JP3100650B2 - X-ray computed tomography system - Google Patents

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 a third generation X-ray CT employing a helical scan system.
It relates to image reconstruction of the device.

[0002]

2. Description of the Related Art Image reconstruction in an X-ray CT apparatus is 36
It is performed independently for each slice position from the projection data of 0 degrees or 180 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. Also, considering that the reconstructed image is stored, the images at a number of slice positions are stored in one slice.
Reconstruction 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 the reconstruction method described in JP-A-58-32748 (U.S. Pat. No. 4,495,645).

However, this conventional continuous slice reconstruction method relates to a continuous scan method at the same slice position, and no method has been proposed for efficiently reconstructing images at continuous slice positions in the helical scan method. .

[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. Is to reconfigure.

Another object of the present invention is to provide an X-ray CT apparatus which employs a helical scan system and which can continuously observe the inside of a body by continuously displaying images at a number of continuous slice positions. To provide.

Another object of the present invention is to reduce the number of images to be stored by reconstructing and displaying images at a large number of continuous slice positions in an X-ray CT apparatus employing a helical scan system. An object of the present invention is to provide an X-ray CT apparatus capable of performing the above.

[0007]

An X-ray CT apparatus according to the present invention uses projection data collected by a helical scan method. The X-ray CT apparatus has 360-degree projection data and 360-degree projection data centered on a predetermined first slice position. Means for obtaining a tomographic image at the slice position by reconstructing the projection data of the first slice position using interpolation processing, and reconstructing the projection data 360 degrees after the first slice position without using interpolation processing. A means for obtaining the first post-image; a means for reconstructing the 360-degree projection data before the first slice position without using the interpolation processing to obtain a first pre-image; Means for reconstructing data obtained by subtracting the projection data after 360 degrees, adding the reconstructed result to the first post-image to obtain a second post-image,
Means for reconstructing data obtained by subtracting the projection data at the first slice position from the previous projection data, adding the reconstructed result to the first previous image to obtain a second previous image, Means for multiplying the difference from the second image by a coefficient and adding the result to the tomographic image to obtain a tomographic image at a second slice position 360 degrees before the first slice position.

Further, another X-ray CT apparatus according to the present invention comprises:
The projection data collected by the helical scan method is used.
Means for reconstructing the 0-degree projection data and the 360-degree projection data using interpolation processing to obtain a tomographic image at the slice position, and interpolating the 360-degree projection data after the first slice position Means for reconstructing a first post-image by using the following, and means for reconstructing projection data of 360 degrees before the first slice position without using an interpolation process to obtain a first pre-image.
The difference between the first pre-image and the first post-image is multiplied by a coefficient, and the result is subtracted from the tomographic image to obtain 360
Means for obtaining a tomographic image of a second slice position after the second slice position.

[0009]

According to the X-ray CT apparatus of the present invention, the tomographic image at the first slice position is obtained by normally reconstructing the projection data for two rotations using interpolation processing, and the tomographic image at the second slice position adjacent thereto is obtained. The tomographic image is obtained by reconstructing only the difference between the different projection data in the first and second slices and multiplying the difference by a coefficient.
It is obtained from the tomographic image at the slice position. This allows
Since images at successive slice positions in the helical scan can be obtained at high speed, the inside of the subject can be continuously observed by changing the slice position continuously according to the operation of an instruction input member such as a mouse. It can provide useful information for diagnosis.

[0010]

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 according to the present invention is projection data collected by the helical scan method. Assuming that an image at a certain slice position is an m-th image (m-th image), the m-th image is obtained by first-order interpolation of projection data in a projection angle range of 720 degrees before and after the slice position as a center, and the slice position. Is obtained by reconstructing this projection data. This reconstruction may be the same as the normal helical scan reconstruction processing.

The image at the slice position adjacent to this 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,
This image is referred to as an mF-th 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.

The (m + 1) B image = the mB image + BP [CONV (ab)] The (m + 1) F image = the 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: Projection data at a position 360 degrees before a 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) Here, DW is a gradient of primary interpolation.

