JP2006034865A - Image reconstruction method and x-ray ct apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To acquire a CT image of a desired cross section even if internal organs of a subject carry out position slippage by inertia according to acceleration or deceleration of table transfer at the time of a helical scan. <P>SOLUTION: Image reconstruction is carried out by rectifying a reconstruction aspect P to a reconstruction plane R according to position slippage by the inertia of the internal organs I of the subject H which rides on a cradle 12 which carries out straight line transfer at the time of the helical scan. According to this invention, the CT image of the desired cross section can be acquired even if the internal organs of the subject carry out the position slippage by the inertia. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an image reconstruction method and an X-ray CT (Computed Tomography) apparatus. More specifically, the present invention relates to a desired cross-section even when the organ of a subject is displaced due to inertia in accordance with acceleration / deceleration of table movement during helical scanning. The present invention relates to an image reconstruction method and an X-ray CT apparatus capable of obtaining a CT image.

  Conventionally, an X-ray CT that guarantees that the table has a substantially constant speed in the imaging area to be helically scanned by moving the table by a distance obtained by adding the table acceleration area and the table deceleration area to both sides of the imaging area to be helically scanned. An apparatus is known (see, for example, Patent Document 1).

JP-A-8-173413

In the above-described prior art, projection data is not collected in the table acceleration region and the table deceleration region, or even if projection data is collected, it is not used for CT image reconstruction.
However, since this is wasteful, the applicant of the present application proposes to collect projection data in the table acceleration region and the table deceleration region and use these projection data for reconstruction of CT images (Japanese Patent Application 2003). -192894).
This is called variable pitch helical scan or variable speed helical scan, and the range that can be scanned is not narrowed when the z-direction width of the X-ray detector is widened, or between the helical scan and the helical scan. It is effective for time saving.

However, it has been found that there is a problem in that a CT image of a desired cross section cannot be obtained if the organ of the subject is displaced due to inertia during the acceleration / deceleration period of table movement.
That is, as shown in FIG. 23, when it is desired to obtain a CT image of the central plane C in the body axis direction of the organ I of the subject H, the reconstruction area P is set in accordance with the central plane C.
However, as shown in FIG. 24 (a), during acceleration, the organ I shifts due to inertia and the center plane C and the reconstruction area P do not coincide with each other. As shown in FIG. 24 (b). While moving at a constant speed, the center plane C and the reconstruction area P coincide with each other. However, as shown in FIG. 24 (c), the center plane C and the reconstruction area are shifted so that the organ I is pulled by inertia during deceleration. P may not match.
Accordingly, an object of the present invention is to provide an image reconstruction method and an X-ray CT apparatus capable of obtaining a CT image of a desired cross section even when the organ of the subject is displaced due to inertia during acceleration / deceleration of table movement during helical scanning. Is to provide.

In the first aspect, the present invention corrects the position of each point on the reconstruction area P in accordance with the positional deviation due to the inertia of the organ of the subject on the table that is linearly moved during the helical scan, Provided is an image reconstruction method characterized by reconstructing each pixel of a CT image along a configuration region R.
In the image reconstruction method according to the first aspect, since the reconstruction area is corrected in accordance with the positional deviation due to the inertia of the organ of the subject on the table, the desired cross section even if the organ of the subject is displaced due to the inertia. CT images can be obtained.

In a second aspect, the present invention provides the image reconstruction method having the above configuration, wherein the linear movement direction of the table is a z-axis direction, a direction perpendicular to the upper surface of the table is a y-axis direction, and the z-axis direction and When the direction orthogonal to the y-axis direction is the x-axis direction, the point on the reconstruction area P before correction is p (x, y, z), and the displacement correction amount is Δz, the reconstruction area after correction Provided is an image reconstruction method characterized in that a point on R is r (x, y, z + Δz).
In the image reconstruction method according to the second aspect, the correction process is simplified because the positional shift due to the inertia of the organ of the subject is limited to the linear movement direction of the table.

In a third aspect, the present invention provides an image reconstruction method, wherein the reconstruction region is corrected in accordance with the acceleration of the table in the image reconstruction method configured as described above.
The displacement due to the inertia of the organ of the subject on the table is caused by the acceleration of the table.
Therefore, in the image reconstruction method according to the third aspect, the reconstruction area is corrected in accordance with the acceleration of the table. Thereby, a CT image of a desired cross section can be obtained even if the organ of the subject is displaced due to inertia.

