JP2003339693A - X-ray ct reconstruction method, and x-ray ct system using the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To obtain a reconstructed image wherein adverse affect owing to a cone angle is reduced, and artifacts are suppressed by processing weighting in accordance with the cone angle. <P>SOLUTION: An angle weighting processing is performed with respect to projection data which is obtained by a plurality of X-ray beams P-R1, Q-R2' passing the same slice position on a rotary center axis C, and, then, correction projection data is provided by obtaining virtual X-ray beams A, B vertically made incident on X-ray multi-string detectors. A position weighting processing is performed with respect to the correction projection data, and, then, the tomographic image of the target slice S is reconstructed. Correction projection data is obtained by the virtual X-ray beams A, B vertically made incident on the X-ray multi-string detectors, so that the influence of the cone angle is reduced so as to reduce the adverse affect by the cone angle of the X-ray beams. <P>COPYRIGHT: (C)2004,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an X-ray CT reconstruction method equipped with an X-ray multi-row detector and an X-ray CT using the same.
The present invention relates to an apparatus, and more particularly to a technique for reconstructing a tomographic image based on projection data collected by helical scanning.

[0002]

2. Description of the Related Art Conventionally, this type of X-ray CT reconstruction method has been
`` Algorithm for image reconstruction in multi-slic
e helical CT (Katsuyuki Taguchi: Med. Phys. 25 (4),
Apr 1998) '' and `` Algorithm for image reconstructio
n in multi-slice helical CT (Hui Hu: Med. Phys. 26
(1), Jan 1999) ”and the like.

Although these differ in detail, the tomographic image of the target slice is reconstructed as follows.

That is, first, with respect to projection data obtained by all X-ray beams included in a predetermined slice range including the target slice, weights corresponding to the positions of each X-ray beam on the target slice are assigned. By multiplying them and obtaining the sum of them, one virtual column X for the target slice
Create projection data for one rotation on the line detector. Then, the tomographic image of the target slice is reconstructed by the usual X-ray CT reconstruction method.

[0005]

However, the conventional method having such a configuration has the following problems. That is, the weight to be multiplied to the projection data obtained by each X-ray beam is determined according to the position of the target slice on the virtual detector including the rotation center axis. However, in an X-ray multi-row detector, a cone-beam type X-ray detector is used.
Since the X-ray irradiates, there is a cone angle in the slice thickness direction in the X-ray multi-row detector in the relative movement direction of the imaging system and the subject, so the sum of the results multiplied by the weight, that is, 1 of the virtual detector. The rotation data is also affected by the cone angle. Furthermore, since the cone angle in one rotation data changes periodically, there is a problem that streak-like artifacts occur in the reconstructed image due to the change.

The present invention uses an X-ray CT reconstruction method and a reconstructed image capable of obtaining a reconstructed image in which the adverse effect of the cone angle is reduced and artifacts are suppressed by performing weighting processing according to the cone angle. An object is to provide an X-ray CT apparatus.

[0007]

The present invention has the following constitution in order to achieve such an object. That is, in the X-ray CT reconstruction method according to claim 1, the object to be examined is rotated while rotating an imaging system including an X-ray source that irradiates a cone-beam-shaped X-ray beam and an n-row X-ray multi-row detector. When processing the projection data acquired by performing a helical scan with a continuous rotation by moving the projection data within a predetermined slice range according to the position with respect to the target slice. In an X-ray CT reconstruction method for reconstructing a tomographic image of a target slice, X-ray CT reconstruction methods for respective projection data obtained by a plurality of X-ray beams passing near the same slice position on the rotation center axis are used.
Performs angle weighting processing according to the cone angle of the line beam,
It is characterized in that corrected projection data obtained by a virtual X-ray beam vertically incident on the X-ray multi-row detector is obtained, and the position weighting processing is performed by using this corrected projection data as projection data. .

