JP2002153455A - X-ray computer tomograph - Google Patents

Abstract

PROBLEM TO BE SOLVED: To a helical scanning type X-ray CT device by which MPR images in which a level difference is suppressed are obtained. SOLUTION: The images of different slice positions are successively reconstructed by using projection data collected by helical scanning and the MRP images are obtained by using the reconstructed images of the respective slice positions.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an X-ray computed tomography apparatus (hereinafter referred to as "X-ray CT apparatus"), and more particularly to an X-ray CT apparatus employing a helical scan system.

[0002]

2. Description of the Related Art FIG. 10 is a schematic block diagram of a processing circuit relating to a reconstruction operation of a conventional X-ray CT apparatus. Projection data from an X-ray detector (not shown) is supplied to a preprocessing circuit 11. The raw data storage device 1 and the central processing unit (CPU) 2 are connected to the output of the preprocessing circuit 11. The output of the preprocessing circuit 11 is a convolution operation (CON
V) The image is stored in the image memory 14 as a reconstructed image via the circuit 12 and the back projection operation (BP) circuit 13. The image data in the image memory 14 is supplied to a contrast scale circuit 15 to be a CT image. CT
The image is also stored in the image memory 14, and the CT image is supplied to a display unit (not shown). Therefore, the image memory 14
May be one, but two are provided when the reconstructed image and the CT image are stored in separate memories. CONV circuit 12,
CP is used for the BP circuit 13 and the contrast scale circuit 15.
The U17 is connected, and the BP circuit 13 is also connected to a constant setting unit 16 for setting various constants for back projection calculation under the control of the CPU 17. Although not shown, in order to perform the reconstruction operation at high speed, CONV
Various buffers are provided in the circuit 12, the BP circuit 13, and the like.

In such a conventional example, the calculation is basically performed as data of one projection image as one unit (unit). Image reconstruction is 360 degrees or 180
It is performed independently for each slice position from the projection data of degrees. Therefore, when reconstructing images at a large number of slice positions, the reconstruction time must be lengthened in proportion to the number of slices. Considering that reconstructed images are stored, reconstructing images at many slice positions one by one has a problem that a large amount of storage area is required. Therefore, a method for efficiently reconstructing images at a large number of continuous slice positions has been considered. An example of this is disclosed in Japanese Patent Application Laid-Open No. 58-32748 (U.S. Pat.
No. 5,645). However, this conventional continuous slice reconstruction method relates to a continuous scan method at the same slice position, and a method for efficiently reconstructing images at consecutive slice positions in the helical scan method has not been proposed.

[0004]

SUMMARY OF THE INVENTION The present invention has been made to address the above-described circumstances, and an object of the present invention is to provide an X-ray CT apparatus employing a helical scan system which can efficiently image a large number of continuous slice positions. By restructuring,
In particular, it is to obtain an MPR image in which a step is suppressed.

[0005]

In order to achieve the above object, an X-ray CT apparatus according to the present invention is an X-ray computed tomography apparatus for performing a helical scan, wherein the projection data collection means collects projection data by the helical scan. And, using projection data collected by the helical scan, a reconstruction means for continuously reconstructing images having different slice positions, using different images having different slice positions reconstructed by the reconstruction means, Means for obtaining an MPR image.

[0006]

According to the present invention, images having different slice positions are continuously reconstructed using projection data collected by helical scan, and an MPR image is obtained using the reconstructed images at each slice position. M
A PR image can be obtained.

[0007]

BRIEF DESCRIPTION OF THE DRAWINGS FIG.
In describing the first embodiment of the apparatus, first, the principle of image reconstruction according to the present invention will be described. Hereinafter, the projection data in the present invention is projection data collected by the helical scan method. The image at a certain slice position is
Assuming that an image (m-th image) is the m-th image, projection data at a slice position is obtained by linearly interpolating projection data in a projection angle range of 720 degrees before and after this slice position as a center. It can be obtained by reconstructing. This reconstruction may be the same as the normal helical scan reconstruction processing. The image at the slice position adjacent to the slice position is obtained by the following processing unique to the present invention, which is different from the above-described normal processing. The image is reconstructed without interpolating the 360-degree projection data collected behind the slice position of the m-th image, and this image is set as the m-th image. Similarly, the image is reconstructed without interpolating the 360-degree projection data collected before the slice position of the m-th image, and this image is set as the m-th F image. Further, the distance that the couch moves while the X-ray tube and the X-ray detector rotate by one projection angle in order to obtain 360-degree projection data is d, and the slice position in front of the m-th image by a distance d. Is an (m + 1) -th image. Here, the traveling direction of the bed is set to the front. Hereinafter, for the sake of simplicity, it is assumed that the slice position matches the projection position.