In a fourth aspect, the present invention provides the image reconstruction method having the above configuration, wherein the linear movement direction of the table is a z-axis direction, a direction perpendicular to the upper surface of the table is a y-axis direction, and the z-axis direction and When the direction orthogonal to the y-axis direction is the x-axis direction and the point on the reconstruction area P before correction is p (x, y, z), the point on the reconstruction area R after correction is r (x , y, z + Δz), and the image reconstruction method is characterized in that Δz increases as the acceleration increases.
The displacement due to the inertia of the organ of the subject on the table increases as the acceleration of the table increases.
Therefore, in the image reconstruction method according to the fourth aspect, the correction processing is simplified by correcting the positional shift due to the inertia of the organ of the subject only in the linear movement direction of the table, and the correction amount Δz is set as the correction amount Δz. Increase as acceleration increases.

In a fifth aspect, the present invention provides the image reconstruction method configured as described above, wherein the linear movement direction of the table is a z-axis direction, a direction perpendicular to the top surface of the table is a y-axis direction, and the z-axis direction and When the direction orthogonal to the y-axis direction is the x-axis direction and the point on the reconstruction area P before correction is p (x, y, z), the point on the reconstruction area R after correction is r (x , y, z + Δz), and Δz increases as the distance from the upper surface of the table increases.
Since the subject is in close contact with the upper surface of the table, the closer to the upper surface of the table, the smaller the displacement due to the constraint on the upper surface of the table. .
Therefore, in the image reconstruction method according to the fourth aspect, the correction processing is simplified by correcting the positional shift due to the inertia of the organ of the subject only in the linear movement direction of the table, and the correction amount Δz is set as the correction amount Δz. Increase the distance from the top surface of the table.

In a sixth aspect, the present invention relates to the image reconstruction method having the above configuration, wherein the acceleration is a, and a function of the distance d from the point p (x, y, z) to the upper surface of the table is D (d). In this case, an image reconstruction method is provided in which a point on the reconstruction area R after correction is set to r (x, y, z + a · D (d)).
In the image reconstruction method according to the sixth aspect, the correction processing is simplified by limiting the displacement due to the inertia of the organ of the subject to the linear movement direction of the table, and the correction amount is set to acceleration a. The product of the function D (d) of the distance d to the upper surface of the table. The function D (d) can be obtained by simulation.

In a seventh aspect, the present invention provides an X-ray tube, an X-ray detector, a table that moves linearly with the subject, and at least one of the X-ray tube or the X-ray detector around the subject. And a helical scan means for collecting projection data while moving the table linearly, an acceleration detection means for detecting the acceleration of the table, and correcting the position of each point on the reconstruction area P based on the acceleration An X-ray CT apparatus comprising: a reconstruction area correction unit; and an image reconstruction unit that reconstructs each pixel of a CT image along the corrected reconstruction area R is provided.
In the X-ray CT apparatus according to the seventh aspect, the image reconstruction method according to the third aspect can be suitably implemented.

In an eighth aspect, the present invention provides an X-ray CT apparatus having the above-described configuration, wherein the linear movement direction of the table is a z-axis direction, a direction perpendicular to the upper surface of the table is a y-axis direction, When the direction orthogonal to the y-axis direction is the x-axis direction and the point on the reconstruction area P before correction is p (x, y, z), the reconstruction area correction means is the corrected reconstruction area Provided is an X-ray CT apparatus characterized in that a point on R is r (x, y, z + Δz), and Δz is increased as acceleration is increased.
In the X-ray CT apparatus according to the eighth aspect, the image reconstruction method according to the fourth aspect can be suitably implemented.

In a ninth aspect, the present invention provides the X-ray CT apparatus having the above-described configuration, wherein the linear movement direction of the table is a z-axis direction, a direction perpendicular to the top surface of the table is a y-axis direction, When the direction orthogonal to the y-axis direction is the x-axis direction and the point on the reconstruction area P before correction is p (x, y, z), the reconstruction area correction means is the corrected reconstruction area Provided is an X-ray CT apparatus characterized in that a point on R is r (x, y, z + Δz), and Δz increases as the distance from the upper surface of the table increases.
In the X-ray CT apparatus according to the ninth aspect, the image reconstruction method according to the fifth aspect can be suitably implemented.