In this manner, at each slice position, a tomographic image can be obtained only by reconstructing only the difference between the projection data and the slice position for which the tomographic image has already been obtained by reconstruction. Since interpolation and reconstruction of the projection data are not required, images at consecutive slice positions in the helical scan can be obtained at high speed, and if the slice position of the m-th image is continuously changed using a mouse or the like. The images at the respective slice positions can be displayed continuously, and the inside of 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 10 and the X-ray detector 14 rotate about the bed 12. In many third-generation CT apparatuses, the rotation direction is reversed in the clockwise direction and the counterclockwise direction for each scan, but recently, there is a method in which the image is rotated continuously in the same direction. Usually, the X-ray tube 10 and the X-ray detector 14 are 360
During the rotation, the bed 12 is stopped and the image is taken. However, in the present invention, an imaging method called a helical scan is employed in which the subject is spirally scanned by continuously moving the bed 12 in synchronization with the rotation of the X-ray tube 12 and the X-ray detector 14. I have. According to this, a multi-slice tomographic image can be captured at high speed.

A pre-processing circuit 1 in which the output of the X-ray detector 14 performs amplification, A / D conversion, various corrections, etc. on the detection signal.
6 to the reconfiguration circuit 18. Reconstruction circuit 1
An input unit 22 including a key switch, a mouse, and the like is also connected to 8. The output of the reconfiguration circuit 18 is displayed on the display unit 20. Table 1 shows a set of data thus photographed (collected). This data is schematically shown in FIG.

[0017]

[Table 1]

Here, d is the X-ray tube 10 and X-ray detector 14
Is the distance that the bed 12 moves while rotating 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 the projection is performed for each rotation of the X-ray tube 10 and the X-ray detector 14. Therefore,
360 sets of projection data are 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 20D
And the obtained projection number is 7200.

The helical scan can take a multi-slice tomographic image at high speed, but cannot obtain a mathematically precise tomographic image. 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)

Here, 2PP2: Interpolated projection data at an angle of 2 with respect to the second image 2P: Projected data 2 362P: Projected data 362 WB2, WF2: Interpolation coefficients, which are expressed as follows: It is. WB2 = d / 360d = 1/360 (2) WF2 = (360d-d) / 360d = 359/360 (3)

Here, m: image number n: interpolation projection data number (1 to 360).

Note that 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 (where 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, assuming 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 the 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, and only one projection is different. Therefore, it can be determined as follows by the method described in Japanese Patent Application Laid-Open No. 58-32748. (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) Here, SMBPm-1 = P (m) -2) -P (m + 358) SMFPm-1 = P (m + 358) -P (m + 718)
It is.

Next, the procedure of this embodiment will be described in detail with reference to the flowchart shown in FIG. In step # 1, projection data is collected by helical scan. In step # 2, an image number (slice position) m to be displayed as the current image is determined as appropriate, and is input from the input unit 22. In step # 3, the current image (m-th image) is reconstructed and displayed according to equation (9).

In step # 4, the current B
The image (the mB-th image) is reconstructed, and (1)
The current F image (mF-th image) is reconstructed by the equation (1). In step # 6, the direction indication switch of the input unit 22 is used to instruct whether to display an image at a slice position before the m-th image or to display an image at a slice position after the m-th image. If it is desired to display a forward image, the process proceeds to step # 11. If it is desired to display a backward image, the process proceeds to step # 21.

In step # 11, the (m + 1) B-th image is reconstructed by the equation (12). Similarly, step # 1
In (2), the (m + 1) F-th image is reconstructed according to the thirteenth equation. In step # 13, the (m + 1) -th image at the slice position (m + 1) is reconstructed as follows. (M + 1) I = mI + DW × [(m + 1) FI− (m + 1) BI] (16) where DW is a coefficient of linear interpolation and is expressed as follows. DW = WB (n + 1) -WBn =-[WF (n + 1) -WFn] = 1/360 (17)

In step # 14, the (m + 1) th image obtained by the equation (16) is displayed. In step # 15, it is instructed whether or not to display an image at an adjacent slice position in the forward direction. If no display is made, the process ends. If display is continued, m = m + 1 is set in step # 16, and then step #
Return to 11. As described above, by sequentially increasing m, images in which the slice position is sequentially shifted in the forward direction can be continuously observed. On the other hand, in step # 21, the (m-1) th image at the slice position (m-1) is obtained as follows. (M−1) I = mI−DW × [mFI−mBI] (18)

