JP5511188B2 - Image reconstruction method and X-ray CT apparatus - Google Patents

Description

  The present invention relates to an image reconstruction method for an X-ray CT (Computed Tomography) image and an X-ray CT apparatus for carrying out the method.

  Conventionally, projection data collected by a helical scan that scans a data (data) collection system including an X-ray tube and an X-ray detector while moving relative to the body axis of the subject. An image reconstruction method for reconstructing a CT image based on this is known.

  Further, in this image reconstruction method, a predetermined centered on a reference view corresponding to a time at which the reference position of the data acquisition system and the position of the reconstruction plane of the CT image coincide with each other in the body axis direction of the subject. A method for setting a view range as a view range of projection data used for reconstruction of the CT image is known (see Patent Document 1, paragraph 22 and the like). The reference position of the data acquisition system is, for example, the position of the intersection of a straight line connecting the X-ray focal point of the X-ray tube and the center of the X-ray detector and the rotation axis of the data acquisition system.

JP 2007-216043 A

  However, when the above helical scan is a so-called variable pitch helical scan in which the data acquisition system is scanned while relatively linearly moving at a variable speed, before and after the time corresponding to the reference view, The speed of the relative linear movement may be different. That is, the predetermined view range used for reconstruction of the CT image includes the view ranges on both sides adjacent to the reference view, but the relative linear movement distance corresponding to the view range on one side and the view range on the other side The distance of the relative linear movement corresponding to may be different. Therefore, among the view ranges on both sides adjacent to the reference view, an X-ray that passes through a view that contributes to the reconstruction of the CT image, that is, an X-ray that passes through the reconstruction plane, is further outside the view range on the slow relative linear movement speed side. The projection data view by the beam remains unused for reconstruction, leaving room for improving the image quality of the CT image.

  In view of the above circumstances, an object of the present invention is to provide an image reconstruction method capable of further improving the image quality of a CT image obtained by variable pitch helical scanning, and an X-ray CT apparatus using the method.

  In a first aspect, the present invention provides a variable pitch helical scan that performs scanning while moving a data acquisition system including an X-ray tube and an X-ray detector relative to the body axis direction of a subject at a variable linear velocity. An image reconstruction method for reconstructing a CT image on the basis of projection data collected by the method, comprising: setting a view range to be used for reconstruction of the CT image, wherein the reference of the data collection system Among the view ranges on both sides adjacent to the reference view corresponding to the time when the position and the position of the reconstruction plane of the CT image coincide with each other in the body axis direction, than the view range on the side with the higher relative linear movement speed A view range composed of the reference view and the view ranges on both sides thereof is used for the reconstruction of the CT image so that the view range on the lower speed side becomes a larger view range. And setting the view range that provides an image reconstruction method and a step of reconstructing the CT image using the projection data of the set view range.

  In a second aspect, the present invention provides the step of setting a view range used for reconstruction of the CT image, specifying a view range for a predetermined same view angle on both sides adjacent to the reference view; Obtaining information on the speed of the relative linear movement of the specified both-side view range; and information on the speed of the relative linear movement of the specified both-side view range based on the information on the speed of the relative linear movement. The step of specifying the magnitude relationship and the reference view and the view ranges on both sides thereof are set such that the side on which the magnitude relationship is smaller than the side on which the magnitude relationship is larger is a larger view range. And providing a view range as a view range to be used for the reconstruction of the CT image.

  In a third aspect, according to the present invention, the step of specifying the magnitude relationship of the relative linear movement speeds of the specified both-side view ranges includes the one-side view range of the specified both-side view ranges. The second aspect includes a step of specifying a magnitude relationship between the distance of the relative linear movement for collecting the projection data of the first and the distance of the relative linear movement for collecting the projection data of the other-side view range. An image reconstruction method is provided.

  In a fourth aspect, the present invention uses a view range corresponding to the predetermined same view angle as the view range on the side where the magnitude relationship is large, and the view range on the side where the magnitude relationship is small is An image reconstruction method according to the second aspect or the third aspect, wherein the image range is made larger than a view range for a predetermined same view angle.

  In a fifth aspect, the present invention has a data acquisition system including an X-ray tube and an X-ray detector, and the data acquisition system is a relative straight line at a variable speed with respect to the body axis direction of the subject. X-ray CT comprising: data collection means for collecting projection data by variable pitch helical scan that performs scanning while moving; and image reconstruction means for reconstructing a CT image based on the projection data collected by the data collection means A view range setting means for setting a view range used for reconstruction of the CT image, wherein a reference position of the data acquisition system and a position of a reconstruction plane of the CT image are in the body axis direction Of the view ranges on both sides adjacent to the reference view corresponding to the matching time, the view range on the side with the lower speed than the view range on the side with the higher relative linear movement speed. Includes a view range setting unit that sets a view range including the reference view and the view ranges on both sides of the reference view as a view range used for reconstruction of the CT image so that a large view range is obtained. An reconstruction means provides an X-ray CT apparatus for reconstructing the CT image using projection data of the set view range.

