is a diagram showing a configuration of the X-ray detector 4. As shown in FIG. 2, the X-ray detector 4 has a structure in which a plurality of detector modules 41 are arranged in an arc along the channel direction (represented by CH). The channel direction is the fan angle (spreading) direction of the fan beam X-rays emitted from the X-ray tube 2. For example, about 60 detector modules 41 are arranged in the channel direction.

  The detector module 41 has a configuration in which the detection elements 42 are two-dimensionally arranged in the channel direction and the column direction (represented by z). The column direction coincides with the body axis direction of the subject 20 or the thickness direction of the fan beam X-ray irradiated from the X-ray tube 2 and is also referred to as a slice direction. The detection element 42 includes, for example, a scintillator that converts X-rays into photons, and a photodiode that converts the photons into electrical signals.

  In the detector module 41, for example, 16 × 64 detection elements 42 are arranged in the channel direction and the column direction. However, in FIG. 2, for the sake of simplification, the number of detection elements is shown smaller than the actual number. The size of the detection element 42 is, for example, about 1 mm × 1 mm in the channel direction and the column direction, and the size of the detector module 41 is, for example, about 16 mm × 64 mm in the channel direction and the column direction.

  The X-ray tube 2 and the X-ray detector 4 are supported so that they can be rotated while maintaining their positional relationship. The rotation center of the X-ray tube 2 and the X-ray detector 4 becomes the center of the imaging field of view FOV and is called an iso-center ISO.

  The X-ray CT apparatus 1 rotates the X-ray tube 2 and the X-ray detector 4 to irradiate the subject 20 with the X-ray 21 from the X-ray focal point f of the X-ray tube 2, and the X-ray detector 4 Scanning is performed by detecting transmitted X-rays.

The X-ray CT apparatus 1 further includes a DAS (Data Acquisition
System) 5, a storage unit 6, and a calculation / control unit 7.

The DAS 5 is an analog data (analog of X-ray intensity detected by the X-ray detector 4).
data) is converted to digital data and collected.

  The storage unit 6 stores various data and programs. In this example, the storage unit 6 stores projection data obtained by scanning, correction data used for various corrections of the projection data, and the like.

  The calculation / control unit 7 irradiates the subject 20 with X-rays from the X-ray focal point f while rotating the X-ray tube 2 and the X-ray detector 4, and detects the transmitted X-rays of the subject 20 with the X-ray detector 4. Each part is controlled to perform scanning. In DAS5, projection data of a plurality of views is collected by performing this scan.

  Further, the calculation / control unit 7 receives the projection data collected by the DAS 5. The calculation / control unit 7 performs image reconstruction calculation such as back projection based on the received projection data, and generates image data. The calculation / control unit 18 accesses the storage unit 6 at the time of image reconstruction, and performs various corrections of the projection data and the image reconstruction algorithm based on the read correction data. Various corrections include detector module misalignment correction and the like in addition to sensitivity correction and scattered radiation correction. Then, the calculation / control unit 7 performs image reconstruction based on the corrected projection data, and generates image data. In this example, the calculation / control unit 18 controls each unit to measure the detector module positional deviation before scanning the subject 20. Then, based on the measurement result, correction data used for detector module positional deviation correction is generated and stored in the storage unit 6.

  Hereafter, the flow of processing of the X-ray CT apparatus according to the first embodiment will be described.

  FIG. 3 is a flowchart showing a process flow of the X-ray CT apparatus according to the first embodiment. Steps S <b> 1 to S <b> 9 are detector module misalignment measurement steps, and are usually performed as part of the calibration of the X-ray CT apparatus 1. Steps S10 to S12 are a subject photographing (scanning) step, in which the detector module positional deviation correction of the projection data is performed.

