X-ray CT apparatus and projection data correction method

Abstract

The present invention provides an X-ray CT apparatus and a projection data correction method that can suppress artifacts generated near the edge of a subject. The X-ray CT apparatus images the subject and comprises: a correction data generator that generates correction data using difference data between measured projection data for each X-ray energy obtained by imaging a known phantom having a known composition and shape and a size smaller than the imaging field of view, and calculated projection data for each X-ray energy calculated based on an X-ray transmission length determined from the shape of the known phantom; and a correction unit that uses the correction data to correct the projection data for each X-ray energy of the subject.

Description

Technical Field

[0001] The present invention relates to an X-ray CT apparatus, and more particularly to a method for correcting projection data obtained by the X-ray CT apparatus. Background Art

[0002] Development of PCCT (Photon Counting Computed Tomography) devices equipped with photon counting detectors, known as photon counting detectors, is progressing. Because photon counting detectors measure the energy of incident X-ray photons, or X-ray energy, PCCT devices can produce medical images that distinguish between materials of different compositions, such as iodinated contrast agents used in angiography and calcified plaques in blood vessels. Furthermore, to obtain medical images that distinguish between materials, it is necessary to obtain calibration data for each detector element, based on the relationship between the output of the photon counting detector and the X-ray energy when measuring a phantom composed of a combination of multiple base materials with known compositions and thicknesses.

[0003] Non-Patent Document 1 discloses acquiring 25 types of calibration data using a stepped phantom consisting of 0 to 4 acrylic resin flat plates with a thickness of 2.54 cm and 0 to 4 aluminum flat plates with a thickness of 0.635 cm.

[0004] Prior art literature

[0005] Non-patent literature

[0006] Non-patent document 1: Taly Gilat Schmidt et al. "A Spectral CT method to directly estimate basis material maps from experimental photon-countingdata", in IEEE Transactions on Medical Imaging, vol.36, no.6, pp.1808-1819, September 2017

[0007] However, Non-Patent Document 1 does not consider artifacts generated near the edges of the subject. Specifically, because the acrylic and aluminum plates that constitute the stepped phantom cover all detector elements, Non-Patent Document 1 cannot account for the effects of penumbras and scattered radiation generated near the edges of the subject. Summary of the Invention

[0008] Therefore, an object of the present invention is to provide an X-ray CT apparatus and a projection data correction method that can suppress artifacts generated near the edge of a subject.

[0009] In order to achieve the above-mentioned object, the present invention is an X-ray CT apparatus for imaging a subject, characterized in that it comprises: a correction data generator that uses difference data between measured projection data for each X-ray energy obtained by imaging a known phantom having a known composition and shape and a size smaller than the imaging field of view, and calculated projection data for each X-ray energy calculated based on the X-ray transmission length obtained from the shape of the known phantom, to generate correction data; and a correction unit that uses the correction data to correct the projection data for each X-ray energy of the subject.

[0010] In addition, the present invention is a method for correcting projection data obtained by an X-ray CT device that images a subject, and the projection data correction method is characterized in that it comprises: a correction data creation step, which uses difference data between measured projection data of each X-ray energy obtained by imaging a known phantom whose composition and shape are known and has a size smaller than the imaging field of view, and calculated projection data of each X-ray energy calculated based on the X-ray transmission length obtained according to the shape of the known phantom, to create correction data; and a correction step, which uses the correction data to correct the projection data of each X-ray energy of the subject.

[0011] Effects of the Invention

[0012] According to the present invention, an X-ray CT apparatus and a projection data correction method capable of suppressing artifacts generated near the edge of a subject can be provided. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Figure 1 It is a diagram showing the overall structure of an X-ray CT apparatus.

[0014] Figure 2 This is a diagram explaining calibration of a photon counting detector.

[0015] Figure 3 This is a diagram showing an example of the process flow of Example 1.

[0016] Figure 4 This is a diagram showing an example of the flow of processing for creating correction data.