  In a sixth aspect, according to the present invention, the view range setting means specifies a view range corresponding to a predetermined same view angle on both sides adjacent to the reference view, and the relative view ranges on the specified both sides are set. A magnitude relation specifying unit that obtains information about the speed of the linear movement and specifies the magnitude relation of the speed of the relative linear movement of the specified view ranges on both sides based on the information about the speed of the relative linear movement; The CT image includes a view range composed of the reference view and the view ranges on both sides of the reference view so that the side on which the magnitude relationship is small is larger than the side on which the magnitude relationship is large. The X-ray CT apparatus according to the fifth aspect, which is set as a view range used for reconstruction of the above, is provided.

  In a seventh aspect, the present invention relates to the relative linear movement distance for the projection of the view range on one side out of the identified view ranges on both sides, and the other An X-ray CT apparatus according to the sixth aspect of the invention that specifies a magnitude relationship with the distance of the relative linear movement for collecting projection data of a side view range is provided.

  In an eighth aspect, according to the present invention, the view range setting unit uses a view range corresponding to the predetermined same view angle as a view range on the side where the magnitude relationship becomes large, and the magnitude relationship becomes small. The X-ray CT apparatus according to the sixth aspect or the seventh aspect, wherein the side view range is made larger than the view range corresponding to the predetermined same view angle.

  In a ninth aspect, the present invention provides the above-described second aspect wherein the predetermined same view angle is half of the fan angle of π / 2, π / 2 + X-ray beam, π, or π + half of the fan angle. An X-ray CT apparatus according to any one of the sixth aspect to the eighth aspect is provided.

  In a tenth aspect, the present invention relates to the distance of the relative linear movement for collecting projection data on one side of the view ranges on both sides adjacent to the reference view constituting the view range used for reconstruction of the CT image. X-ray CT apparatus according to any one of the fifth to ninth aspects, wherein the distance of the relative linear movement for collecting projection data on the other side of the view ranges on both sides is the same I will provide a.

  In an eleventh aspect, the present invention provides that the reference position of the data acquisition system is an intersection of a straight line connecting the X-ray focal point of the X-ray tube and the center of the X-ray detector and the rotation axis of the data acquisition system. The X-ray CT apparatus according to any one of the fifth to tenth aspects is provided.

  According to the present invention, the relative linear movement speed of the data acquisition system in the view ranges on both sides adjacent to the reference view corresponding to the time at which the reference position of the data acquisition system and the position of the reconstruction plane of the CT image coincide with each other. The view range consisting of the reference view and the view ranges on both sides is set as the view range used for CT image reconstruction so that the view range on the slow side is larger than the view range on the fast side Therefore, it is not used when a view range including a reference view and a view range corresponding to a predetermined view angle on both sides of the reference view is set as a view range to be used for CT image reconstruction as in the past, Effective use of projection data of useful views can reduce the noise component of projection data used for reconstruction. The image quality of the obtained CT image can be further improved.

It is a block diagram which shows the X-ray CT apparatus concerning this embodiment. It is a flowchart which shows the data collection process concerning this embodiment. It is a time chart showing "projection data collection", "rotation", "linear movement", and "relative position of X-ray tube and X-ray detector". It is a figure which shows an example of the projection data collected in this embodiment. It is a flowchart which shows the image reconstruction process concerning this embodiment. It is a flowchart which shows the view range setting process for reconstruction concerning this embodiment. It is a figure for demonstrating the 1st specific example of the view range setting process for a reconstruction in this embodiment. It is a figure for demonstrating the 2nd specific example of the view range setting process for a reconstruction in this embodiment.

  Hereinafter, the present invention will be described in more detail with reference to embodiments shown in the drawings. Note that the present invention is not limited thereby.

  FIG. 1 is a configuration diagram showing an X-ray CT apparatus 100 according to the present embodiment.

  The X-ray CT apparatus 100 includes an operation console 1, a bed apparatus 10, and a scanning gantry 20.

  The operation console 1 includes an input device 2 that receives user input, and a central processing unit that executes control for data collection processing, image reconstruction processing, view range setting processing for reconstruction included in the processing, and the like. 3, a data collection buffer (buffer) 5 that collects projection data acquired by the scanning gantry 20, a monitor 6 that displays a CT image reconstructed from the projection data, a program, data, and a CT image And a storage device 7 for storing.

  The bed apparatus 10 includes a table 12 on which an imaging target 40 is placed and put into and out of the opening B of the scanning gantry 20. The table 12 is moved up and down and horizontally moved by a motor built in the bed apparatus 10. Here, the linear movement direction of the table 12 is the z direction, the vertical direction is the y direction, and the horizontal direction perpendicular to the z direction and the y direction is the x direction.

