JP2748346B2 - X-ray CT system - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an X-ray CT apparatus for performing a spiral scan. 2. Description of the Related Art A conventional example of an X-ray CT apparatus performing a helical scan includes "application of digital image processing to medical equipment and problems" (a seminar "Digital signal of medical image" organized by the Giken Center). Papers on Processing Techniques and Their Problems in Clinical Applications, "October 26, 1981. Isao Horiba, II42.
-II page 44) (referred to as Conventional Example A), and
-1111738 (referred to as Conventional Example B). Conventional example A
Is a document showing the principle of a spiral scan CT apparatus.
The principle of a spiral scan having two features of rotating the source around the subject and moving the subject in the body axis direction with the rotation is disclosed. Further, Conventional Example A discloses reconstructing data collected by the spiral scan. Thus, the principle of the spiral scan X-ray CT apparatus has been described. A conventional example B discloses a spiral scan X-ray CT apparatus as in the conventional example A. Further, Conventional Example B discloses various methods for reconstructing data from data acquired by spiral scanning and for reducing artifacts. [0004] In each of the above conventional examples, the bed is moved only in one direction. As a result of the movement in only one direction, the projection data necessary for reconstruction must be large in view of the width in the body axis direction of the subject. Further, when the width in the body axis direction becomes large, there is a problem in the reliability of the reconstructed image itself. [0005] An object of the present invention is to provide an X-ray CT apparatus that enables reconstruction by moving in both directions instead of moving in only one direction. According to the present invention, an X-ray source that rotates around a subject at a first predetermined speed while generating X-rays,
An X-ray detector provided to face the X-ray source and detecting X-rays transmitted through the subject, and the subject can be continuously moved at a second predetermined speed on both the forward path and the return path with the subject mounted thereon A patient bed, a unit for performing a line scan after simultaneously rotating the X-ray source and moving the patient bed, a data collecting unit for collecting X-ray data transmitted through the subject, In the X-ray CT apparatus having the image reconstruction means to be constituted, the position of the X-ray source in the axial direction of the subject at the end of the forward scan, the X-ray emission end angle of the X-ray source, and the start of the return scan An X-ray CT apparatus characterized by comprising control means for controlling an X-ray source axial position of the X-ray source and an X-ray emission start angle of the X-ray source in a predetermined relationship at the time. FIG. 2 is an external view of an RR type CT apparatus.
The X-ray CT device includes an X-ray tube device (X-ray generator) 1,
It comprises an X-ray detector 2, a high-voltage generator for an X-ray tube (not shown), and a patient bed 3. The X-ray tube apparatus 1 and the X-ray detector 2 are in a positional relationship to face each other with the subject on the bed 3 interposed therebetween. The X-ray tube apparatus 1 and the X-ray detector 2 are continuously rotated under the opposed positional relationship. For continuous rotation, high voltage from the high voltage device to the X-ray tube device 1 was supplied via a slip ring. This rotation speed is a constant speed as can be seen from a sine wave locus of FIG. 5 described later. [0008] The X-ray tube device 1 and the X-ray detector 2 are integrally mounted on a frame. A high voltage is supplied by attaching a slip ring mechanism to the frame (scanner). The patient bed 3 can move at a constant speed in a direction (arrow) perpendicular to the rotation plane of the scanner. Patient bed 3
Movement and X-ray irradiation by the X-ray tube device 1 and the X-ray tube device 1
Are synchronized with each other. By moving and rotating the patient bed and the X-ray tube apparatus 1 at a constant speed determined respectively, there is an advantage that management of acquired data in a spiral scan becomes easy.
If the movement of the patient bed and the rotation of the X-ray tube apparatus 1 are not constant, it is not easy to set the coefficients a and b in the interpolation processing described later, and it is not easy to extract two data having the same projection angle. Now, the scanner rotates continuously and at a high speed on a fixed rotating surface. At this time, the patient bed 3 is inserted into the gantry opening 4 at a constant speed, and scanning is performed in a range including a desired tomographic plane. Prior to this scanning, scanning positioning is performed. In FIG. 3, the position of the first tomographic plane 6 as a reference for starting the imaging is set at a certain distance a from the scanner rotation plane A.
