JP2002320607A - X-ray ct apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an computed tomographic apparatus in which X-ray detection elements for reference are used for the measurement of data for the correction of an X-ray beam shift in an examee's body axis direction in a multi-slice X-ray detector. SOLUTION: The distribution of X-ray doses in a slice direction of X-ray beam 5 radiated from a focus 2C of an X-ray tube 2 is measured by eight X-ray detection elements composing a detector bloc 21 of the reference channels arranged in the terminal of the channel direction of the multi-slice type X-ray detector 4. The eight measurement data are added up by four elements each on the cathode and anode sides by an adder 36 after they are amplified and digitized by an amplifier 11 and A/D converter 12. The both elements are compared by a comparator 38 when the former is treated as a and the latter as b, and the X-ray tube drive mechanism 42 is controlled by a drive control device 40 when there exists any difference between them, while the X-ray tube 2 is shifted to a direction where the difference becomes smaller. The elements are shifted to the cathode side, for example, when they are proved to be a>b.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a multi-slice X-ray CT apparatus capable of simultaneously measuring image data of a plurality of slices, and more particularly to a body axis of an X-ray beam using a reference detecting element of an X-ray detector. The present invention relates to an X-ray CT apparatus that corrects directional movement.

[0002]

2. Description of the Related Art In an X-ray CT apparatus, an X-ray source and an X-ray detector are rotated around a body axis of a subject in a circumferential direction.
X-rays are emitted from an X-ray source, and an X-ray detector having a plurality of detection elements arranged in an arc shape facing the X-ray source detects the amount of X-rays transmitted through the subject, and outputs the attenuation data. The tomographic image is reconstructed by the image processing apparatus based on the image.

In such an X-ray CT apparatus, CT imaging (hereinafter, referred to as C) in which an X-ray beam is rotated to scan a subject.
If the intensity of the X-ray beam from the X-ray source fluctuates during T-scan or simply scan), an error occurs in measurement data detected by the X-ray detector. FIG. 7 shows an example of a measurement error of the X-ray detector. In FIG. 7, the horizontal axis represents time, and the vertical axis represents measurement data values of the X-ray detector. The graph 101 is the measurement data before correction, the graph 102 is the intensity fluctuation of the X-ray source, and the graph 100 is the measurement data after correction, that is, the measurement data after correcting the intensity fluctuation of the X-ray source. If the measurement error of such measurement data increases, artifacts occur in the reconstructed tomographic image, which causes deterioration in image quality.

In order to reduce the measurement error, the X-ray CT apparatus is provided with reference detecting elements at both ends in the circumferential direction (hereinafter referred to as a channel direction) of the X-ray detector. The device measures X-ray X-ray dose (hereinafter referred to as reference X-ray dose) data that does not pass through the subject, and normalizes the X-ray dose data measured by the X-ray detection element in the normal measurement channel using the reference X-ray dose data. X-ray intensity correction is generally performed to remove the influence of X-ray intensity fluctuations.

On the other hand, in the X-ray CT apparatus, it is desired to shorten the inspection time in order to improve the throughput of the apparatus. Therefore, by arranging a plurality of X-ray detectors in which a large number of X-ray detection elements are arranged one-dimensionally in the channel direction until now in a slice direction, a plurality of X-ray detectors are arranged in a plurality of slices during one scan. An X-ray CT apparatus equipped with a multi-slice type X-ray detector capable of measuring X-ray dose data has been put to practical use.

The applicant of the present application disclosed an improved example of an X-ray CT apparatus (hereinafter, referred to as a multi-slice X-ray CT apparatus) provided with the multi-slice type X-ray detector in Japanese Patent Application Laid-Open No. 2000-316841. I have. Japanese Patent Laid-Open No. 2000-316841 has an X-ray detector having a small characteristic variation between X-ray detection elements,
A multi-slice X-ray CT apparatus capable of coping with various slice configurations is disclosed.

FIG. 8 shows an example of a configuration of a main part of the invention disclosed in Japanese Patent Application Laid-Open No. 2000-316841. FIG. 8 shows a relationship between an X-ray detection element group in the slice direction for one channel of the multi-slice X-ray detector and its input side and output side. 8, the slice direction of one channel of the X-ray detector is composed of eight X-ray detection elements 105, and eight X-ray detection elements 10
A switch circuit 107 for switching the output from 5, an amplification circuit 109 for amplifying the output signal of the X-ray detection element 105, an image reconstruction circuit 110 for reconstructing a tomographic image based on the output of the amplification circuit 109, An image addition circuit 112 for adding an output signal of the image reconstruction circuit 110 is connected.

The X-rays input to the X-ray detector having such a structure
The line beam 114 is narrowed down by the collimator 116 and input to all eight X-ray detection elements 105 in the slice direction, and then, by switching the subsequent wiring 108 and the switch circuit 107, two adjacent data are combined. As a result, the data is combined into four sets of data, amplified by the amplifier circuit 109, and then a four-slice tomographic image is generated by the image reconstruction circuit 110.
The four-slice tomographic image is output without any processing by the image adding circuit 112 without any processing. In this configuration, X
The ray beam 114 is divided into four X-ray beams 114A to 114D,
This means that a 4-slice tomographic image has been obtained. The measurement data of the two X-ray detection elements 105 is captured in one slice tomographic image.

In this example, a plurality of X-ray detecting elements 105 are arranged in the slice direction, and a switch circuit 107 for switching the output is provided. The outputs of a plurality of adjacent X-ray detection elements 105 are added and output so as to have a desired slice thickness. In the present invention, by using the multi-slice method for the X-ray detector, measurement data for a plurality of slices can be collected in one scan, and the inspection time is reduced. However, since the amount of measurement data is large, the time required for image processing is long. To improve this, the switch circuit 107 selects and aggregates the measurement data so that only the required measurement data is processed. Thus, the processing time is reduced.

[0010]

Japanese Patent Application Laid-Open No. 2000-316841 discloses that a multi-slice X-ray detector is used to acquire image data for a plurality of slices during one rotation of an X-ray source. However, no sufficient study has been made on the correction of the variation over time of the X-ray intensity or the variation of the X-ray dose due to the thermal movement of the focal position of the X-ray source, and the occurrence of artifacts in the reconstructed image has not been conducted. Therefore, it is necessary to take effective correction measures.