  FIG. 4 is a diagram for explaining the installation location of the pins. In step S <b> 1, as shown in FIG. 4, a pin 9 that is a columnar body is installed in the imaging field of view FOV of the X-ray CT apparatus 1. In this example, the pins 9 have a cylindrical shape extending in the column direction. Further, the pin 9 is installed in the vicinity of the outer edge in the imaging field of view FOV, which is far away from the isocenter ISO. In this example, in the imaging field plane, that is, the scan plane, the left and right positions are aligned so that the pin 9 is positioned on an imaginary straight line passing through the isocenter ISO and extending in the vertical direction. It is fixed to the cradle (not shown). Further, the height position of the cradle is adjusted so that the pin is disposed at a position shifted from the isocenter ISO by a distance substantially the same length as the radius of the imaging field of view FOV. As a result, the projection position of the pin 9 in the projection data profile of a plurality of views obtained by scanning the pin 9 moves in almost the entire region from one end to the other end in the channel direction of the X-ray detector 4.

  The pin 9 is made of a heavy metal having a high X-ray absorption rate, such as tungsten or molybdenum. The diameter of the pin 9 takes into account the size of the detection element 42 and the projection magnification of the subject 20 in the X-ray detector 4 so that the projection position of the pin 9 in the projection data profile can be detected with high contrast and high resolution. Determined. In the case of a standard X-ray CT apparatus, the diameter φ of the pin 9 is suitably about 1 to 10 times the width of the detection element 42. For example, when the size of the detection element 42 is 1 mm, the diameter φ of the pin 9 is determined within a range of 1 mm to 10 mm, but empirically, about 1 mm to 3 mm is preferable. In this example, the diameter φ of the pin 9 is about 2 mm. The position of the pin 9 is, for example, a position where the radius of the imaging visual field FOV is 20 cm and a position slightly higher than the isocenter ISO by 20 cm.

  In step S2, scanning of the pin 9 is performed once or a plurality of times to collect projection data of a plurality of views. In this example, a full scan for one rotation of the X-ray tube is performed once by an axial scan method. The number of views is about 1000 for one rotation of the X-ray tube. Since the pin 9 is away from the isocenter ISO, the projection position of the pin 9 by the X-ray in the X-ray detector 4 changes according to the view angle.

  In step S3, the measured projection position chpm of the pin 9 in the projection data profile is specified for each view based on the collected projection data of the plurality of views of the pin 9. The measurement projection position chpm of the pin 9 is specified as follows, for example.

  FIG. 5 is a diagram for explaining a method of specifying the actually measured projection position chpm of the pin 9. First, correction for obtaining an appropriate contrast such as sensitivity correction is performed on projection data of a plurality of views of the pin 9. Next, projection data for one view to be processed is selected, and as shown in FIG. 5, the horizontal axis indicates the channel direction coordinates of each detection element 42 (hereinafter referred to as channel coordinates), and the vertical axis indicates the detection element 42. The projection data values are taken and graphed. In this graph G0, curve fitting is performed with a function that draws a symmetrical mountain-shaped curve. The position corresponding to the fitted curve, that is, the apex of the fitting curve FC is determined as the measured projection position (measured position) chpm of the pin 9. Such processing is performed for each of the plurality of views. As a result, the measured projection position chpm of the pin 9 can be specified for each view with a high resolution finer than the size of the detection element 42.

  When the actual projection position chpm of the pin 9 in each view can be specified, a graph representing the relationship between the view number view and the actual projection position chpm of the pin 9 in the projection data profile of the view (hereinafter referred to as the actual projection position graph of the pin). Create G1.

  FIG. 6 shows an example of the actually measured projection position graph G1 of the pin. In this graph G1, the horizontal axis represents the view number view, and the vertical axis represents the actually measured projection position chpm of the pin in the projection data profile (represented by channel coordinates). The view number view corresponds to the view angle. Since the pin 9 is set in the vicinity of the outer edge of the imaging field of view FOV, which is far away from the isocenter ISO, the measured projection position graph G1 of the pin 9 generally draws a smooth curve close to a sine curve. However, in practice, as shown in the enlarged view in the upper right of FIG. 6, it can be seen that changes such as minute unevenness and variation are included when enlarged. The main causes of the change such as unevenness and variation include the positional shift of the detector module 41 constituting the X-ray detector 4, the change in the position of the X-ray focal point f in the X-ray tube 2, and the X-ray tube. 2 and the distortion of the data acquisition system including the X-ray detector 4 and the vibration at the time of scanning can be considered.