The following image is obtained to obtain the (m + 1) th image. (M + 1) B image = mB image + BP [CONV (a
-B)] (m + 1) F image = mF image + BP [CONV (c
−a)] where BP: back projection operation CONV: convolution operation a: projection data at the (m + 1) th slice position b: projection data at a position 360 degrees after a c: c: 360 degrees before a It is projection data at a position. From the above, the (m + 1) th image is obtained as follows. (M + 1) th image = mth image + DW × ((m + 1) F
(Image- (m + 1) B image) where DW: slope of primary interpolation. Similarly, the (m-1) th image is obtained as follows. (M−1) image = mth image−DW × (mF image−mB image) In this way, the difference between the projection data and the slice position at which a tomographic image has already been obtained by reconstruction at each slice position Tomographic images can be obtained simply by reconstructing only the slice data, and interpolation and reconstruction of the projection data need not be performed for each slice position, so images at consecutive slice positions in helical scan can be obtained at high speed. If the slice position of the m-th image is continuously changed using a mouse or the like, images at each slice position can be displayed continuously, and the body can be continuously observed.

An embodiment of the present invention based on such a principle will be described. FIG. 1 is a block diagram showing a schematic configuration of the X-ray CT apparatus according to the first embodiment. Here, the third generation X-ray CT apparatus will be described as an example, but the present invention can be applied to other generation apparatuses. In the third generation, imaging is performed while the X-ray tube and the X-ray detector rotate around the subject.
In the third generation CT apparatus, the rotation direction is often reversed in a clockwise direction and a counterclockwise direction in each scan, but recently there is a method in which the image is continuously rotated in the same direction.
Usually, while the X-ray tube and the X-ray detector rotate 360 degrees, the bed is stopped to perform imaging. However, in the present invention, X
An imaging method called a helical scan in which a subject is spirally scanned by moving a bed continuously in synchronization with the rotation of the X-ray detector and the X-ray detector is adopted. According to this, a multi-slice tomographic image can be captured at high speed. The projection data output from the X-ray detector is amplified,
It is supplied to a pre-processing circuit 11 for performing D conversion, various corrections and the like. The raw data storage device 1 and the CPU 2 are connected to the output of the preprocessing circuit 11. The output of the preprocessing circuit 11 is applied to a convolution operation (C
ONV) circuit 12. Interpolation (difference) circuit 21
Is a difference circuit by setting the interpolation constants to −1 and +1. The output of the CONV circuit 12 is supplied to a B image memory 24 via a back projection operation (BP) circuit 23,
It is supplied to the F image memory 25 and the reconstructed image memory 26. The definitions of the B image and the F image are as described above. Difference circuit 21, convolution operation circuit 12,
The CPU 22 is also connected to the back projection arithmetic circuit 23. In addition, the back projection arithmetic circuit 2
A constant setting unit 30 is connected to 3. B image memory 2
4. The outputs of the F image memory 25 and the reconstructed image memory 26 are combined by the image combining unit 27, and the combined result is supplied to the reconstructed image memory 26 again. The output of the reconstructed image memory 26 is supplied to the CT image memory 29 via the contrast scale circuit 28. The output of the CT image memory 29 is displayed on a display unit (not shown). The constant setting unit 30 also has an input unit including a key switch, a mouse, and the like.

A set of projection data photographed (collected) by helical scan is shown in Table 1. This data is schematically shown in FIG.