[0017] Figure 5 It is a diagram illustrating measured projection data of a phantom whose composition and shape are known.

[0018] Figure 6 It is a diagram illustrating differential data.

[0019] Figure 7 This is a diagram showing an example of explanatory variables.

[0020] Figure 8 This is a diagram showing an example of correction data.

[0021] Figure 9 1 is a diagram showing an example of the flow of processing for correcting projection data.

[0022] Description of Reference Signs

[0023] 101: X-ray CT apparatus, 102: Gantry, 103: X-ray tube, 104: X-ray, 105: Bow-tie filter, 106: Bed, 107: Subject, 108: Detector panel, 109: Calculation device, 110: Input device, 111: Display device, 112: Opening, 201: Combination of base materials, 202: First base material, 203: Second base material, 204: Calibration data, 501: Known phantom, 502: Imaging field of view, 701: Distance between edge element and correction element, 702: X-ray transmission length in the correction element, 703: Alternative X-ray transmission length DETAILED DESCRIPTION

[0024] Hereinafter, embodiments of the present invention will be described using the drawings.

[0025] [Example 1]

[0026] exist Figure 1 The overall configuration of an X-ray CT apparatus 101 according to this embodiment is shown. The horizontal direction of the paper is designated as the X-axis, the vertical direction is designated as the Y-axis, and the direction perpendicular to the XY plane is designated as the Z-axis. The X-ray CT apparatus 101 includes a gantry 102, an X-ray tube 103, a bow-tie filter 105, a bed 106, a detector panel 108, a computing device 109, an input device 110, and a display device 111.

[0027] A subject 107 is placed on a bed 106 and positioned within an opening 112 provided in the gantry 102. X-rays 104 emitted from the X-ray tube 103 are shaped into a beam suitable for the size of the subject 107 by a bowtie filter 105, then irradiate the subject 107. After passing through the subject 107, they are detected by the detector panel 108. The X-ray tube 103 and the detector panel 108 are mounted on the gantry 102 so as to face each other with the subject 107 sandwiched between them. The gantry 102's rotational drive unit rotates around the subject 107. By repeating X-ray irradiation from the X-ray tube 103 and X-ray measurement by the detector panel 108 in conjunction with the rotation of the rotational drive unit, projection data at various projection angles is acquired.

[0028] The acquired projection data is reconstructed in the computing device 109 to generate a tomographic image of the subject 107 and display it on the display device 111. Furthermore, when projection data is acquired while the bed 106 carrying the subject 107 and the gantry 102 are relatively moved in the Z-axis direction, a volumetric image of the subject 107 is generated. Furthermore, the X-ray dose emitted by the X-ray tube 103, the rotation speed of the gantry 102, and the relative movement speed between the gantry 102 and the bed 106 are set based on the scanning conditions input by the operator via the input device 110. The computing device 109 has the same hardware configuration as a typical computer device, including a CPU (Central Processing Unit), memory, and an HDD (Hard Disk Drive). It performs correction processing on projection data and controls various components.

[0029] The detector panel 108 is constructed by arranging multiple detection elements in an arc shape centered on the X-ray focal point of the X-ray tube 103. The detection elements are photon counting detectors that measure the energy of incident X-ray photons, i.e., the X-ray energy, and output an output corresponding to the X-ray energy.

[0030] The X-ray CT apparatus 101 equipped with a photon-counting detector can acquire an X-ray energy spectrum associated with projection data of the subject 107. This allows the generation of medical images that distinguish materials of different compositions and medical images separated into multiple energy components. Furthermore, to obtain medical images that distinguish materials of different compositions, it is necessary to preliminarily calibrate the relationship between the output and X-ray energy of each detector element when measuring a combination of multiple base materials of known composition and thickness using the photon-counting detector.

