JP2000102532A - X-ray ct scanner - Google Patents

Abstract

PROBLEM TO BE SOLVED: To surely correct the projection data of high resolution free from the distortion by excluding the influence of the displacement of the effective pass caused by the lag of the correction timing of the projection data while keeping the practical circuit scale of a data acquisition system(DAS) without specially elongating a scanning time when the scanning is executed by a cone beam by using the continuous X-ray. SOLUTION: An X-ray CT scanner comprises an X-ray tube for continuously exposing the X-ray, and a two-dimensional detector 11. The X-ray is formed into the cone beam. A data acquisition system(DAS) 24 correcting the intensity of transmitted X-ray detected by the detector 11 as the projection data, is mounted. The DAS 24 reads the projection data of one of the sequence direction and the channel direction with priority. A reconfiguration unit determines the vertual effective focal spot position of the X-ray tube by the X-ray effective pass relative to the collection of a plurality of X-ray detecting elements, to reconfigurate the image data on the basis of the projection data and the vertual effective focal spot position.

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 scanner for obtaining image data in a subject by a reconstruction process, and more particularly, to converting a continuous X-ray irradiated from an X-ray source into a cone beam. The present invention relates to an X-ray CT scanner that reconstructs an image by reconstructing the intensity of transmitted X-rays due to the irradiation, that is, projection data.

The term "cone beam" used here
Collectively refers to a beam whose irradiating surface extends not only in the channel direction of the detector but also in the slice direction orthogonal thereto, and refers to only the "cone", which is necessarily a direct meaning. Not something.

[0003]

2. Description of the Related Art Generally, an X-ray CT scanner has an X-ray tube and an X-ray detector arranged in a gantry so as to sandwich the subject, and emits an X-ray beam emitted from the X-ray to the subject. The geometry is to be transmitted and incident on the X-ray detector. During scanning, for example, the X-ray tube and the X-ray detector are rotated around the subject in synchronization. The X-ray detector is connected to a DAS (data collection device), and obtains intensity data (that is, projection data) of transmitted X-rays for a predetermined number of views for each scan. By reconstructing the projection data, image data (slice data or volume data) in the subject is obtained.

[0004] In the field of X-ray CT scanners, there has been a demand from clinical sites and research institutions to generate high-resolution three-dimensional images at high speed. As one attempt to meet this demand, a so-called cone beam CT for performing scanning using a cone beam has been studied.

[0005] The first of the cone beam scanning methods
In the conventional example, a cone beam X-ray source and a two-dimensional detector are rotated once around a subject in synchronization with each other, and a three-dimensional image reconstruction is approximated by a three-dimensional convolution method proposed by Feldkamp et al. ("Feldkamp LA et al.,:" Practi
cal cone-beam algorithm ", J.Opt.Sac.Am.A, 1.pp.612-
619 (1984) ").

The second conventional example is a method proposed by Kudo et al. By further improving the above-described three-dimensional convolution method (“Hiroyuki Kudo, Tsuneo Saito:“ 3. -Dimensional helical scan CT ”, IEICE (D-1
1), Vol. J74-D-II, No. 8, pp-11
08-1114, August 1191 "). In the first conventional example described above, three-dimensional image reconstruction is performed by approximating that X-rays obliquely incident on the body axis are regarded as parallel ones. In a plane far from the plane including the image, the accuracy of the approximation is reduced, and the reconstruction error is increased. In the second conventional example, a conventional helical scan using a fan beam projection is adapted to a measurement system using a conical beam projection to prevent such an increase in error and to enable high-speed data collection. .

[0007]

The X-ray tube X
The radiation exposure mode includes a mode in which pulsed X-rays are emitted and a mode in which continuous X-rays are emitted. Most conventionally known cone beam CTs are configured on the premise of pulsed X-rays, and specific proposals have been made on how to read out projection data when continuous X-rays are used. It has not been done yet. The proposals of the first and second conventional examples described above are both based on pulsed X-rays and do not focus on continuous X-rays.

From the viewpoint of further increasing the speed of scanning, it is required that an X-ray tube be irradiated with a predetermined dose of X-rays within a shorter time. In the case of pulsed X-rays, continuous X-rays are more advantageous because of the technical limitations of the tube itself to meet this demand. For this reason, in the future, continuous X-rays tend to be used heavily.

In the cone beam CT, if high-speed scanning is performed using continuous X-rays to obtain a high-resolution image, a problem related to reading of projection data becomes a bottleneck. In order to ensure the resolution, it is necessary to increase the number of channels and the number of columns of the detector. In such a case, it takes time to read the intensity of the transmitted X-ray from the detector, that is, read the projection data. During the reading, the X-ray tube and the detector are moving (rotating), and continuously emit X-rays.