  The scanning gantry 20 includes a rotating portion 15 and a main body portion 20a that rotatably supports the rotating portion 15. The rotating unit 15 includes an X-ray tube 21, an X-ray controller 22 that controls the X-ray tube 21, and a collimator that shapes cone beam X-rays generated from the X-ray tube 21. 23, an X-ray detector 24 in which a plurality of detector arrays in which a plurality of X-ray detection elements (detectors) are arranged in the channel direction are arranged in the z direction, and the output of the X-ray detector 24 is converted into projection data A DAS (Data Acquisition System) 25 that collects data and an X-ray controller 22, a collimator 23, and a rotating unit controller 26 that controls the DAS 25 are mounted. The X-ray tube 21 and the X-ray detector 24 constitute a data collection system 41. The main body 20 a includes a control controller 29 that exchanges control signals and the like with the operation console 1 and the couch device 10. The rotating part 15 and the main body part 20 a are electrically joined via a slip ring 30.

  Note that the DAS 25 attaches to the collected projection data the view angle position β on the circular orbit of the X-ray tube 21 when the projection data is collected and the position TL in the z direction of the table 12 for each view. To the data collection buffer 5. Thereby, based on the geometrical positional relationship of the data acquisition system 41, the positional relationship between the projection data corresponding to each detection element of the X-ray detector 24 in the imaging space is determined by the X-ray beam. be able to.

  The scanning gantry 20 and the central processing unit 3 are examples of X-ray data collection means in the present invention. The central processing unit 3 is an example of a view range setting unit and an image reconstruction unit in the present invention.

  FIG. 2 is a flow diagram showing the data collection process.

  Here, projection data is collected by variable pitch helical scanning. This scan is a helical scan that collects projection data even while the helical pitch changes. Here, the scanning gantry 20 moves relatively linearly with acceleration / deceleration from the linear movement start point Zs in the z direction with respect to the table 12 to the linear movement end point Zf, and the linear movement start point Zs and the linear movement end. Projection data is collected by providing a dwell time of the scanning gantry 20 at the point Zf.

  In step A1, a scan condition including a residence time τ is set based on a parameter set by the user. The residence time τ is, for example, τ1 corresponding to one rotation time R of the rotating unit 15.

  In step A2, for example, as shown at time t0 in FIG. 3, “projection data collection” is started.

  In step A3, for example, as shown at time t0 in FIG. 3, “rotation” of the rotating unit 15 on which the data acquisition system 41 including the X-ray tube 21 and the X-ray detector 24 is mounted is started.

  In step A4, the linear movement direction of the rotating unit 15 with respect to the table 12 is set to the forward direction (here, the + z direction).

  In step A5, for example, as shown at time t0 to t1 in FIG. That is, the projection data is collected only for the dwell time τ1, without only the “rotation” of the rotating unit 15 and the linear movement.

  In step A6, for example, as shown at time t1 in FIG. 3, “straight line movement” of the table 12 is started.

  In step A7, for example, as shown at times t1 to t2 in FIG. 3, the table 12 is accelerated. During this time, projection data is collected.

  In step A8, for example, as shown at times t2 to t3 in FIG. 3, the table 12 is linearly moved at a constant speed V1. During this time, projection data is collected.

  In step A9, it is determined whether or not the vicinity of the start point or end point of the linear movement has been reached. If not reached, the linear movement is continued until it reaches, and if it has reached, the process proceeds to step A10.

  In step A10, for example, as shown at times t3 to t4 in FIG. 3, the table 12 is decelerated. During this time, projection data is collected.

  In step A11, for example, as shown at time t4 in FIG.

  In step A12, for example, as shown at times t4 to t5 in FIG. That is, the projection data is collected only for the residence time τ1 without rotating the rotating unit 15 and moving it linearly.

  In step A13, if the scheduled data collection has not been completed, the process proceeds to step A13. If completed, the process proceeds to step A14.

  In step A14, the moving direction of the table 12 is reversed. And it returns to step A6 and continues data collection. That is, the previous end point is the current start point, the previous start point is the current end point, and projection data is collected while the table 12 is linearly moved in the opposite direction to the previous time.

  In Step A15, for example, as shown at time t9 in FIG. 3, the “rotation” of the rotating unit 15 on which the data acquisition system 41 including the X-ray tube 21 and the X-ray detector 24 is mounted is terminated.

  In Step A16, for example, as shown at time t9 in FIG.

  FIG. 4 is a diagram schematically showing an example of projection data collected by the data collection process. The upper part of FIG. 4 schematically shows the X-ray tube 21 and the X-ray detector 24 at the linear movement start time and the linear movement end time, and the lower part of FIG. 4 shows the collected projection data of all views. An example of PD is schematically represented. In the lower part of FIG. 4, the horizontal axis represents the position Z in the z direction, and the vertical axis represents the view number (proportional to time) View. For the projection data, for each view number, a detector width (scan width) W, that is, a plurality of detector rows, is collected around the reference position TL with respect to the table 12 of the data collection system 41. Here, the reference position TL of the data acquisition system 41 is the position of the intersection of the straight line connecting the X-ray focal point F of the X-ray tube 21 and the center of the X-ray detector 24 and the rotation axis Ic of the data acquisition system 41. It is.