[0031] use Figure 2 To illustrate the calibration of the photon counting detector. In the calibration of the photon counting detector, calibration data 204 is used, and the calibration data 204 is obtained using a combination 201 of two substrate materials, such as a first substrate material 202 and a second substrate material 203, of a plurality of substrate materials with known compositions and thicknesses. For the first substrate material 202, for example, acrylic resin, polyethylene, or the like is used. For the second substrate material 203, for example, aluminum, hydroxyapatite, a calcium mixture, an iodine mixture, tin, a tin mixture, or the like is used. For the combination 201 of substrate materials, a plurality of plates of different thicknesses can be used for each substrate material. For example, if the thickness of the first substrate material 202 is J type and the thickness of the second substrate material 203 is K type, a combination 201 of substrate materials of J×K types is used, and for each combination, an X-ray energy spectrum is obtained for each detector element. In Figure 2In FIG, since J=3 and K=3, nine types of X-ray energy spectra are shown as the correction data 204. The acquired correction data 204 is stored in the storage unit of the calculation device 109 and is used to correct the projection data of the subject 107.

[0032] Furthermore, using only the correction data 204 obtained using the combination 201 of the base material cannot suppress artifacts generated near the edges of the subject 107. Therefore, in Example 1, artifacts generated near the edges are suppressed by correcting the projection data of the subject 107 using pre-created correction data. The correction data is created using difference data between measured projection data for each X-ray energy obtained by imaging a phantom with a known composition and shape, and calculated projection data for each X-ray energy calculated based on the X-ray transmission length determined from the phantom's shape.

[0033] use Figure 3 , an example of the processing flow of Example 1 is described step by step.

[0034] (S301)

[0035] The calculation device 109 controls each unit based on the set scanning conditions to obtain projection data of the subject 107. Since the detector panel 108 is a photon counting detector, projection data is obtained for each X-ray energy.

[0036] (S302)

[0037] The calculation device 109 reads the correction data used to correct the projection data obtained in S301. The correction data is pre-created for each X-ray energy based on, for example, measured projection data for each X-ray energy obtained by imaging a phantom with a known composition and shape, and is stored in the storage unit of the calculation device 109.

[0038] use Figure 4 , an example of the process of creating correction data is described step by step.

[0039] (S401)

[0040] The calculation device 109 acquires measurement projection data for each X-ray energy by imaging a phantom whose composition and shape are known.

[0041] use Figure 5The following describes the measured projection data for a phantom with a known composition and shape. A phantom with a known composition and shape, namely a known phantom 501, is placed in the center of opening 112 and imaged. A material with a relatively low effective atomic number is used for known phantom 501 to accommodate soft tissue in the human body. Furthermore, known phantom 501 is preferably elliptical, similar in cross-section to the subject 107. Furthermore, to simulate the head of subject 107, the outer surface of known phantom 501, which has an elliptical shape, may be made of a material with a relatively high effective atomic number.

[0042] By rotating and imaging the known phantom 501, measured projection data is acquired, which maps projection values ​​onto a plane whose coordinate axes are the projection angle θ and the channel number ξ of the detector panel 108. Since the detector panel 108 is a photon-counting detector, measured projection data is acquired for each X-ray energy E. Furthermore, the size of the known phantom 501 is smaller than the imaging field of view 502. Because the known phantom 501 is smaller than the imaging field of view 502, the measured projection data includes the effects of penumbra and scattered radiation generated near the edges of the subject 107.

[0043] Furthermore, the measured projection data can be used to determine the errors Δx and Δy between the center of the known phantom 501 and the rotation center of the gantry 102, as well as the inclination Δθ of the known phantom 501 relative to the horizontal. For example, the tomographic image obtained by reconstructing the measured projection data is divided into an air region and an area outside the air region, and Δx, Δy, and Δθ are determined by applying an elliptical approximation to the boundary between the two regions. Δx and Δy can also be determined based on the center positions of the maximum and minimum widths in the ξ direction in the measured projection data, and Δθ can be determined based on the projection angle θ that gives the maximum or minimum width in the ξ direction.