It is positioned only in front. The distance a has a margin until the moving speed of the patient bed becomes constant, and after the scanner and the patient bed start rotating and moving, the X-ray pulse is synchronized with the plane B (distance b) where the speed becomes steady. Start exposure of. The distance b in this case is a distance for which extra data must be measured after the start and end of the measurement because projection data is obtained using interpolation in the direction in which the patient bed moves. When the patient bed reaches the plane B 'which has passed the final tomographic plane 7 by the distance b, the X-ray irradiation is stopped, and the patient bed decelerates and stops at the plane A'. In this manner, by moving the patient bed during scanning, the patient is scanned spirally as shown in FIG. FIG. 5 shows the trajectory of the X-ray tube apparatus at this time by one-way scanning of the subject. As shown in FIG. 4, projection data obtained by helical scanning (hereinafter referred to as helical data) is obtained when a scanner is helically rotated around an object and scanned. It is equivalent to projection data. The spiral data SR is determined by the projection angle β and the position X of the subject in the body axis direction. Here, the position X ₀ at the start of scanning and the distance that the bed (and the subject) moves during one rotation of the scanner are D. To reconstruct a tomographic image of a slice plane (tomographic plane) S (X _n ) at the subject position X _n , projection data R (β, X _n ) (where β = 0 ゜ −36)
0 °) is required. Then, projection data R (β, X _n ) of the desired tomographic plane S (X _n ) is obtained from the spiral data SR by interpolation, and an image is reconstructed from the projection data. To obtain a tomographic image, the projection data R (β, X _n ) on the tomographic plane (where β = 0 ゜ 360
I) can be obtained. The spiral data is represented by the following general formula. SR = (β _i , X _j ) where the projection angle β _i is a value in the range of 0 ° ≦ β _i ≦ 360 °, and the position X _j satisfies X ₀ ≦ X _j ≦ X _e It is an arbitrary turn to do. X ₀ is the scanning start position, the X _e is a scan end position.
The projection angle β _i corresponds to the X-ray tube height on the vertical axis in FIG. Therefore, in one-way scanning, X _j
= R _n (X _i ) at the projection angle β _i on the slice plane S (X _n ) at the position X _{n at} the position X _n is R (β _i , X _n ) for one rotation before and after X _n (± D) (Ie, X _n −D
<X _n <X _n + D section) and the projection angle R (β _i , X _n ) from the spiral data of the same projection angle β _i
Is obtained by interpolation. This interpolation is a two-point linear interpolation, for example, the projection angle β shown in FIG.
_{In FIG. 1} , the front and rear positions of X _n that match the projection angle β ₁ are X _e and X _m , and the projection data at that time is SR (β ₁ ,
X _e), a _{SR (β 1, X m)} , and the distance b between the X _e and X _n b = X _n -X _e, the distance a between X _m and X _n is a = X _m -
Since it is X _n , the interpolation equation is [Equation 3]. R (β ₁ , X _n ) = ｛SR (β ₁ , X _e ) × a +
SR (β ₁ , X _m ) × b｝ / (a + b) If this processing is performed in all directions (all projection angles) of 360 ° of 0 ° ≦ β _i ≦ 360 ° with X _n fixed, the position X
_n projection data are obtained. FIG. 1 shows an embodiment of an X-ray CT apparatus for one-way scanning. The X-ray CT apparatus includes an X-ray generator 1, an X-ray detector 2, a data acquisition circuit 2A, a buffer memory 12,
It comprises an interpolation circuit 13, a filter correction circuit 14, a back projection operation circuit 15, and a CRT 16. X-ray generator 1 generates a fan-shaped X-ray beam. X-ray detector 2 is composed of a multi-channel detection element for detecting a transmission fan-shaped X-ray beam. Data collection circuit 2A: fetches the detection values of the multi-channel detector 2 and performs processing such as preamplification and AD conversion to obtain spiral projection data SR. Two-dimensional buffer: a buffer having an address i × j. The helical projection data SR is stored. That is, the buffer 12 is determined by the projection number i and the scanner rotation number (number of rotations) j. Scanner 1
Assuming that the number of projections in rotation is pp and the total number of rotations of the scanner (how many scans) is JC, the parameters of the spiral data SR are obtained as follows by the arguments i and j of the two-dimensional array. ## EQU4 ## Projection angle β _i = β ₀ + (i-1) × Δθ Position X _ij = X ₀ + (j-1) × D + (i-1) × ΔD where: ΔD = 360 ° / Pp ΔD = D / pp. Interpolation circuit 13: When the position X _n is designated,
Projection data R in the X _n and (beta _i, X _n) created by interpolation. That is, the helical projection data SR is converted into projection data R. Filter correction circuit 14 performs blur correction.