As a means for correcting the variation over time of the X-ray intensity of the X-ray source, reference detection elements are provided at both ends in the channel direction of the single-slice type X-ray detector as described above. A method of normalizing and correcting measurement data of each detection element of the main X-ray detector based on a reference X-ray dose measured by the detection element has been performed. However, an X-ray CT apparatus using a multi-slice X-ray detector has not been sufficiently studied.

On the other hand, various studies have been made on the correction of the fluctuation of the X-ray dose distribution due to the thermal movement of the focal position of the X-ray source in a single slice type X-ray CT apparatus.
For this reason, a representative example studied in a single-slice X-ray CT apparatus will be described below.

First, Japanese Patent Application Laid-Open No. 4-227238 discloses that even if the focal position moves due to a rise in the temperature of the X-ray tube, the shift amount of the focal position is detected and the shift amount is offset. An example of a correction method is disclosed. In this correction method, position control is performed on a collimator as a means for obtaining an X-ray fan beam so as to offset the amount of movement. Further, in order to offset the moving amount of the focal position by this correction method, it is necessary to accurately detect the moving amount.

Japanese Patent Application Laid-Open No. 9-201352 discloses a method of correcting an X-ray intensity distribution shift due to a magnitude of an X-ray tube voltage and an X-ray beam shift due to a shift of a focal position due to a rise in the temperature of the X-ray tube. ing. The X-ray CT apparatus disclosed in Japanese Patent Application Laid-Open No. 9-201352 has a drive mechanism that moves an X-ray tube in the body axis direction, and an X-ray that has two detection elements and detects a shift of the X-ray intensity distribution in the body axis direction. A detector, and means for adjusting an X-ray intensity distribution shift error depending on the magnitude of the X-ray tube voltage of the X-ray detector. Detect with detector
The X-ray tube is moved and corrected by the driving mechanism.

Japanese Patent Application Laid-Open No. 11-70103 discloses that the main X
An FMS reference channel for directly receiving X-rays that does not pass through the subject is provided at both ends of the X-ray detector. The FMS reference channel detects a change in the X-ray dose due to the movement of the focal point of the X-ray source. A collimator provided immediately before the X-ray detector, or a table on which the subject rests, is moved in accordance with the focal point movement of the X-ray source, and the aperture width of the collimator is increased or decreased. An X-ray imaging device that makes the dose the same is disclosed.

Japanese Patent Application Laid-Open No. 11-128217 discloses that the main X
A reference channel for measuring the X-ray intensity that does not transmit through the subject and a monitor detector for detecting the focal point movement of the X-ray tube are provided at both ends of the X-ray detector, and the reference channel is provided based on the detector of the monitor detector. Output data, and the corrected reference channel data is
An X-ray imaging apparatus that corrects data of each channel of a line detector is disclosed.

As described above, various methods for correcting the fluctuation of the X-ray dose distribution due to the thermal movement of the focal position of the X-ray source have been disclosed, but they have advantages and disadvantages and are applicable to a multi-slice type X-ray CT apparatus. In such a case, there is a problem as described below.

First, in Japanese Patent Application Laid-Open No. 4-227238, the position of a collimator is controlled in order to correct a shift of an X-ray beam. At this time, noise is superimposed on a signal for detecting a shift amount. There is a problem that the shift amount cannot be detected correctly.

In Japanese Unexamined Patent Publication No. 9-201352, an X-ray detector for detecting the shift amount of an X-ray beam is provided separately from the main X-ray detector, and is provided separately from the main X-ray detector. Since it is installed at a position, an extra space and a supporting mechanism for the installation are required, and there is a problem that the cost is increased.

Further, X-ray CT disclosed in JP-A-11-70103
In the apparatus, the position of a collimator or the like is drive-controlled based on the amount of increase or decrease with respect to the initial value of the X-ray dose measured by the FMS reference channel due to the movement of the X-ray beam.
There are problems such as that an error due to a change over time in the X-ray dose is likely to occur, and that it is difficult to apply to a multi-slice X-ray detector.

Further, in the X-ray imaging apparatus disclosed in Japanese Patent Application Laid-Open No. H11-128217, since the reference channel and the monitor detector are separately provided, an extra installation space is required and the cost is increased. There are problems such as things.

As described above, in the conventional X-ray CT apparatus, there are obstacles to downsizing of the scanner and factors that increase the cost of the product. Therefore, in the present invention,
In a multi-slice X-ray detector, an X-ray CT having a configuration in which an X-ray detection element for reference correction can also perform data measurement for X-ray beam movement correction in the body axis direction
It is intended to provide a device.

[0023]

In order to achieve the above object, an X-ray CT apparatus according to the present invention comprises an X-ray tube, a collimator for converting the X-rays emitted from the X-ray tube into a fan-shaped X-ray beam. A multi-slice X-ray detector (hereinafter, referred to as an X-ray detector) that is arranged to face the X-ray tube, measures X-rays transmitted through the subject, and outputs measurement data of a plurality of channels and a plurality of slices. An X-ray CT apparatus comprising image processing means for reconstructing an image from measurement data of an X-ray detector, wherein at least one channel is arranged at an end of the X-ray detector in a channel direction, and a plurality of X-ray detectors are provided. An element is arranged in the slice direction in the same manner as the detector block of the measurement channel, and a detector block of a reference channel that outputs a plurality of X-ray dose data for correcting the measurement data of the measurement channel, Based on the X-ray dose data output from the detector block of the reference channel, whether or not the X-ray beam center moves in the slice direction due to the focus movement of the X-ray tube and the direction of the movement (hereinafter referred to as X-ray beam center movement data) are determined. X-ray beam center movement detecting means for detecting, an X-ray beam moving mechanism for moving the X-ray tube or the collimator in the slice direction to move the X-ray beam in the slice direction, and an X-ray beam center movement data. X-ray beam position control means for controlling the X-ray beam moving mechanism such that the amount of movement of the X-ray beam center on the detector block of the reference channel is reduced based on the reference channel (claim 1).

With this configuration, the X-ray dose data output from the detector block of the reference channel arranged at the end of the X-ray detector in the channel direction is used to correct the X-ray dose of the measurement data output from the measurement channel, Since it can be used for detecting the X-ray beam movement in the slice direction, it is not necessary to separately provide a dedicated X-ray detector for detecting the X-ray beam movement. It also contributes to cost reduction.