Specific Examples of Conventional Problems (Part 1) FIGS. 18 to 20 show examples of inconveniences related to the circumstances in this area. As shown in FIG. 18, as an X-ray detector, N pieces of X-rays in a channel direction × M pieces of X-rays in a column direction (slice direction) are used.
A two-dimensional detector 101 in which the line detecting elements are arranged in a matrix
Is formed. For this two-dimensional detector 101, one row of M X-ray detecting elements arranged in the column direction are sequentially read out, and thereafter, one row of M X-ray detecting elements arranged in the next column direction A reading order in which the detection elements are sequentially read is adopted. In this case, in a certain view, the read-out projection data for one row of the 0th channel is the data shown in FIG.
A value obtained by integrating the detector output over the period drawn by the arrow in FIG. The same reading as channel 1 and channel 2 is proceeding, and the last N-1 channel 1
The projection data from which the rows have been read out has a value obtained by integrating the detector output over the period indicated by the arrow in FIG.

As can be seen at a glance by comparing FIGS. 1A and 1B, the view angles are different even though the views are the same. That is, as shown in FIG. 20, unlike the original hardware geometry, the position of the effective focal point F is farther from the position of the actual focal point, and the situation is equivalent to a situation where the detector pitch is fine. On the other hand, in the same view, in the same view, one row of element columns of the last N-1 channel are sequentially read, and finally, 1 element of the first 0th channel is read.
When sequentially reading the element columns of the rows, the position of the effective focal point F is closer to the actual focal position, and the angle (fan angle) of the cone beam in the channel direction is larger than the actual value. Situation.

As described above, since the effective focus position is different even for the same view, the image quality deteriorates if the conventional image reconstruction method is performed as it is.

Another example of the conventional problem (part 2) Another example is shown in FIGS. In this example, as shown in FIG. 21, projection data is sequentially read out from the N X-ray detection elements in a row arranged in the channel direction for each row in the two-dimensional detector 101 configured as described above. In DAS,
For example, as shown in FIG. 7 of JP-A-8-140962, switching means for reading data is provided for each channel. For this reason, in the case of reading for each column along the channel direction in FIG. 21, a time difference required for reading between elements in each column can be ignored. Therefore, the effective fan angle and effective focal track radius for each row can be considered almost equivalent to the actual hardware geometry.

[0014] However, the projection angle position differs for each row. This projection angle position can be determined, for example, as the source position at the center time in the integration period of the detection output. It is also assumed that scanning is controlled so as to obtain a predetermined number K of views per rotation of the X-ray tube and the detector. That is, it is assumed that the reading of all the columns is completed with some margin (but exactly) during "one rotation time / K". In this situation, the effective focal position at the time of reading the projection data in the 0th column for all views K = 0 to 7 is shown in FIG. 22A, that in the first column is shown in FIG. (C) of FIG.
Are shown below.

Also in this case, as is apparent from the figure, the effective focal position changes for each column even within the same view. This is not a serious problem in the case of plane reconstruction of independent slices, but in the case of cone beam CT, it is a cone angle reconstruction, and each plane is not independent.
If Feldkamp reconstruction is performed ignoring that the source position is shifted, the image cannot be simply rotated.

For example, the image reconstruction area Rr shown in FIG.
For the voxel indicated by a black circle in ec, when projection is performed in the state of the view (U) in which the effective focus F is on the upper side of the drawing, the data of the column to be read out last contributes, and the lower view ( In the projection in the state of L), the data of the column to be read relatively first contributes. Read timing
Since the degree of deviation from the nominal time differs for each column, the projection angle pitch for the voxel of interest slightly changes for each view. The projection data contributing to the voxels indicated by black circles in FIG. 23 is, for example, data collected at the source position for each view shown in FIG. The projection angle pitch is not uniform.

The non-uniform pitch itself is not preferred. If the number of views itself is in short supply, but there is a sparse projection direction (coarse projection angle pitch), artifacts resulting from insufficient view are likely to appear. If one tries to deal with this artifact, the number of correction measures increases accordingly.

On the other hand, it is also conceivable to adopt a configuration of simultaneous reading of all elements in which a DSA collection circuit is connected to all X-ray detecting elements forming a two-dimensional detector. However, as described above, the cone beam CT is a two-dimensional detector, and the number of X-ray detection elements increases. For example, since the number of elements in the channel direction is usually about 1000, the total number of elements in the slice direction is 100.
It becomes 0 × n. All these elements are connected in parallel to the DAS,
The installation of a parallel acquisition circuit is not practically practical because the circuit scale of the DAS becomes too large, the component cost becomes extremely high, and the DAS itself requires high-speed processing. In the situation.

The present invention has been made in view of the above-mentioned problems encountered with the cone beam CT, and maintains a practical circuit scale of the DAS when scanning with a cone beam using continuous X-rays. , Without prolonging the scan time, eliminating the effect of the deviation of the effective path due to the deviation of the projection data acquisition timing,
An object of the present invention is to reliably collect high-resolution projection data.

[0020]

In order to achieve the above object, an X-ray CT scanner according to the claimed invention has an X-ray source for continuously irradiating X-rays, and this X-ray is conical. X-ray irradiating means for shaping into X-rays in the form of a beam and irradiating the X-ray onto the subject, and two-dimensional detection in which a plurality of X-ray detecting elements for detecting X-rays transmitted through the subject are two-dimensionally arranged. Device, scanning means for relatively moving between at least one of the X-ray irradiating means and the two-dimensional detector and the subject to scan the X-rays, and wherein the two-dimensional detector detects Collecting means for collecting the intensity of transmitted X-rays as projection data; and obtaining a virtual effective focal position of the X-ray source based on an effective X-ray path relating to collection of the plurality of X-ray detection elements. Image data based on the virtual effective focus position. Characterized by comprising a reconstructing means for reconstructing a.