  FIG. 5 is a flowchart showing image reconstruction processing.

  In step B1, the phase Pi of the CT image to be reconstructed is designated. Here, the phase Pi means a phase or pass when the helical scan is continuously performed, and the first outbound path is the phase P1, the first backward path is the phase P2, and the second outbound path is the phase P3. The second return path is defined to be phase P4. In Step B1, for example, each time Step B1 is executed, the phases are specified one by one in time series order. Note that the user may set one or a plurality of arbitrary phases or phase ranges, and sequentially specify the phases therein.

  In step B2, a slice position Zj of the CT image to be reconstructed is designated. For example, the entire region of the image extension region Rs on the linear movement start point Zs side, the linear movement range Rm, and the image extension region Rf on the linear movement end point Zf side is set as a slice position setting region. Each time this step B2 is executed, the slice positions are sequentially designated at predetermined slice intervals in the z direction from the end in the slice position setting area. The user may set one or a plurality of arbitrary slice positions or slice ranges, and sequentially specify them.

  In step B3, the view range of the projection data PDpi, zj used to reconstruct the CT image at the slice position Zj in the phase Pi is set by the following method, for example.

  FIG. 6 is a flowchart showing the reconstruction view range setting process.

  In step B31, the reference view CVpi, zj corresponding to the position of the reconstruction plane of the CT image to be reconstructed is specified. The reference view CVpi, zj is a view of projection data collected at a time when the reference position TL of the data collection system 41 coincides with the slice position Zj in the phase Pi.

  In step B32, a view range of a predetermined same view angle F / 2 minutes (= half of the view angle F) is specified on both sides of the reference view CVpi, zj specified in step B31, and the reference view CVpi, zj And the identified view range on both sides are provisionally determined as a view range VRpi, zj for reconstruction. The view range VRpi, zj is defined as a range having both ends of the first view β1pi, zj having the smaller view number View and the second view β2pi, zj having the larger view number View. The view angle F may be, for example, the fan angle of π, π + X-ray beam, 2π, 2π + X-ray beam. That is, the view angle F / 2 can be, for example, half the fan angle of π / 2, π / 2 + X-ray beam and half the fan angle of π, π + X-ray beam. Here, the view angle F / 2 is π.

  In step B33, the data acquisition system 41 moves relative to the z-direction while the projection data of the view corresponding to the first range VR1pi, zj from the reference view CVpi, zj to the first view β1pi, zj is acquired. A first distance K1pi, zj that is the calculated distance is calculated. Further, the distance that the data collection system 41 has moved in a relative straight line in the z direction while the projection data of the view corresponding to the second range VR2pi, zj from the reference view CVpi, zj to the second view β2pi, zj is collected. A second distance K2pi, zj is calculated.

  In Step B34, it is determined whether or not the first distance K1pi, zj is equal to the second distance K2pi, zj. If the determination condition is not satisfied, the process proceeds to step B35. If the determination condition is satisfied, the process proceeds to step B39.

  In step B39, the current reconstruction view range VRpi, zj is determined, and the reconstruction view range setting process is terminated.

  In step B35, the magnitude relationship between the first distance K1pi, zj and the second distance K2pi, zj is specified, and a shift target view is determined. When the first distance K1pi, zj is relatively small, the first view β1pi, zj is the shift target view, and when the second distance K2pi, zj is relatively small, the second view β2pi , zj is the shift target view.

  In step B36, the shift target view is shifted and updated by a minute view Δβ in a direction in which the reconstruction view range VRpi, zj becomes larger. For example, Δβ = 1 view.

  In step B37, the first distance K1pi, zj or the second distance K2pi, zj, which is the distance corresponding to the range from the reference view CVpi, zj to the shift target view, is calculated again.

  In Step B38, it is determined whether or not the first distance K1pi, zj is equal to the second distance K2pi, zj. If the determination condition is satisfied, the process proceeds to step B39. If the determination condition is not satisfied, the process proceeds to step B36.

  Here, a specific example of the reconstruction view range setting process will be described.

  A first specific example will be described. In this example, the phase of the CT image to be reconstructed is the first phase P1, and the slice position is a position Z6 near the center of the linear movement range Rm.

  FIG. 7 is a diagram for explaining a second specific example of the reconstruction view range setting process. The lower part of FIG. 7 shows the correspondence between the phase View P1 in FIG. 4 near the center of the linear movement range Rm and the reference position TL in the z direction of the data acquisition system 41. The upper part of FIG. A state example of the data collection system 41 when the TL is at each predetermined position is schematically shown.

  In step B31, for example, as shown in FIG. 7, a view in which the reference position TL of the data collection system 41 matches the position Z6 is specified as the reference view CVp1, z6.

