JP2008212744A - X-ray ct device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To optimize the photographing condition more finely according to a helical pitch. <P>SOLUTION: This X-ray CT device adopting a helical scan system and coping with multi-scan for collecting projection data for a plurality of slices at the same time includes a tube current control means for controlling a tube current of an X-ray tube according to the moving speed by a driving means for moving an X-ray detection means and the X-ray tube relatively to a subject to draw a spiral trajectory. Thus, the photographing condition can be optimized more finely according to a helical pitch. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

The present invention relates to a multi-scan X-ray computed tomography apparatus that employs a scanning method that collects projection data in a spiral form from a subject, generally called a helical scan, and that simultaneously collects projection data for a plurality of slices.

An X-ray computed tomography apparatus (hereinafter referred to as an X-ray CT apparatus) is an X-ray target irradiated from the periphery of a subject by means of an X-ray tube and an X-ray detector, which are opposed to each other with the subject in between. The X-ray dose transmitted through the specimen is measured as projection data, and the data is reconstructed using a computer to obtain a tomographic image of the subject.

This X-ray CT apparatus has made tremendous progress since it was invented. In recent years, the X-ray CT apparatus is spirally moved with respect to the subject by continuously rotating the X-ray tube in one direction while moving the subject in the body axis direction. Multi-slice X-ray CT that simultaneously performs multi-slice tomography by providing a helical scan that collects projection data in a spiral form by irradiating X-rays in a spiral manner and a plurality of rows of X-ray detectors The device has been put into practical use. It is no exaggeration to say that such advances in X-ray CT apparatus are the result of tireless challenges to improving image quality, shortening imaging time and image reconstruction time, reducing X-ray exposure, etc. Has been.

In a known X-ray CT apparatus called a third generation, an X-ray tube and a multi-channel X-ray detector are arranged opposite each other with a subject interposed therebetween, and these are arranged at 360 ° around the subject. The subject is irradiated with an X-ray beam from the X-ray tube while being rotated, and the X-ray transmitted through the subject is detected by an X-ray detector. The X-ray intensity at this time is constant (that is, the tube voltage and tube current of the X-ray tube are constant). X-rays emitted from the focal point of the X-ray tube are collimated into a fan-shaped X-ray beam. Further, the spread width of the X-ray beam is determined according to the slice thickness or the like.

A set of projection data detected by the X-ray detector at a certain angle with respect to the subject is called a view, and a plurality of views are rotated while the X-ray tube and the X-ray detector are rotated around the subject. Measuring the transmitted X-ray dose in the direction is called a scan. A tomographic image of the subject can be obtained by reconstructing the projection data of a plurality of views obtained by scanning using a high-speed arithmetic device or the like.

By the way, when the subject is a person, the cross section from the chest to the waist is considered to be almost elliptical as shown in FIG. Accordingly, the amount of X-ray transmission differs between when X-rays are irradiated to the subject P from the angular position θ1 (ellipse major axis direction) and when the X-rays are irradiated from the angle position θ2 (ellipse minor axis direction). It will be. In particular, in the pelvis portion, the amount of X-ray transmission differs greatly between the plane side and the side surface of the subject P.

FIG. 6 shows the relationship between the angle position of the X-ray tube with respect to the subject and the transmitted X-ray dose, and the horizontal axis represents the X-ray irradiation angle with respect to the subject P (that is, the angle position of the X-ray tube), and the vertical axis. The axis is the transmitted X-ray dose. Therefore, as is apparent from FIG. 6, the transmitted X-ray dose is small in the major axis direction θ1 of the ellipse, and the transmitted X-ray dose is increased in the minor axis direction θ2 of the ellipse. Looking at the circumference, the transmitted X-ray dose changes periodically.

Therefore, when the periphery of the subject P is scanned with a constant X-ray intensity, the S / N ratio in the portion where the transmitted X-ray dose is large is high, but the S / N ratio in the portion where the transmitted X-ray dose is small is low, and the image The S / N ratio as a whole will decrease. Therefore, in order to increase the S / N ratio of the entire image, it is necessary to increase the transmitted X-ray dose as a whole so that the S / N ratio of the portion having a low S / N ratio is increased. However, when this is done, excessive X-rays are originally irradiated on the portion having a high S / N ratio, which leads to a problem of increasing the exposure dose as a subject.