  In step S4, image reconstruction is performed based on the collected projection data of the plurality of views of the pin 9, and an image representing the pin 9 in the imaging field of view FOV is obtained. For the image reconstruction, for example, a filter back projection method, a three-dimensional image reconstruction method, or the like is used.

  In step S5, the exact position of the pin 9 in the real space is specified based on the reconstructed image of the pin 9. The coordinates of the imaging field of view FOV in real space can be obtained in advance. Therefore, by checking the relative position of the pin 9 in the imaging field of view FOV based on the reconstructed image of the pin 9, the exact position of the pin 9 in real space can be specified. The exact position of the pin 9 may be specified using other methods.

  In step S6, the ideal projection position (calculated position) of the pin 9 in the projection data profile for each view based on the geometrical positional relationship between the pin 9 and the X-ray tube 2 and the X-ray detector 4 in each view. ) Calculate chpc virtually. At this time, the geometry of the data acquisition system including the positions of the X-ray tube 2, the X-ray focal point f, the X-ray detector 4, each detector module 41, etc. is all as designed and is in an ideal state. Assume. Then, a graph (hereinafter referred to as an ideal projection position graph of the pin) G2 representing the relationship between the view number view and the ideal projection position chpc of the pin 9 in the projection data profile of the view is created.

  In FIG. 7, an example of the ideal projection position graph G2 of the pin 9 is shown. This graph G2 draws a smooth curve even when enlarged as shown in the partially enlarged view in the upper right of FIG.

  In step S7, for each view, that is, for each channel, the difference between the actual projection position chpm of the pin 9 on the actual projection position graph G1 and the ideal projection position chpc of the pin 9 on the ideal projection position graph G2 ( chpc−chpm). Then, a graph (hereinafter referred to as a channel unit projection position deviation graph) G3 representing the relationship between the ideal projection position chpc of the pin 9 and the difference between the ideal projection position chpc and the actual measurement projection position chpm for each view is created.

  FIG. 8 shows an example of the channel unit projection position deviation graph G3. In this example, the channel unit projection position shift graph G3 has the ideal projection position chpc (represented by channel coordinates) of the pin 9 as the horizontal axis, and the difference between the ideal projection position chpc and the actually measured projection position chpm corresponding thereto is represented by the vertical axis. It is expressed as an axis.

  FIG. 9 is a diagram illustrating a state in which the displacement of the detector module 41 is reflected on the projection position of the pin 9 in the projection data profile. Here, it is assumed that the detector module 41 is shifted from the ideal position 41 'by + d in the + CH direction. In this case, the projection image 9P of the pin 9 on the detector module 41 spreads around a virtual straight line passing through the center of the pin 9 starting from the X-ray focal point f as shown in FIG. Is a position C on the straight line. However, the projection data profile is generated as if the detector module 41 is located at its ideal position 41 '. Therefore, the projection position of the pin 9 in the projection data profile is recognized as a position M that is shifted by Δd from the position C in the −CH direction. Therefore, when there is a position shift in the detector module 41, the position shift appears as a difference between the ideal projection position chpc of the pin 9 and the actually measured projection position chpm. On the contrary, when there is no position shift of the detector module 41, the ideal projection position chpc of the pin 9 and the actually measured projection position chpm coincide, and the difference does not appear.

  In step S8, the difference (chpc−chpm) between the ideal projection position chpc and the actual measurement projection position chpm of the pin 9 is grouped for each corresponding detector module 41 in the created channel unit projection position deviation graph G3. A representative value of the difference is obtained for each detection module 41. Then, a graph (referred to as a module unit projection position deviation graph) G4 representing the relationship between the detector module number m and the representative value in the detector module 41 is created.