[0011]

[Table 1] Here, d is the distance the bed moves while the X-ray tube and the X-ray detector rotate by one projection angle. Although the bed position and the projection angle are strictly continuous, they are represented by the central value of the data collection time. Table 1 shows an example in which projection is performed for each rotation of the X-ray tube and the X-ray detector. Therefore, 360 sets of projection data can be obtained in one rotation. Meanwhile, the bed moves by D = 360d. This is, for example, continuously photographed 20 times. In that case, the shooting length is 2
0D, and the obtained projection number is 7200. Helical scan can capture multi-slice tomographic images at high speed, but cannot obtain mathematically precise tomographic images. This is because, in order to reconstruct a tomographic image from a projection image, a projection image of 180 ° or more is required on the same tomographic plane, but in the case of helical scanning, the tomographic plane to be projected moves continuously. This is because it is not possible to obtain a projection image of 180 ° or more on the same tomographic plane. However, since the projected image has a constant slice thickness, a practically sufficient tomographic image can be obtained unless the bed moves fast enough. However, in order to reconstruct a tomographic image in a helical scan, a projection image that is insufficient at a position to be reconstructed needs to be artificially created from a projection image at another slice position. There are two typical methods for this. The first method is to interpolate two projection data projected at the position closest to the slice position and having the same projection angle. The second is a method in which reflection data is created by a so-called reflection method, and interpolation is similarly performed. The present invention provides a method for efficiently executing the first method when reconstructing a continuous image.

Hereinafter, the reconstruction method according to the present embodiment will be described with reference to Table 1 and FIG. Here, as an example, a case where the second image of the image number 2 is reconstructed will be described. The slice position of the second image is 361d when represented by the position of the bed. Since the projection data 361 is a projection at this position, projection data at angle 1 exists, but projection data at other angles does not exist. The non-existent projection data has the same angle, and is obtained by interpolation from two projection data whose slice positions are close to each other. For example, the interpolation projection data at the angle 2 can be obtained from the projection data 2 and the projection data 362 by interpolation. Interpolation is often performed by primary interpolation of the distance between the position of the projection data and the slice position of the image. Therefore, the interpolation projection data at the angle 2 with respect to the second image is obtained as follows. 2PP2 = 2P × WB2 + 362P × WF2 (1) where 2PP2: Interpolated projection data at an angle of 2 with respect to the second image 2P: Projection data 2 362P: Projection data 362 WB2, WF2: Interpolation coefficient , Which are represented as follows: WB2 = d / 360d = 1/360 (2) WF2 = (360d-d) / 360d = 359/360 (3) where, m: image number n: interpolation projection data number (1 to 360) It is.

Here, n is the projection data at the same angle as the projection data at the slice position of the image, that is, n = 1, that is, the first interpolation projection data. For example, in the case of the image 2, the interpolated projection data of the angle 1 is the first interpolated projection data in the case of the image 3, and the interpolated projection data of the angle 2 is the first interpolated projection data. On the basis of this, n also increases in the direction in which the number of projection data increases. Therefore, for example, in the case of image 2, the interpolation projection data of angle 2 is used.
If the data is image 3, the interpolation projection data at angle 3 is n
= 2, that is, the second interpolated projection data. mPPn: n-th interpolated projection data of the m-th image Pj: j-th projection data (however, Po = 0) Pk: k-th projection data (where k> Pk when k> number of projections)
= 0) j = m + 358 + n-360 (4) When k = j + 360 (5), mPPn = Pj × WBn + Pk × WFn (6) (n = 1 to 360) Here, WBn and WFn are interpolation coefficients of the n-th interpolated projection data, which are expressed as follows. WBn = (n-1) / 360 (7) WFn = (361-n) / 360 (8) Here, if CONV: convolution BP: back projection mI: m-th image mQQ: set of mPPn (n = 1 to 360), the m-th image mI can be obtained as follows.