In step # 22, the (m-1) th image obtained by the equation (18) is displayed. In step # 23, (1
The (m-1) B-th image is reconstructed by the expression 4). Similarly, in step # 24, the (m-1) F
Reconstruct the image. In step # 25, it is instructed whether or not to display the image at the adjacent slice position in the backward direction. If it is not displayed, the process ends.
After m = m−1 at 26, the process returns to step # 21. As described above, by sequentially reducing m, images in which the slice position is sequentially shifted in the backward direction can be continuously observed. Next, the equation (18) is proved. Equation (18) is modified as follows. mI = (m−1) I + DW × [mFI−mBI] (19) In the equation (9): mI = BP [CONV (mQQ)], if m = m−1, the following equation is obtained. (M-1) I = BP [CONV {(m-1) QQ}] (20) From equations (9) and (20), mI- (m-1) I = BP [CONV} (m- 1), mRR｝] ... (21)

Is obtained. Here, (m-1), mRR is a difference set between the set mQQ and the set (m-1) QQ. m
Since QQ is a set of mPPn (n = 1 to 360), it is expressed as follows. mPP1 = P (m-1) × WB1 + P (m + 359) × WF1 mPP2 = Pm × WB2 + P (m + 360) × WF2 mPP360 = P (m + 358) × WB360 + P (m + 718) × WF360 Similarly, the set (m-1) QQ is represented as follows. (m-1) PP1 = P (m-2) × WB1 + P (m + 358) × WF1 (m-1) PP2 = P (m-1) × WB2 + P (m + 359) × WF2 ・ ・ ・ ( m-1) PP360 = P (m + 357) .times.WB360 + P (m + 717) .times.WF360 Therefore, the difference set (m-1), mRR is expressed as follows. -P (m-2) × WB1 -P (m + 358) × WF1 P (m-1) × (WB1-WB2) + P (m + 359) × (WF1-WF2) ・ ・ ・ P (m + 357 ) × (WB359-WB360) + P (m + 717) × (WF359-WF360) P (m + 358) × WB360 + P (m + 718) × WF360 From equation (17), WB (n) −WB (n + 1) = Since WF (n) −WF (n + 1) = − DW, the difference set (m−1) and mRR are as follows by substituting this into the above set and collecting projection data of the same angle. . −P (m-2) × WB1 −DW × P (m-1) + DW × P (m + 359) ・ ・ ・ −DW × P (m + 357) + DW × P (m + 717) −P (m +358) × (WF1-WB36
0) + P (m + 718) × WF360

Here, (WF1-WB360) = 1/3
60 = DW, WB1 = 0, WF360 = 1/360 = D
Since it is W, by substituting it into the above set and rearranging it, the difference set (m-1) and mRR are as follows. −DW {P (m−1),... P (m + 358)} + DW
{P (m + 359),... P (m + 718)} = − DW × mBPJ + DW × mFPK By substituting this equation into equation (21), the following equation is obtained. mI− (m−1) I = BP [CONV ｛(m−1), mRR｝] = BP [CONV ｛−DW × mBPJ + DW × mFP
K] = DW × BP [CONV (mFPK)] − DW × BP
[CONV (mBPJ)] = DW × [mFI−mBI] (22) By transforming equation (22), (m−1) I = mI−DW ×
[MFI−mBI], and the equation (18) is obtained. Similarly, equation (16) is obtained by adding m = m + 1 to equation (19).
It is required by setting.

As described above, according to the first embodiment, the tomographic image at one slice position is obtained by normal reconstruction processing by interpolating projection data for two rotations, and the slice position at the adjacent slice position is obtained. The tomographic image is obtained by reconstructing only the difference between the projection data at both slice positions, multiplying by a coefficient, and adding. This eliminates the need to perform interpolation and reconstruction of projection data for each slice position, so that images at successive slice positions in helical scan can be obtained at high speed, and the adjacent slice positions can be successively determined using a mouse or the like. By changing to, images at each slice position can be displayed continuously, and the inside of the body can be continuously observed.

In the first embodiment, the forward or backward direction is designated in step # 6, and the process proceeds to step # 11 or step # 21. However, the operation may be terminated here depending on the case. , May be returned to step # 2.

Hereinafter, another embodiment of the present invention will be described. In other embodiments, the device configuration is the same as that of the first embodiment. Therefore, the description thereof will be omitted, and only the operation will be described. In the operation, the same parts as in the first embodiment will not be described.

In the first embodiment, adjacent images are sequentially reconstructed,
Although they are sequentially displayed, the display may be performed for each of a plurality of slice positions. A second embodiment for doing this will now be described. FIG. 4 is a flowchart thereof. Steps up to step # 5 for reconstructing the mF-th image are the same as those in the first embodiment, and are not shown. In step # 5a, a display slice interval n (n ≧ 1) is specified.