In a tenth aspect, the present invention relates to an X-ray CT apparatus having the above configuration, wherein the acceleration is a, and a function of a distance d from a point p (x, y, z) to the upper surface of the table is D (d). Then, the reconstruction area correction means provides an X-ray CT apparatus characterized in that a point on the reconstruction area R after correction is set to r (x, y, z + a · D (d)). .
In the X-ray CT apparatus according to the tenth aspect, the image reconstruction method according to the sixth aspect can be suitably implemented.

  According to the image reconstruction method and the X-ray CT apparatus of the present invention, a CT image of a desired cross section can be obtained even if the organ of the subject is displaced due to inertia during acceleration / deceleration of table movement during helical scanning. .

  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 block diagram showing an X-ray CT apparatus according to Embodiment 1 of the present invention.
The X-ray CT apparatus 100 includes an operation console 1, a table apparatus 10, and a scanning gantry 20.

  The operation console 1 includes an input device 2 that receives input from an operator, a central processing unit 3 that executes image reconstruction processing, a data collection buffer 5 that collects projection data acquired by the scanning gantry 20, and a projection data. A display device 6 that displays the reconstructed CT image and a storage device 7 that stores programs, data, and X-ray CT images are provided.

  The table device 10 includes a cradle 12 that puts a subject and puts it in and out of a bore (cavity) of the scanning gantry 20. The cradle 12 is moved up and down and linearly moved by a motor built in the table apparatus 10.

  The scanning gantry 20 includes an X-ray tube 21, an X-ray controller 22, a collimator 23, a multi-row X-ray detector 24, a DAS (Data Acquisition System) 25, and an X that rotates around the body axis of the subject. A rotary controller 26 that controls the line tube 21 and the like, a tilt controller 27 that performs control when the scanning gantry 20 is tilted forward or backward of the rotary shaft, and control signals and the like are exchanged with the operation console 1 and the table device 10. A controller 29 and a slip ring 30 are provided.

FIG. 2 is an explanatory diagram of the geometric arrangement of the X-ray tube 21 and the multi-row X-ray detector 24.
The X-ray tube 21 and the multi-row X-ray detector 24 rotate around the rotation center IC. When the vertical direction is the y direction, the linear movement direction of the cradle 12 is the z-axis direction, and the direction orthogonal to the z-axis direction and the y-axis direction is the x-axis direction, the X-ray tube 21 and the multi-row X-ray detector 24 The rotation plane is an xy plane.
The X-ray tube 21 generates an X-ray beam called a cone beam CB. When the central axis direction of the cone beam CB is parallel to the y direction, the view angle is 0 °.
The multi-row X-ray detector 24 has first to J-th detector rows, for example, J = 256. Each detector row has channels from the first channel to the I-th channel, for example, I = 1024.

Projection data collected by irradiation with X-rays is A / D converted by the DAS 25 from the multi-row X-ray detector 24 and input to the data collection buffer 5 via the slip ring 30.
The projection data input to the data collection buffer 5 is processed by the central processing unit 3 by a program stored in the storage device 7, converted into a CT image, and displayed on the display device 6.

FIG. 3 is a flowchart showing an outline of the operation of the X-ray CT apparatus 100.
In step S1, the helical scan operation is performed while rotating the X-ray tube 21 and the multi-row X-ray detector 24 around the subject and moving the cradle 12 linearly, and the table linear movement position z, the view angle view, , Projection data D0 (z, view, j, i) represented by detector row number j and channel number i is collected. For the table linear movement position z, z-axis direction position pulses are counted by an encoder built in the table apparatus 10, converted to z-axis coordinates by the controller 29, and converted to z-projection data of the DAS 25 via the slip ring 30. It is added as axis coordinate information.
FIG. 4 shows a projection data format of a view to which z-axis coordinate information is added.
This data collection process will be described later with reference to FIG.

In step S2, pre-processing including offset correction, logarithmic conversion, X-ray dose correction, and sensitivity correction is performed on the projection data D0 (z, view, j, i), and projection data Din (z, view, j, i).
In step S3, beam hardening processing is performed on the preprocessed projection data Din (z, view, j, i). The beam hardening process is expressed by the following polynomial, for example. Here, B ₀ , B ₁ and B ₂ are beam hardening coefficients.
Dout (z, view, j, i) = Din (z, view, j, i) × (B ₀ (j, i) + B ₁ (j, i) × Din (z, view, j, i) + B ₂ (j, i) × Din (z, view, j, i) ² )
At this time, independent beam hardening correction can be performed for each column of the detector, so that if the tube voltage of each data acquisition system differs depending on the imaging conditions, the difference in characteristics for each column of the detector can be corrected. .