(Operation / Effect) By performing angle weighting processing on projection data obtained by a plurality of X-ray beams passing near the same slice position on the rotation center axis,
The cone angle of the X-ray beam is compensated, and the corrected projection data obtained by the virtual X-ray beam that is vertically incident on the X-ray multi-row detector is obtained. Then, after the corrected projection data is treated as projection data and subjected to position weighting processing, a tomographic image of the target slice is reconstructed. Corrected projection data is X
The effect of the cone angle is small because it is obtained by the virtual X-ray beam vertically incident on the line multi-row detector. Therefore, a reconstructed image with suppressed artifacts can be obtained.

The X-ray beam passes not only at the same slice position and the same position on the rotation center axis, that is, at the same slice position on the rotation center axis, but also at the same slice position. In addition, those passing near the same slice position are also included so as to intersect at the upper side and the lower side of the rotation center axis.

Further, when the relative movement amount with respect to the object during one rotation of the image pickup system, that is, the pitch is m, which is an integer rounded down to the decimal point of (n + 1) / 2, virtual detection including the rotation center axis is performed. When the detector row width on the detector is multiplied by m (claim 2) or n-1 is m,
It is preferable to multiply the width of one detector row on the virtual detector including the rotation center axis by m times (claim 3).

Further, when the relative movement amount with respect to the object during one rotation of the imaging system, that is, the pitch is n-1 where m is one detector row width on the virtual detector including the rotation center axis. However, it is preferable that the value is greater than 0 and equal to or less than n (claim 4).

An X-ray CT apparatus according to a fifth aspect is
An imaging system equipped with an X-ray source that radiates a cone-beam-shaped X-ray beam and an n-row X-ray multi-row detector is rotated and moved relative to the subject, and helical rotation is performed by continuous rotation. When the projection data collected and processed is processed, position-weighting processing according to the position with respect to the target slice is performed on the projection data within a predetermined slice range, and then a data processing unit for reconstructing a tomographic image of the target slice is provided. In the line CT apparatus, the data processing unit weights the projection data obtained by a plurality of X-ray beams passing near the same slice position on the rotation center axis, according to the cone angle of each X-ray beam. The corrected projection data obtained by the virtual X-ray beam perpendicularly incident on the X-ray multi-row detector is calculated, and the corrected projection data is obtained. It is characterized in that performing the location weighting processing as.

(Operation / Effect) The data processing means performs an angle weighting process on projection data obtained by a plurality of X-ray beams passing through the same slice position on the rotation center axis, and calculates a cone angle of the X-ray beam. To compensate. Then, the corrected projection data obtained by the virtual X-ray beam vertically incident on the X-ray multi-row detector is obtained, and the corrected projection data is treated as the projection data and subjected to the position weighting processing, and then the target slice of Reconstruct a tomographic image. Therefore, it is possible to obtain a reconstructed image in which the adverse effect of the cone angle of the X-ray beam is reduced and artifacts are suppressed.

[0014]

BEST MODE FOR CARRYING OUT THE INVENTION An embodiment of the present invention will be described below with reference to the drawings. 1 to 3 relate to an embodiment of the present invention, FIG. 1 is a block diagram showing a schematic configuration of an X-ray CT apparatus using an X-ray CT reconstruction method according to the embodiment, and FIG. 3A is a side view of the X-ray CT apparatus, FIG. 3A schematically shows the X-ray beam when the imaging system makes one rotation, and FIGS. 3B and 3C are for explaining the angle weighting process. It is a figure.

The top plate 1, on which the subject M is placed, is made of a material that transmits X-rays. This top plate 1
Is attached to the drive table 3, and is driven back and forth in the body axis direction of the subject M, and is moved up and down in a direction orthogonal to the body axis of the subject M.

As shown in FIG. 2, a gantry 5 is arranged on the right side of the drive table 3. The gantry 5 is provided with an opening 7 having an inner diameter large enough to allow the top plate 1 together with the subject M to move back and forth near the center thereof. An X-ray tube 9 for radiating an X-ray beam diverging in an arc shape around and around the body axis of the subject M is provided on the inner peripheral portion of the opening 7.
(X-ray source) is deployed.