In the X-ray CT apparatus according to the present invention, the X-ray of the detector block of the reference channel of the X-ray detector is further provided.
The line detecting element is based on the center position in the slice direction,
A cathode side adder which is arranged on the cathode side and the anode side of the X-ray tube in the same number, and wherein the X-ray beam center movement detecting means adds the output of the cathode side X-ray detecting element to obtain a cathode side addition value; An anode-side adder that adds the outputs of the anode-side X-ray detection elements to obtain an anode-side addition value, and a comparator that compares the magnitudes of the cathode-side addition value and the anode-side addition value are provided. In this configuration, the detector block of the reference channel has an even number of X-ray detection elements, and outputs of half of the X-ray detection elements with respect to the center position of the detector block are output to the cathode side adder and the anode side. Since the addition is performed by the adder, it can be determined that the center position of the X-ray beam has moved to the side of the larger added value. For this reason, the control for returning the X-ray beam position to the original position can be easily performed based on the magnitude comparison result between the cathode-side addition value and the anode-side addition value.

In the X-ray CT apparatus according to the present invention, the X-ray beam moving mechanism is a mechanism for moving the X-ray tube in the slice direction, and the X-ray beam position control means determines the direction in which the X-ray beam center has moved. Control is performed so that the X-ray tube moves in the same direction. In this configuration, when the focal position of the X-ray tube moves to the cathode side, the center position of the X-ray beam moves to the anode side. Therefore, by controlling the X-ray tube to move to the anode side, the X-ray tube is controlled. It is possible to reduce the amount of movement of the line beam center position.

Further, in the X-ray CT apparatus according to the present invention, the X-ray beam moving mechanism is a mechanism for moving the collimator in the slice direction, and the X-ray beam position control means is in a direction opposite to the direction in which the X-ray beam center is moved. The collimator is controlled to move in the direction. In this configuration, when the collimator is moved to the cathode side, the center position of the X-ray beam moves to the cathode side. Therefore, when the focal position of the X-ray tube moves to the cathode side and the X-ray beam center position moves to the anode side, By controlling the collimator to move in the opposite direction to the cathode side, the amount of movement of the X-ray beam center position can be reduced.

[0028]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the X-ray CT apparatus according to the present invention will be described below with reference to the accompanying drawings. FIG. 1 shows an example of the overall configuration of an X-ray CT apparatus according to the present invention. In FIG. 1, an opening 1A is formed at the center of the gantry 1, and a subject 6 placed on the bed 7 is inserted into the opening 1A as the bed 7 is driven. On a scanner 1B rotatably supported by a gantry 1, an X-ray tube 2 and a multi-slice X-ray detector (hereinafter, also referred to as X-ray detector) 4 are arranged to face each other across a subject 6. Have been. On the front surface of the X-ray tube 2, a collimator 3 for controlling the spread of the X-ray beam 5 is arranged. The X-ray tube 2 emits X-rays under the control of the X-ray controller 8. The collimator 3 converts the conical X-ray beam emitted from the focal point of the anode of the X-ray tube 2 into a fan-shaped X-ray beam 5 having a thickness (slice width) in the body axis direction of the subject 6.
Collimate. The fan-shaped X-ray beam 5 that has passed through the collimator 3 passes through the subject 6. The arc-shaped multi-slice X-ray detector 4 detects X-rays that have passed through the subject 6. The multi-slice X-ray detector 4 has a plurality of X-ray detection elements arranged in the circumferential direction (also referred to as a channel direction) and the body axis direction (also referred to as a slice direction). It is structured so that it can be obtained.

The X-ray tube 2 and the X-ray detector 4 are arranged opposite to each other, and rotate around the subject 6 together with the scanner 1B. The rotation of the scanner 1B is performed by a scanner driving device 9 driven by a scanner control device 10. The scanner control device 10 is driven by an output signal from the image processing device 13. Further, the measurement data acquired by the X-ray detector 4 is amplified by the amplifier 11, digitized by the A / D converter 12, and input to the image processing device 13. The measurement data is subjected to image reconstruction processing by the image processing device 13, and a tomographic image of a plurality of slices is obtained. This tomographic image is displayed on the display
The image data of the tomographic image is stored in the storage device 14 upon being displayed on the display 15.

Next, a first embodiment of the X-ray CT apparatus according to the present invention will be described. In the apparatus of the present embodiment, the configuration of the multi-slice X-ray detector 4 is different from that of the conventional apparatus,
The method of correcting the measurement data output from the X-ray detector 4 differs from the method of compensating the focus movement of the X-ray tube for the measurement data of the X-ray detector 4. For this reason, regarding the contents of the first embodiment, the configuration of the X-ray detector 4, the method of correcting the measurement data, and the method of compensating the focal point movement of the X-ray tube are the main parts. Will be described.

First, the configuration of the multi-slice X-ray detector according to the present invention will be described. FIG. 2 shows a schematic structural diagram of an example of the multi-slice X-ray detector of the present invention. Figure 2
2A is a structural diagram in a channel direction, and FIG. 2B is a structural diagram in a slice direction. In the multi-slice type X-ray detector 4 according to the present invention, a plurality of X-ray detection elements are arranged in the channel direction and the slice direction, respectively.
It constitutes a line detector. FIG. 2B shows an X-ray detection element group for one channel (hereinafter, referred to as a detector block). In this embodiment, the detector blocks are No. 1 to No. 8
Each of the X-ray detection elements 23 includes a scintillator 24 and a light detection element 25 such as a silicon photodiode. Each X-ray detection element 23 is arranged and fixed on a substrate 26 made of plastic or the like.

FIG. 2A shows the arrangement of the detector blocks 20 and 21 in the channel direction. The detector blocks 20, 21 are arranged on an arc at a distance of a radius R from the focal point of the X-ray tube 2. As the detector block, X passing through the subject 6 is used.
The detector block 20 of the measurement channel that measures the dose and outputs the data of the amount of X-ray attenuation as measurement data, and the X-ray dose that is disposed at both ends of the X-ray detector 4 in the channel direction and does not pass through the subject 6 There is a reference channel detector block 21 that outputs data for measuring and correcting X-ray dose as reference data. The detector blocks 20 of the measurement channels are arranged, for example, in 512 channels or 1024 channels. On the other hand, the reference channel detector blocks 21 are arranged, for example, at one end or two channels at both ends. Regarding the reference channels, in some cases, only one channel may be arranged on one side, but it is better to arrange them at both ends for safety.