MI = BP [CONV (mQQ)] (9) Next, in order to explain reconstruction of a continuous slice image according to the present invention, images named as a B image and an F image are defined as follows. mBI = BP [CONV (mBPJ)] (10) mFI = BP [CONV (mFPK)] (11) where, mBI: B image related to m-th image (back image) mFI: F image related to m-th image ( (Previous image) mBPJ: set of Pj (j = (m−1) to (m + 35)
8)) mFPK: a set of Pk (k = j + 360). Next, from the B image mBI and the F image mFI, the B image (m +
1) A method for obtaining the BI, F image (m + 1) FI will be described. mBI, mFI, (m + 1) BI, (m + 1) FI
Is an image reconstructed from 360-degree projection data,
Only one projection differs. Therefore, Japanese Patent Application Laid-Open No.
According to the method of No. 32748, it can be obtained as follows. (M + 1) BI = mBI + BP [CONV (SBPm + 1)] (12) (m + 1) FI = mFI + BP [CONV (SFPm + 1)] (13) where SBPm + 1 = P (m + 359) -P (m-1) SFPm + 1 = P (m + 719) -P (m + 359). Similarly, (m-1) BI and (m-1) FI are obtained from mBI and mFI as follows.

(M-1) BI = mBI + BP [CONV (SMBPm-1)] (14) (m-1) FI = mFI + BP [CONV (SMFPm-1)] (15) where SMBPm-1 = P (m-2) -P (m + 358) SMFPm-1 = P (m + 358) -P (m + 718)
It is. Next, with reference to the flowchart shown in FIG. 3, a description will be given of a case where the image of the present embodiment is reconstructed by one projection earlier (in a direction in which m increases). The current image number m is designated from the constant setting unit 30. At first, since no image is reconstructed, the current image (mI), B image (mBI), and F image (mFI) are reconstructed in step # 1. This procedure only needs to be performed once. For example, in the case where image reconstruction in the forward direction is performed after continuous image reconstruction in the backward direction, mI, mBI, and mFI are already stored in the respective memories 26. , 24, 25, there is no need to execute step # 1. These overall controls are executed by a program of the CPU of the CT apparatus, and can be implemented by a known technique. Step #
1 will be described later in detail, and here, mI, mB
It is assumed that I and mFI are stored in the reconstructed image memory 26, the B image memory 24, and the F image memory 25, respectively.

In step # 2, it is necessary to generate information indicating that it is a reconstruction of an image adjacent in the forward direction, a gradient DW of primary interpolation, and constants necessary for back projection (two types for B image and F image). Values, convolution function, and other necessary information from CPU 2 to each circuit 2
1, 12 and 23 for storage. It is not necessary to transfer information already stored in the reconstruction arithmetic device. In step # 3, the CPU 2 reads out three projection images of the (m + 359) th projection data, the (m−1) th projection data, and the (m + 719) th projection data from the raw data storage device 1, and sends them to the interpolation (difference) circuit 21. Supply. Here, each projection image data is PC1, PC2, and PC3. Step #
At 4, the interpolation (difference) circuit 21 calculates PC1-PC2,
The operation result is set to SBPC, and the convolution circuit 12
Transfer to In step # 5, the convolution circuit 12
Performs a convolution operation on the difference result SBPC using a convolution operation function. For the convolution operation, a known method can be appropriately used. Let the convolution result be SBPCV. In step # 6, the back projection circuit 23 stores the SBPCV in the B image memory 2
4 is superimposed (added) on the B image mBI in 4 and back-projected. The back projection result is stored in the B image memory 24 as mBI. A known method can be appropriately used for the back projection. The constant required for back projection of the B image is C
Created by the constant setting unit 30 under the control of the PU 22,
The data is transferred to the back projection circuit 23.

In step # 7, the interpolation (difference) circuit 21
C3-PC1 is calculated, the calculation result is set to SFPC, and the result is transferred to the convolution circuit 12. In step # 8, the convolution circuit 12 performs a convolution operation on the difference result SFPC using a convolution operation function. Let the convolution result be SFPCV. In step # 9, the back projection circuit 23
Is superimposed on (added to) the F image mFI in the F image memory 25 and back-projected. The back projection result is stored in the F image memory 25 as mFI. Note that constants necessary for back projection of the F image are created by the constant setting unit 30 under the control of the CPU 22 and transferred to the back projection circuit 23.
Note that a part or all of Steps # 4 to # 6 and Steps # 7 to # 9 may be multiplexed and executed by separate circuits, printed circuit boards, or separate devices. In step # 10, the image combining device 27 reconstructs a new reconstructed image (m + 1) I using the B image, the F image, and the current reconstructed image mI, and stores the reconstructed image as a new mI. This procedure will be described later in detail.