  FIG. 10 shows an example of the module unit projection position deviation graph G4. In this example, the detector module 41 has 16 detection elements 42 arranged in the channel direction. Therefore, the module unit projection position deviation graph G4 in FIG. 10 is the same as the channel unit projection position deviation graph G3 in FIG. 32 areas are grouped in the form of detector module number m = 2,..., And a representative value of the difference (chpc−chpm) between the ideal projection position chpc and the actual measurement projection position chpm is obtained for each group. , They are plotted.

  By the way, the detection elements 42 are not arranged independently of each other, but are arranged in units of detector modules 41 in which a plurality of detection elements 42 are arranged with high precision at regular intervals. Therefore, by dividing the difference (chpc−chpm) between the ideal projection position chpc of the pin 9 and the actually measured projection position chpm for each detector module 41 in this way, the deviation from the ideal position of each detector module 41 is achieved. Can be estimated.

  It should be noted that the difference between the ideal projection position chpc of the pin 9 and the actual measurement projection position chpm when viewed in channel units may not be expected to be high individually. By obtaining the value, the error of the difference can be suppressed and the calculation accuracy can be increased.

  The representative value is most commonly an average value, but may be an intermediate value, median value, maximum value, minimum value, or the like. In this example, the representative value is an average value. Further, in this example, the module unit projection position deviation graph G4 represents the coordinate representing the center position in the channel direction of the detector module 41 as the horizontal axis, and the average value of the differences in the detector module 41 as the vertical axis. Is.

  In step S9, a high frequency component graph G5 representing a high frequency component in the module unit projection position deviation graph G4 is created.

  As described above, the deviation between the ideal projection position chpc of the pin 9 and the actually measured projection position chpm is not only the positional deviation of the detector module 41 but also the change in the position of the X-ray focal point f in the X-ray tube 2 and the data acquisition system. It also changes depending on uncertain factors such as distortion and vibration. However, in the module unit projection position deviation graph G4, of the deviation between the ideal projection position chpc and the actual measurement projection position chpm, the deviation caused by these uncertain elements is compared with the deviation caused by the position deviation of the detector module 41. Therefore, it is expected to appear as a low frequency component. Therefore, by removing the low frequency component and extracting the high frequency component in the module unit projection positional deviation graph G4, it is possible to acquire robust positional deviation information mainly due to the positional deviation of the detector module 41. That is, it can be considered that the graph G5 representing the high frequency component of the module unit projection position deviation graph G4 represents the amount of displacement from the ideal position of the detector module 41.

  As a method for obtaining a high frequency component in a graph curve and a method for obtaining a low frequency component on the contrary, a method using a moving average, various fittings, a low pass filter, or the like can be considered. In this example, a curve corresponding to a low frequency component is obtained by fitting a curve in the module unit projection position deviation graph G4, and this is subtracted from the original module unit projection position deviation graph G4 to obtain a high frequency component. For example, a least square method, a smoothing method, or a bootstrap method can be used for curve fitting.

  FIG. 11 shows an example of a graph (hereinafter referred to as a projection position deviation high-frequency component graph) G5 representing a high-frequency component of the module unit projection position deviation graph G4. In this example, the projection position deviation high-frequency component graph G5 is obtained by subtracting the low-frequency component from the representative value of the difference in the detector module 41, with the channel coordinate representing the position of the detector module 41 as the horizontal axis. It is expressed as an axis. The deviation amount from the ideal position for each detector module 41 obtained here is stored in the storage unit 6 and used as correction data for performing image reconstruction with high accuracy.

  In step S10, the subject 20 is scanned to collect projection data for a plurality of views.