In the X-ray detector 4 of the present embodiment, the detector block 21 of the reference channel has a role of correcting the X-ray dose and also a role of detecting the focus movement of the X-ray tube 2. In the detector block 21 shown in FIG. 2B, eight X-ray detection elements 23 of No. 1 to No. 8 are arranged in the slice direction. The positional relationship with X-ray tube 2 is usually No. 1
The side is the cathode side, and the No. 8 side is the anode side. X-ray detector 23
The number of arrays is not limited to eight, but may be other numbers such as 12 or 16.

Reference channel detector block
Reference numeral 21 denotes a plurality of X-ray detection elements 23 arranged in the slice direction.
, The X-ray dose distribution in the slice direction can be measured. On the other hand, when the focal position of the X-ray tube 2 moves, the X-ray distribution in the slice direction changes. Therefore, the X-ray tube 2 is measured by measuring the X-ray distribution in the slice direction with the detector block 21 of the reference channel. Can be detected. For example, if the focal point of the X-ray tube 2 moves to the cathode side, the peak point of the X-ray dose distribution moves to the anode side, and if the focal point moves to the anode side, the peak point moves to the cathode side.

Next, the procedure of measuring the X-ray dose data in the slice direction by the X-ray detector 4 and processing the measured data will be described. In the multi-slice type X-ray detector 4 of the present invention, as shown in FIG. 2A, detector blocks 20 of measurement channels occupying most of the channels are arranged in an arc shape, and one end is provided at each end. Detector blocks 21 for each reference channel are arranged. In the slice direction, as shown in FIG. 2B, X-ray detection elements 23 corresponding to the number of slices are arranged. The detector blocks 20 and 21 are functionally divided into those for the measurement channel and those for the reference channel, but usually have the same structure. In the following description, X in the slice direction
A description will be given of a case where the number of arrayed line detecting elements 23 is eight as a typical example.

In the present invention, the detector block 21 of the reference channel of the X-ray detector 4 has two functions, namely, the first function is to correct X-ray dose data measured in the measurement channel, and the second function is to It is responsible for detecting whether or not the focal point of the tube 2 has moved or the amount of movement. First, the procedure of measurement of X-ray dose data in the measurement channel of the X-ray detector 4 and the correction processing procedure using the measurement data of the reference channel will be described with reference to FIG.

FIG. 3 is a diagram for explaining a procedure for correcting the measurement data of the X-ray detector. FIG. 3A shows an example of the flow of X-ray data measured by the detector block 20 of the measurement channel, and FIG. 3B shows an example of the flow of X-ray data measured by the detector block 21 of the reference channel.
Each is shown. On the measurement channel side in FIG. 3A, the X-ray beam 5 emitted from the X-ray tube 2
After passing through, the N of the detector block 20 of the measurement channel
It is measured by eight X-ray detection elements 23 of o.1 to No.8.
Similar measurement is performed in other measurement channels other than the illustrated one, but the measurement channel is represented by one illustrated one (the same applies to the following).

On the reference channel side shown in FIG. 3B, the X-ray beam 5 emitted from the X-ray tube 2 does not pass through the subject 6, and the detector block 21 of the reference channel.
No. 1 to No. 8 are measured by eight X-ray detection elements 23. When the subject 6 is large, the X-rays incident on the detector block 21 of the reference channel may be transmitted through the subject 6, but the distance between the subject 6 and the focal point of the X-ray tube 2 is increased. The adjustment is performed so that the X-ray transmitted through the subject 6 does not enter the detector block 21 of at least one reference channel.

The multi-slice type X-ray detector 4 of this embodiment
Then, the detector block 20 of the measurement channel in FIG.
A switch circuit 30 is provided at a stage subsequent to the eight X-ray detection elements 23, and by switching of the switch circuit 30,
Measurement data (output) of the X-ray detection element 23 of the measurement channel
Switch the distribution of In the example of FIG.
The output of the X-ray detection element 23 of o.4 and the output of the X-ray detection element 23 of No.5 to No.8 are combined into one each, and the output of the eight X-ray detection elements 23 is divided into two. Summarized in data. By compiling the measurement data of the X-ray detection element 23 in this manner,
Image data for a slice is obtained. That is, one slice corresponds to the X-ray detecting elements 23 of No. 1 to No. 4, and the other one slice corresponds to the X-ray detecting elements 23 of No. 5 to No. 8.
It corresponds to. As a result, the slice thickness becomes X
The thickness is equivalent to four line detection elements 23.

Various operation examples of the switch circuit 30 are possible. In addition to the above, first, the outputs of the eight X-ray detection elements 23 of No. 1 to No. 8 are directly output to the subsequent stage. There is. In this case, since image data for eight slices is obtained, it is effective to obtain an image of a thin slice, but there is a problem that it takes a long time to process the measurement data because the measurement data increases.

Next, there are cases where the outputs of the two X-ray detection elements 23, such as No. 1 and No. 2, No. 3 and No. 4, etc., are put together and output to the subsequent stage. In this case, image data for four slices is obtained. Although the number of slices is reduced to one half as compared with the above case, the processing time of the measurement data is shortened because the measurement data to be processed is halved.

In the case where the width of the X-ray beam 5 in the slice direction is limited to be smaller than the total length of the eight X-ray detection elements 23 in the slice direction by the collimator 3 disposed in front of the X-ray tube 2. , The X-ray beam 5 is incident only on the X-ray detection element 23 at the center, and is not incident on the X-ray detection element 23 at the end. As described above, when the width of the X-ray beam 5 in the slice direction is small, the switch circuit 30 sets the X-ray beam 5 incident X-ray in accordance with the width dimension of the X-ray beam 5 in the slice direction.
Only the X-ray detection element 23, for example, the measurement data of the six X-ray detection elements 23 of No. 2 to No.
X-ray detecting element 23 at the end where light does not enter, for example, No. 1 and No. 8
It operates so as not to output the measurement data of the X-ray detection element 23 of FIG.