In order to solve such a problem, the transmitted X-ray dose at one tomographic plane of the subject P is constant by controlling the tube voltage or tube current of the X-ray tube according to the angular position of the X-ray tube. It was proposed to scan to be. This proposal is effective in the case of so-called single scan for obtaining a cross-sectional image of one cross section, but as in recent years, along the body axis direction of the subject P by helical scan (helical scan). However, when a large number of tomographic scans are continuously performed, the objective of reducing the exposure dose has become unsatisfactory. The reason is,
The subject P has a different tissue structure depending on parts such as the chest, abdomen, waist, and legs, and has a non-uniform thickness in the body axis direction.
This is because, since the imaging range covers a wide range along the body axis direction of the subject P, a portion with a large amount of transmitted X-rays and a portion with a small amount of transmitted X-rays are mixed.

In view of this, in the X-ray CT apparatus of the type that performs helical scanning, the present applicant has previously proposed a technique for making the transmitted X-ray dose almost constant for each position in the angular direction and body axis direction around the subject. Is described in, for example, Japanese Patent No. 2768932.

The X-ray CT apparatus described in Japanese Patent No. 2768932 discloses an X-ray CT apparatus in a state where an X-ray tube is fixed at a predetermined angular position with respect to a subject prior to tomography by a helical scan (spiral scan). The so-called scano imaging, in which the X-ray detector detects the transmitted X-ray dose with respect to the two directions (for example, the plane direction and the side surface direction) with different angles, moves horizontally relative to the body axis of the subject while irradiating a line. To implement. Then, based on the transmitted X-ray dose of the subject at the time of the first scanography and the transmitted X-ray dose of the subject at the time of the second scanography, an appropriate X-ray generation amount pattern during the helical scan is calculated. When the helical scan is performed, the X-ray dose generated from the X-ray tube is equal to the pattern determined in advance according to the angle of the X-ray tube with respect to the subject and the position in the body axis direction. It controls the tube current of the X-ray tube.

However, this X-ray CT apparatus has a problem that it is necessary to carry out scanography twice in order to obtain an appropriate X-ray generation amount pattern at the time of helical scanning, and the exposure amount of the subject increases accordingly. . Further, when the tube current of the X-ray tube is controlled based on a pattern obtained by scanography, there has been a fundamental problem that if the subject moves after scanography, it does not make sense.

As a means for solving such problems, a proposal has been made to dynamically change the X-ray output during the rotation based on the projection data collected one or several rotations before in the helical scan. . However, in the helical scan, the top plate on which the subject is placed moves in the direction of the body axis with the rotation of the X-ray tube, so the portion of the subject from which projection data was collected before one rotation is projected at the current rotation. It is difficult to make the X-ray output appropriate, particularly in the chest where the bones are complicated and complicated, especially from the part of the subject from which data is to be collected.

In order to solve such a problem, the applicant of the present invention has made it possible to collect projection data for a plurality of slices at the same time by using an X-ray detector in which multi-channel X-ray detection elements have been configured in a row.
A so-called multi-slice X-ray detector having a plurality of line detection elements is used to adjust the X-ray detection element array for collecting projection data for image reconstruction and the X-ray output of the X-ray tube. And an X-ray detection element array that collects data for the purpose of use, and moreover on the spiral trajectory for helical scanning than the X-ray detection element array selected to acquire projection data for image reconstruction. The X-ray output of the X-ray tube is dynamically adjusted based on the output of another X-ray detection element array preceding this (this is an X-ray detection element array that was not selected for image reconstruction). The content of which is proposed in Japanese Unexamined Patent Publication No. 2000-262512.
Japanese Patent No. 2768932 JP 2000-262512 A

This proposal eliminates the deviation between the projection data collection position for image reconstruction in the slice direction and the basic data collection position for dynamically adjusting the X-ray output, thereby reducing exposure and optimizing the X-ray output. Has contributed. However, there has been a strong demand for further optimizing the imaging conditions in more detail according to the region to be imaged and the helical pitch.

  The present invention has been made to meet such a demand.

In order to solve the above-described problem, the invention described in claim 1 detects an X-ray tube that irradiates a subject with X-rays, and X-rays that are irradiated from the X-ray tube and transmitted through the subject. An X-ray detector having a plurality of rows of multi-channel X-ray detectors arranged in a row, and the X-ray detector and the X-ray tube draw a spiral trajectory relative to the subject. Drive means for moving the image data, data collection means for amplifying the output of the X-ray detection means and converting it into a digital signal, and image reconstruction means for reconstructing a tomographic image based on the projection data collected by the data collection means The X-ray CT apparatus includes a tube current control unit that controls a tube current of the X-ray tube according to a moving speed of the driving unit.