[0044] (S402)

[0045] The computing device 109 calculates the X-ray transmission length based on the shape of the known phantom 501. The X-ray transmission length is the length that an X-ray incident on any detection element travels across the known phantom 501. It is calculated for each projection angle θ and channel number ξ of the detector panel 108. The X-ray transmission length can be calculated using the following formula.

[0046]

Mathematical formula 1

[0047]

[0048]

Mathematical formula 2

[0049] y=px+q

[0050]

Mathematical formula 3

[0051]

[0052] Here, Mathematical Formula 1 represents the surface of the known phantom 501 having an elliptical shape, and is an equation in an xy coordinate system with the center of the ellipse as the origin. When A>B, A is the major axis radius and B is the minor axis radius.

[0053] Equation 2 represents the straight line connecting the X-ray focal point and an arbitrary detection element. The values ​​of the slope p and slice q of the straight line are calculated for each projection angle θ and channel number ξ. Furthermore, the errors Δx and Δy between the center of the known phantom 501 and the rotation center of the gantry 102, as well as the known slope Δθ of the phantom 501 relative to the horizontal, can be included in the slope p and slice q in Equation 2.

[0054] Equation 3 is an equation for finding the x-coordinate of the intersection of Equations 1 and 2. Substituting the x-coordinate found in Equation 3 into Equation 2 yields the y-coordinate of the intersection of Equations 1 and 2. When the coordinates of the two intersection points are (x1, y1) and (x2, y2), the X-ray transmission length is calculated as ((x1-x2)^2+(y1-y2)^2)^0.5. If Equation 3 does not yield two real number solutions, the X-ray transmission length is zero because the ellipse and the line do not intersect. Furthermore, when p→∞, that is, when the line is parallel to the y-axis, Equations 2 and 3 cannot be used as an exception. However, since the variable y disappears, the X-ray transmission length can be easily calculated.

[0055] (S403)

[0056] The calculation device 109 calculates projection data based on the X-ray transmission length calculated in S402. The calculation of the projection data uses, for example, calibration data 204, which is the X-ray energy spectrum for each thickness of the substrate material, and attenuation coefficient values ​​from literature. Using calibration data 204 improves the accuracy of the calculated projection data.

[0057] Since the vertical axis of the X-ray energy spectrum represents the number of X-ray photons, the X-ray transmission length is converted to the number of X-ray photons per X-ray energy using the X-ray energy spectrum corresponding to the thickness of the underlying material equivalent to the X-ray transmission length. The converted value is then compared with the number of X-ray photons in the X-ray energy spectrum when the underlying material thickness is zero. This determines the projection value (calculated projection data) for each projection angle θ and channel number ξ of the detector panel 108 for each X-ray energy. Since the calculated projection data is based on the X-ray transmission length, the length of the X-ray traversal through the known phantom 501, it does not include the effects of penumbra and scattered radiation generated near the edges.

[0058] (S404)

[0059] The computing device 109 obtains differential data by performing a differential operation between the measured projection data obtained in S401 and the calculated projection data calculated in S403. Since the effects of penumbra and scattered rays generated near the edge are included in the measured projection data but not in the calculated projection data, the differential data only includes the effects of penumbra and scattered rays generated near the edge.

[0060] use Figure 6 The differential data is explained below. The differential data shows a maximum value at the edge element, which is the boundary between the air region and the phantom region, that is, the detection element where the X-rays passing through the edge are incident, and decreases as it moves from the edge element toward the center of the known phantom 501. Since the X-ray focus has a finite size and produces a penumbra, the X-rays passing through the air region are incident on the detection element near the edge, thus becoming Figure 6 The differential data is shown.

[0061] Furthermore, the difference data values ​​for the edge elements are larger for Section 1, which includes the edge along the minor axis of the ellipse, than for Section 2, which includes the edge along the major axis. Furthermore, the change in the difference data in the ξ direction is also larger for Section 1 than for Section 2. This is because the greater the attenuation change in the ξ direction, the greater the penumbra's influence.