Further, the switch circuit 30 outputs the six measurement data of the X-ray detection elements 23 of No. 2 to No. 7 as image data for six slices as they are, or collectively outputs three data at a time. It is possible to change the way of outputting the measurement data, such as outputting as image data for a slice, or outputting the image data for two slices collectively by three.

Next, the measurement data output from the switch circuit 30 is amplified by the amplifier 11, converted into a digital signal by the A / D converter 12, and then input to the correction processing circuit. The correction processing circuit 34 corrects (normalizes) the X-ray dose based on the reference data obtained by the detector block 21 of the reference channel. The measurement data subjected to the X-ray dose correction processing is sent to the image processing device 13 and used for reconstructing a tomographic image of the subject 6.

Also, in the present embodiment, the reference channel detector block 21 shown in FIG.
The amplifier 11, the A / D converter 12, and the addition processing circuit 32 are provided after the eight X-ray detection elements 23 of O.8. The width of the X-ray beam 5 incident on the detector block 21 of the reference channel in the slice direction is a fixed width, and is a dimension capable of covering all the X-ray detection elements 23 (eight in the figure) in the slice direction. And Normally, the width of the X-ray beam 5 in the slice direction is set to be equal to or slightly larger than the length in the slice direction of the eight X-ray detection elements 23 constituting the detector block 21 of the reference channel.

Reference block detector block
From 21, measurement data of the X-ray dose is output from all the X-ray detection elements 23, and these measurement data are amplified by the amplifier 11 and converted into digital signals by the A / D converter 12. The digitized measurement data is input to the addition processing circuit 32,
The addition processing of the measurement data is performed so that the combination of the numbers of the X-ray detection elements 23 is the same as the combination of the numbers of the X-ray detection elements 23 at the output of the switch circuit 30 on the detector block 20 side of the measurement channel. . For example, on the detector block 20 side of the measurement channel, the switch circuit 30 is configured to combine the measurement data of the X-ray detection elements 23 of No. 1 and No. 2, No. 3 and No. 4,.
Operates, No. 1, No. 2, and No. 3 are added to the addition processing circuit 32 on the detector block 21 side of the reference channel.
The X-ray dose data output from the X-ray detection elements 23 of Nos. 4,... Are added and output, and for example, on the detector block 20 side of the measurement channel, No. 1 to No. 4, No. 5 When the measurement data of the X-ray detecting elements 23 of No. to No. 8 are summarized, the No. 1 to No.
4. The X-ray dose data output from the X-ray detection elements 23 of No. 5 to No. 8 are respectively added.

As described above, by performing the addition processing of the X-ray dose data on the detector block 21 side of the reference channel, the image data acquired on the detector block 20 side of the measurement channel is compared with the object data as reference data. X-ray dose data that has not passed through 6 is added to the processing circuit 32
Output from These X-ray dose data are processed by the correction processing circuit.
The data is input to the reference 34 and used as reference data for correcting the X-ray dose of the measurement data.

The correction processing of the measurement data of the detector block 20 of the measurement channel by the correction processing circuit 34 is performed by a switch circuit for the outputs of the X-ray detection elements 23 of No. 1 to No. 8 of the detector block 20 of the measurement channel. The correction of the X-ray dose is performed for each measurement data of one slice compiled by switching the 30 switches. The measurement data that has been subjected to the X-ray dose correction processing is sent to the image processing device 13 and reconstructed as a tomographic image.

Next, with reference to FIGS. 4 and 5, a first example of a method of compensating the focal point movement of the X-ray tube in the apparatus of the present embodiment (hereinafter referred to as the first method).
Method). In the apparatus of the present embodiment, the detector block 21 of the reference channel of the multi-slice type X-ray detector 4 uses the X-ray tube by moving the focal point.
It is used as an X-ray detector for detecting the movement of the center position of the beam 5 in the slice direction, and the X-rays on the detector block 21 based on the X-ray dose data output from the detector block 21 in the subsequent stage. X-ray beam center movement detecting means for detecting the presence or absence and movement direction (X-ray beam center movement data) of the center position of the beam 5 in the slice direction, and an X-ray beam movement mechanism for moving the X-ray beam in the slice direction. X-ray beam position control means for controlling the X-ray beam moving mechanism so as to reduce the amount of movement of the X-ray beam center based on the X-ray beam center movement data is provided.

FIG. 4 is a diagram showing the overall configuration of a first example of a method of compensating the focal point movement of the X-ray tube in the apparatus of the present embodiment, and FIG.
FIG. 9 is a diagram for explaining the flow of processing of X-ray dose data acquired by the detector block 21 of the reference channel at the time of X-ray tube focal point movement compensation.

First, the overall structure of the first method will be described with reference to FIG. In FIG. 4, in an X-ray tube 2, an electron beam emitted from a cathode 2A collides with a target of a rotating anode 2B, and an X-ray beam 5 is generated from a focal point 2C. The X-ray beam 5 is collimated by the collimator 3 into a fan-shaped X-ray beam 5 having a certain thickness (slice width) in the slice direction, passes through the subject, and enters the multi-slice type X-ray detector 4. , X-ray detector 4. X
Detector block of reference channel of X-ray detector 4 in both ends of channel direction of X-ray detector 4
The X-ray dose data detected by 21 does not pass through the subject, and is used not only for correcting the above-mentioned measurement data, but also for the center position of the X-ray beam 5 in the slice direction accompanying the X-ray tube focal point movement. It is also used to detect movement.

Reference Channel Detector Block
2 has a plurality (eight in the figure) of X-ray detecting elements 23 arranged in the slice direction as shown in FIG. 2B, and thus detects the X-ray dose distribution of the X-ray beam 5 in the slice direction. be able to. As described above, the X-ray tube focal shift changes the X-ray dose distribution in the slice direction, and appears as a shift of the center position of the X-ray dose distribution.
By detecting the X-ray dose distribution in the slice direction 5, the amount of movement of the center position of the X-ray beam 5 in the slice direction (at least whether or not the center position of the X-ray beam 5 moves in the slice direction) due to the focal point movement of the X-ray tube is determined. Can be detected.

