JP2003159243A - X-ray computer tomography apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a multislice-X-ray computer tomography apparatus capable of securing high-quality images in total slices and yet minimizing the exposure dose of radiation as well. <P>SOLUTION: This X-ray computer tomography apparatus comprises an X-ray tube 1, aperture-variable collimater 2, X-ray detector 4, reconstructor 11 and a frame controller 8. The X-ray tube 1 generates X-rays conically expanding. The collimater 2 controls the conical angles of X-rays. In the X-ray detector 4, detection elements to detect X-rays passing through a specimen are arranged in a plurality of rows in the slice-direction. The reconstructor 11 reconstructs image data based on the output of the X-ray detector 4. The frame controller 8 controls the aperture of the collimator 2 correspondingly to the width and diameter of FOV (field of vision) of the region of interest. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an X-ray computed tomography apparatus equipped with a multi-slice type X-ray detector in which a plurality of detection elements for detecting X-rays transmitted through an object are arranged in the slice direction. Regarding

[0002]

2. Description of the Related Art Recent technological developments related to X-ray computed tomography apparatus are remarkable. The main examples are high-speed continuous scan by slip ring, helical scan that realizes high-speed volume scan,
CT fluoroscopy that allows you to observe fluoroscopic images on the spot while scanning like an X-ray television system, multi-slice X-ray detector with multiple channels in the body axis direction (slice direction) (also called cone-beam X-ray detector) There is a multi-slice scan which realizes simultaneous scanning of a plurality of slices using.

In the above multi-slice type X-ray detector, X
The development of a semiconductor detection element capable of directly converting a line into a signal charge and its mounting technology are progressing, and there is a tendency that the number of rows is further advanced.

However, as the multi-slice type X-ray detector has multiple rows, it is required to increase the cone angle of X-rays. This X
The expansion of the cone angle of the line causes the deterioration of the image quality of the slices at both ends. This problem will be described below.

First, adjustment of the X-ray diaphragm opening in the case of a single slice will be described with reference to FIG. In FIG. 5, the Z axis coincides with the rotation center axis of the X-ray tube 101 and the X-ray detector 103. Prior to photographing, as a photographing condition, in addition to the tube voltage, the tube current, the scan time, etc., the diameter (indicated by a radius or a diameter) of a circular, actually thin cylinder-shaped photographing area (FOV). , Which will be described here as a radius), and the slice thickness is set by the operator.

In the gantry controller on the device side, the X-ray diaphragm 10
The opening degree of 2 is adjusted so that the thickness of the X-ray flux matches the set slice thickness on the Z axis. At this time, in the edge portion of the shooting area FOV indicated by the diagonal lines in FIG.
When the line is not illuminated, that is, when viewed for each view, the projection data of that part is missing. In the opposite view,
Although the data is collected, the lack of the data affects the deterioration of the image quality at the peripheral portion of the tomographic image. However, in actuality, in the single slice, the volume ratio of the data missing portion to the imaging area FOV is very limited, and it did not cause much problem.

However, this problem is exposed in multi-slice. In FIG. 6, as an example of multi-slice,
The geometrical relationship between the X-ray irradiation range and the imaging region when three slices are set with the same slice thickness is schematically shown. Regarding the central slice S2 of the three slices, there is almost no data missing portion. But 2 at both ends
With regard to the slices S1 and S3, most of the edge portions indicated by diagonal lines lack data. That is, when viewed in slice units, the volume ratio of the data missing portion to the imaging region FOV becomes extremely high in the slices S1 and S3 at both ends, and the image quality deterioration of the two slices S1 and S3 at both ends is not suitable for interpretation. Falls to.

Therefore, as shown in FIG. 7, in order to prevent the image quality deterioration of the slices S1 and S3 at both ends, the X-ray diaphragm 102 is arranged in accordance with a virtual slice width which is expanded by ΔW before and after the actual slice width. It is under study to set the opening degree of a little wider.

With this opening degree setting method, it is possible to suppress deterioration of image quality to some extent, but as shown in FIG. 8, when the shooting area (FOV) is set to a small diameter, it is outside the shooting area (FOV). Since X-rays are also excessively irradiated to the area (2), a new problem of increasing the exposure dose occurs. That is, in the case of the conventional studied method, since the opening of the X-ray diaphragm is determined so that the X-ray is irradiated wider than the actual slice width, and the diameter of the imaging region is not taken into consideration. Depending on the situation, there is a risk of giving extra radiation.