Preferably, the two-dimensional detector arranges the plurality of X-ray detecting elements in a matrix, sets one direction of the matrix arrangement in a column direction, and sets a direction orthogonal to this direction to a channel direction. A detector having a structure set in the direction, wherein the collecting means sequentially reads out detection signals of the elements and the X-ray detection elements belonging to one row along the column direction, and repeats the reading for each row. This is a means for performing priority reading.

In this case, for example, the collecting means performs the column direction preferential reading in parallel in each of a plurality of regions divided in the channel direction among the plurality of X-ray detecting elements arranged in the matrix. Means. The collection means may be means for performing the column-direction priority reading of each of the plurality of regions symmetrically between the regions along the same column direction. The reconstruction means may combine full-channel projection data by combining projection data between different views, which is a combination in which the effective focus position is substantially located from a projection data group collected from the plurality of regions over a plurality of views. May be provided. For example, the scanning unit is a unit that performs an overscan that scans the scan extra to a predetermined range beyond a certain range.

As another preferred example, the two-dimensional detector arranges the plurality of X-ray detection elements in a matrix, sets one direction of the matrix arrangement in a column direction, and A detector having a structure in which a direction perpendicular to the direction is set in a channel direction, wherein the collecting means sequentially reads out detection signals of the X-ray detection elements belonging to one row of the elements and the X-ray detection elements belonging to the row along the channel direction. This is a means for performing channel direction priority reading that is repeated for each column. For example, the collecting means is means for performing the channel direction priority reading in parallel in each of the plurality of regions divided in the column direction among the plurality of X-ray detecting elements arranged in the matrix.

As still another preferred example, the scanning means irradiates the X-rays in a substantially parallel beam state, and the reconstructing means responds to the X-rays by the collecting means. This is means for creating parallel view image data from the collected projection data.

As still another preferred example, the reconstructing means performs parallel processing when an X-ray path projected on each of the plurality of X-ray detection elements is projected on a plane perpendicular to a scan rotation axis. Means for performing the reconfiguration.

[0026]

Embodiments of the present invention will be described below with reference to the accompanying drawings.

First Embodiment An X-ray CT scanner according to a first embodiment will be described with reference to FIGS.

The X-ray CT scanner shown in FIGS. 1 to 3 includes a gantry 1, a bed 2, a control cabinet 3, a power supply 4, and various controllers 31 to 33, and is driven by, for example, an RR system. I have. As shown in FIG. 3, the controller includes a high-voltage controller 31, a gantry controller 33, and a bed controller 32.

Here, as shown in FIGS. 1 and 2, the longitudinal direction of the bed 2 is defined as a row direction (or a rotation axis direction or a slice direction) Z, and two directions orthogonal thereto are a channel direction X and a beam irradiation direction. Each is defined as Y.

A top plate 2a is provided on the upper surface of the bed 2 so as to be slidably supported in the longitudinal direction (row direction Z), and the subject P is placed on the upper surface of the top plate 2a. . The top board 2a is inserted into a diagnostic opening (not shown) of the gantry 1 so as to be able to advance and retreat by driving a bed driving device 2b represented by a servomotor. Bed drive 2
A drive signal is supplied to b from the bed controller 32. The couch 2 also includes a position detector (not shown) such as an encoder that detects the position of the couchtop 2a in the couch longitudinal direction by an electric signal, and sends this detection signal to the couch controller 32 as a couch control signal. It has become.

The gantry 1 is, as shown in FIGS.
A substantially cylindrical rotating frame 9 is provided therein. The above-described diagnostic opening is located inside the rotating frame 9. Further, the rotating frame 9 is provided with an X-ray tube 10 and a two-dimensional detector 11 as an X-ray detector so as to face each other across the subject P inserted into the diagnostic opening. Further, at a predetermined position of the rotating frame 9, as schematically shown in FIG.
2. Collision elimination collimator 2 as post collimator
3. A data acquisition device (DAS) 24 and a gantry driving device 25 are provided.

Among them, an X-ray tube 10 functioning as an X-ray source
Has a structure of a rotating anode X-ray tube, for example, and a high voltage generator 2
A continuous flow of current from 1 to the filament heats the filament and emits thermoelectrons toward the target. These thermoelectrons collide with the target surface to form an effective focal point, and X-rays extend from the effective focal point on the target surface.
The line beam is continuously exposed with a divergence.

The high-voltage generator 21 is supplied with low-voltage power from the power supply unit 4 via the low-pressure slip ring 26 and controls the X-ray irradiation from the high-voltage controller 31 via the optical signal transmission system 27. A signal is provided. For this reason, the high voltage generator 21 generates a high voltage from the supplied low voltage power supply, generates a continuous tube voltage according to the control signal from the high voltage, and supplies this to the X-ray tube 10. .