In this first method, the center of the X-ray beam 5 in the slice direction is calculated from the X-ray dose data measured by the eight X-ray detection elements 23 of No. 1 to No. 8 of the detector block 21 of the reference channel. Which side the position 22 has moved with respect to the center position 45 in the slice direction of the detector block 21 (the boundary between the No. 4 X-ray detecting element and the No. 5 X-ray detecting element shown) 45 (that is, X (Cathode side or anode side of the tube).
The detection of the movement of the X-ray beam 5 at the center position 22 in the slice direction is performed by the X-ray beam center movement detecting means. In FIG. 4, the X-ray beam center movement detecting means comprises an adder 36 and a comparator 38. Is done.

The adder 36 adds the output from the cathode-side X-ray detection element and the output from the anode-side X-ray detection element to the slice position center position 45 of the detector block 21. The adder 36a and the adder 36b are provided. The comparator 38 compares the output from the cathode-side X-ray detection element with the output from the anode-side X-ray detection element.
Whether the output on the anode side is large is output as a comparison result. The comparison result of the comparator 28 is output as a detection result of the X-ray beam center movement detecting means. This detection result indicates the moving direction of the X-ray beam center position 22. When the output on the cathode side is large, the one has moved to the cathode side, and when the output on the anode side is large, it has moved to the anode side. The X-ray beam center movement detecting means outputs X-ray beam center movement data to the next stage X-ray beam position control means.

In FIG. 4, an X-ray beam position control means controls the movement of the X-ray tube 2 in the slice direction.
The X-ray beam moving mechanism comprises an X-ray tube driving mechanism 42 for moving the X-ray tube 2 in the slice direction. Drive control device
Numeral 40 denotes X-ray beam center movement data, in which the moving direction of the X-ray beam center position 22 is known, and the direction in which the X-ray tube 2 should move, that is, whether to move to the cathode side or the anode side. An instruction and an instruction of a moving distance (usually a minimum moving distance) are given to the X-ray tube driving mechanism 42. Since the X-ray tube driving mechanism 42 moves the X-ray tube 2 by driving a motor or the like, the instruction of the moving direction and the moving distance of the X-ray tube 2 is given after being converted into the rotating direction and the number of rotations of the motor. Can be In accordance with this instruction, the X-ray tube driving mechanism 42 moves the X-ray tube 2 in the slice direction. As a result, the amount of movement of the X-ray beam center position 22 decreases. As a result of performing the above series of operations, if the movement of the X-ray beam center position 22 still remains,
The above series of operations is repeated until the movement amount of the line beam center position 22 becomes substantially zero.

Next, a method of processing the X-ray dose data acquired by the reference channel detector block 21 will be described with reference to FIG. In FIG. 5, in the initial state of load application to the X-ray tube 2, slices of the X-ray beam 5 are applied to the X-ray detection elements 23 of No. 1 to No. 8 of the detector block 21 of the reference channel of the present embodiment. The X-ray irradiation is performed so that the center line 22 in the direction substantially hits the boundary between the No. 4 element and the No. 5 element which is the center position 45 of the detector block 21 in the slice direction. The position of the center line 22 of the X-ray beam 5 in the slice direction corresponds to the center position of the X-ray beam 5 in the slice direction. Therefore, in the initial state of load application to the X-ray tube 2,
Since the focal position of the X-ray tube 2 does not move, the No.
The four elements No. 1 to No. 4 and the four elements No. 5 to No. 8 on the anode side are irradiated with substantially equal amounts of X-rays. For this reason,
The sum of the X-ray dose data output from the four devices No. 1 to No. 4 is almost equal to the total of the X-ray dose data output from the devices No. 5 to No. 8.

On the other hand, CT imaging is performed and the X-ray tube
When the load application to the X-ray tube 2 is repeated,
B is heated, the focal position moves, and the center line 22 of the X-ray beam 5 in the slice direction moves, so that the X-ray dose distribution in the slice direction of the X-ray beam 5 received by the detector block 21 of the reference channel changes. Change. Where the X-ray tube
The center line of the focus movement direction of 2 and the slice direction of the X-ray beam 5
There is an opposite relationship between the direction of movement of 22 and the X-ray tube
If the focal point 2 moves to the cathode side, the center line 22 of the X-ray beam 5 in the slice direction moves to the anode side. For this reason, X-ray tubes
When the focal point shift due to the application of the load 2 occurs, the focal point position moves to the cathode side, so that the center line 22 of the X-ray beam 5 in the slice direction moves to the anode side.

In FIG. 5, the center line 22 of the X-ray beam 5 in the slice direction is moved to the anode side by the focal point movement of the X-ray tube 2.
That is, when moving to the element side of No. 5 to No. 8, N on the anode side
o The X-ray dose incident on the four elements No. 5 to No. 8 is No. 1 on the cathode side.
X-rays that are incident on the four elements No. 4 to No. 4 and as a result, the total value of the measurement data output from the four elements No. 5 to No. 8 on the anode side (hereinafter, output on the anode side) Is larger than the total value of the measurement data output from the four elements No. 1 to No. 4 on the cathode side (hereinafter, referred to as cathode side output).

As described above, when the center line 22 of the X-ray beam 5 in the slice direction is moved by the focal point movement of the X-ray tube 2, the cathode-side output and the cathode-side output from the X-ray unit block 21 of the reference channel are changed. Since there is a difference between them, by detecting this difference, the moving direction of the center line 22 in the slice direction of the X-ray beam 5 can be known. For this reason, in the present embodiment, the X-ray beam center movement detecting means includes an adder 36 and a comparator 38. The adder 36 obtains a cathode side output and an anode side output. The output of the cathode side and the output of the anode side are compared, and the moving direction of the center line 22 in the slice direction of the X-ray beam 5 is detected.

In FIG. 5, eight X-ray detecting elements 23 of No. 1 to No. 8 of the detector block 21 of the reference channel are shown.
The eight measurement data output from the
, And digitized by the A / D converter 12, and then X
The addition processing is performed by an adder 36 included in the line beam center movement detecting means. The adder 36 includes two adders, a cathode side adder 36a and an anode side adder 36b. The cathode side adder 36a adds four measurement data output from the four elements No. 1 to No. 4 on the cathode side, and the anode side adder 36b adds No. 5 to No. 8 on the anode side. Of 4
The four measurement data output from the elements are added.
The former addition value 37a corresponds to the above-described cathode side output, and the latter addition value 37b corresponds to the above-described anode side output. Both addition values 37a and 37b are amplified and digitized by the amplifier 11 and the A / D converter 12, but will be referred to by the same names in the following description.