This eliminates the difference between the projection data collection position for image reconstruction in the slice direction and the basic data collection position for dynamically adjusting the X-ray output, contributing to low exposure and optimization of the X-ray output. In addition, the imaging conditions can be optimized more finely according to the part to be imaged and the helical pitch.

Hereinafter, an X-ray CT apparatus according to an embodiment of the present invention will be described in detail with reference to FIGS.

  FIG. 1 is a system diagram showing a schematic configuration of an embodiment of an X-ray CT apparatus according to the present invention.

As shown in FIG. 1, the X-ray CT apparatus according to the present invention includes an X-ray tube 2 that irradiates a subject P with X-rays toward a subject P, and an X-ray with the subject P in between. An X-ray detector 4 that is disposed so as to face the tube 2 and detects an X-ray dose transmitted through the subject P is provided, and these are configured to rotate together with the gantry 1. The X-ray tube 2 is given a tube voltage and a tube current by the high voltage generator 5. Further, the gantry 1 is rotationally driven by the rotational driving unit 6. In addition, X
The line detector 4 includes, for example, 800 channels of X-ray detection elements arranged in a line in accordance with the spread of the X-ray beam emitted from the X-ray tube 2 as schematically shown in a plan view in FIG. Is configured as a multi-slice-compatible X-ray detector 4 arranged in a plurality of rows in parallel in the slice direction (that is, the body axis direction of the subject P).

Then, in the space between the X-ray tube 2 and the X-ray detector 4 arranged to face each other, the bed top 7
A bed driving unit 8 is provided that positions the subject P placed on the bed and moves the bed top 7 horizontally in the body axis direction of the subject P. Note that the rotational speed of the gantry 1 that is rotationally driven by the rotational driving unit 6 (that is, the rotational speed of the X-ray tube 2 and the X-ray detector 4) is normally constant. The distance that the subject P moves during one rotation of the gantry 1 (that is, the X-ray tube 2 and the X-ray detector 4) according to the speed at which the couchtop 7) is moved in the body axis direction (this is referred to as a helical pitch). The helical pitch becomes smaller as the moving speed of the subject P is slower.

A data collection unit 9 for collecting X-ray transmission data of the subject detected by the X-ray detector 4;
An image reconstruction unit 10 for reconstructing an image based on the transmission data collected by the data collection unit 9;
A display unit 11 for displaying the reconstructed image is provided. Furthermore, the data collection unit 9
Is provided with a detector selection unit 13 for selecting transmission data corresponding to a desired column and / or channel of the X-ray detector 4.
The transmission data selected in is weighted appropriately and fed back to the high voltage generator 5.

A system controller 15 having a computer or a memory that performs a central function of organically controlling each unit constituting the X-ray CT apparatus, and an operator makes various settings for the system controller 15. An operation unit 16 for inputting values and instructions is also provided.

Note that the multi-slice X-ray detector 4 has a C in the channel direction as shown in FIG.
1 to Cn (for example, n = 800) and m columns (S1 to Sm in the slice direction)
For example, m = 32) X-ray detection elements are arranged in a matrix. And the operator
By specifying the slice thickness and the number of slices via the operation unit 16, the system control unit 1
5 selects at least one X-ray detection element array in the slice direction from the X-ray detector 4 according to the specified content. Accordingly, the image reconstruction unit 10 reconstructs an image based on the X-ray transmission data detected by the X-ray detection element array of the selected X-ray detector 4 and collected by the data collection unit 9.

For example, if the slice thickness is 1 mm and the number of slices is specified as 4, the four X-ray detection element rows in the center in the slice direction from the X-ray detector 4 as shown in FIG. Based on the X-ray transmission data selected, detected by the X-ray detection elements of each column, and collected by the data collection unit 9, the image reconstruction unit 10 reconstructs four adjacent slice images with a thickness of 1 mm. . Further, when the slice thickness is 2 mm and the number of slices is designated as 4, the center 8 in the slice direction from the X-ray detector 4 as shown as pattern B in FIG.
X-ray detection element rows in a row are selected, and X collected by two adjacent X-ray detection elements
In the image reconstruction unit 10, the line transmission data is converted into 2 pieces of X-ray transmission data for one slice.
Four adjacent slices of mm thickness are reconstructed.