At step # 11, a contrast scale conversion is performed for all the pixels of the reconstructed image by the contrast scale device 28 for each pixel, and a CT image is created. The CT image is stored in the CT image memory 29. The contrast scale conversion is a known technique, and is generally performed by the following equation. PV = a × RPV + b (20) where, PV: pixel value of CT image RPV: pixel value of reconstructed image a, b: constants, a and b are generally 0 for water and −1000 for air Is determined according to the apparatus, the X-ray tube voltage, and the like. In step # 12, the CT image is transferred to an image display device (not shown), and the image is displayed. By the above operation, one of the slice images normally reconstructed is obtained.
The image at the slice position just before the projection can be easily reconstructed and displayed. Furthermore, when displaying images of a plurality of slices continuously by shifting the slice position in the forward direction,
In step # 14, it is sufficient to set m = m + 1 and repeat steps # 3 to # 12. In addition, continuous reconstruction can be performed in any of the front and rear directions by combining with continuous image reconstruction in a later (direction in which m decreases) image described later. These controls are performed by the CPU 22.

Next, the details of step # 1 will be described with reference to FIGS.
This will be described with reference to the flowchart of FIG. The image number to be reconstructed is m, and the number of interpolated projection images required for reconstructing the reconstructed image is RPJN. The following steps are controlled by programs of the CPU 2 and the CPU 22. Step #
At 110, a value necessary for generating constants necessary for back projection (three types for reconstructed image, B image, and F image), a convolution function, and other constants necessary for reconstruction are provided from the CPU 2 to each circuit 21. , 12, and 23 for storage. In step # 120, the CPU 2 notifies each of the circuits 21, 12, and 23 that the first reconstructed image is to be reconstructed. CPU in step # 130
Reference numeral 22 initializes the reconstructed image memory 26 and sets the reconstructed image data to 0. In step # 140, n = 1, in step # 150, j = m + 358 + n-360, and in step # 160, k = j + 360. Step # 17
At 0, the CPU 2 reads the j-th projection data Pj (Po = 0) from the raw data storage device 1 and transfers it to the interpolation circuit 21.
In step # 180, the CPU 2 reads the k-th projection data Pk from the raw data storage device 1 (where k> the total number of projections,
Pk = 0) and transfers it to the interpolation circuit 21.

In step # 190, the n-th interpolation projection data PP of the m-th image is obtained as follows. PP = Pj × WBn + Pk × WFn (21) where WBn and WFn are interpolation coefficients of the n-th interpolated projection data. In step # 220, the convolution circuit 12 performs a convolution operation on the output PP of the difference circuit 21 and a convolution function. Step # 230
Then, the back projection circuit 23 superimposes the convolution result PPCV on the image in the reconstructed image memory 26 and performs back projection. The result of the back projection is stored in the reconstructed image memory 26. Step #
At 240, n = n + 1, j = j + 1, k = k + 1, and it is determined at step # 270 whether n ≦ RPJN. n ≦ R
In the case of PJN, the process returns to step # 170, where n> RPJ
If N, the process proceeds to the next step # 280. At this time, the reconstructed image memory 26 stores the reconstructed image.
In step # 280, the contrast scale circuit 28 performs contrast scale conversion for all pixels of the reconstructed image for each pixel to create a CT image.
It is stored in the image memory 29. In step # 290, the CT image is transferred to an image display device (not shown), and the image is displayed. Thereby, the current image (m-th image) is displayed.

At step # 300, the CPU 2
1, 12, and 23 are notified that this is the first B image reconstruction. In step # 310, the CPU 22 initializes the B image memory 24 and sets the B image data to 0. In step # 320, n = 1, and in step # 330, j =
In step # 340, the CPU 2 reads the j-th projection data Pj from the raw data storage device 1 and transfers it to the interpolation circuit 21. Here, since the interpolation coefficients 1 and 0 are set in the interpolation circuit 21, in step # 350, the interpolation circuit 21 does nothing and simply passes the data.
Store in P. In step # 360, the convolution circuit 12 performs a convolution operation on the output PP of the interpolation circuit 21 and a convolution function. Step # 3
At 70, the back projection circuit 23 superimposes the convolution result PPCV on the image in the B image memory 24 and performs back projection. The result of the back projection is stored in the B image memory 24. Step # 38
In step # 390, j is set to j = j + 1, and in step # 395, it is determined whether n ≦ RPJN.
If n ≦ RPJN, the flow returns to step # 340. n> R
In the case of PJN, the process proceeds to the next step # 400. At this time, the current B image mBI is reconstructed and stored in the memory 24.