The pre-collimator 22 is located between the X-ray tube 10 and the subject P, and the scattered radiation removing collimator 2 as a post-collimator is located between the subject P and the two-dimensional detector 11. The pre-collimator 22 forms, for example, a slit-shaped opening having a constant width in the column direction Z, for example. Thus, the width of the X-ray beam emitted from the X-ray tube 10 in the column direction Z is limited, and a cone beam having a desired slice width corresponding to, for example, a plurality of detection element rows of the two-dimensional detector 11 is formed. .

The X-ray tube 10 and the two-dimensional detector 11 are rotatable in the gantry 1 by rotation of the rotating frame 9 so as to be opposed to the surroundings of the axial rotation center axis in the diagnostic opening.

The two-dimensional detector 11 is a detector in which a plurality of rows of detection elements 11a having a plurality of detection channels are arranged in the "row" direction (the "slice" direction) (see FIG. 1). As an example, the detection unit of each detection element 11a includes a solid state detector of a scintillator and a photodiode, which once converts an incident X-ray into an optical signal and converts the optical signal into an electric signal. Each of the detection elements 11a is provided with a charge storage section (sample hold). For this reason, this two-dimensional detector 11 uses this DAS as the DAS
The charge is read out by sequentially selecting the switches with the 24 switch groups, thereby detecting a signal (projection data) representing the intensity of the transmitted X-ray.

It should be noted that each of the detection elements 11a has an incident X
Sensors that directly convert wires into electrical signals (II.
Etc.) may be used.

The DAS 24 is switched by switching a switch group.
A detection signal is sequentially read from each detection element 11a, and A / D
It has a structure of a so-called filter DAS that performs conversion (converts to voltage and performs sampling).

To do this, the DAS 24 detects the detector 1
Considering that 1 is a two-dimensional (flat) detector, the column selecting units 24A1... 24AN for N channels, one channel selecting unit 24B, and one A / D converter 24C
And a control circuit 24D.

The control circuit 24D supplies various switching control signals to the column selecting sections 24A1... 24AN, the channel selecting section 24B, and the A / D converter 24C, as described later. The storage control signal Scont is also supplied to the charge storage section of the detection element 11a. As a result, as shown in FIG. 5, when each of the detection elements 11a is not selected, the first operation mode in which charge accumulation (that is, data measurement) is performed during a period other than the reset period, and the detection element 11a is selected. In this case, the second operation mode in which charge is transmitted (that is, data is read) can be alternatively executed.

The column selection units 24A1... 24AN each receive a signal from an "N channel.times.M element" detection signal (N and M are positive integers greater than "1") in accordance with a row selection signal Sselc. Alternatively, the detection signal of the column is selected. The column selection signal Sselc is supplied from the control circuit 24D. The detected projection data of each channel is sent to the channel selection unit 24B. Since the channel selection unit 24B is supplied with the channel selection signal Schan from the control circuit 24D, only the projection data of the channel specified by this selection signal is sent to the next A / D converter 24C.

The A / D converter 24C has a control circuit 24
D supplies various control signals including a read signal Sread. Therefore, the A / D converter 24C outputs the read signal S
At the time of read, the detection signal of the selected row and column is set to A
/ D conversion and sequentially output from the DAS 24 as projection data. The output projection data is sent to the data transmission unit 28.

The control circuit 24D includes, for example, a CPU, and generates and outputs a control signal required for data collection under the control of the main controller 30.

Returning to FIG. 3, the data transmission unit 28 connects the signal path on the rotating side and the signal path on the fixed side in the gantry 1.
Here, as an example, an optical transmission system that transmits signals in a non-contact manner is used. The data transmission unit 28 may have a slip ring structure. The digital amount of projection data taken out via the data transmission unit 28 is sent to a correction unit of the control cabinet 3 described later.

Further, the gantry driving device 25 includes a motor and a gear mechanism for rotating the rotating frame 9 around the central axis of the entire rotating element in the gantry 1. A drive signal is provided from the gantry controller 33 to the gantry drive device 25.

The high-voltage controller 31, the bed controller 32, and the gantry controller 33 are interposed between the gantry 1 and the bed 2 and the control cabinet 3 in a signal manner, and respond to a control signal from a main controller described later. , Each of which drives a load element.

The control cabinet 3 includes a main controller 30 that controls the entire system, a correction unit 34 connected to the main controller 30 via a bus,
It includes a data storage unit 35, a reconstruction unit 36, a display processor 37, a display 38, and an input device 39.

The correction unit 34 performs various correction processes such as offset correction and calibration correction on the digital projection data sent from the DAS 24 in response to a processing command from the main controller 30. The collected data subjected to the correction processing is transmitted to the main controller 30.
Is temporarily stored and stored in the data storage unit 35 in response to the write command. The stored data is read from the data storage unit 35 in response to a read command at a desired timing from the main controller 30 and transferred to the reconstruction unit 36. Under the control of the main controller 30, the reconstruction unit 36 performs reconstruction processing on the cone beam based on, for example, a convolution back-projection method at the time when the acquired data for reconstruction has been transferred, and performs image processing in a three-dimensional area. Generate data.