At the opposite positions of the X-ray tube 9 with the top plate 1 sandwiched between them,
An X-ray in which a plurality of detectors 11 arranged in an arc shape around the body axis of the subject M and having sensitivity to X-rays are linearly stacked in a row in the body axis direction (slice direction) of the subject M. A detection array 13 is provided. The imaging system 14 including the X-ray tube 9 and the X-ray detection array 13 is rotated about the rotation center axis C along the body axis of the subject M by the rotating unit 15 while maintaining the facing arrangement state. .

In this embodiment, an X-ray detection array 13 in which four rows of detectors 11 are laminated will be described as an example.
The X-ray detection array 13 corresponds to the X-ray multi-row detector in this invention. Further, in the following description, the four X-ray detectors 11 will be appropriately referred to by the reference numerals 11a, 1 from the top plate 1 side.
1b, 11c and 11d.

The top plate 1 and the drive base 3 are moved forward and backward and moved up and down by the drive unit 17, and the X-ray tube 9 is moved to the high voltage control unit 19.
The X-ray intensity and the like are controlled by. Control of the rotation unit 15, the drive unit 17, and the high-voltage control unit 19 is performed via the operation control unit 21 operated by the operator. The X-rays emitted from the X-ray tube 9 and transmitted through the subject M are detected by the X-ray detection array 13 and then collected by the data collection unit 23. The drive unit 17 moves the top plate 1 in conjunction with the continuous rotation of the imaging system 14 by the rotating unit 15,
So-called "helical scan" is performed.

The data memory 25 uses the signals collected by the data collecting unit 23 as projection data associated with different irradiation angles of the image pickup system 14 and relative irradiation positions of the image pickup system 14 and the tabletop 1. Store.

Once the projection data has been collected and stored,
A data processing section 27 corresponding to the data processing means of the present invention.
However, first, for the projection data obtained by a plurality of X-ray beams passing near the same slice position on the rotation center axis C, "angle weighting processing" is performed according to the cone angle of each X-ray beam, The corrected projection data obtained by the virtual X-ray beam that is vertically incident on the detector 11 is obtained.
Next, this corrected projection data is treated as projection data, and "position weighting processing" according to the position within a predetermined slice range including the target slice to be reconstructed is applied to the projection data, and then based on them. Then, a process of reconstructing a tomographic image of the target slice is performed. The tomographic image thus reconstructed is displayed on a monitor 29 such as a CRT or a liquid crystal display device.
Is displayed in.

Next, X described above with reference to FIGS.
The operation of the line CT device will be described. In addition, in FIG.
6A and 6B are schematic diagrams of a line beam, FIG. 9A is an X-ray beam when the image pickup system makes one rotation, and FIGS. 9B and 9C are diagrams for explaining the angle weighting process. In addition, Fig. 4 shows X-ray CT
It is a flowchart which shows the process of reconstruction.

First, the operator operates the operation control section 21 to set the X-ray having a predetermined intensity to be emitted from the X-ray tube 9, and at the same time, the imaging system 14 and the object to be examined during one rotation of the imaging system 14. The amount of movement relative to M (top 1), that is, the pitch is set. For example, when the number of columns of the detectors 11 in the X-ray detection array 13 is n and the integer m is obtained by rounding down the decimal point of (n + 1) / 2, here, the pitch is a virtual detector including the rotation center axis C. It is m times the width of one detector row above. Specifically, the number of columns n =
Since it is 4, the pitch m = 2. This makes it possible for the X-ray beam to pass through the vicinity of the same slice position on the rotation center axis C and become redundant. Imaging is started after the above setting. Here, as shown in the schematic view of FIG. 3A, it is assumed that the X-ray tube 9 and the X-ray detection array 13 of the imaging system 14 make one rotation.