When the forward direction is instructed in step # 6, i = 0 is set in step # 10, and steps # 11 to # 11 are performed.
Step # 13 is executed. Next, in step # 16, m = m +
It is set to 1 and i = i + 1 in step # 16a, and it is determined in step # 16b whether i <n. If i <n, the process returns to step # 11. If i <n, the m-th image is displayed at step # 14. If the operation is to be continued, the process returns to step # 3.

If the backward direction is designated in step # 6, i = 0 is set in step # 20, and steps # 21 to # 21 are set.
Step # 24 is executed. Next, in step # 26, m = m-
It is set to 1, i = i + 1 in step # 26a, and it is determined in step # 26b whether i <n. If i <n, the process returns to step # 21. If i <n, the m-th image is displayed at step # 22. If the operation is to be continued, the process returns to step # 3.

In the first and second embodiments, a tomographic image is obtained for each slice position regardless of the presence or absence of display. Next, a third embodiment for obtaining a tomographic image for each of a plurality of slice positions will be described. This embodiment can be applied to the first and second embodiments. Here, an embodiment adapted to the second embodiment is shown in FIG.
This will be described with reference to the flowchart shown in FIG. If the forward direction is designated in step # 6, the process proceeds to step # 40. If the backward direction is designated, the process proceeds to step # 60. Otherwise, the process returns to step # 3.

In the case of the forward direction, in step # 40, i =
1, and in step # 41, the (m +
1) The B image is reconstructed, and in step # 42, the (m + 1) F-th image is reconstructed according to the equation (13). Step # 43
Stores the (m + 1) B-th image in the register SIGB, and stores the (m + 1) F-th image in the register SIGF in step # 44.
To be stored. In step # 45, m = m + 1 is set, and in step # 46, it is determined whether n = 1.

If n = 1, the process proceeds to step # 55. n =
If it is not 1, the (m + 1) B-th image is reconstructed by the equation (12) in step # 48, and (1) is obtained in step # 49.
The (m + 1) F-th image is reconstructed by equation (3). In step # 50, the data of the register SIGB is added to the (m + 1) B
The sum of the images is newly stored in the register SIGB, and in step # 51, the sum of the data of the register SIGF and the (m + 1) F-th image is newly stored in the register SIGF. In step # 52, m = m + 1, in step # 53, i = i + 1, and in step # 54, i <i <
It is determined whether it is n.

If i <n, the flow returns to step # 48, where i
If not <n, an image (m-th image) mI to be displayed next is obtained in step # 55 according to the following equation. Where (m
-N) I is the currently displayed image. mI = (mn) I + DW (SIGF-SIGB) (23) The m-th image is displayed in step # 56, and the process returns to step # 3.

If the backward direction is designated in step # 6, i = 1 is set in step # 60, and the mB-th image is stored in the register SIGB in step # 61.
In step 2, the mF-th image is stored in the register SIGF. In step # 63, the (m-1) B-th image is reconstructed according to equation (14).
Reconstruct the F image. At step # 65, m = m−1, and at step # 66, it is determined whether n = 1.

If n = 1, the process proceeds to step # 74, where n
If not = 1, the result of adding the mB-th image to the data in the register SIGB in step # 67 is stored in the register SIGB.
And the data obtained by adding the mF-th image to the data of the register SIGF in step # 68 is stored in the register SIGF.
Is stored anew. In step # 69, the (m-1) B-th image is reconstructed by the equation (14), and in step # 70, (1)
The (m−1) F-th image is reconstructed by the expression 5). In step # 71, m = m−1, and in step # 72, i = i +
It is determined at step # 73 whether i <n.

If i <n, the process returns to step # 67, where i
If not <n, an image (m-th image) mI to be displayed next is obtained by the following equation in step # 74. Where (m +
n) I is the currently displayed image. mI = (m + n) I-DW (SIGF-SIGB) (24) The m-th image is displayed in step # 75, and the process returns to step # 3. Next, the equation (23) is proved. If m = m + n-1 is set in the expression (16), the expression (23) is obtained as follows. Similarly, equation (24) is obtained by calculating m = m−n in equation (18).
It can be obtained by expanding similarly in the case of +1.