Therefore, in the present invention, the X-ray detector 4 for multi-slice in which a plurality of channels and a plurality of rows of X-ray detection elements are arranged in a matrix form a desired X according to the imaging region and helical pitch of the subject P. Specify the line detection element, extract the output of the specified X-ray detection element,
The extracted output is fed back to the high voltage generator 5 so as to control the tube current of the X-ray tube 2 so as to obtain an optimum imaging condition. The X-ray detection element array to be selected is not used for collecting X-ray transmission data for image reconstruction, but an empty X-ray detection element array provided in the X-ray detector 4 is used. It is good. In the following description, when individual X-ray detection elements need to be specified, column numbers S1 to Sm in the slice direction
And the channel numbers C1 to Cn in the channel direction, for example, the X-ray detection element of the third channel in the second column is displayed as 4 (S2C3).

FIG. 3 is an explanatory diagram showing the relationship of the moving direction of the subject P with respect to the X-ray detector 4. It is assumed that the X-ray detector 4 is stationary and X-rays are irradiated under the condition that the subject P is moving from the right side to the left side of the drawing as indicated by the arrow. In FIG. 3, only five rows of X-ray detection element rows S1 to S5 are shown as the X-ray detector 4 for convenience. Therefore, the X-ray detection element array S1 precedes the subject P, and the X-ray obtained by the X-ray detection element array S1 is obtained.
If the tube current of the X-ray tube 2 is adjusted as the setting of the X-ray condition for the subsequent X-ray detection element rows S2 to S5 based on the line transmission data, any slice position in the body axis direction of the subject P can be obtained. Also, X-ray transmission data for good image reconstruction can be collected.

When the subject P moves in the direction opposite to the arrow shown in FIG. 3, the X-ray detection element array S5
It goes without saying that the X-ray conditions for the subsequent X-ray detection element arrays S4 to S1 may be set based on the X-ray transmission data obtained in the X-ray detection element array S5. . As described above, in the helical scan type X-ray CT apparatus including the multi-slice-compatible X-ray detector 4, an empty X-ray detection preceding the X-ray detection element array for collecting data for image reconstruction is performed. By using the element array, it is possible to optimize the X-ray output.

Therefore, in the present invention, this is further advanced, and among the multi-slice compatible X-ray detectors 4,
For example, with respect to the X-ray detection element row in the forefront row as shown by hatching in FIG. 2, the X-ray detection element 4 (S1C3) on the left peripheral side and the X-ray detection element 4 (S1Cj) in the substantially central portion, The X-ray detection element 4 (S1Cn-2) on the right peripheral side is set in advance so that it can be selected according to the target region of the subject P. Further, the X-ray detection element 4 on the left peripheral side with respect to the central X-ray detection element array
(SiC3), X-ray detection element 4 (SiCj) at the substantially central portion, X-ray detection element 4 (S
iCn-2), the X-ray detection element 4 on the left peripheral side with respect to the last X-ray detection element array.
(SmC3), X-ray detection element 4 (SmCj) in the substantially central portion, X-ray detection element 4 (S
mCn-2) and the like are also set in advance so that they can be selected according to the region of interest of the subject P.
These X-ray detection elements are not only set in advance, but may be selected whenever necessary.

That is, for example, when diagnosing the lung portion of the subject P, it is preferable to optimize the imaging conditions in the peripheral portion of the subject P. For this reason, for example, the X-ray detection element 4 (S1C3) on the left peripheral side The output is fed back to the high voltage generator 5 so that the tube current is determined by the output of the X-ray detection element 4 (S1Cn-2) on the right peripheral side. When diagnosing the heart, the subject P
It is preferable to optimize the imaging conditions in the central part of the high-voltage generating unit 5 so that, for example, the tube current is determined by the output of the X-ray detection element 4 (S1Cj) in the substantially central part. Feedback to Furthermore, when diagnosing the liver, it is preferable to optimize the imaging conditions for the entire abdomen of the subject P. For this reason, for example, the X-ray detection element 4 (S1C3) on the left peripheral side, X-ray detection element 4 (S1Cj) and right peripheral X-ray detection element 4 (S1Cn)
-2), or this output is fed back to the high voltage generator 5 so that the tube current is determined by the output of the entire X-ray detection element array S1.

Instead of selecting the X-ray detection element on the peripheral side or the center part according to the diagnosis site, for example, the above three types of X-ray detection elements in the channel direction or the output of the entire X-ray detection element array Thus, the weight may be appropriately weighted according to the diagnosis site and fed back to the high voltage generator 5. Furthermore, although the description has been given here on the assumption that the X-ray detection element row in the foremost row is used, the same applies to the case where the X-ray detection element row in the center or the last row is used.