  In step S4, a Z filter convolution process for applying a filter in the z direction (column direction) to the projection data Dout (z, view, j, i) subjected to beam hardening correction is performed. That is, the projection data Dcor (z, view, j, i) multiplied by the column direction filter coefficient Wk (i) as shown in FIG. j, i).

The slice thickness can be controlled by the column direction filter coefficient wk (i).
As shown in FIG. 6, in the slice SL, the peripheral slice thickness is generally thicker than the reconstruction center.
Therefore, as shown in FIG. 7, the column direction filter coefficient wk (i of the center channel) having a wide width is used for the center channel, and the column direction filter coefficient wk having a large width is used for the peripheral channel. (Peripheral channel i) is used. As a result, as shown in FIG. 8, a slice SL having a uniform slice thickness can be obtained at the reconstruction center and the periphery.

  When the slice thickness is slightly reduced with the column direction filter coefficient wk (i), both artifact and noise are greatly improved. Thereby, the artifact improvement degree and the noise improvement degree can also be controlled. That is, it is possible to control the image quality of the CT image reconstructed from the three-dimensional image.

  As shown in FIG. 9, a CT image having a thin slice thickness can be realized by using a column-direction filter coefficient wk (i) as a deconvolution filter.

Returning to FIG. 3, in step S5, reconstruction function superimposition processing is performed. That is, the Fourier transform is performed, the reconstruction function is multiplied, and the inverse Fourier transform is performed. When the projection data after the reconstruction function superimposing process is Dr (z, view, j, i), the reconstruction function is Kernel (j), and the convolution operation is represented by *, the reconstruction function superimposing process is as follows. It is expressed in
Dr (z, view, j, i) = Dcor (z, view, j, i) * Kernel (j)
Since the independent reconstruction function superimposing process can be performed using the independent reconstruction function Kernel (j) for each column of the detector, the difference in the noise characteristic and the resolution characteristic for each column of the detector can be corrected.

  In step S6, a three-dimensional backprojection process is performed on the projection data Dr (z, view, j, i) to obtain backprojection data D3 (x, y). This three-dimensional backprojection process will be described later with reference to FIG.

In step S7, post-processing such as image filter convolution processing and CT value conversion processing is performed on the backprojection data D3 (x, y) to obtain a CT image.
In the image filter convolution process, if the data after the image filter convolution process is D4 (x, y), the detector column number corresponding to the center pixel of the CT image is j, and the image filter is Filter (j),
D4 (x, y) = D3 (x, y) × Filter (j)
It becomes. That is, since independent image filter convolution processing can be performed for each slice position of the CT image, the difference in noise characteristics and resolution characteristics for each slice position can be corrected.

FIG. 10 is a flowchart showing details of the data collection process (step S1 in FIG. 3).
In step A1, the X-ray tube 21 and the multi-row X-ray detector 24 are rotated around the subject.
In step A2, the cradle 12 is linearly moved at a low speed to the linear movement start position.
In step A3, the moving direction of the cradle 12 is set to one direction (+ z direction or -z direction).

In step A4, the linear moving speed of the cradle 12 is accelerated based on a predetermined function, and the tube current is increased accordingly. The predetermined function may be a function that is linear with respect to time or a function that is non-linear with respect to time. The X-ray density in the linear movement direction, that is, the X-ray dose per unit thickness is proportional to “tube current / linear movement speed”. Therefore, “tube current / linear movement speed” can be made constant by increasing the tube current in accordance with the increase of the linear movement speed, thereby making the X-ray density in the linear movement direction constant even during acceleration. I can do it.
In step A5, projection data D0 (z, view, j, i) during acceleration is collected.
In step A6, if the linear moving speed of the cradle 12 reaches a predetermined speed, the process proceeds to step A7, and if it does not reach the predetermined speed, the process returns to step A4 and further accelerates.

In step A7, constant-speed projection data D0 (z, view, j, i) is collected with the linear movement speed of the cradle 12 maintained at a predetermined speed.
In Step A8, when the cradle 12 reaches the constant speed end position, the process proceeds to Step A9. When the cradle 12 does not reach the constant speed end position, the process returns to Step A7 and the constant speed projection data collection is continued.