The filter function is determined by the details of the blur correction. Back-projection operation circuit 15 Back-projects the output of the filter correction circuit 14 after filtering. Thereby, a tomographic image is obtained. CRT16 ... Displays a tomographic image. The operation will be described. The X-ray generator 1 and the X-ray detector 2 are continuously rotating on a predetermined plane. In this state, the bed 3 on which the subject is placed advances at a constant speed. In the process of moving forward, the X-ray
The line is exposed. This irradiation is performed by helical scanning. The transmitted X-ray obtained by helical scanning is
The data is detected by the X-ray detector 2 and subjected to various preprocessing and AD conversion by the data acquisition circuit 2A. Thus, spiral data SR
Get. The spiral data SR is stored in the two-dimensional buffer 12 having the addresses of the arguments i and j. Similarly, spiral data SR is obtained over the entire measurement range of the subject and stored in the two-dimensional buffer 12. After the spiral data is filled in the two-dimensional buffer 12, the interpolation circuit 13 obtains projection data R of a desired tomographic plane from the spiral data SR. That is, to identify the desired tomographic plane by specifying the position X _n, projected at the position X _n data R
Create (i, X _n ). Specifically, as is clear from [Equation 4] and [Equation 5], when the sample position of the spiral data SR in the body axis direction of the subject is set on the horizontal axis, and the projection number i is set on the vertical axis, 6 is obtained. Therefore, the projection data R (i,
X _n ) can be obtained by the following equation. R (i, X _n ) = ｛SR (i, J1) × a + S
R (i, J1 + 1) × b｝ / (a + b) Next, the obtained projection data R (i, X _n )
Undergoes blur correction processing in the filter correction circuit 14. The projection data after the blur correction processing is back-projected by the back-projection operation circuit 15 to obtain a tomographic image at the position _Xn . CRT1
6 displays a tomographic image. According to the present invention, a reciprocating bidirectional scan is performed instead of the above one-way scan, and a reconstructed image is obtained from the reciprocal scan by interpolation. An embodiment of the present invention will be described. The patient bed is moved not only in one direction but also in the opposite direction, and is scanned from plane B to plane B 'in FIG. At this time, if the scanning is performed so that the trajectory 9 of the forward movement (forward direction) and the trajectory 8 of the backward movement (reverse direction) cross each other,
Scanning is performed as shown in (b). A case where only one tomographic image is obtained will be described with reference to FIGS. FIG. 7A is a time chart showing the relationship among the scanner rotational speed 16, the patient bed moving speed 17, and the X-ray pulse 18. From FIG. 7 (b), a scan 9 of 180 ° (or more) necessary for obtaining one tomographic image is performed in the forward direction, and the movement of the patient bed and the X-ray scan are performed until the scanner is further rotated by 180 °. After stopping the irradiation and shifting the phase by 180 ° (in this way, the trajectory in the forward direction and the trajectory in the reverse direction intersect), a scan 8 of 180 ° (or more) is performed in the reverse direction. The trajectory 9 of the X-ray tube apparatus with respect to the subject is as shown in FIG. However, the broken line indicates the movement of the patient bed.