At step # 400, the CPU 2
1, 1, and 23 are notified that this is the first F-image reconstruction. In step # 410, the CPU 22 initializes the F image memory 25 and sets the F image data to 0. In step # 420, n = 1, and in step # 430, k =
In step # 440, the CPU 2 reads the k-th projection data Pk from the raw data storage device 1 and transfers it to the interpolation circuit 21. Here, since the interpolation coefficients 1 and 0 are set in the interpolation circuit 21, step # 4
At 50, the interpolation circuit 21 does nothing and simply passes the data and stores it in the PP. In step # 460, the convolution circuit 12 performs a convolution operation on the output PP of the interpolation circuit 21 and a convolution function. In step # 470, the back projection circuit 23 superimposes the convolution result PPCV on the image in the F image memory 25 and performs back projection. The result of the back projection is stored in the F image memory 25. In step # 480, n = n + 1, and in step # 490, k =
In step # 495, it is determined whether n ≦ RPJN. If n ≦ RPJN, the flow returns to step # 440. If n> RPJN, the process ends. At this time, the current F image mFI is reconstructed and stored in the memory 25.

Next, the details of step # 10 will be described with reference to the flowchart of FIG. Here, the matrix size of the image is set to MX (pixel) × MX (pixel). In step # 1010, j = 1, and in step # 1020, i = 1. In step # 1030, the F image memory 25
The pixel value at the address (i, j) is read out, and is defined as FPV. At step # 1040, (i,
j) The pixel value at the address is read, and this is set as BPV. In step # 1050, (i, j) in the reconstructed image memory 26
The pixel value at the address is read out, and this is set as mRPV. At step # 1060, the following calculation is performed. (M + 1) RPV = mRPV + DW × (FPV−BPV) (22) where DW is the slope (difference) of the primary interpolation coefficient of the helical scan. (M + 1) RP in step # 1070
V is written into the reconstructed image memory 26 at the address (i, j). In step # 1080, i = i + 1 is set, and step #
At 1090, it is determined whether or not i ≦ MX. If i ≦ MX, the process returns to step # 1030. If i> MX, j = j + 1 is set in step # 1100, and j is set in step # 1110.
It is determined whether or not ≤MX. Step # 1 if j ≦ MX
Return to 020. If j> MX, the process ends.

Next, referring to the flow chart shown in FIG. 8, after one projection (the direction in which m decreases) according to this embodiment.
Will be described. The current image number m is designated from the constant setting unit 30. At first, since no image is reconstructed, the current image (mI), B image (mBI), F image (mFI) are determined in step # 21.
Reconfigure. This procedure only needs to be performed once. For example, in the case where image reconstruction is performed continuously in the forward direction and then image reconstruction is performed in the backward direction, mI, mBI, mF
Since I is already stored in each of the memories 26, 24 and 25, there is no need to execute step # 21. These overall controls are executed by a program of the CPU of the CT apparatus, and are executed by a known technique. In step # 22, information indicating that it is reconstruction of an image adjacent in the backward direction, gradient DW of primary interpolation, values required to generate constants (two types for B image and F image) required for back projection, And the convolution function and other necessary information from the CPU 2 to the respective circuits 21, 12, 23
Transfer to and memorize. It is not necessary to transfer information already stored in the reconstruction arithmetic device.