  In step B32, for example, as shown in FIG. 7, a view having a view number smaller than the reference view CVp1, z6 by the view angle F / 2 is set as the first view β1p1, z6 and the view angle F / from the reference view CVp1, z6. A view having a view number larger by 2 minutes is set as a second view β2p1, z6. Thereby, the range from the first view β1p1, z6 to the second view β2p1, z6 is provisionally determined as the view range VRp1, z6 for reconstruction.

  In step B33, a first distance K1p1, z6 corresponding to the first range VR1p1, z6 from the reference view CVp1, z6 to the first view β1p1, z6 is calculated, and the second view is calculated from the reference view CVp1, z6. A second distance K2p1, z6 corresponding to the second range VR2p1, z6 up to β2p1, z6 is calculated.

  The reference position TL of the data collection system 41 at the time when the projection data of the first view β1p1, z6 is collected is, for example, the position Z5 in FIG. 7, and the time when the projection data of the second view β2p1, z6 is collected. The reference position TL of the data collection system 41 is, for example, a position Z7 in FIG. Therefore, the first distance K1p1, z6 is a distance between the position Z6 and the position Z5, and the second distance K2p1, z6 is a distance between the position Z6 and the position Z7.

  When the reference position TL of the data collection system 41 is located near the center of the linear movement range Rm, projection data is collected with the relative linear movement speed of the data collection system 41 being constant, that is, with the helical pitch being constant. . Therefore, here, as shown in FIG. 7, the first distance K1p1, z6 = the second distance K2p1, z6 = ΔZ1.

  In step B34, it is determined whether or not the first distance K1p1, z6 is equal to the second distance K2p1, z6. Here, since the determination condition is satisfied as described above, the process proceeds to step B39.

  In step B39, the tentatively determined view range for reconstruction VRp1, z6 is determined as it is, and the reconstruction view range setting process is terminated.

  In the case of the first specific example, the outermost X-ray beam Xb5 passing through the reconstruction plane RP when the reference position TL of the data acquisition system 41 is the position Z5, and the reference position TL of the data acquisition system 41 is the position Z7. Sometimes the outermost X-ray beam Xb7 passing through the reconstruction plane RP is symmetric with respect to the reconstruction plane RP with respect to the z direction, as shown in FIG. As can be seen from this, in a situation where the helical pitch is constant, the CT image is reconstructed even if the view range for the same view angle is assigned to both sides of the reference view and the view range for reconstruction is set. In this case, the projection data by the X-ray beam passing through the reconstruction plane RP is evenly used on both sides of the reconstruction plane RP in the z direction in terms of space, and unused useful projection data remains only on one side. There is no such thing.

  Next, a second specific example will be described. In this example, the phase of the CT image to be reconstructed is the first phase P1, and the slice position is a position Z3 near the linear movement start point Zs.

  FIG. 8 is a diagram for explaining a second specific example of the reconstruction view range setting process. The lower part of FIG. 8 shows the correspondence between the phase P1 in FIG. 4 and the view View near the linear movement start point Zs and the reference position TL in the z direction of the data collection system 41. The upper part of FIG. A state example of the data collection system 41 when the TL is at each predetermined position is schematically shown.

  In step B31, for example, as shown in FIG. 8, a view in which the reference position TL of the data collection system 41 matches the position Z3 is specified as the reference view CVp1, z3.

  In step B32, for example, as shown in FIG. 8, a view having a view number smaller than the reference view CVp1, z3 by the view angle F / 2 is set as the first view β1p1, z3, and the view angle F / from the reference view CVp1, z3. A view having a larger view number by 2 minutes is set as a second view β2p1, z3. Thereby, the range from the first view β1p1, z3 to the second view β2p1, z3 is provisionally determined as the view range VRp1, z3 for reconstruction.

  In step B33, a first distance K1p1, z3 corresponding to the first range VR1p1, z3 from the reference view CVp1, z3 to the first view β1p1, z3 is calculated, and the second view is calculated from the reference view CVp1, z3. A second distance K2p1, z3 corresponding to the second range VR2p1, z3 up to β2p1, z3 is calculated.

  The reference position TL of the data collection system 41 at the time when the projection data of the first view β1p1, z3 is collected is, for example, the position Z2 in FIG. 8, and the projection data of the second view β2p1, z3 is collected. The reference position TL of the data collection system 41 at is, for example, the position Z4 in FIG. Accordingly, the first distance K1p1, z3 is a distance between the position Z2 and the position Z3, and the second distance K2p1, z3 is a distance between the position Z3 and the position Z4.

  When the reference position TL of the data acquisition system 41 is located near the linear movement start point Zs, the projection data is in a state where the speed of the relative linear movement of the data acquisition system 41 is changing, that is, the helical pitch is changing. Are collected. Therefore, here, as shown in FIG. 8, the first distance K1p1, z3 = ΔZ2, and the second distance K2p1, z3 = ΔZ1 (ΔZ2 <ΔZ1).

  In step B34, it is determined whether or not the first distance K1p1, z3 is equal to the second distance K2p1, z3. Here, since the determination condition is not satisfied as described above, the process proceeds to step 35.