This image data is stored in the main controller 3
Under the control of 0, the data is stored in the data storage unit 35 as necessary, while being sent to the display processor 37. The display processor 37 performs necessary processing such as colorization processing, superimposition processing of annotation data and scan information on the image data, and supplies the processed data to the display 38. The image data is D / A converted by the display 38 and displayed as a tomographic image. The input device 39 is used to give commands such as scan conditions (including a scan part and position, a slice thickness, an X-ray tube voltage and current, and a scan direction to a subject), image display conditions, and the like to the main controller 30. .

In this X-ray CT scanner, the X-ray tube 1
The 0 and 2D detectors 11 are rotationally driven by the RR method, and X-ray projection is performed by a scanning method such as multi-scan or helical scan. During this rotation drive, the X-ray tube 1
From 0, X-rays are continuously emitted toward the subject. This continuous X-ray is shaped into a cone by the pre-collimator 22 and irradiated to the subject as a cone beam. X-rays transmitted through the subject are detected by the two-dimensional detector 11 and read out as follows.

Now, as shown in FIG. 6, the two-dimensional detector 11
Is assumed to be formed by arranging a plurality of M X-ray detection elements in a matrix in N channels × M columns (slice direction). For this two-dimensional detector 11, one row of M X-ray detection elements arranged in the column direction are sequentially read out, and then, one X-ray detection element arranged in the next column direction is read out.
It is assumed that a “column direction priority reading method” is adopted, in which M X-ray detecting elements for a row are sequentially read. In this column-direction priority reading method, in the DAS 24, control signals Scont, Sselc, Schan, Sre supplied to the respective parts by the control circuit 24D are provided.
This is achieved by controlling the timing of ad so that reading in the column direction has priority over that in the channel direction.

More specifically, as shown in FIG. 6, the column selection section 24A1 of the first channel selects the first row, and at the same time, the channel selection section 24B selects the first column. Next, the first channel is selected by the column selecting section 24A1 of the first channel, and at the same time, the second column is selected by the channel selecting section 24B. Thereafter, reading in the column direction is sequentially performed in the same manner. Then, the first channel column selecting section 24A1 sends the first
Select a row and, at the same time,
When the column is selected and the reading of the first row and the M-th column is completed, the second channel is selected by the column selecting section 24A2 of the second channel, and at the same time, the first column is selected by the channel selecting section 24B. . Thereafter, reading in the column direction of the second row is performed in the same manner. This is repeated in the channel direction.

As a result, as shown by arrows in FIG. 6, detection signals (projection data) are sequentially read out in the row direction for each channel of the two-dimensional detector 11. FIG. 7 shows the relationship between the time and the ray angle in the column direction priority reading method. As shown in FIG. 8, the ray angle is the angle of the X-ray path from the effective focal point F to each X-ray detection element, and is represented by ray angle = β + γ. The coordinate origin O in FIG. 8 is set to the center of rotation of the gantry, and FIG. 8 shows coordinate axes viewed along the column direction.

The read projection data is sent to the correction unit 34 through the data transmission unit 28. After receiving various corrections in the correction unit 34, the data storage unit 3
5 for each view. The reconstruction unit 36 performs a three-dimensional reconstruction process on the stored data based on, for example, a three-dimensional CBP method or various reconstruction methods based on the CBP method. At the time of this reconstruction processing, back projection is performed not from the actual effective focal position determined by hardware geometry but from a virtual effective focal position (effective ray position). The reason is as follows.

As shown in FIG. 7, in this column-direction priority reading method, each of the reading times of the first column to the M-th row of a certain view spans all the channels and requires a long time. Therefore, as described in the above 19 and 20,
The effective focus position is farther (fan angle is smaller than actual) or closer (fan angle is larger than actual), which is different from the original hardware geometry. For this reason, if the reconfiguration is performed as it is, the same problem as in the related art occurs.

Therefore, the reconstruction unit 36 performs a reconstruction process in the following procedure.

(1) The virtual effective focal position for each column is calculated in accordance with the readout time of the detection elements A to N (see FIG. 6) in each column. The elements in the first column in the channel direction are A1, B1
,... N1, the elements in the second column are A2, B2,.
Since the elements in the Mth column are Am, Bm,... Nm, the virtual effective focal position for each column is calculated in accordance with the readout time of each element. (2) For each column, the collection position of each element is corrected and defined using the virtual effective focal position and the virtual fan angle. (3) Convolution is executed for each column. (4) A virtual effective focus position calculated for each voxel by a predetermined rule (for example, a rule according to the weight used for the calculation) from a plurality of virtual effective focus positions corresponding to a plurality of detection element arrays used for calculating backprojection data And back projection at the virtual detector position.

As a result, the virtual effective focal position different for each column is corrected, and the back projection process is performed. Therefore, it is possible to reliably collect high-resolution projection data with little distortion without prolonging the scan time, eliminating the effect of a shift in the effective path caused by a shift in the collection timing of the projection data. . Further, in the DAS, the collecting circuits are not provided independently for all the elements, so that a practical circuit scale can be suppressed and the cost of circuit components can be suppressed. In addition, since continuous X-rays are used, a predetermined dose can be emitted in a shorter time than pulsed X-rays.