It indicates that the scanner is rotating only after stopping the exposure. When scanning is performed in this manner, the projection data is obtained by interpolating projection data when the patient bed is moving in the forward direction and projection data when the patient bed is moving in the reverse direction. In the one-way scan, the error caused by the interpolation is the same on any tomographic plane. However, in the two-way scan, the plane including the intersection and perpendicular to the bed moving direction has the least error due to the interpolation. Therefore, when the scanning shown in FIG. 7B is performed, the tomographic plane 19 is obtained. In FIG. 8, (a) is from above, (b)
Is the projection of the trajectory from the side. The tomographic plane 19 in FIG. 7B corresponds to the plane S in FIG. To obtain a tomographic image of the surface S, projection data on the surface S may be obtained. Therefore,
Consider projection data P1 and P2 having the same projection angle β. P
Reference numeral 1 denotes forward data, and P2 denotes projection data at the time of backward movement.
The projection data P on the surface S is obtained from P1 and P2.
By using linear interpolation, the projection data P (i, j) can be expressed as follows: P (i, j) = (P1 (i, j) + P2 (i, j)) / 2 i = 1,2,. CN: total number of channels j = 1, 2,... NP NP: total number of views When this processing is performed for 0 ≦ β ≦ 180 °, projection data of the first half of the half scan is obtained. 1 obtained
If one tomographic image is obtained from the projection data for 80 °, and this data is subjected to blur correction and back-projection, a desired tomographic image can be obtained. FIG. 9 shows the bidirectional scan of FIG.
The corresponding diagram is shown. The horizontal axis indicates the sample position of the object in the body axis direction of the spiral data, and the vertical axis indicates the projection number. Compare with FIG. To slice position X _n in FIG. 6, to obtain a reconstructed image of two loci L ₁ and L ₂ Metropolitan on both sides. 9, to obtain a reconstructed image from the locus L ₃ on both sides L ₄ Prefecture. L
₁ and the L ₂ are completely separated on either side of the position X _n, greater its body axis direction width. That is, in order to obtain a reconstructed image, it is necessary to obtain the reconstructed image by interpolation from a position in the body axis direction having a large width. On the other hand, in FIG. 9, the locus L ₃ and L4 cross with X _n, is small body axis direction of the width of the L ₃ and L _4. That is, reconstruction can be performed with data at a position closer to the slice position _Xn . As a result, the reliability of the reconstructed image increases. In the above embodiment, one tomographic image is obtained from the obtained 180 ° projection data. In another embodiment, a method of obtaining 360 ° projection data and obtaining a tomographic image will be described. In FIG. 10, 360 ° scanning is required in both forward and reverse directions to obtain 360 ° projection data.
Assuming that the range of the second embodiment is 0 ° to 180 °, the present embodiment requires scanning in the range of −90 ° to 280 °. The first half scan is obtained in the same manner as the first embodiment, and the second half scan is obtained by obtaining Q ( _n ) similarly from Q1 ( _n ) and Q2 ( _n ) shown in FIG. One tomographic image can be reconstructed from all projection data. However, in the second embodiment, it is considered that when obtaining the second half scan, the projection data used for the interpolation is far away from the distance, and the error due to the interpolation becomes larger than when obtaining the first half scan. There must be. However, the error is small compared to the one-way scan. FIG. 11 shows a control system diagram of the present invention. The X-ray control unit 101 controls the high voltage generator 110 to generate a high voltage. This is so-called X-ray irradiation control. The rotating frame control unit 102 is an X-ray tube device (X-ray generator)
In addition to control for continuously rotating the X-ray detector and the X-ray detector in opposition, control taking into account the projection angle is also possible, and the projection angle can be arbitrarily controlled. The bed movement control unit 103 controls the bed movement direction and speed. However, the speed is constant during scanning. The synchronization device 100 in the system control unit
Using the position information from the bed position detector 113 and the projection angle information from the projection angle detector 114, the X-ray controller 10
1, rotating frame controller 102, bed movement controller 10
Synchronize 3. [0032] More specifically, in each embodiment, so that the previously designated slice position is [Equation 8] _{X (n) = X s +3} (D / 4) + (D / 2) n, scan to determine the starting position X _s. However,
D is determined by the bed moving speed and the rotating speed of the rotating frame. In addition, the position X _e at the end of the forward scan and the projection angle θ _e are stored, and the start position of the backward scan is X _e and the start projection angle is θ _e + 180 °.