In step A9, the linear movement speed of the cradle 12 is decelerated based on a predetermined function, and the tube current is reduced accordingly. The predetermined function may be a function that is linear with respect to time or a function that is non-linear with respect to time. The X-ray density in the linear movement direction, that is, the X-ray dose per unit thickness is proportional to “tube current / linear movement speed”. Therefore, the “tube current / linear movement speed” can be made constant by reducing the tube current in accordance with the decrease in the linear movement speed, and thereby the X-ray density in the linear movement direction can be made constant even during deceleration. I can do it.

In step A10, the projection data D0 (z, view, j, i) during deceleration is collected.
In step A11, if the linear movement speed of the cradle 12 reaches the stoppable speed, the process proceeds to step A12. If not, the process returns to step A9 and further decelerates.

In step A12, the linear movement of the cradle 12 is stopped.
In step A13, when the scheduled data collection is completed, the process is terminated; otherwise, the process proceeds to step A14.
In step A14, the moving direction of the cradle 12 is set in the reverse direction. And it returns to step A4 and continues data collection. That is, data is continuously collected while the subject is reciprocated.

FIG. 11 is a graph showing a change with time of the linear movement speed v of the cradle 12.
The symbol + represents a linear movement in the + z direction, and the symbol-represents a linear movement in the -z direction.

FIG. 12 is a graph showing the time change of the acceleration a of the cradle 12.
The sign + represents acceleration in the + z direction, and the sign-represents acceleration in the -z direction.

As shown in FIG. 13, when it is desired to obtain a CT image of the center plane C in the body axis direction of the organ I of the subject H, the reconstruction area P is set in accordance with the center plane C. And the set reconstruction area | region P is used as it is during the period when the linear movement of the cradle 12 is constant speed.
On the other hand, as shown in FIG. 14, if the organ I is displaced due to inertia during acceleration or deceleration, the center plane C and the reconstruction area P do not coincide with each other. Therefore, the reconstruction area P is corrected without using the reconstruction area P. Region R is used.

As shown in FIG. 15, when the point on the reconstruction area P before correction is p (x, y, z), the central processing unit 3 sets the point on the reconstruction area R after correction to r (x , y, z + Δz). Then, Δz is increased as the acceleration increases, and Δz is increased as the distance from the upper surface of the cradle 12 increases.
That is, the central processing unit 3 detects the acceleration a of the cradle 12, and when the function of the distance d from the point p (x, y, z) to the upper surface of the cradle 12 is D (d),
Δz = a × D (d)
And The function D (d) is, for example,
D (d) = d ²
It is. The function D (d) is obtained in advance by simulation.

  If the displacement amount Δz of the organ I is not constant in the x direction, Δz is a function of x and y.

  FIG. 16 is a flowchart showing details of the three-dimensional backprojection process (step S6 in FIG. 3).

  In step S61, attention is paid to one view in all views necessary for CT image reconstruction (ie, a view of 360 ° or a view of “180 ° + fan angle”), and Projection data Dr corresponding to the pixel is extracted. When the deformation of the organ I due to inertia can be ignored, the reconstruction area R = the reconstruction area P is set.

  As shown in FIG. 17, a 512 × 512 pixel square reconstruction area P parallel to the xy plane is corrected to a reconstruction area R, and pixel rows L0, y = 63 parallel to the x axis where y = 0. Pixel row L63, pixel row L127 of y = 127, pixel row L191 of y = 191, pixel row L255 of y = 255, pixel row L319 of y = 319, pixel row L383 of y = 383, pixel row of y = 447 Taking the pixel column L511 of L447, y = 511 as an example, these pixel columns L0 to L511 are projected on the surface of the multi-row X-ray detector 24 in the X-ray transmission direction on the lines T0 to T511 as shown in FIG. If these projection data are extracted, they become projection data Dr of the pixel columns L0 to L511.

  Although the X-ray transmission direction is determined by the X-ray focal point of the X-ray tube 21 and the geometric position of each pixel and the multi-row X-ray detector 24, the z-coordinate of the projection data D0 (z, view, j, i) Therefore, the X-ray transmission direction can be accurately obtained even with the projection data D0 (z, view, j, i) during acceleration / deceleration.

  For example, when a part of the line goes out of the plane of the multi-row X-ray detector 24, such as a line T0 in which the pixel row L0 is projected on the surface of the multi-row X-ray detector 24 in the X-ray transmission direction. The corresponding projection data Dr is set to “0”.