Next, the cathode side output 37a and the anode side output 37b are X
The data is input to the comparator 38 included in the line beam center movement detecting means, and the magnitude is compared. In the comparator 38, the cathode side output
The difference between 37a and the output 37b on the anode side is calculated. For example, the anode output 37b is subtracted from the cathode output 37a. The code data of the difference indicates the moving direction of the center line 22 in the slice direction of the X-ray beam 5. That is, when the cathode side output 37a is large (the sign is +), the center line 22 of the X-ray beam 5 moves to the cathode side, and when the anode side output 37b is large (the sign is-), the X-ray beam 5 The center line 22 has moved to the anode side. Therefore, by outputting the difference code data from the comparator 38, the center line 22 of the X-ray beam 5 is output in the next stage.
Can be compensated for restoring.

Next, the output of the comparator 38 is sent to the drive control device 40 of FIG. The drive control device 40 receives the code data of the difference from the comparator 38, identifies whether the moving direction of the center line 22 in the slice direction of the X-ray beam 5 is the cathode side or the anode side, and drives the next stage of the X-ray tube. By controlling the mechanism 42, the X-ray beam 5
X in the direction in which the moving amount of the center line 22 in the slice direction of
The tube 2 is moved.

The X-ray tube driving mechanism 42 is a mechanism for moving the X-ray tube 2 by a small distance in the slice direction, that is, in the body axis direction, and is constituted by a combination of a worm gear and a motor. Moving direction is anode side (A → B) or cathode side (B → A)
And the movement distance is 1 mm or less at the maximum. Since the focal distance of the X-ray tube 2 is usually about 500 μm,
An appropriate moving distance in one compensation operation is 20 to 100 μm. The moving direction can be driven by rotating the motor forward or backward.

A control example of the X-ray tube driving mechanism 42 by the drive control device 40 will be described. From the output of the comparator 38 as the means for detecting the movement of the center of the X-ray beam, 3 (1) the cathode side is large (the sign is +), (2) there is no difference, and (3) the anode side is large (the sign is-).
One of the two cases is detected. First, (1)
In the case of (1), since the center line 22 of the X-ray beam 5 in the slice direction is on the cathode side, it is determined that the focal point of the X-ray tube 2 is moving to the anode side. The mechanism 40 is controlled so that the X-ray tube 2 is moved to the cathode side, for example, by 50 μm. By this compensating operation, the position of the center line 22 in the slice direction of the X-ray beam 5 on the detector block 21 of the reference channel is several hundred μm (50 μm × (distance between collimator and X-ray detector) / (focus and collimator). Distance)) is returned. With just one compensation operation, X
Since the position of the center line 22 in the slice direction of the line beam 5 may not return to the original position, in such a case, the same compensation operation is repeated.

Next, the case (2) indicates that the center line 22 of the X-ray beam 5 in the slice direction is at the original position, so that the compensation operation is not required. Next, in the case of (3), since the center line 22 of the X-ray beam 5 in the slice direction is moving in the opposite direction to that of (1), the X-ray tube 2 is moved in the opposite direction. What is necessary is just to control it. Other operations are the same as in the case of (1).

As described above, by controlling the movement of the X-ray tube 2, the focal shift amount of the X-ray tube 2 is compensated, and the X-ray beam 5
Since the position of the center line 22 in the slice direction is returned to the original,
The position of the center line 22 in the slice direction of the X-ray beam 5 is N
The X-ray dose distributions for the elements No. 1 to No. 8 substantially coincide with the boundary 45 of the element No. o.5, and the distribution similar to the distribution in the initial state is maintained.

Next, a second example (hereinafter, referred to as a second method) of a method of compensating the focal point movement of the X-ray tube in the apparatus of the present embodiment will be described with reference to FIG. The following description focuses on the differences from the first method. In this method, the X-ray beam center movement detecting means is different from the first method. The X-ray beam center movement detecting means in the present method detects the maximum position of the X-ray dose which detects the position of the X-ray detecting element indicating the maximum value from the X-ray detecting data output from the X-ray detecting element of the detector block 21 of the reference channel. Means.

FIG. 6 shows an example of the X-ray dose distribution measured by the eight X-ray detecting elements 23 of the detector block 21 of the reference channel. The horizontal axis indicates the number of the X-ray detection element, and the vertical axis indicates the X-ray dose data measured by the X-ray detection element. X1 to X8 are X-ray dose data measured by the X-ray detection elements 23 of No. 1 to No. 8, and a broken line 46 shows an X-ray dose distribution. In the case shown, the center line 22 of the X-ray beam 5 in the slice direction moves toward the anode, and the position of the center line 22 is the position of the No. 5 element that outputs the maximum X-ray dose data X5.

From the above, according to the present method, the X-ray maximum value position detecting means is disposed after the A / D converter 12 in FIG. 5, and the X-ray maximum value position detecting means detects the reference channel. No.1 to No.8 X-ray detection element 23 of block 21
Are sequentially compared to detect the position (or number) of the X-ray detecting element 23 that indicates the maximum value. The position (or number) of the X-ray detecting element 23 that has measured the maximum X-ray dose is output from the X-ray maximum value position detecting means. In the example of FIG. 6, the position of the No. 5 element is output.

In this method, the operation of the drive control device 40 is also the first.
The method is slightly different from the method described above, and will be described below. In the case of this method, the X-ray beam center movement data includes an X-ray detection element which measures the maximum X-ray dose from the X-ray dose maximum value position detecting means.
Since the position (or number) of the X-ray beam 23 is output, the moving direction and the moving distance of the center line 22 of the X-ray beam 5 can be calculated from this data. The moving direction and moving distance of the mechanism 42 are controlled. Therefore, in the case of the present method, the position of the center line 22 of the X-ray beam 5 can be returned to almost the original position by one compensation operation.