The X-ray beam at the initial position is shown by a solid line in the figure, and the symbols P-R0 and P-R are shown.
1, P-R2, P-R3. Each X-ray beam P
-R0, P-R1, P-R2, and P-R3 do not enter perpendicularly to the rotation center axis C but are inclined.
In other words, the X-ray beam has four X-ray detectors 11a,
Each has an angle (cone angle) with respect to 11b, 11c, and 11d. The X-ray beam in the case where the imaging system 14 makes one rotation and the X-ray tube 9 moves from the position P to the position Q while rotating is shown by the line segment shown by the dotted line in the drawing, and the symbol Q- R0 ', Q-R1', Q-R2
It is shown by ', Q-R3'.

Step S1 The projection data obtained by a plurality of X-ray beams passing near the same slice position on the rotation center axis C is subjected to "angle weighting processing" according to the cone angle of each X-ray beam. .

In this case, the X-ray beams P-R2 and Q-
Since R0 'passes through the same slice position, the projection data obtained from them are processed. Specifically, first, a weight determined by the ratio of the respective cone angles is multiplied to create a virtual X-ray beam having a cone angle of 0 degree.

For example, the ratio of the cone angles of the X-ray beam P-R2 and the X-ray beam Q-R0 'is atan (D /, where D is the detector row interval on the rotation center axis C).
2): Since it is atan (3D / 2), the processing coefficient is W
Let W = atan (D / 2) / (atan (3
D / 2) + atan (D / 2)). This represents the ratio of the X-ray beam P-R2 to the total cone angle of the X-ray beam.

Then, the projection data obtained by the X-ray beam P-R2 is multiplied by 1-W, and the X-ray beam Q-
By multiplying the projection data obtained by R0 ′ by W and adding these results, a virtual X-ray beam having a cone angle of 0 degrees as shown by the chain double-dashed line in FIG. The corrected projection data obtained by A is generated.

Similarly, the ratio of the cone angles of the X-ray beam P-R3 and the X-ray beam Q-R1 'is atan (3D /, where D is the detector row interval on the rotation center axis C).
2): atan (D / 2) (see FIG. 3 (b)), where W is atan (3D / 2) / (atan (3D / 2) +
atan (D / 2)). This represents the ratio of the X-ray beam P-R3 to the total cone angle of both.

Then, as described above, the X-ray beam PR
Multiply the projection data obtained by 3 by 1-W,
By multiplying the projection data obtained by the X-ray beam Q-R1 ′ by W and adding these results,
Corrected projection data obtained by the virtual X-ray beam B having a cone angle of 0 degree is generated as shown by the chain double-dashed line in FIG.

As described above, the corrected projection data is obtained by subjecting the projection data obtained by the X-ray beam passing through the same slice position to the angle weighting processing by multiplying the processing coefficient W according to the cone angle.

Step S2 For example, assuming that the target slice S is located between the virtual X-ray beams A and B obtained as described above, as shown in FIG. A position weighting process is performed on the target slice S based on the corrected projection data.

Specifically, the virtual X-ray beams A and B are applied to the corrected projection data of the virtual X-ray beams A and B, respectively.
Weighting is performed according to the position of the target slice S with respect to. Thereby, the projection data of the target slice S can be obtained based on the corrected projection data obtained by the X-ray beams A and B.

The position weighting process is not performed only on the corrected projection data of each of the virtual X-ray beams A and B. For example, the X-ray beam P-R having a cone angle is used.
The projection data of 1, Q-R2 'may be used depending on the position. Even when a part of the projection data of the X-ray beam having the cone angle is used as described above, the correction projection data of the vertically incident virtual X-ray beams A and B is used, so that the adverse effect of the cone angle is more exerted than in the conventional case. Can be suppressed.

Step S3 The corrected projection data and the projection data obtained by the above-mentioned processing for the target slice S are added.

Step S4 If the above "angle weighting process" and "position weighting process" are not completed for the projection data within the predetermined slice range, the process returns to step S1. If completed, the process proceeds to the next step S5. .