[0062] (S405)

[0063] The computing device 109 generates correction data based on the difference data acquired in S404 for use in correcting the projection data of the subject 107. For example, the correction data is generated by associating the difference data value with the distance from the edge element, which is the detection element incident on the edge portion of the subject 107, to the correction element, which is the detection element to be corrected.

[0064] Furthermore, to improve correction accuracy, steps S401 to S404 can be performed on phantoms 501 of known sizes to obtain multiple differential data sets, and the obtained differential data sets can be used as correction data corresponding to subjects 107 of arbitrary sizes. In this case, the correction data sets are also associated with the distance from the edge element to the correction element.

[0065] Alternatively, the differential data obtained using a known phantom 501 of a single size may be compared with the Figure 7 The distance 701 between the edge element 108A and the correction element 108B is established in correspondence with the X-ray transmission length 702 in the correction element 108B. Figure 6As shown, the greater the attenuation change in the ξ direction, the greater the influence of the penumbra. Therefore, by associating the distance 701 and the X-ray transmission length 702 with the differential data, the influence of the penumbra can be accurately reflected in the differential data.

[0066] Furthermore, when the distance 701 between the edge element 108A and the correction element 108B approaches zero, the X-ray transmission length 702 of the correction element 108B also approaches zero, causing a bias in the correction data. Therefore, the difference data can be associated with an alternative X-ray transmission length 703, which is the X-ray transmission length of a detection element closer to the center of the known phantom 501 than the correction element 108B, to replace the X-ray transmission length 702 of the correction element 108B. The distance between the detection element corresponding to the alternative X-ray transmission length 703 and the correction element 108B is preferably 1 to 3 mm.

[0067] use Figure 8 The correction data generated by associating the difference data with the distance from the edge element to the correction element and the X-ray transmission length in the correction element will be described. Figure 8 In the left graph, the vertical axis represents the X-ray transmission length in the correction element, the horizontal axis represents the distance from the edge element to the correction element, and the axis perpendicular to the paper represents the differential data. The oblique lines in the figure are the areas with differential data. In addition, the differential data at sections A and B in the left graph are Figure 8 The figure is shown on the right side of . For both cross-sections A and B, the differential data values ​​are large near the edge, where the distance from the edge element to the correction element is zero. Furthermore, cross-section B, where the X-ray transmission length of the correction element is large, exhibits larger differential data values ​​near the edge than cross-section A.

[0068] Above, by executing Figure 4 The correction data is created according to the processing flow described above, and the arithmetic device 109 functions as a correction data creation unit that creates the correction data.

[0069] Back to Figure 3 Description.

[0070] (S303)

[0071] The calculation device 109 uses the correction data acquired in S302 to correct the projection data of the subject 107. That is, the calculation device 109 functions as a correction unit that corrects the projection data of the subject using the correction data.

[0072] use Figure 9 An example of the flow of processing for correcting projection data will be described step by step.

[0073] (S901)

[0074] The calculation device 109 performs image reconstruction on the projection data acquired in S301 to generate a cross-sectional image.

[0075] (S902)

[0076] The arithmetic device 109 extracts the subject region by performing image binarization processing on the cross section generated in S901 .

[0077] (S903)

[0078] The computing device 109 obtains the position of the edge element and the X-ray transmission length in the correction element based on the X-ray transmission length for each coordinate (ξ, θ) obtained by performing a forward projection operation on the subject region extracted in S902. Furthermore, when the correction data is associated with the substitute X-ray transmission length 703, the substitute X-ray transmission length 703 is obtained based on the X-ray transmission length obtained by the forward projection operation.