Also, an X-ray detecting element for measuring the maximum X-ray dose
The following procedure is used to calculate the moving direction and moving distance of the center line 22 of the X-ray beam 5 from the number 23. First, the moving direction is the cathode side if the number of the X-ray detection element 23 belongs to No. 1 to No. 4, and the anode side if the number belongs to No. 5 to No. 8. Next, regarding the moving distance, the center position of the detector block 21
Since it is a distance from 45, when the width dimension in the slice direction of the X-ray detection element 21 is d, 0.5% is obtained for the elements No. 4 and No. 5.
d, 1.5d for No. 3 and No. 6 devices. Furthermore,
In controlling the X-ray tube driving mechanism 42, a conversion coefficient = (distance between focus and collimator) / (collimator and X-ray detector, in order to convert this moving distance into a moving amount of the focal position of the X-ray tube 2. Distance).

Next, a description will be given of a third example (hereinafter, referred to as a third method) of a method for compensating the focal point movement of the X-ray tube in the apparatus of the present embodiment. Hereinafter, description will be made with reference to FIG. This method differs from the first and second methods in the X-ray beam moving mechanism and the X-ray beam position control means. In this method, in FIG. 4, the X-ray tube driving mechanism 42 is a collimator driving mechanism,
The drive control device 40 is a collimator drive control device.

As in the case of the X-ray tube driving mechanism 42, the collimator driving mechanism is constituted by a combination of a motor and a worm gear. The collimator drive control device is substantially the same as the drive control device 40 for the X-ray tube 2, but the position of the X-ray tube 2 and the collimator 3 with respect to the X-ray detector 4 changes. 5) Collimator moving direction and X-ray beam
(2) The ratio of the moving distance of the collimator to the moving distance of the center line 22 of the X-ray beam 5 ((the distance between the focal point and the X-ray detector) / (the focal point The distance between the collimators)). For this reason, when driving the collimator 3, it is necessary to perform control in consideration of this point.

In the first to third methods, the X-ray tube 2
In the case where the focal shift amount is small, it is difficult to move the X-ray tube 2 or the collimator 3 by the minute distance due to mechanical accuracy. In such a case, there is often no need for correction at the time of image reconstruction. For this reason, when the moving amount of the center line 22 of the X-ray beam 5 is small, for example, 20
When the value is smaller than the value corresponding to μm, or when the difference between the cathode side output and the anode side output in the example of FIG. 5 is small and smaller than a predetermined value, the center line 22 of the X-ray beam 5 does not move. It is preferable that a mechanism for determining that the X-ray beam is centered be incorporated in the X-ray beam center movement detecting means.

[0075]

As described above, according to the X-ray CT apparatus of the present invention, the X-ray dose arranged at the end of the multi-slice type X-ray detector in the channel direction or output from the detector block of the reference channel is obtained. Since the data can be used for correcting the X-ray dose of the measurement data output from the measurement channel and detecting the X-ray beam movement in the body axis direction, a dedicated X-ray for detecting the X-ray beam movement This eliminates the need to provide a separate detector, increases the space in the scanner, and contributes to a reduction in the cost of the apparatus.

Since the position where the X-ray beam movement detecting means is located is the end of the X-ray detector in the channel direction and the distance from the focal point is the same as the detector block of the measurement channel, More ideal measurement of the amount of X-ray beam movement is possible.

[Brief description of the drawings]

FIG. 1 is an example of the overall configuration of an X-ray CT apparatus according to the present invention.

FIG. 2 is a schematic structural diagram of an example of a multi-slice X-ray detector according to the present invention.

FIG. 3 is a diagram for explaining a procedure for correcting measurement data of an X-ray detector.

FIG. 4 is a diagram showing an overall configuration of a first example of a method of compensating for a focal point movement of an X-ray tube in the apparatus of the present embodiment.

FIG. 5 is a view for explaining a flow of processing of X-ray dose data acquired by a detector block of a reference channel at the time of X-ray tube focal point movement compensation of FIG. 4;

FIG. 6: Reference channel detector block 8
6 is an example of an X-ray dose distribution measured by X-ray detection elements.

FIG. 7 is an example of a measurement error of the X-ray detector.

FIG. 8 shows a relationship between an X-ray detection element group in a slice direction for one channel of a multi-slice X-ray detector and its input side and output side.

[Explanation of symbols]

 2 X-ray tube 2A Cathode 2B Anode 2C Focus 3 Collimator 4 Multi-slice X-ray detector (X-ray detector) 5 X-ray beam 6 Subject 9 Scanner drive 10 Scanner control Device 11 Amplifier 12 AD converter 13 Image processing device 14 Storage device 20 Measurement channel detector block 21 Reference channel detector block 22 Center position (center line) 23 X-ray detection element 24 Scintillator 25 Photodetector 26 Substrate 30 Switch circuit 32 Addition circuit 34 Correction circuit 36 Adder 36a Cathode adder 36b Anode adder 37a Cathode output 37b Anode output 38 ... Comparator 40 ... Drive control unit 42 ... X-ray tube drive mechanism 45 ... Detector block center position 46 ... X-ray dose distribution

Claims (1)

[Claims] 1. An X-ray tube, a collimator for converting X-rays emitted from the X-ray tube into a fan-shaped X-ray beam, and an X-ray tube arranged opposite to the X-ray tube to measure X-rays transmitted through a subject. Multi-slice X-ray detector (hereinafter, referred to as X-ray detector) that outputs measurement data of a plurality of channels and a plurality of slices
And an X-ray CT apparatus comprising image processing means for performing image reconstruction from measurement data of the X-ray detector, wherein at least one channel is arranged at an end of the X-ray detector in the channel direction, and a plurality of X-rays are arranged. A detection element arranged in the slice direction in the same manner as the measurement channel detector block, a reference channel detector block for outputting a plurality of X-ray dose data for correcting measurement data of the measurement channel, and detection of the reference channel X-ray beam for detecting the presence / absence of the X-ray beam center in the slice direction due to the focal point movement of the X-ray tube and the direction of the movement (hereinafter referred to as X-ray beam center movement data) based on the X-ray dose data output from the detector block Center movement detecting means, and slicing the X-ray tube or the collimator to move the X-ray beam in the slice direction. An X-ray beam moving mechanism for moving the X-ray beam center in the direction of the reference channel on the detector block based on the X-ray beam center moving data. An X-ray CT apparatus comprising: an X-ray beam position control means for controlling.