Step S5 If the above-mentioned steps S1 to S4 have not been completed for all the X-ray beams, that is, all the projection data obtained by one rotation of the X-ray beam, the process returns to step S1 and is completed. If so, the process proceeds to step S6.

Step S6 The corrected projection data and projection data for one rotation are treated as projection data in the normal CT reconstruction processing by the data processing unit 29 to reconstruct a tomographic image of the target slice.

Step S7 The tomographic image reconstructed in step S6 is displayed on the monitor 29.

The "angle weighting process" is applied to the projection data obtained by a plurality of X-ray beams passing through the same slice position on the rotation center axis C to compensate the cone angle of the X-ray beam, and the X-ray detector Corrected projection data obtained by the virtual X-ray beam incident perpendicularly on the array 13 is obtained. Then, after performing the “position weighting process” on the corrected projection data, the tomographic image of the target slice is reconstructed. Since the corrected projection data is obtained by the virtual X-ray beam that is vertically incident on the X-ray detector array 13, the influence of the cone angle is small, and the adverse effect of the cone angle of the X-ray beam can be reduced. Therefore, a reconstructed image in which streak artifacts are suppressed can be obtained.

Note that FIG. 5 shows an example in which the number of columns of the X-ray detection array 13 is n and (n-1) is the pitch. That is, this is the case where the pitch m = 3. Here, FIG. 5A schematically shows the X-ray beam when the image pickup system is rotated once, and FIG. 5B is a diagram for explaining the angle weighting process.

In this case, a plurality of X-ray beams passing through the same slice position on the rotation center axis C do not exist for all X-ray beams, but the X-ray beams P-R3 and Q-R0 'are present. It passes through the same slice position. Therefore, the corrected projection data obtained by the virtual X-ray beam A is generated from these X-ray beams as shown in FIG. And the virtual X by the previous or next one revolution
A tomographic image of the target slice is reconstructed based on the corrected projection data obtained by the line beam. Alternatively, the tomographic image of the target slice is reconstructed based on the corrected projection data of the virtual X-ray beam and the projection data obtained by the X-ray beam having the cone angle.

FIG. 6 shows an example in which the number of columns of the X-ray detection array 13 is n and the pitch m is "0 <m≤n". That is, this is an example of the case where the pitch m = 3.5. By controlling the pitch in the above range,
A plurality of X-ray beams can pass through the same slice position to make the irradiation of the X-ray beams redundant. Here, FIG. 6A schematically shows the X-ray beam when the image pickup system is rotated once, and FIG. 6B is a diagram for explaining the angle weighting process.

In this case, there are no plural X-ray beams passing through the same slice position on the rotation center axis C, but there are X-ray beams passing near the same slice position. Therefore, in this case, the X-ray beams P-R3 and Q-R0 are
Since ′ passes near the same slice position, corrected projection data obtained by the virtual X-ray beam A is generated from these X-ray beams as shown in FIG. 6B.

As described above, even when the X-ray beam does not pass through the same slice position on the rotation center axis C, the angle weighting processing according to the cone angle can be performed in the same manner as the above-mentioned processing. Therefore, X passing near the same slice position
The tomographic image of the target slice can be reconstructed based on the corrected projection data obtained for the line beam.

The present invention is not limited to the above-mentioned embodiments, but the present invention can be applied if the X-ray detection array 13 has two or more detector rows.

[0047]

As is apparent from the above description, according to the present invention, angle weighting processing is performed on projection data obtained by a plurality of X-ray beams passing through the same slice position on the rotation center axis. The cone angle of the X-ray beam is compensated, and the corrected projection data obtained by the virtual X-ray beam that is vertically incident on the X-ray multi-row detector is obtained. And
After performing the position weighting process on the corrected projection data, the tomographic image of the target slice is reconstructed. Since the corrected projection data is obtained by the virtual X-ray beam that is vertically incident on the X-ray multi-row detector, the influence of the cone angle is small, and the adverse effect of the cone angle of the X-ray beam is reduced to reduce artifacts. It is possible to obtain a reconstructed image in which

[Brief description of drawings]

FIG. 1 is a block diagram showing a schematic configuration of an X-ray CT apparatus using an X-ray CT reconstruction method according to an embodiment.