[0079] (S904)

[0080] The computing device 109 calculates the position of the edge element obtained in S903 and the X-ray transmission length in the correction element. Figure 8 The correction data shown in the example is differential data. Specifically, differential data corresponding to the distance between the edge element and the correction element and the X-ray transmission length in the correction element is read. Furthermore, when the correction data is associated with the substitute X-ray transmission length 703, the differential data is read based on the edge element position and the substitute X-ray transmission length 703.

[0081] (S905)

[0082] The computing device 109 corrects the projection data acquired in S301 based on the differential data read out in S904. For example, if the differential data read out in S904 is obtained by subtracting the measured projection data from the calculated projection data, correction is performed by adding the differential data to the subject's projection data. Corrected projection data, or corrected projection data, is acquired for each X-ray energy.

[0083] Back to Figure 3 Description.

[0084] (S304)

[0085] The computing device 109 decomposes the corrected projection data obtained in S303 into base materials. For example, the correction data 204 is used for the decomposition into base materials. The corrected projection data obtained for each X-ray energy has an X-ray spectrum for each coordinate (ξ, θ). Therefore, the X-ray spectrum in the correction data 204 is searched for an X-ray spectrum with a shape closest to the shape of the X-ray spectrum for each coordinate (ξ, θ), and the thickness combination of the base material combination 201 corresponding to the searched X-ray spectrum is obtained. Specifically, if the first base material 202 is acrylic resin and the second base material 203 is tin, the thickness of the acrylic resin and the thickness of the tin are obtained for each coordinate (ξ, θ), and the projection data for the acrylic resin and the projection data for the tin are obtained.

[0086] (S305)

[0087] The calculation device 109 performs image reconstruction on the projection data for each base material decomposed in S304 to generate a tomographic image for each base material.

[0088] Above, by executing Figure 3 The processing flow described above can generate a tomographic image in which artifacts near the edge of the subject are suppressed.

[0089] The above describes embodiments of the X-ray CT apparatus and projection data correction method of the present invention. Furthermore, the X-ray CT apparatus and projection data correction method of the present invention are not limited to the above embodiments; the components may be modified and embodied without departing from the spirit of the invention. Furthermore, the multiple components disclosed in the above embodiments may be appropriately combined. Furthermore, some components may be deleted from all the components shown in the above embodiments.

Claims

1. An X-ray CT apparatus for imaging a subject, the X-ray CT apparatus comprising: a correction data generator that generates correction data using difference data between measured projection data for each X-ray energy obtained by imaging a known phantom having a known composition and shape and a size smaller than an imaging field of view, and calculated projection data for each X-ray energy calculated based on an X-ray transmission length determined from the shape of the known phantom; and A correction unit corrects the projection data of the subject for each X-ray energy using the correction data.

2. The X-ray CT apparatus according to claim 1, wherein: The correction data is associated with a distance from an edge element, which is a detection element incident with X-rays that have passed through an edge portion of the subject, to a correction element, which is a detection element to be corrected.

3. The X-ray CT apparatus according to claim 2, wherein: The correction data is associated with an X-ray transmission length of the X-ray incident on the correction element.

4. The X-ray CT apparatus according to claim 1, wherein: The correction data generating unit calculates the calculated projection data using calibration data for each X-ray energy obtained by imaging a plate material having a known composition and thickness.

5. The X-ray CT apparatus according to claim 1, wherein: The known body mold has an oval shape.

6. The X-ray CT apparatus according to claim 5, characterized in that: The known phantoms include: a first base material having an oval shape; and The second base material has an effective atomic number greater than that of the first base material and covers the periphery of the first base material.

7. A method for correcting projection data, the projection data being acquired by an X-ray CT apparatus for imaging a subject, the method comprising: a correction data creating step of creating correction data using difference data between measured projection data for each X-ray energy obtained by imaging a known phantom having a known composition and shape and a size smaller than an imaging field of view and calculated projection data for each X-ray energy calculated based on an X-ray transmission length determined from the shape of the known phantom; and The correction step corrects the projection data of the subject for each X-ray energy using the correction data.

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