  Thus, as shown in FIG. 19, projection data Dr (view, x, y) corresponding to each pixel in the reconstruction area R can be extracted.

Returning to FIG. 16, in step S62, the projection data Dr (view, x, y) is multiplied by the cone beam reconstruction load coefficient to create projection data D2 (view, x, y) as shown in FIG.
Here, the cone beam reconstruction load coefficient is set such that the distance from the focal point of the X-ray tube 21 to the detector row j and the channel i of the multi-row X-ray detector 24 corresponding to the projection data Dr is r0. (R1 / r0) ² where r1 is the distance from the focal point to the pixel on the reconstruction area R corresponding to the projection data Dr.

In step S63, as shown in FIG. 21, the projection data D2 (view, x, y) is added to the back projection data D3 (x, y) that has been cleared in advance in correspondence with the pixels.
In step S64, steps S61 to S63 are repeated for all views necessary for reconstruction of the CT image (ie, views of 360 ° or views of “180 ° + fan angle”), as shown in FIG. , Back projection data D3 (x, y) is obtained.

  As shown in FIG. 22, the reconstruction area R may be a circular area.

  According to the X-ray CT apparatus 100 described above, projection data is collected not only when the linear movement speed is constant, but also during acceleration / deceleration of linear movement, and the collected projection data is used for image reconstruction. For this reason, the linear movement distance for acceleration / deceleration out of the entire linear movement distance can be used for image reconstruction. Further, since the reconstruction area P is corrected to the reconstruction area R in accordance with the displacement due to the inertia of the organ I of the subject H on the cradle 12, even if the organ I of the subject H is displaced due to the inertia, it is desirable. A CT image of a cross section can be obtained.

  The image reconstruction method may be a three-dimensional image reconstruction method by a conventionally known Feldkamp method. Furthermore, JP 2003-334188 A, JP 2004-41675 A, JP 2004-41474 A, JP 2004-73360 A, JP 2003-159244 A, JP 2004-41675 A. You may use the three-dimensional image reconstruction method proposed by the gazette.

  The present invention can be applied to an X-ray CT apparatus using a single detector instead of the multi-row X-ray detector 24.

  The image reconstruction method and the X-ray CT apparatus of the present invention can be used to obtain, for example, a brain perfusion image.

1 is a configuration block diagram showing an X-ray CT apparatus according to Embodiment 1. FIG. It is explanatory drawing which shows the geometric arrangement of an X-ray tube and a multi-row X-ray detector. FIG. 3 is a flowchart showing a schematic operation of the X-ray CT apparatus according to the first embodiment. It is explanatory drawing which shows a data structure. It is explanatory drawing which shows a column direction filter coefficient. It is explanatory drawing which shows a slice whose slice thickness is thick in the periphery from the center of a reconstruction area. It is explanatory drawing which shows the column direction filter coefficient which changes with channels. It is explanatory drawing which shows a slice with equal slice thickness at the center of a reconstruction area, and its periphery. It is explanatory drawing which shows the column direction filter coefficient for making slice thickness thin. It is a flowchart which shows the detail of a data collection process. It is a graph which shows the change of the speed in the case of reciprocating linear movement. It is a graph which shows the change of the acceleration in the case of reciprocating linear movement. It is explanatory drawing which shows the reconstruction area | region P at the time of constant speed. It is explanatory drawing which shows the reconstruction area | region R at the time of acceleration / deceleration. It is explanatory drawing which shows the method of correct | amending the reconstruction area | region P to the reconstruction area | region R. FIG. It is a flowchart which shows the detail of a three-dimensional image reconstruction process. It is a conceptual diagram which shows the state which projects the pixel line on the reconstruction area | region R to a X-ray transmissive direction. FIG. 5 is a conceptual diagram showing lines obtained by projecting pixel lines on a reconstruction area R onto a detector surface. 5 is a conceptual diagram showing a state in which projection data Dr at view = 0 ° is projected onto a reconstruction area R. FIG. It is a conceptual diagram which shows the backprojection pixel data D2 on the reconstruction area | region R in view = 0 degree. It is explanatory drawing which shows the state which obtains backprojection data D3 by adding all the views to backprojection pixel data D2 corresponding to a pixel. It is a conceptual diagram which shows the circular reconstruction area | region R. FIG. It is explanatory drawing which shows an organ and the reconstruction area | region P. FIG. It is explanatory drawing which shows the position shift of the organ at the time of acceleration / deceleration.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Operation console 2 Input device 3 Central processing unit 5 Data collection buffer 6 Display device 7 Storage device 10 Table device 12 Cradle 20 Scanning gantry 21 X-ray tube 22 X-ray controller 23 Collimator 24 Multi-row X-ray detector 25 DAS
26 Rotation unit controller 29 Control controller 100 X-ray CT apparatus C Center plane H Subject I Organ P, R Reconstruction region