FIG. 2 is a side view of the X-ray CT apparatus in FIG.

FIG. 3A is a schematic view of an X-ray beam when the imaging system makes one rotation, and FIGS. 3B and 3C are diagrams for explaining an angle weighting process.

FIG. 4 is a flowchart showing an X-ray CT reconstruction process.

5A is a diagram schematically showing an X-ray beam when the image pickup system is rotated once at a pitch different from that in FIG. 3, and FIG. 5B is a diagram for explaining an angle weighting process.

FIG. 6A schematically shows an X-ray beam when the imaging system is rotated once at a pitch different from those in FIGS. 3 and 5.
(B) is a figure offered for description of an angle weighting process.

[Explanation of symbols]

M ... Subject 1… Top plate 9 ... X-ray tube (X-ray source) 11 ... Detector 13 ... X-ray detection array (X-ray multi-row detector) C… rotation center axis 14 ... Imaging system 27 ... Data processing unit (data processing means) D ... Detector row spacing on the rotation center axis A, B ... Virtual X-ray beam

Claims (5)

[Claims] 1. An imaging system equipped with an X-ray source for irradiating a cone-beam-shaped X-ray beam and an n-row X-ray multi-row detector is rotated and moved relative to a subject, and is continuous. An X-ray for reconstructing a tomographic image of a target slice after subjecting the projection data within a predetermined slice range to position weighting processing according to the position with respect to the target slice when processing the projection data acquired by performing a helical scan by rotation. In the CT reconstruction method, projection data obtained by a plurality of X-ray beams passing near the same slice position on the rotation center axis is subjected to angle weighting processing according to the cone angle of each X-ray beam, Corrected projection data obtained by a virtual X-ray beam incident perpendicularly to the X-ray multi-row detector is obtained, and the corrected weighted projection data is used as projection data to perform the position weighting process. X-ray CT characterized by performing
Reconstruction method. 2. The X-ray CT reconstruction method according to claim 1, wherein when an integer obtained by rounding down the decimal point of (n + 1) / 2 is m, the object is detected during one rotation of the imaging system. An X-ray CT reconstruction method, wherein the relative movement amount is m times the width of one detector row on a virtual detector including the rotation center axis. 3. The X-ray CT reconstruction method according to claim 1, wherein when n−1 is m, a relative movement amount with respect to the subject during one rotation of the imaging system is represented by a rotation center axis. An X-ray CT reconstruction method characterized in that the width of one detector row on the virtual detector including the detector is m times. 4. The X-ray CT reconstruction method according to claim 1, wherein the relative movement amount with respect to the subject during one rotation of the imaging system is one detector on a virtual detector including a rotation center axis. An X-ray CT reconstruction method, wherein the column width is set to be larger than 0 and not larger than n. 5. An imaging system provided with an X-ray source for irradiating a cone-beam-shaped X-ray beam and an n-row X-ray multi-row detector is rotated and moved relative to the subject, and is continuous. When processing projection data collected by performing helical scan by rotation, data processing that performs position weighting processing according to the position with respect to the target slice on the projection data within a predetermined slice range and then reconstructs a tomographic image of the target slice In the X-ray CT apparatus provided with the means, the data processing means is arranged so that, with respect to projection data obtained by a plurality of X-ray beams passing near the same slice position on the rotation center axis, cone angles of the respective X-ray beams are obtained. The corrected projection data obtained by the virtual X-ray beam perpendicularly incident on the X-ray multi-row detector is obtained by performing an angle weighting process according to The X-ray CT apparatus is characterized in that the position weighting process is performed using projection data as projection data.