[0010]

SUMMARY OF THE INVENTION An object of the present invention is to provide an X-ray computed tomography apparatus having an X-ray detector in which a plurality of detector elements for detecting X-rays transmitted through an object are arranged in the slice direction. That is, both ensuring high image quality in the entire imaging region and minimizing exposure dose.

[0011]

An X-ray computed tomography apparatus according to the present invention includes an X-ray tube for generating X-rays that spread in a cone shape, and a collimator with a variable opening for limiting the cone angle of the X-rays. An X-ray detector in which a plurality of detection elements for detecting X-rays transmitted through an object are arranged in a slice direction, a reconstruction device for reconstructing image data based on the output of the X-ray detector, and the collimator. And a control device that controls the opening degree of the control unit according to the width and the diameter of the imaging region.

[0012]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An X-ray computed tomography apparatus according to the present invention will be described below with reference to the accompanying drawings with reference to the accompanying drawings. The scanning method of the X-ray computed tomography apparatus is a rotate / rotate type in which an X-ray tube and an X-ray detector are integrally rotated around the subject, and a large number of detection elements arranged in a ring shape are fixed. There are various types such as a stationary / rotate type in which only the X-ray tube rotates around the subject, and the present invention can be applied to any type. Here, the rotate / rotate type will be described as an example. Further, the present invention is not limited to one pair of the X-ray tube and the X-ray detector, and is a so-called multi-tube type in which a plurality of pairs of the X-ray tube and the X-ray detector are provided at mutually different angles. However, one pair will be described here as an example. Furthermore, in order to reconstruct one tomographic image, one round of the circumference of the subject is about 360 °.
Minute projection data, and also the half scan method requires 180 ° + fan angle projection data.
The present invention can be applied to either method.

FIG. 1 is a view showing the arrangement of an X-ray computed tomography apparatus according to an embodiment of the present invention. The X-ray tube 1 and the multi-slice X-ray detector 4 are a bed 3 and a bed 3
The rotating frames (not shown) are provided in a positional relationship facing each other with a distance required to secure a space for inserting the upper subject P. The multi-slice X-ray detector 4 is defined as an X-ray detector having a function of simultaneously collecting projection data of a plurality of slices, and typically, as shown in FIG. It has a structure in which a plurality of rows of arrayed detecting elements are connected in the slice direction, or a structure in which a plurality of detecting element modules in an n × m array are connected in two directions of the channel and the slice or in one direction of the channel.

A collimator (also referred to as an X-ray diaphragm device) 2 is attached to the front surface of the X-ray emission window of the X-ray tube 1. The collimator 2 has a structure that individually changes the opening degree in the channel direction and the opening degree in the slice direction. Typically, a plurality of X-ray shield plates and a mechanism for individually movably holding the X-ray shield plates are further provided. It is composed of an X-ray shield driving unit for individually moving the X-ray shield electrically. The X-ray fan angle is limited by the opening of the collimator 2 in the channel direction.
The cone angle of the X-ray is limited by the opening degree in the slice direction.

A signal having a peak value corresponding to the X-ray intensity output from each channel of the multi-slice type X-ray detector 4 is passed through a slip ring to a data acquisition unit (also called DAS) 5 for example. It is captured. The data collecting unit 5 amplifies the detection signal individually for each channel,
It is converted into a digital signal and output as projection data to the image reconstruction device 11. In some cases, the data collection device 5 may bundle the detection signals of a plurality of channels to perform amplification and digital conversion. The image reconstruction device 11 reconstructs tomographic image data in multi-slices based on the projection data. The tomographic image data is displayed on the image display device / operation console 12 and sent to the image storage device 10 to be stored in a large-capacity storage medium such as a magneto-optical disk device.

A series of operations from data acquisition to reconstruction, image display and storage are performed under the control of the computer 9. In addition to these signal processing controls, the computer 9 functions as a control center for setting the scanning environment of the device according to the imaging conditions set via the image display device / operation console 12 and a control of the entire device such as a gantry control during scanning. .

The photographing conditions set via the image display device / operation console 12 include, in addition to the tube voltage, the tube current, the scanning time, etc., a cylindrical photographing area (FO) having a thickness.
The diameter of V) (represented by a radius or a diameter, which will be described here as a radius), the slice thickness, and the number of slices are set by the operator. The size of the shooting area FOV is
It is defined by the radius and the slice width. further,
The slice width is determined by the slice thickness × the number of slices.