  In step B35, the magnitude relationship between the first distance K1p1, z3 and the second distance K2p1, z3 is specified, and the shift target view is determined. Here, as described above, the first distance K1p1, z3 (= ΔZ2) <the second distance K2p1, z3 (= ΔZ1) is specified, and the first distance K1p1, z3 is relatively small. View β1p1, z3 is the view to be shifted.

  In step B36, the first view β1p1, z3, which is the view to be shifted, is shifted by Δβ in the direction in which the view range VRp1, z3 increases, that is, the view number decreases, and the first view β1p1, z3 is updated. .

  In step B37, the first distance K1p1, z3 that is the distance corresponding to the first range VR1p1, z3 from the reference view CVp1, z3 to the updated first view β1p1, z3 is calculated again.

  In step B38, it is determined whether or not the first distance K1p1, z3 is equal to the second distance K2p1, z3. If the determination condition is satisfied, the process proceeds to step B39. If the determination condition is not satisfied, the process proceeds to step B36.

  In this specific example, steps B36 to B38 are repeatedly executed until the first distance K1p1, z3 becomes equal to the second distance K2p1, z3, that is, until the first distance K1p1, z3 changes from ΔZ2 to ΔZ1. . As a result, the first view β1p1, z3 is shifted to β1′p1, z3, which is a view at a time when the reference position TL of the data collection system 41 coincides with the position Z1 whose distance from the position Z2 is ΔZ1.

  Thereafter, in step B39, the view range VR'p1, z3 for reconstruction at this time, that is, the time when the first range becomes VR1'p1, z3, is determined, and the process ends.

  In the case of the second specific example, the outermost X-ray beam Xb2 passing through the reconstruction plane RP when the reference position TL of the data acquisition system 41 is the position Z2, and the reference position TL of the data acquisition system 41 is the position Z4. Sometimes, the outermost X-ray beam Xb4 passing through the reconstruction plane RP is asymmetric with respect to the z direction with respect to the reconstruction plane RP as shown in FIG. As can be seen from this, in a situation where the helical pitch changes, the CT image can be reconstructed simply by assigning a view range for the same view angle on both sides of the reference view and setting a view range for reconstruction. In this case, the projection data by the X-ray beam passing through the reconstruction plane RP is used unevenly on both sides of the reconstruction plane RP in the z direction in terms of space, and unused projection data is unused only on one side. May remain.

  On the other hand, the outermost X-ray beam Xb1 passing through the reconstruction plane RP when the reference position TL of the data acquisition system 41 is the position Z1, and the reconstruction plane RP when the reference position TL of the data acquisition system 41 is the position Z4. As shown in FIG. 8, the outermost X-ray beam Xb4 that passes through is symmetrical with respect to the z direction with respect to the reconstruction plane RP. Therefore, when the reconstruction view range setting process according to the present embodiment is executed and the view range for reconstruction is set, when the CT image is reconstructed, the projection by the X-ray beam passing through the reconstruction plane RP is performed. The data is used evenly on both sides of the reconstruction plane RP in the z direction in terms of space, leaving no unused useful projection data on only one side.

  Returning to FIG. 5, in step B <b> 4, one pixel P is designated from all the pixels on the reconstruction plane RP. When step B4 is executed for the second time and thereafter, pixels that have not been designated are designated.

この式において、ｆ（ｘ，ｙ，ｚ）は再構成面ＲＰ上の画素Ｐ（ｘ，ｙ，ｚ）の画素データ、ｓ（α，β，γ）は再構成面ＲＰ上の画素Ｐ（ｘ，ｙ，ｚ）を通るＸ線ビームによる投影データ、ｇ（γ）は再構成関数、×を○で囲んだ記号はコンボリューション(convolution)演算子、αはＸ線焦点とＸ線検出器のスライス方向の中央線とを通る面からのコーン(cone)角、βはＸ線焦点のビュー角度位置、γはファンビーム中心線からのチャネル方向の角度、ＲはＸ線焦点から回転中心軸Ｉcまでの距離、Ｚは画素Ｐのｚ座標に依存する所定の値、ω_3d（α，β，γ）は投影データｓ（α，β，γ）に乗算する、コーン角を考慮した３次元の加重係数である。従来のハーフリコン(half reconstruction)ではβmax−βmin＝π＋ファン角、従来のフルリコン(full reconstruction)ではβmax−βmin＝２πとなる。ここでは、βmin＝第１のビューβ１pi,zj、βmax＝第２のビューβ２pi,zjとなる。 In step B5, projection data by an X-ray beam passing through a path passing through the reconstruction plane corresponding to the slice position Zj is extracted from the projection data of the reconstruction view range VR1pi, zj. Then, the extracted projection data is subjected to filtered back-projection processing or the like to reconstruct the pixel data of the designated pixel. For example, the pixel data is reconstructed according to the following equation.