  In step S11, projection data of a plurality of views of the subject 20 is obtained on the basis of the positional deviation (shifted direction and amount) from the ideal position (designed position) of each detector module 41 stored in the storage unit 6. to correct. For example, with respect to the projection data of each view, correction is performed for each detector module 41 so that the data value of each channel in the detector module is shifted in the direction of canceling the error component according to the position shift of the detector module. Further, for example, in processing in a preparation stage before image reconstruction such as fan-pala conversion, weights used for interpolation when output data of adjacent or adjacent detection elements 42 are interpolated to obtain projection data. The coefficient is adjusted according to the positional deviation, and the error component corresponding to the positional deviation is canceled. In this example, an X-ray path from the focal point of the X-ray tube to each detection element is as designed, and an approach for correcting data corresponding to the path based on the above-described deviation amount is used. take. Alternatively, an approach may be taken in which the corresponding data is not changed and the path itself is corrected to correct the image reconstruction algorithm.

  In step S12, image reconstruction is performed based on the corrected projection data of a plurality of views.

(Second Embodiment)
In the second embodiment, when extracting a high-frequency component in the module-based projection position deviation graph, the pin pin reconstruction image is obtained from the pin reconstructed image and the pin ideal projection position is not obtained. A method of directly extracting a high frequency component from a graph based on the measured projection position of the pin is used.

  FIG. 12 is a flowchart showing a process flow of the X-ray CT apparatus according to the second embodiment.

  In step T1, a pin is placed near the outer edge in the imaging field of view FOV in the X-ray CT apparatus 1.

  In step T2, a scan of the pin 9 is performed to collect projection data for a plurality of views.

  In step T3, an actually measured projection position graph G1 of the pin 9 is created from the collected projection data of a plurality of views of the pin.

  In step T4, curve fitting is performed in the actually measured projection position graph G1, and the obtained curve, that is, the low frequency component is converted into a theoretical projection position graph G6 that represents a change component due to uncertain elements other than the positional deviation of each detector module 41. Asking. FIG. 13 shows an example of the theoretical pin projection position graph G6.

  In step T5, based on the actual projection position graph G1 and the theoretical projection position graph G6, the difference between the actual projection position and the theoretical projection position (calculation position) chpt is obtained for each pin projection position in the projection data profile for each view. . Then, for each view, a second projection position deviation graph G7 representing the relationship between the theoretical projection position chpt of the pin 9 and the difference (chpt−chpm) between the theoretical projection position chpt and the actual measurement projection position chpm is created. FIG. 14 shows an example of the second channel unit projection position deviation graph G7. In this example, the second projection position deviation graph G7 uses the theoretical projection position chpt of the pin 9 (represented by channel coordinates) as the horizontal axis, and the difference between the theoretical projection position chpt and the actually measured projection position chpm corresponding thereto ( chpt−chpm) is represented as a vertical axis.

  In step T6, a representative value of the difference (chpt−chpm) between the theoretical projection position chpt of the pin 9 and the actual measurement projection position chpm is obtained for each detector module 41 in the generated second projection position deviation graph G7. A second module unit projected position deviation graph G8 representing the relationship between the position of the detector module 41 and its representative value is created. FIG. 15 shows an example of the second module unit projection position deviation graph G8. In this example, the representative value is an average value. The deviation amount from the theoretical position for each detector module 41 obtained here is stored in the storage unit 6 and used as correction data for performing image reconstruction with high accuracy.

  In step T7, the subject 20 is scanned to collect projection data for a plurality of views.

  In step T8, the projection data of a plurality of views of the subject 20 is corrected based on the amount of deviation from the theoretical position of each detector module 41 stored in the storage unit 6.

  In step T9, image reconstruction is performed based on the corrected projection data of a plurality of views.

  As described above, according to the above-described embodiment, the pin 9 is installed at a position deviated from the rotation center of the X-ray tube 2 and the X-ray detector 4 in the X-ray CT apparatus 1, and the pin 9 is scanned to display a plurality of views. Projection data is collected, and an actual measurement position based on the collected projection data and a calculation position based on the calculation are obtained as the projection positions of the pins 9 in the projection data profiles of a plurality of views, and the difference between the actual measurement position and the calculation position is obtained. The amount of misalignment of each detector module can be detected. In other words, no special tool other than the pin and the pin installation means is required, and the projection position of the pin can be specified with a resolution finer than the width of the detection element from the angle-shaped signal intensity distribution in the projection data profile. it can. Thereby, the exact position of each detector module which comprises the radiation detector in a radiation tomography apparatus can be measured comparatively easily.