In step # 23, the CPU 2 sends the (m + 358) th projection data from the raw data storage device 1 to the (m-2) th projection data.
The three projection images of the projection data and the (m + 718) th projection data are read out and supplied to the difference circuit 21. Here, each projection image data is PC1, PC2, and PC3. Step #
At 24, the new reconstructed image (m−m) is generated by the image combining device 27 using the B image, the F image, and the current reconstructed image
1) Reconstruct I and store it as the new mI. This procedure is the same as that of FIG. 7 except that the operation formula executed in step # 1060 in the flowchart shown in FIG. 7 is changed to the following formula. (M-1) RPV = mRPV-DW.times. (FPV-BPV) (23) In step # 25, the contrast scale device 28 performs contrast scale conversion for all pixels of the reconstructed image for each pixel. A CT image is created and stored in the CT image memory 29. The contrast scale conversion is performed by the aforementioned equation (20). In Step # 26, C
The T image is transferred to an image display device (not shown), and the image is displayed. By the above operation, the image at the slice position one projection after the normally reconstructed slice image can be easily reconstructed and displayed.

Further, when displaying a plurality of slice images continuously by shifting the slice position backward, m = m-1 in step # 28, and the difference circuit 21 calculates PC2-PC1 in step # 29. And the calculation result is SBPC
And transfers it to the convolution circuit 12. In step # 30, the convolution circuit 12 outputs the difference result SBP
A convolution operation is performed on C by a convolution operation function. For the convolution operation, a known method can be appropriately used. SBP of convolution result
CV. In step # 31, the back projection circuit 23 stores the SBPCV in the B image m in the B image memory 24.
Back-projection (addition) over BI. The back projection result is stored in the B image memory 24 as mBI. A known method can be appropriately used for the back projection. Note that constants necessary for back projection of the B image are created by the constant setting unit 30 under the control of the CPU 22 and transferred to the back projection circuit 23. In step # 32, the difference circuit 21 calculates PC1-PC3, and calculates the calculation result as SF.
The PC is transferred to the convolution circuit 12. In step # 33, the convolution circuit 12 outputs the difference result S
The convolution operation is performed on the FPC by the convolution operation function. SFPCV for convolution result
And In step # 34, the back projection circuit 23 stores the SFPCV in the F image mFI in the F image memory 25.
Back projection (addition). The back projection result is stored in the F image memory 25 in mF
It is stored as I. The constants necessary for the back projection of the F image are created by the constant setting unit 30 under the control of the CPU 22, and the back projection circuit 2
3 is transferred.

A part or all of Steps # 29 to # 31 and Steps # 32 to # 34 may be multiplexed and implemented by separate circuits, printed circuit boards, or separate devices. . By returning to step # 23 after step # 34, an image shifted by one projection in the backward direction can be continuously reconstructed and displayed. Also,
By combining with the continuous reconstruction in the forward direction described in FIG. 3, continuous reconstruction can be performed in both the forward and backward directions. These controls are performed by the CPU 2. As described above, according to this embodiment, a tomographic image at one slice position is obtained by normal reconstruction processing by interpolating projection data for two rotations, and a tomographic image at a slice position adjacent thereto is obtained by both reconstruction data. Only the difference between the projection data at the slice position is reconstructed, multiplied by a coefficient, and added. Therefore, interpolation of projection data for each slice position,
Since it is not necessary to perform reconstruction, images at consecutive slice positions in the helical scan can be obtained at high speed. If the adjacent slice positions are continuously changed using a mouse or the like, the images at each slice position can be continuously obtained. Can be displayed
The body can be observed continuously. The present invention is not limited to the embodiments described above, but can be implemented with various modifications. Modifications will be described below. Since the device configuration of the modified example is the same as that of the embodiment, the description thereof will be omitted, and only the operation will be described. Also,
In the operation, the same parts as those of the embodiment are not described.