Control the system. In a third-generation (RR type) CT apparatus, an algorithm for reconstructing a tomographic image from projection data includes a direct method for back-projecting the detected fan beam data as it is and a parallel method for projecting the fan beam data to a parallel beam. Arrangement method of back projection after converting to data is known, but the present invention can be applied to various applications such as rotary scan and electronic scanning type using cone beam, regardless of those algorithms and generations. It is effective. Further, as an interpolation method, in addition to linear interpolation, 2
Next-order, third-order, and other higher-order interpolation (for several slices) is also possible. According to the present invention, spiral scanning can be performed in both reciprocating directions, and high-speed continuous scanning can be performed. Furthermore, for data acquired by spiral scanning in both directions, accurate projection data can be obtained by interpolation processing using relatively close continuous data before and after the data at an arbitrary slice position. The reliability of the reconstructed image can be improved. Furthermore, the slice position
It can be used anywhere, and a tomographic image can be freely generated at any position.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an embodiment of a CT apparatus according to the present invention. FIG. 2 is an external view of an RR type CT apparatus. FIG. 3 is an explanatory diagram of one-way scanning. FIG. 4 is an explanatory diagram of one-way scanning and two-way scanning. FIG. 5 is a diagram illustrating a relationship between a rotational position and spiral data. FIG. 6 is a relationship diagram between a position and a projection number. FIG. 7 is a diagram showing a time chart and a trajectory according to the embodiment of the present invention. FIG. 8 is a diagram showing a trajectory in the embodiment of the present invention. FIG. 9 is a diagram illustrating a relationship between a position and a projection number in bidirectional scanning according to the present invention. FIG. 10 is an explanatory diagram of another embodiment of the present invention. FIG. 11 is a system configuration diagram of the present invention. [Description of Signs] 1 X-ray tube device 2 X-ray detector

──────────────────────────────────────────────────続 き Continued on front page (58) Field surveyed (Int.Cl. ⁶ , DB name) A61B ^6/ 03-6/04

Claims (1)

(57) [Claims] An X-ray source that rotates around the subject at a first predetermined speed while generating X-rays, and an X-ray detector that is provided opposite to the X-ray source and detects X-rays transmitted through the subject A patient bed capable of continuously moving at a second predetermined speed on both the forward path and the return path with a subject placed thereon, and a means for performing line scanning after simultaneously rotating the X-ray source and moving the patient bed An X-ray CT apparatus having data collection means for collecting X-ray data transmitted through the subject, and image reconstruction means for reconstructing an image from the data, wherein an X-ray source at the end of the forward scan is provided. The predetermined relationship between the subject axial position and the X-ray emission end angle of the X-ray source, the subject axial position of the X-ray source at the start of the return scan, and the X-ray emission start angle X-ray CT having control means for controlling
apparatus. 2. The X-ray CT apparatus according to claim 1, wherein the forward scan and the backward scan are distinguished by the same rotation direction of the X-ray source and the different moving direction of the patient bed. 3. In the forward scan and the backward scan, the body axial position of the X-ray source with respect to the subject at the end of the forward scan and the body axial direction position of the subject with the X-ray source at the start of the backward scan are set to be the same. 2. The X-ray CT according to claim 1, wherein
apparatus. 4. In the forward scan and the backward scan, the X-ray emission end angle of the X-ray source at the end of the forward scan and the X-ray emission start angle of the X-ray source at the start of the backward scan are set differently. The X-ray CT apparatus according to claim 1, wherein