  The invention is not limited to the above-described embodiment, and various embodiments can be adopted without departing from the spirit of the invention.

  As described above, the computer measures the position deviation of the detector module, and further performs correction on the projection data to remove the fluctuation component due to the position deviation based on the measurement result. The program for doing this is also an embodiment of the invention.

1 X-ray CT system (radiation tomography system)
2 X-ray tube (radiation source)
f X-ray focus 3 Aperture 4 X-ray detector (radiation detector)
41 Detector module 41 ′ Ideal position of detector module 42 Detector element 5 DAS
6 Storage unit 7 Calculation / control unit (calculation means, reconstruction means)
9 pins (columnar body)
20 subjects

Claims (12)

A radiation tomography apparatus that includes a radiation source and a radiation detector in which a plurality of detector modules are arranged in a channel direction, and scans by rotating the radiation source and the radiation detector with a rotation axis interposed therebetween. A first step of collecting projection data of a plurality of views by scanning a columnar body that is disposed at a position deviated from the rotation axis and extends in a direction perpendicular to the imaging field plane;
For each of the plurality of views, an actual measurement position based on the collected projection data and a calculation position based on the calculation are obtained as the projection position of the columnar body in the projection data profile of the view, and the actual measurement position and the calculation position are obtained. And a second step of obtaining a difference from the detector module position measuring method. A radiation tomography apparatus comprising a radiation source and a radiation detector in which a plurality of detector modules are arranged in a channel direction, wherein the radiation source and the radiation detector are rotated with a rotation axis interposed therebetween and scanned. And
Using projection data of a plurality of views collected by scanning a columnar body that is arranged at a position off the rotation axis and extends in a direction perpendicular to the imaging field plane, for each of the plurality of views, As the projection position of the columnar body in the projection data profile of the view, an actual measurement position based on the collected projection data and a calculation position based on the calculation are obtained, and a difference between the actual measurement position and the calculation position is obtained, A radiation tomography apparatus comprising arithmetic means for generating correction data used for image reconstruction based on a difference.   The radiation tomography apparatus according to claim 2, further comprising a reconstruction unit that performs image reconstruction based on projection data obtained by scanning a subject and the correction data.   The radiation according to claim 3, wherein the calculation unit extracts a high-frequency component in a curve representing a relationship between the calculation position and the difference in each of the plurality of views, and generates the correction data based on the high-frequency component. Tomography equipment.   The radiation tomography apparatus according to claim 4, wherein the calculation unit extracts the high-frequency component by removing a low-frequency component of the curve from the curve.   The calculation means obtains the position of the columnar body based on the image of the columnar body reconstructed based on the projection data of the plurality of views, and obtains the calculated position based on the position of the columnar body. The radiation tomography apparatus according to any one of claims 2 to 5.   The calculation unit performs curve fitting on a graph representing a relationship between a view and a projection position of the columnar body in a projection data profile of the view, and obtains a position on the fitted curve as the calculation position. The radiation tomography apparatus according to claim 2 or claim 3.   The radiation tomography apparatus according to claim 7, wherein the fitting includes processing by any one of a least square method, a smoothing method, and a bootstrap method.   The calculation means obtains a representative value of a difference between the calculated position and the actually measured position for each detector module, and obtains a displacement amount of the detector module based on the representative value. Item 9. The radiation tomography apparatus according to any one of Items 8 to 9.   The radiation tomography apparatus according to claim 9, wherein the representative value is an average value.   The radiation tomography apparatus according to any one of claims 2 to 10, wherein the columnar body is disposed in the vicinity of an outer edge of the imaging visual field.   The program for functioning a computer as a calculating means in the radiation tomography apparatus as described in any one of Claims 2-11.

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