Further, in the body axis direction of the subject, as shown in FIG. 4, the collection of structural diagnostic information is emphasized in the chest Ch, and the collection of systematic diagnostic information is emphasized in the abdomen Ab. . Similarly, there is diagnostic information that is similarly emphasized in the lower abdomen La, the waist Wa, and the like, and it is necessary to collect information accordingly. Therefore, it is effective to finely set the optimum shooting conditions even in accordance with the scan range and the helical pitch at the time of scanning. Therefore, the X-ray detection elements 4 (S1C3) and 4 (S1Cj) that are selected to determine the above tube current
4 (S1Cn-2), or X-ray detection element 4 (SiC3), 4 (SiCj), 4 (S
iCn-2), X-ray detection element 4 (SmC3), 4 (SmCj), 4 (SmCn-
2) or the like, the slice direction (column direction of the X-ray detection element) according to the scan range of the chest Ch, abdomen Ab, lower abdomen La, waist Wa, etc., or according to the size of the helical pitch. Is appropriately weighted and fed back to the high voltage generator 5.

FIG. 4 shows an example of a scano screen acquired for scanning positioning.

It goes without saying that the present invention is not limited to the above-described embodiment, and can be implemented in various forms. The X-ray detection element selected to determine the tube current for obtaining the optimum imaging condition is not limited to a specific one element, for example, the X-ray detection element 4 (S1Cj) in the first column j channel. A plurality of elements located on one side or both sides of the element may be regarded as one group. Furthermore, the X-ray detection element to be selected is an empty X-ray.
The X-ray detection element array used for collecting X-ray transmission data for image reconstruction may be used instead of selecting from the line detection element array.

(The invention's effect)
As described above in detail, according to the present invention, the projection for image reconstruction in the slice direction in the multi-scan X-ray CT apparatus that employs the helical scan method and simultaneously collects projection data for a plurality of slices. The gap between the data collection position and the basic data collection position for dynamically adjusting the X-ray output is eliminated, contributing to low exposure and optimization of the X-ray output, and depending on the region to be imaged and the helical pitch. Therefore, the shooting conditions can be optimized more finely.

1 is a system diagram showing a schematic configuration of an embodiment of an X-ray CT apparatus according to the present invention. It is the top view shown in order to demonstrate an example of the element arrangement | sequence in the X-ray detector corresponding to a multi slice. It is explanatory drawing which showed the relationship of the moving direction of the subject with respect to an X-ray detector. It is explanatory drawing of the scanning range demonstrated and shown as an example of a scan screen. It is explanatory drawing which showed the cross section of the subject typically. It is the characteristic view shown in order to demonstrate the relationship between the irradiation angle of the X-ray tube with respect to a subject, and transmitted X-ray dose.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Stand 2 X-ray tube 4 X-ray detector 5 High voltage generation part 6 Rotation drive part 7 Bed top plate 8 Bed drive part 9 Data collection part 10 Image reconstruction part 11 Display part 13 Detector selection part 15 System control part 16 Operation part

Claims (3)

An X-ray tube that irradiates the subject with X-rays, and an X-ray that is irradiated from the X-ray tube and passes through the subject
X-ray detection means for detecting a line and having a plurality of rows of multi-channel X-ray detection elements arranged in a row, and the X-ray detection means and the X-ray tube spirally relative to the subject Driving means for moving the image so as to draw a trajectory, data collection means for amplifying the output of the X-ray detection means and converting it into a digital signal, and an image for reconstructing a tomographic image based on projection data collected by the data collection means In an X-ray CT apparatus comprising a reconstruction means,
An X-ray CT apparatus comprising: a tube current control unit that controls a tube current of the X-ray tube in accordance with a moving speed of the driving unit. In the output of the data collection means corresponding to a desired X-ray detection element in the X-ray detection means,
Weighting means for weighting according to the spiral pitch based on the moving speed by the driving means,
The tube current control means controls the tube current of the X-ray tube while moving so as to draw the spiral trajectory based on the output of the data collection means weighted by the weighting means. The X-ray CT apparatus according to claim 1. A selection unit that selects an output of the data collection unit corresponding to a desired X-ray detection element in the X-ray detection unit according to an imaging region of the subject;
The weighting unit weights a plurality of outputs of the data collection unit selected by the selection unit according to a helical pitch based on a moving speed by the driving unit,
The tube current control means is selected by the selection means while moving so as to draw the spiral trajectory, and based on the output of the data collection means weighted by the weighting means, the X-ray tube The X of claim 2, wherein the tube current is controlled.
Line CT device.

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