In the above-described embodiment, the slice position is sequentially changed in the front-rear direction by the switch. However, a desired slice position may be designated in conjunction with a pointer such as a mouse or a track ball. In this case, instead of step # 6 of each embodiment, a signal from a pointer such as a mouse is read, and if there is no change in the signal, the process returns to step # 3. If there is a change, the amount of change is multiplied by a constant coefficient. To determine the number n of slices to be changed. When n> 0, the process in the front direction is executed, and when n <0, the process in the rear direction is executed.

In the above description, the slice position is changed in conjunction with a pointer such as a mouse or a track ball. A fourth embodiment in which this is further modified will be described. Usually, the display device of the X-ray CT apparatus displays a scanogram (a projection image obtained by moving only the bed without rotating the X-ray tube) to determine a slice position, and displays a vertical line ROI on the scanogram. A cursor is superimposed and displayed, and the display position of the ROI cursor is moved with a mouse or the like. In the fourth embodiment, the reconstructed slice is adjusted to the position of the vertical line ROI on the scanogram. FIG. 6 is a flowchart thereof.

At step # 81, a scanogram is photographed, and at step # 82, a helical scan is performed to collect projection data. In step # 83, a scanogram is displayed on a part of the image display device. In step # 84, the vertical line ROI cursor is superimposed and displayed on the scanogram.
In step # 85, the position RP of the vertical line ROI cursor is stored, and in step # 86, the image number m corresponding to the cursor position RP is obtained.

At step # 87, the current image (m-th image) is reconstructed and displayed according to equation (9). Step # 88
In step # 89, the current B image (mB image) is reconstructed by equation (10), and in step # 89, the current F image (mF image) is reconstructed by equation (11). In step # 90, the display position of the vertical line ROI cursor on the scanogram is moved using a mouse or the like.

In step # 91, the cursor position RP is updated, the amount of change is obtained, and the number of changes n is obtained by multiplying the amount of change by a constant coefficient. In step # 92, it is determined whether n> 0. If n> 0, step # 11 or step # 40 is executed in the same manner as in the case where the forward direction is instructed in step # 6 of the above-described embodiment. If n> 0, step # 11 of the above-described embodiment is performed. Step # 21 or # 60 is executed in the same manner as when the backward direction is designated in Step 6. Note that the helical scan may be used as a prescan for positioning, and the function of this embodiment may be used for a scan plan on a scanogram. Although the reconstructed image is not stored in the above-described embodiment, the designated image may be stored in the image storage device as needed. Further, the image at the slice position of the designated image may be reconstructed, displayed, and stored by another method as necessary, for example, by using reflection data.

Next, a fifth embodiment will be described. In the fifth embodiment, for a plurality of axial images, only a part of one image is reconstructed, and an MPR (coronal, sagittal, oblique) image is created from this image and displayed. Things. In the conventional MPR display, Ax
Since the interval between the ial images is larger than the pixels of the Axial image, there is a problem that a step is seen on the vertical axis (horizontal axis) of the MPR image. However, in this embodiment, since the interval between the axial images can be shortened, an MPR image having no step can be reconstructed. FIG. 7 is a flowchart of the fifth embodiment when reconstructing a coronal image. The same applies to sagittal and oblique images.

In step # 101, an appropriate Axial image is displayed according to any of the above-described embodiments. Step # 10
Axial reconstructed to create coronal image in 2
Specify the slice interval n of the image. In step # 103
Axial A horizontal line ROI cursor is displayed at the position of the coronal image to be reconstructed on the display image. M in step # 104
= 1. In the following operation, the back projection and the storage of the image are executed only for the pixels of one line on which the horizontal line ROI cursor is displayed.

At step # 105, the image (first image) is reconstructed by equation (9) and stored. Step # 106
Reconstructs the B image (first B image) by equation (10),
In step # 107, the F image (first F image) is reconstructed by equation (11). In step # 108, it is determined whether or not (m + n) is equal to or larger than the maximum slice number of the image.

If (m + n) is not equal to or larger than the maximum slice number taken, the (m + n) th is performed in steps # 40 to # 55 of the third embodiment shown in FIG.
n) Reconstruct and store the image and return to step # 108. However, also in this case, reconstruction is performed only for the pixels of one line of the horizontal line ROI cursor. If it is determined in step # 108 that m + n is equal to or larger than the maximum slice number, the coronal image is reconstructed from the images stored so far by a known method in step # 110.
indicate.

In the fifth embodiment, one MPR image is created. An Axial image (including a line ROI cursor) and an MPR image are displayed at the same time, and the line ROI cursor is moved with a mouse. The MPR image may be reconstructed and displayed according to the position of the ROI cursor.
In this way, the MPR image can be continuously observed.