FIG. 9 shows a modification of this embodiment. In the above embodiment, when the projection angle pitch is large, the beam approaches a parallel beam, and when extremely large, the beam becomes a parallel beam. Even if the projection angle pitch is realistic, if the data of the next view or the next view and the data of a distant view are used, a geometric arrangement close to a parallel beam without interpolation becomes possible. In the state where the parallel view can be considered, for example, as proposed in Japanese Patent Application Laid-Open No. H10-43175, the weighting function is represented by a simple Fourier series based on the weight depending on the interval between the beam and the slice. Approximation and parallel reconstruction may be used.

[0060] The X-ray CT scanner according to a second embodiment the second embodiment 10-11
It will be described based on.

The configuration of the X-ray CT scanner of this embodiment is the same as that of the first embodiment. However, a configuration is adopted in which the reading of the detection data from the two-dimensional detector 11 is changed from the “column direction priority reading method” to the “channel direction priority reading method”.

This channel-direction priority reading method uses the two-dimensional detector 11 as shown in FIG.
As shown by an arrow 0, this is a method of sequentially reading projection data from N X-ray detection elements in a column arranged in the channel direction for each column.

This “channel direction priority reading method”
The DAS 24 is executed in the following procedure. The first column is selected in the column selection section 24A1 of the first channel, and at the same time, the first row is selected in the channel selection section 24B. Next, the second channel column selecting section 24A2
, The first column is selected, and at the same time, the second row is selected in the channel selection unit 24B. Hereinafter, the data is sequentially collected up to the Nth row in the first column. Therefore, the first column of projection data is sequentially output from the A / D converter 24C for each channel. The second column, the third column,..., The M-th column are sequentially and similarly repeated.

In the case of the “channel direction priority reading method”,
One row of elements along the channel direction is read almost simultaneously, and the row of elements along the "column" direction is only slightly different in timing for each element. It can be considered that there is no time difference as a whole. Therefore, the effective fan angle and the effective focal track radius for each row can be regarded as almost equivalent to the actual hardware geometry.

FIG. 11 shows the relationship between the time T and the ray angle (= β + γ: see FIG. 8) in the case of priority reading in the channel direction, using the reading of each column as a parameter.

However, the projection angle position differs for each row. Therefore, if the projection data is reconstructed as it is, the projection angle pitch becomes uneven as described with reference to FIGS. 22 to 24, and an artifact resulting from insufficient view is likely to appear.

Therefore, in the present embodiment, a reconstruction method conforming to the CBP method, such as the Feldkamp reconstruction method, is modified as follows to perform reconstruction processing. In other words, independent calculations for each column other than backprojection, such as convolution, are processed for each column, and processes involving a plurality of columns, such as backprojection (normally, data interpolation is performed for a total of four points in two channels and two columns). In some cases, interpolation is performed using data of three or more columns and six or more points.) Is performed in consideration of the X-ray source position at the time of data acquisition of each column.

That is, unlike the conventional reconstruction method, back projection is performed from a slightly different source position for each column. Specifically, a virtual effective focal point is obtained for each column, and weighted in a direction along the effective path, and backprojected. Alternatively, the back projection may be performed by calculating the effective focal position from a plurality of virtual effective focal points corresponding to a plurality of data to be back projected. For the calculation of the effective focus position, for example, there is a method of obtaining a virtual effective focus as a position weighted and interpolated in the angular direction according to the weight of data.

Instead of this, similarly to the first embodiment, (1) for each column, the collection position of each element is corrected and defined using the virtual effective focal position and the virtual fan angle. ,
(2) Execute convolution for each column, (3)
From a plurality of virtual effective focus positions corresponding to a plurality of detection element rows used for calculating backprojection data, a predetermined rule (for example,
The reconstruction may be performed in such a manner that back projection is performed at the virtual effective focus position and the virtual detector position calculated for each voxel according to the rule according to the weight used for the calculation.

As a result, the virtual effective focal position different for each column is corrected, and the back projection process is performed. Therefore, the same operation and effect as those of the first embodiment can be obtained.

Third Embodiment A third embodiment will be described with reference to FIGS.

This embodiment is a further development of the "column direction priority reading method" described in the first embodiment, and employs the "split / column direction priority reading method" to achieve higher speed. .

As an example, as shown in FIG. 12, for a plurality of detection elements arranged in a matrix of the two-dimensional detector 11, the first half and the second half of the two-dimensional detectors are simultaneously read in two systems at the same time. . The projection data is read out row by row starting from the first element row and the G-th element row in the column direction at the same time. This reading is achieved by forming the same switch circuit group as in the first embodiment for each half channel in the DAS 24, and controlling these two switch circuit groups in the same manner.

In this case, the effective source position of the first half channel and the source position of the second half channel are different from each other (see FIG. 13). Even if the back projection is performed from each source position, since the data of all the channels is discontinuous, a problem occurs in the convolution.