In this equation, f (x, y, z) is the pixel data of the pixel P (x, y, z) on the reconstruction plane RP, and s (α, β, γ) is the pixel P ( projection data by an X-ray beam passing through x, y, z), g (γ) is a reconstruction function, a symbol surrounded by ○ is a convolution operator, α is an X-ray focal point and an X-ray detector The cone angle from the plane passing through the center line in the slice direction, β is the view angle position of the X-ray focus, γ is the channel direction angle from the fan beam center line, and R is the rotation center axis from the X-ray focus The distance to Ic, Z is a predetermined value depending on the z coordinate of the pixel P, and ω _3d (α, β, γ) is multiplied by the projection data s (α, β, γ). Is a weighting factor. In the conventional half reconstruction, βmax−βmin = π + fan angle, and in the conventional full reconstruction, βmax−βmin = 2π. Here, βmin = first view β1pi, zj, βmax = second view β2pi, zj.

This formula can be expressed by cone beam filtered back projection (Cone
Although it is an expression corresponding to an image reconstruction algorithm (algorithm) of Beam FBP, an expression corresponding to a two-dimensional filtered back projection (2D FBP) image reconstruction algorithm can also be used.

  In step B6, it is determined whether or not the pixel data reconstruction of the planned pixel is completed. If not completed, the process proceeds to step B4. If completed, the image reconstruction of the CT image at the slice position Zj in the phase Pi is performed. Is completed, and the process proceeds to Step B13.

  In step B7, it is determined whether the image reconstruction of the CT image has been completed for the scheduled slice position. If not completed, the process proceeds to step B2, and if completed, the process proceeds to step B8.

  In Step B8, it is determined whether or not the image reconstruction of the CT image has been completed for the scheduled phase. If not completed, the process proceeds to Step B1, and if completed, the image reconstruction process is terminated. .

  Thus, according to the X-ray CT apparatus 100 according to the present embodiment, the view ranges on both sides adjacent to the reference view corresponding to the time at which the reference position of the data acquisition system and the position of the reconstruction plane of the CT image coincide with each other. Among them, the view range consisting of the reference view and the view ranges on both sides is set so that the view range on the slow side of the data collection system has a larger view range than the view range on the fast side. Since it is set as a view range used for CT image reconstruction, a view range consisting of a reference view and a view range corresponding to a predetermined angle of view on both sides of the reference view is set as a view range used for CT image reconstruction as in the past. In that case, the noise data of the projection data used for reconstruction can be reduced by effectively using the projection data of the useful view outside that. And the image quality of the CT image obtained by the variable pitch helical scan can be further improved.

  In this embodiment, since the view ranges having the same distance of the relative linear movement of the data acquisition system are set on both sides adjacent to the reference view as the view ranges for reconstruction, the view ranges on both sides thereof are set. Can be reconstructed with the spatial lengths carried equal to each other and the spatial information balanced in the z direction.

  In this embodiment, in order to specify the magnitude relationship between the relative linear movement speeds in the view ranges on both sides adjacent to the reference view, the relative linear movement distance magnitude for collecting projection data of the view range is specified. Although the relationship is specified, the relationship may be specified based on other information related to the speed of relative linear movement in the view ranges on both sides of the relationship.

  Further, in the present embodiment, the magnitude relation of the speed of relative linear movement in the view ranges on both sides adjacent to the reference view is specified, and the size of the view ranges on both sides is adjusted based on the specified magnitude relationship. However, the size of the view ranges on both sides may be adjusted based on other information without specifying this magnitude relationship. For example, when the movement speed at each position in the relative linear movement range of the data acquisition system is known in advance, the position of the slice of the CT image to be reconstructed and the view range to be set as the view range for reconstruction Is uniquely determined. Therefore, a table and calculation formulas for determining the correspondence relationship are prepared in advance. Based on the position of the slice of the CT image to be reconstructed, refer to the table or use the calculation formula for reconstruction. You may set the view range.

  When setting the view range consisting of the reference view and the view ranges on both sides as the view range for reconstruction, the entire view range for reconstruction is set to the view angle F, and the views on both sides adjacent to the reference view are displayed. The range may be a view range in which the distance of the relative linear movement of the data collection system 41 is the same. For example, the entire view range consisting of the reference view and the view ranges on both sides of the view range on both sides adjacent to the reference view is maintained while maintaining the relative linear movement distance of the corresponding data acquisition system at one to one. Increase until the view range is F. A view range composed of the reference view obtained as a result and the view ranges on both sides thereof is set as a view range for reconstruction. In this case, since the CT image can be reconstructed using projection data by an X-ray beam having a relatively small incident angle on the reconstruction surface of the CT image, the image quality of the CT image can be improved. .