Further, as a modification of the fifth embodiment, a plurality of Axial images may be reconstructed, and a three-dimensional display image (surface display or the like) may be created from these images and displayed. In the conventional three-dimensional display image, the interval between the Axial images is larger than the pixels of the Axial image, so that there is a problem that a step appears in the slice thickness direction. According to this modification, the interval between Axial images can be shortened, so that a three-dimensional display image without steps can be reconstructed. Therefore, in the flowchart shown in FIG. 7, a three-dimensional display image may be created and displayed by a known method from the stored image instead of reconstructing the coronal image in step # 110.

In the above-described embodiment, the slice and the projected image are located at the same position. However, even when the position is shifted, the present invention can be implemented by modifying the mathematical formula.

Further, even when the interval between the slice positions is not an integral multiple of the interval between the projected images, the present invention can be implemented by similarly modifying the mathematical expressions. for that reason,
In the fifth embodiment and its modification, the interval between the slice positions can be adjusted to the pixel pitch of the Axial 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.

[0063]

As described above, according to the present invention,
An image n slices apart can be reconstructed by only 2n convolutions and back projections and image addition and subtraction. Since the execution time of image addition and subtraction is short, the reconstruction time is the time required for approximately 2n convolutions and back projections. On the other hand, the time required for normal reconstruction
This is the time required for convolution and back projection for several 360 ° projections. Usually, 2n is 3
Since the number of projections is far less than 60 degrees, the present invention can reconstruct an image n slices away in a short time. Therefore, continuous images captured by helical scanning can be observed almost in real time. Also, since only necessary images can be stored, the image storage capacity is small. Further, it is possible to reconstruct an MPR image having a small level difference.

[Brief description of the drawings]

FIG. 1 is a block diagram showing the configuration of a first 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 first embodiment.

FIG. 3 is a flowchart showing the operation of the first embodiment.

FIG. 4 is a flowchart showing the operation of the second embodiment.

FIG. 5 is a flowchart showing the operation of the third embodiment.

FIG. 6 is a flowchart showing the operation of the fourth embodiment.

FIG. 7 is a flowchart showing the operation of the fifth embodiment.

[Explanation of symbols]

10 X-ray tube, 14 X-ray detector, 18 Reconstruction circuit,
20: display unit, 22: input unit.

Claims (3)

(57) [Claims] 1. An X-ray computed tomography apparatus for performing a helical scan, wherein 360 degrees front projection data and 360 degrees rear projection data are re-interpolated by interpolation using a predetermined first slice position as a center. Means for determining a tomographic image at the slice position, and 36
By reconstructing the 0-degree projection data without using interpolation processing,
A means for obtaining a post-image; a means for reconstructing projection data at 360 degrees before the first slice position without using interpolation processing to obtain a first pre-image; and a means for calculating 360 degrees from the projection data at the first slice position. The data obtained by subtracting the post-projection data is reconstructed, the reconstructed result is added to the first post-image, and the second
Reconstructing data obtained by subtracting the projection data at the first slice position from the projection data 360 degrees before the projection at the first slice position, and adding the reconstruction result to the first previous image; Means for obtaining a second previous image;
Means for multiplying the difference between the second previous image and the second rear image by a coefficient and adding the result to the tomographic image to obtain a tomographic image at a second slice position 360 degrees before the first slice position. X-ray computed tomography apparatus. 2. An X-ray computed tomography apparatus for performing a helical scan, wherein projection data of 360 degrees before and 360 degrees after a center of a predetermined first slice position are reproduced by interpolation processing. Means for determining a tomographic image at the slice position, and 36
By reconstructing the 0-degree projection data without using interpolation processing,
Means for obtaining a post-image, means for reconstructing the 360-degree projection data before the first slice position without using interpolation processing to obtain a first pre-image, and a difference between the first pre-image and the first post-image. Means for multiplying by a coefficient and subtracting the result from the tomographic image to obtain a tomographic image at a second slice position 360 degrees after the first slice position. 3. Reconstructing data obtained by subtracting the projection data one projection before the projection at the first slice position from the projection data (one projection angle + 360 degrees) before the projection at the first slice position, and the reconstruction result 3. The X-ray computed tomographic imaging apparatus according to claim 2, further comprising: a unit for adding the first post-image to the first post-image to update the first post-image.