Therefore, in this embodiment, the time margin (read timing margin) from the end of reading of a certain projection data to the shift to the reading of the next projection data is set sufficiently shorter than the reading period. in this case,
The effective focus position of the first half channel of one view and the effective focus position of the second half channel of the next view are considerably closer in time than those of the same view (FIG. 14).
And the dotted line in FIG. 15).

Then, the reconstruction unit 36 sets the projection data of the first half channel of a certain view and the second half channel of the next view as one set (projection data connected by dotted lines in FIG. 14 as one set), It is used for convolution and back projection based on the above-mentioned Feldkamp reconstruction method or a reconstruction method based on it. As a result, the same operation and effect as those of the first embodiment can be obtained. In addition, since the two-partition / column-direction priority reading is performed, the data reading can be speeded up correspondingly, and a plurality of time-dependent data groups can be stored. It is possible to prepare projection data for a full channel in which the difference in the effective source position is small by combining the groups and the combination, thereby enabling a suitable image reconstruction.

Various modifications can be made to the third embodiment. For example, instead of the above-described two-partition priority reading in the column direction (parallel reading), three or four divisions may be used. As a result, a set of projection data for a full channel near the effective focal position is obtained in combination with the neighboring view data.

Further, even when the read timing margin and the read period are almost equal, an appropriate combination is advantageous.

Yet another variation relates to the number of views. The above-described embodiment is configured on the assumption that projection data of a specific number of views is obtained in one rotation, so that the existence of a read timing margin is considered. However, in the case where the number of views is short, it is also possible to adopt a configuration in which reading is continuously repeated as much as possible as long as the circuit is in time, without being restricted by scanning at a fixed number of views. The number of views per rotation becomes uncertain due to fluctuations in the rotation speed of the gantry, etc., but the system determines the indefinite number of views.
Thereby, the effective radiation source positions of the projection data to be combined can be overlapped well.

Still another modification relates to overscan. When the projection data is combined to create a full channel as in the above-described embodiment, the latter half of the projection data read in the first view is usually combined with the former half of the projection data read in the next view. This is advantageous in terms of time difference. However, if the subject moves during one rotation, a discontinuity will occur in the middle of the projection data used for convolution. In order to avoid this discontinuity, overscan may be performed little by little. Here "overscan"
This means that scanning is performed slightly beyond the necessary rotation range, for example, slightly more than one rotation, and overlapping projection data is collected. The projection data obtained by this overscan may be used for weighted interpolation, or any appropriate data may be selected. This overscan applies only to multiscan,
The main controller 30 and the gantry controller 33 work together.

Fourth Embodiment A fourth embodiment will be described with reference to FIGS.

This embodiment is a further development of the "channel direction priority reading method" described in the second embodiment, and employs the "split / channel direction priority reading method" to achieve higher speed. .

As an example, as shown in FIG. 16, for a plurality of detection elements arranged in a matrix of the two-dimensional detector 11, the first half and the second half of the two-dimensional detector are simultaneously read in two systems at the same time. . Starting simultaneously from the first element row and the G-th element row along the channel direction, the projection data is read out for each row. This reading is achieved in the DAS 24 by a two-circuit configuration employing the same switch circuit group and control method as in the second embodiment for each half channel.

In this case, for example, the collection timing of the column contributing to the voxel indicated by the black circle in FIG.
As shown in FIG. 17, although the unevenness of the projection angle pitch becomes large (FIG. 17 schematically shows the unevenness itself, and does not show the calculated view position). , Can be dealt with by the same reconstruction processing as in the second embodiment. Thereby, high-speed scanning can be performed.

It is needless to say that various modifications and changes can be made in addition to the above-described embodiment and its modifications without departing from the gist of the invention described in the claims of the present invention. .

[0086]

As described above, the X-ray CT of the present invention
In a scanner, the detection signal is read out with priority given to the row direction or channel direction of the two-dimensional detector, and the virtual effective X-ray source based on the effective X-ray path involved in the collection of a plurality of X-ray detection elements is read. Since the focus position is determined and the image data is reconstructed based on the projection data and the virtual effective focus position, the continuous X-rays are reduced from the X-ray reduction, and when scanning with a cone beam, the DAS is used. Practical circuit scale can be maintained, scan time is not lengthened significantly, and the effect of effective path shift due to shift in projection data acquisition timing is eliminated to obtain high-resolution projection data with little distortion. Can be collected and the quality of the reconstructed image can be increased.

[Brief description of the drawings]

FIG. 1 is a diagram illustrating a positional relationship between an X-ray tube in a gantry and a two-dimensional detector in an X-ray CT scanner according to an embodiment of the present invention.

FIG. 2 is a schematic configuration diagram of an X-ray CT scanner.

FIG. 3 is a schematic block diagram of an electric system of the X-ray CT scanner according to the embodiment.

FIG. 4 is a block diagram showing a schematic configuration of a DAS.

FIG. 5 is a timing chart showing charge accumulation and charge delivery for each element of the DAS.

FIG. 6 is a view for explaining column-direction priority reading of the two-dimensional detector according to the first embodiment;

FIG. 7 is a diagram showing changes in time and ray angle in the case of column-direction priority reading.