DESCRIPTION OF SYMBOLS 100 X-ray CT apparatus 1 Operation console 2 Input device 3 Central processing unit 5 Data collection buffer 6 Monitor 7 Memory | storage device 10 Bed apparatus 12 Table 15 Rotating part 20 Scanning gantry 20a Main part 21 X-ray tube 22 X-ray controller 23 Collimator 24 X Line detector 25 DAS
26 Rotation unit controller 29 Control controller 30 Slip ring 40 Imaging target 41 Data collection system B Opening

Claims (11)

CT based on projection data collected by variable-pitch helical scan that scans while moving the data acquisition system including the X-ray tube and X-ray detector relative to the body axis direction of the subject at a linear velocity at a variable speed An image reconstruction method for reconstructing an image,
A step of setting a view range used for reconstruction of the CT image, the reference view corresponding to a time at which a reference position of the data acquisition system and a position of a reconstruction plane of the CT image coincide with each other in the body axis direction Of the reference view and its both sides so that the view range on the side where the speed is smaller than the view range on the side where the relative linear movement speed is larger among the view ranges on both sides adjacent to Setting a view range consisting of the view range as a view range used for reconstruction of the CT image;
Reconstructing the CT image using projection data of the set view range. The step of setting a view range used for reconstruction of the CT image includes:
Identifying a view range for a predetermined same view angle on both sides adjacent to the reference view;
Obtaining information about the speed of the relative linear movement of the identified view ranges on both sides;
Identifying a magnitude relationship between the relative linear movement speeds of the identified view ranges on both sides based on information on the relative linear movement speeds;
The view range including the reference view and the view ranges on both sides of the reference view is set so that the side on which the magnitude relationship is small is larger than the side on which the magnitude relationship is large. The image reconstruction method according to claim 1, further comprising a step of setting as a view range used for reconstruction.   The step of identifying the magnitude relationship of the relative linear movement speeds of the identified two-side view ranges includes the relative for collecting projection data of one of the identified two-side view ranges. The image reconstruction method according to claim 2, further comprising: specifying a magnitude relationship between a distance of the linear movement and a distance of the relative linear movement for collecting projection data of the other view range.   The view range corresponding to the predetermined same view angle is used as the view range on the side where the magnitude relationship is large, and the view range on the side where the magnitude relationship is small is compared to the view range corresponding to the predetermined view angle. The image reconstruction method according to claim 2 or 3, wherein the image reconstruction method is increased. A variable pitch helical scan having a data acquisition system including an X-ray tube and an X-ray detector, and performing scanning while moving the data acquisition system relative to the body axis direction of the subject at a variable linear speed An X-ray CT apparatus comprising: data collection means for collecting projection data by means of; and image reconstruction means for reconstructing a CT image based on the projection data collected by the data collection means,
View range setting means for setting a view range used for reconstruction of the CT image, corresponding to a time at which the reference position of the data acquisition system and the position of the reconstruction plane of the CT image coincide with each other in the body axis direction Among the view ranges on both sides adjacent to the reference view, the reference view is set such that the view range on the side with the lower speed is larger than the view range on the side with the higher relative linear movement speed. And a view range setting means for setting a view range including the view ranges on both sides thereof as a view range used for the reconstruction of the CT image,
The X-ray CT apparatus, wherein the image reconstruction unit reconstructs the CT image using projection data of the set view range. The view range setting means includes:
Information on the relative linear movement speed is obtained by specifying a view range corresponding to a predetermined same view angle on both sides adjacent to the reference view, obtaining information on the relative linear movement speed of the specified view ranges on both sides. And a magnitude relation specifying means for specifying the magnitude relation of the speed of the relative linear movement of the specified view ranges on both sides based on
The view range including the reference view and the view ranges on both sides of the reference view is set so that the side on which the magnitude relationship is small is larger than the side on which the magnitude relationship is large. The X-ray CT apparatus according to claim 5, wherein the X-ray CT apparatus is set as a view range used for reconstruction.   The magnitude relation specifying means collects the distance of the relative linear movement for collecting projection data of one view range and the projection data of the other view range among the specified view ranges on both sides. The X-ray CT apparatus according to claim 6, wherein a magnitude relation with the distance of the relative linear movement for the purpose is specified.   The view range setting means uses the view range corresponding to the predetermined same view angle as the view range on the side where the magnitude relationship is large, and the view range on the side where the magnitude relationship is small is the same as the predetermined range. The X-ray CT apparatus according to claim 6, wherein the X-ray CT apparatus is larger than a view range corresponding to a view angle.   9. The device according to claim 6, wherein the predetermined same view angle is a half of a fan angle of π / 2, π / 2 + X-ray beam, π, or π + half of the fan angle. X-ray CT system.   The distance of the relative linear movement for collecting projection data on one side of the view range on both sides adjacent to the reference view constituting the view range used for reconstruction of the CT image, and the other side of the view range on both sides The X-ray CT apparatus according to claim 5, wherein a distance of the relative linear movement for collecting the projection data is the same.   6. The reference position of the data acquisition system is a position of an intersection of a straight line connecting an X-ray focal point of the X-ray tube and a center of the X-ray detector and a rotation axis of the data acquisition system. The X-ray CT apparatus according to any one of 10.

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