The computer 9 supplies the set tube voltage and tube current data to the X-ray controller 7. The X-ray controller 7 controls the high voltage generator 6 so that the X-ray is irradiated from the X-ray tube 1 with this tube voltage and tube current. Also,
The computer 9 supplies the set scan time data to the gantry control device 8. The gantry control device 8 controls a rotary frame drive device (not shown) so that the X-ray tube 1 makes one revolution around the subject during the scan time.

Further, the computer 9 supplies to the gantry control device 8 the data of the set slice thickness and the number of slices, and the set radius of the imaging region FOV.
The gantry controller 8 controls the X-ray shield drive unit of the collimator 2 so that the opening degree of the collimator 2 in the slice direction becomes a distance according to the supplied slice thickness, the number of slices, and the radius of the imaging region FOV. To do.

In FIG. 3, the radius of the photographing area FOV is R1.
2 schematically shows the geometrical relationship between the X-ray irradiation range and the imaging region when the three slices S1, S2, S3 are set with the same slice thickness. In FIG. 4, the shooting area FO
The radius of V is R2 (R2 <R1), and three slices S
1 schematically shows the geometrical relationship between the X-ray irradiation range and the imaging region when 1, S2 and S3 are set with the same slice thickness. Please refer to these FIG. 3 and FIG. 4 in comparison with the conventional FIG. Conventionally, as described above, the collimator 2
The opening degree of is adjusted so that the thickness of the X-ray flux matches the slice width on the Z axis. Therefore, as shown by the diagonal lines in FIG. 5, data is missing at the peripheral portion of the shooting area FOV. That is, conventionally, the opening of the console is adjusted based on the slice width (slice thickness × the number of slices).

On the other hand, in this embodiment, not only the slice width (slice thickness × the number of slices) but also the photographing area F
The opening of the collimator 2 is adjusted in consideration of the radius of the OV.

Specifically, the opening degree of the collimator 2 is adjusted so that X-rays are irradiated to the entire photographing area FOV.
More specifically, the opening degree of the collimator 2 is adjusted so that the thickness of the X-ray flux in the slice direction matches the slice width at a position closer to the X-ray tube 1 by the radius of the imaging region FOV from the Z axis. To be done. As a result, data loss does not occur over the entire shooting area FOV, in other words, projection data is collected over the entire shooting area FOV. Therefore, not only the central slice S2 but also the slices S1 and S3 at both ends do not cause image quality deterioration due to data loss. Moreover, since the X-rays are not radiated to the outside of the imaging region FOV, it is possible to minimize the exposure dose.

As a preferable mounting method, slice thickness,
A table that inputs three kinds of data of the number of slices and the radius of the imaging area FOV and outputs the opening of the collimator 2 corresponding to the ROM is installed in the gantry control device 8 (or the computer 9). Of course, the opening degree of the collimator 2 corresponding to the three parameters of the slice thickness, the number of slices, and the radius of the imaging region FOV may be calculated by the gantry control device 8 (or the computer 9) by geometric calculation. Good.

The cone angle of X-rays has an upper limit depending on the width of the multi-slice type X-ray detector 4 in the slice direction. That is, when the imaging area FOV is set to be relatively large, the opening of the collimator 2 is adjusted so that the entire area of the imaging area FOV is irradiated with X-rays. It extends beyond the sensitivity region in the slice direction of the container 4 to the outside thereof. In order to avoid this situation, the gantry control device 8
The opening when the X-ray irradiation field coincides with the sensitivity region in the slice direction of the multi-slice X-ray detector 4 is set as a control limiter for the collimator 2 or the shield plate of the collimator 2 is moved. The range is physically limited.

The situation where this limiter works and X-rays are not irradiated to a partial area of the imaging area FOV is as follows: slice width (slice thickness × number of slices), radius of imaging area FOV, The maximum width of the sensitivity area in the slice direction, and if it is variable, the X-ray tube 1
It can be easily determined by geometric calculation based on the distance between the X-ray focal point and the multi-slice X-ray detector 4. The gantry control device 8 or the computer 9 has a function of determining the above-mentioned situation, a function of displaying a message to that effect or a message prompting the resetting on the image display device / operation console 12 when the above-mentioned situation is determined, and further the set photographing. It is equipped with a function to calculate the volume ratio of the area not irradiated with X-rays to the area FOV and display the value. As an operator, based on this volume ratio, he / she can determine forcibly to perform the scan with the setting and reset and can input a command according to the determination.