FIG. 8 is a diagram illustrating a ray angle.

FIG. 9 is a projection state diagram illustrating a parallel view according to a modification of the first embodiment.

FIG. 10 is a view for explaining channel-direction priority reading of a two-dimensional detector according to the second embodiment.

FIG. 11 is a diagram showing changes in time and ray angle in the case of channel direction priority reading.

FIG. 12 is a view for explaining division / column-direction priority reading of the two-dimensional detector according to the third embodiment;

FIG. 13 is a projection state diagram illustrating division / column direction priority reading.

FIG. 14 is a projection state diagram illustrating division / column direction priority reading.

FIG. 15 is a view for explaining a time difference for each view based on the division / column direction priority reading method.

FIG. 16 is a diagram illustrating division / channel direction priority reading of the two-dimensional detector according to the fourth embodiment.

FIG. 17 is a view of a view projection angle for explaining division / channel direction priority reading.

FIG. 18 is a diagram illustrating column-direction priority reading according to a conventional example.

FIG. 19 is a view for explaining the problem of the column direction priority reading.

FIG. 20 is a view for explaining a problem of column-direction priority reading;

FIG. 21 is a view for explaining channel direction priority reading according to a conventional example.

FIG. 22 is a diagram illustrating a problem of channel direction priority reading.

FIG. 23 is a view for explaining the problem of channel direction priority reading.

FIG. 24 is a view for explaining the problem of channel direction priority reading.

[Description of Signs] 1 Gantry 3 Control cabinet 10 X-ray tube 11 Two-dimensional detector (X-ray detector) 11a X-ray detector 24 DAS 24A Row selector 24B Channel selector 24C A / D converter 24D Control circuit 30 Main controller 31-33 Controller

Claims (10)

[Claims] 1. An X-ray source for continuously irradiating X-rays,
X-ray irradiating means for shaping the X-rays into cone-beam X-rays and irradiating the X-rays toward the subject, and a plurality of X-ray detecting elements for detecting the X-rays transmitted through the subject are two-dimensionally arranged. An arrayed two-dimensional detector; scanning means for scanning the X-ray by relatively moving at least one of the X-ray irradiating means and the two-dimensional detector and the subject; Collecting means for collecting the intensity of the transmitted X-rays detected by the detector as projection data, and obtaining a virtual effective focal position of the X-ray source based on an effective X-ray path relating to collection of the plurality of X-ray detection elements. An X-ray CT scanner, comprising: reconstructing means for reconstructing image data based on the projection data and the virtual effective focus position. 2. The invention according to claim 1, wherein the two-dimensional detector arranges the plurality of X-ray detection elements in a matrix, sets one direction of the matrix arrangement in a column direction, and sets the direction in a column direction. A collecting structure, wherein the collecting means sequentially reads out detection signals of the X-ray detection elements belonging to one row of the elements along the column direction,
An X-ray CT scanner, which is a means for performing a column direction priority read which repeats this read for each row. 3. The invention according to claim 2, wherein said collection means comprises a plurality of Xs arranged in a matrix.
An X-ray CT scanner, which is means for performing the column direction priority reading in parallel in each of a plurality of regions divided in the channel direction among the line detecting elements. 4. The X-ray CT scanner according to claim 3, wherein the collection unit is a unit that performs the column-direction priority reading of each of the plurality of regions symmetrically between the regions along the same column direction. . 5. The method according to claim 3, wherein the reconstructing means is a combination in which the effective focal position is substantially located in a group of projection data collected from the plurality of regions over a plurality of views. An X-ray CT scanner having means for creating projection data of a full channel by combining the projection data of the above. 6. An X-ray CT scanner according to claim 5, wherein said scanning means is means for performing an overscan in which said scan is extra-scanned to a predetermined range beyond a predetermined range. 7. The two-dimensional detector according to claim 1, wherein the two-dimensional detector arranges the plurality of X-ray detection elements in a matrix, sets one direction of the matrix arrangement in a column direction, and sets the direction in a column direction. A detector having a structure in which a direction perpendicular to the channel direction is set in a channel direction, wherein the collecting means sequentially reads out detection signals of the X-ray detection elements belonging to one row of the elements and the X-ray detection elements belonging to the column along the channel direction. An X-ray CT scanner, which is a means for performing channel direction priority reading that is repeated for each column. 8. The invention according to claim 7, wherein said collection means includes a plurality of Xs arranged in a matrix.
An X-ray CT scanner, which is means for performing the channel direction priority reading in parallel in each of the plurality of regions divided in the column direction among the line detecting elements. 9. The invention according to claim 1, wherein said scanning means irradiates said X-rays in a substantially parallel beam state, and said reconstructing means responds to said X-rays to acquire said X-rays. X-ray CT scanner, which is means for creating parallel view image data from projection data collected by a computer. 10. The invention according to claim 1, wherein the reconstructing means becomes parallel when an X-ray path projected on each of the plurality of X-ray detection elements is projected on a plane perpendicular to a scan rotation axis. X-ray CT scanner as a means for performing the reconstruction as described above.