Claims (10)

  The position of each point on the reconstruction area P is corrected in accordance with the positional deviation due to the inertia of the organ of the subject on the table that is linearly moved during the helical scan, and each of the CT images along the corrected reconstruction area R is corrected. An image reconstruction method comprising reconstructing an image of pixels.   2. The image reconstruction method according to claim 1, wherein a linear movement direction of the table is a z-axis direction, a direction perpendicular to the upper surface of the table is a y-axis direction, and a direction orthogonal to the z-axis direction and the y-axis direction. Is the x-axis direction, the point on the reconstruction area P before correction is p (x, y, z), and the amount of misalignment correction is Δz, the point on the reconstruction area R after correction is r ( x, y, z + Δz).   3. The image reconstruction method according to claim 1, wherein the position of each point on the reconstruction area P is corrected based on the acceleration of the table.   4. The image reconstruction method according to claim 3, wherein a linear movement direction of the table is a z-axis direction, a direction perpendicular to the upper surface of the table is a y-axis direction, and a direction orthogonal to the z-axis direction and the y-axis direction. Is the x-axis direction, and the point on the reconstruction area P before correction is p (x, y, z), the point on the reconstruction area R after correction is r (x, y, z + Δz), An image reconstruction method, wherein Δz is increased as acceleration is increased.   5. The image reconstruction method according to claim 1, wherein a linear movement direction of the table is a z-axis direction, a direction perpendicular to the upper surface of the table is a y-axis direction, and the z-axis direction and When the direction orthogonal to the y-axis direction is the x-axis direction and the point on the reconstruction area P before correction is p (x, y, z), the point on the reconstruction area R after correction is r (x , y, z + Δz), and Δz is increased as the distance from the upper surface of the table increases.   5. The image reconstruction method according to claim 3, wherein the acceleration is a and the function of the distance d from the point p (x, y, z) to the upper surface of the table is D (d). An image reconstruction method characterized in that a point on the reconstruction area R after correction is represented by r (x, y, z + a · D (d)).   An X-ray tube, an X-ray detector, a table on which a subject is placed and moved linearly, and at least one of the X-ray tube or the X-ray detector is rotated around the subject and the table is moved linearly. A helical scan means for collecting projection data while detecting an acceleration of the table, a reconstruction area correction means for correcting the position of each point on the reconstruction area P based on the acceleration, and a post-correction An X-ray CT apparatus comprising image reconstruction means for reconstructing each pixel of a CT image along the reconstruction region R.   8. The X-ray CT apparatus according to claim 7, wherein the linear movement direction of the table is a z-axis direction, a direction perpendicular to the upper surface of the table is a y-axis direction, and a direction orthogonal to the z-axis direction and the y-axis direction. Is the x-axis direction and the point on the reconstruction area P before correction is p (x, y, z), the reconstruction area correction means sets the point on the reconstruction area R after correction to r ( x, y, z + Δz), and the larger the acceleration, the larger the Δz is.   9. The X-ray CT apparatus according to claim 7, wherein a linear movement direction of the table is a z-axis direction, a direction perpendicular to an upper surface of the table is a y-axis direction, and the z-axis direction and the y-axis direction are Is the x-axis direction and the point on the reconstruction area P before correction is p (x, y, z), the reconstruction area correction means is on the reconstruction area R after correction. An X-ray CT apparatus characterized in that a point is set to r (x, y, z + Δz), and Δz is increased with increasing distance from the upper surface of the table.   9. The X-ray CT apparatus according to claim 8, wherein the reconstruction is performed when the acceleration is a and the function of the distance d from the point p (x, y, z) to the upper surface of the table is D (d). An X-ray CT apparatus characterized in that the area correction means sets a point on the reconstruction area R after correction as r (x, y, z + a · D (d)).

Images