The present invention is not limited to the above-described embodiment, and can be carried out in various modifications without departing from the scope of the invention in an implementation stage. For example, in the above-described embodiment, the opening of the collimator is determined so that the shooting area is entirely included in the main shadow, but the opening of the collimator is determined so that the shooting area includes a slight penumbra other than the main shadow. May be.

[0027]

According to the present invention, high image quality can be ensured over the entire imaging region, and because X-rays can be irradiated to only the minimum necessary region for that purpose, the exposure dose can be minimized. You can

[Brief description of drawings]

FIG. 1 is a configuration diagram of an X-ray computed tomography apparatus according to an embodiment of the present invention.

FIG. 2 is a perspective view of the multi-slice X-ray detector of FIG.

FIG. 3 is a schematic diagram showing a geometrical relationship between an X-ray irradiation range and an imaging region depending on the opening of a collimator controlled by the gantry control device of FIG.

FIG. 4 is a schematic diagram showing a geometrical relationship between an X-ray irradiation range and an imaging region depending on the opening of a collimator controlled by the gantry control device of FIG. 1 when the radius of the imaging region is shorter than the example of FIG. Fig.

FIG. 5 is a schematic diagram showing a geometrical relationship between an X-ray irradiation range and an imaging region in a conventional single slice.

FIG. 6 is a schematic diagram showing a geometrical relationship between an X-ray irradiation range and an imaging region when the conventional single-slice X-ray aperture opening control is applied to multi-slices.

FIG. 7 is a schematic diagram showing conventional improved X-ray aperture opening control.

FIG. 8 is a schematic diagram showing a problem of the conventional improved X-ray aperture opening control.

[Explanation of symbols]

1 ... X-ray tube, 2 ... Collimator, 3 ... sleeper, 4 ... Multi-slice X-ray detector, 5 ... Data collection unit, 6 ... High voltage generator, 7 ... X-ray control device, 8 ... Frame control device, 9 ... Computer, 10 ... Image storage device, 11 ... Image reconstruction device, 12 ... Image display device / operation console.

Claims (7)

[Claims] 1. An X-ray tube that generates X-rays that spread in a cone shape, a collimator with a variable opening that limits the cone angle of the X-rays, and a detection element that detects the X-rays that have passed through the subject are in the slice direction. An X-ray detector arrayed in a plurality of rows, a reconstruction device that reconstructs image data based on the output of the X-ray detector, and an opening of the collimator is controlled according to a width and a diameter of an imaging region. An X-ray computed tomography apparatus, comprising: a control device. 2. The X-ray computed tomography apparatus according to claim 1, wherein the control device controls the opening degree of the collimator so that the X-ray is irradiated to the entire imaging region. 3. The controller controls the thickness of the X-ray flux in the slice direction to match the width of the imaging region at a position closer to the X-ray tube from the Z axis by the radius of the imaging region. The X-ray computed tomography apparatus according to claim 1, wherein the opening degree of the collimator is controlled. 4. The X-ray computed tomography apparatus according to claim 1, wherein the control device has a storage unit that stores the relationship between the width and the diameter of the imaging region and the opening degree of the collimator. 5. The width and diameter of the imaging region are set to values that cause a situation in which X-rays cannot irradiate the entire region of the imaging region due to physical size restrictions of the X-ray detector. The X-ray computed tomography apparatus according to claim 2, further comprising means for outputting a message prompting the resetting of the situation or the width and diameter of the imaging region. 6. An X-ray tube that generates X-rays that spread in a cone shape, a collimator with a variable opening that limits the cone angle of the X-rays, and a detection element that detects the X-rays that have passed through the subject are in the slice direction. An X-ray detector arranged in a plurality of rows, a reconstruction device for reconstructing image data based on the output of the X-ray detector, and a console for inputting conditions relating to slice thickness, the number of slices, and the diameter of the imaging region. An X-ray computed tomography apparatus comprising: a controller that controls the opening of the collimator based on the input slice thickness, the number of slices, and the diameter of the imaging region. 7. An X-ray computed tomography apparatus for reconstructing image data based on data collected from an imaging region by an X-ray tube and an X-ray detector, wherein the X-ray is determined according to a width and a diameter of the imaging region. An X-ray computed tomography apparatus, characterized in that the spread angle of X-rays emitted from a X-ray tube is adjusted.