In the above-described embodiment, an image shifted for each projection data is reconstructed and all the images are displayed. However, in practice, it is not necessary to observe an image with a very small slice interval. Therefore, although the reconstructed image is continuously reconstructed, the creation and display of the CT image and the image display may be performed at intervals of several projection positions.
FIG. 9 shows a flowchart of a modified example in the case of reconstructing a continuous image in the forward direction. However, the same can be applied to the case of reconstructing an image in the backward direction. Display nn
It shall be performed at every projection data (bed position) interval. In step # 5010, steps # 1 and # 2 in FIG. 3 are performed, and mI, mBI, and mFI are stored. Step #
In step 5020, s = 1 is set. In step # 5030, steps # 3 to # 10 in FIG. 3 are performed, and (m + 1) I
Reconfigure. In step # 5040, s = s + 1 is set,
In step # 5050, it is determined whether or not s ≦ nn. s ≦ n
If n, the process returns to step # 5030. If s> nn, the process proceeds to the next step # 5060. Step # 5060
Perform steps # 11 and # 12 of FIG.
Display the T image. Hereinafter, when the operation is to be continued, m = m + 1 is set in step # 5080, and then step # 502.
Steps 0 to # 5060 are repeated.

Thus, a CT image can be displayed at every interval of nn projection data (bed position). Next, another modified example will be described. In the conventional MPR (coronal, sagittal, oblique) display, there is a problem that a step appears in an image because a slice interval is larger than a pixel size of a tomographic image, and interpolation is performed to prevent this. And other image processing. Since the helical scan provides a continuous image, an MPR image with no visible step is obtained. However, in the conventional reconstruction method, since a large amount of calculation is required to obtain one image, it is not practical to reconstruct a large number of images. However, according to the present invention,
Since images can be reconstructed at high speed, a large number of images can be reconstructed. However, storing a large number of images requires a large storage capacity. Also, 1
The image used for one MPR is only a part of the tomographic image. When time is required for reconstruction, it is advantageous to reconstruct all the pixels of the tomographic image in consideration of creating a large number of MPR images, but the reconstruction can be performed at high speed as in the present invention. In this case, it is more advantageous to reconstruct only the pixels of the tomographic image necessary to create one MPR.

Therefore, in the embodiment, all pixels are reconstructed. However, in the case of coronal, only one line is reconstructed, in the case of sagittal, only one column is reconstructed, and in the case of oblique, two pixels along it are reconstructed. It may be configured.
In this case, it is necessary to control the constant setting unit of the back projection accordingly. Further, it is obvious that the image reconstruction apparatus of the present invention can reconstruct a conventional helical scan image or a normal scan image. The present invention is not limited to the above-described embodiments, and can be implemented with various modifications without departing from the spirit thereof.

[0031]

As described above, according to the present invention,
An MPR image with few steps can be obtained.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a configuration of an embodiment of an X-ray computed tomography apparatus according to the present invention.

FIG. 2 is a diagram showing a state of a helical scan according to the embodiment.

FIG. 3 is a flowchart showing an operation of reconstructing a continuous image in the forward direction according to the embodiment.

FIG. 4 is a flowchart showing details of an m-th image reconstruction process in the flowchart of FIG. 3;

FIG. 5 is a flowchart showing details of an mB-th image reconstruction process in the flowchart of FIG. 3;

FIG. 6 is a flowchart showing details of an mF-th image reconstruction process in the flowchart of FIG. 3;

FIG. 7 is a flowchart showing details of the (m + 1) -th image reconstruction process in the flowchart of FIG. 3;

FIG. 8 is a flowchart illustrating an operation of reconstructing a continuous image in the backward direction according to the first embodiment.

FIG. 9 is a flowchart showing an operation of a modification.

FIG. 10 is a block diagram showing a reconstruction operation circuit of a conventional X-ray CT apparatus.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Raw data storage device, 11 ... Preprocessing circuit, 12 ... Convolution operation circuit, 21 ... Interpolation circuit, 23 ... Back projection operation circuit, 24 ... B image memory, 25
... F image memory, 26 ... Reconstructed image memory, 26 ... Image synthesis unit, 28 ... Contrast scale circuit, 29 ... CT
Image memory.

Claims (2)

[Claims] 1. An X-ray computed tomography apparatus for performing a helical scan, wherein: a projection data acquisition unit for acquiring projection data by the helical scan; and a slice position different by using projection data acquired by the helical scan. X-ray computed tomography comprising: reconstruction means for continuously reconstructing images; and means for obtaining an MPR image by using images at different slice positions reconstructed by the reconstruction means. Image capturing device. 2. An X-ray computed tomography apparatus according to claim 1, wherein said reconstructing means reconstructs only pixels of a tomographic image necessary for creating said MPR image.