JP2000070254A - X-ray detector - Google Patents

Abstract

PROBLEM TO BE SOLVED: To remove the influence of scattered X-rays without deteriorating detecting efficiency of a main X-ray compared with a conventional one- dimensional array detector by allowing an X-ray detecting element string at both end parts of plurally arrayed slice direction X-ray detecting element string to detect scattered X-rays. SOLUTION: Since a collimator board 5 is not placed in the direction of separating a slice though it is placed in the direction of separating a channel, the scattered X-rays incident in parallel with the board 5 is not attenuated by the board 5 but is made to enter a scintillator to be a part of an output current. At this time, in the X-ray detecting element string in a slicing direction, the elements at both of the ends are used as elements for measuring the scattered X-rays but not used as elements for measuring the X-ray transmission of an examinee 10. Thus, an opening restricting collimator 14 restricting X-rays radiated from an X-ray tube in the slicing direction prohibits incidence to an outermost slice scintillator 1a. Consequently, the scintillator 1a makes only the scattered X-rays from the examinee 10 incident.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an X-ray detection technique for an X-ray CT apparatus, and more particularly to an X-ray detection technique for an X-ray CT apparatus capable of simultaneously measuring a plurality of slices of an image.

[0002]

2. Description of the Related Art In recent years, solid-state detectors using scintillators have been widely used as X-ray detectors used in X-ray CT apparatuses. This CT solid-state detector includes a scintillator that converts incident X-rays into light, and a photoelectric conversion element such as a silicon photodiode that detects light converted by the scintillator and outputs it as an electric signal. And a large number of channels arranged in an arc shape.

[0003] In the X-ray CT apparatus, it is desired to reduce the time required for one scan in order to improve the throughput of the apparatus. There are the following two methods for shortening the time. (1) Reduction of time required per rotation (2) Increase in tomographic images that can be captured per rotation

Regarding (1), the rotation speed has been improved by reducing the weight of an X-ray tube as an X-ray generator.

[0005] On the other hand, the case (2) is achieved by arranging a plurality of detectors which have been arranged one-dimensionally in a row in the slice direction as shown in FIG. 9 (multi-slice X-ray detection). vessel). In the method (1), the acceleration applied to the scanner components increases with an increase in the rotation speed, so that it is necessary to increase the strength of each component used. Further, in order to maintain the same measurement accuracy as before even when the rotation speed is increased, it is necessary to increase the irradiation X-ray dose within the data sampling interval by an amount corresponding thereto. This is because, when the tube voltage of the X-ray tube is the same, the amount of X-ray photons that affects the image quality of the CT image is increased or decreased by (tube current) × (irradiation time). Even if the rotation speed is increased, the ratio of the X-rays emitted from the X-ray tube that contributes to the measurement of the subject (X-ray use efficiency) does not change. (2)
A multi-slice detector in which a plurality of detector elements are arranged in the slice direction can perform a wide range of measurement in a shorter time than a conventional device without increasing the rotation speed, and furthermore, the X-rays radiated from the X-rays can be measured. Usage efficiency is improved. The improvement in X-ray utilization efficiency means that an X-ray tube, which is an expensive consumable, can be used efficiently, which means that the amount of tube deterioration per inspection is reduced. From such a viewpoint, the multi-slice detector (2) is excellent not only in improving the patient throughput but also in improving the use efficiency of the X-ray tube.

[0006] There are also some problems caused by using this multi-slice detector in which X-ray detecting elements are two-dimensionally arranged. One of them is the influence of scattered radiation. X-rays emitted from the X-ray tube pass through the subject
The light enters the line detecting element and is converted into an electric signal according to the intensity. However, part of the X-rays incident on the subject is then scattered. Therefore, X-rays incident on the X-ray detection element are:
This means that main X-rays transmitted through the subject from the X-ray tube focal point and scattered X-rays scattered from the entire range of the subject irradiated with the X-rays are mixed.

Here, the main X-ray transmitted through the subject from the focal point of the X-ray tube has information corresponding to the amount of X-ray attenuation of the subject at the position of the X-ray detecting element. Does not have information corresponding to the amount of X-ray attenuation of the subject.

[0008]

As described above, in the above-mentioned prior art, it is necessary to install a collimator on the X-ray incident side of the X-ray detecting element in order to eliminate the influence of scattered X-rays. The collimator is made by arranging plates made of a material having a small X-ray transmittance along the detection element pitch along the radiation centered on the focal point of the X-ray tube. By arranging such plates, the main X-rays incident on the X-ray detector from the focal point of the X-ray tube slightly attenuate by the thickness of the collimator plate and reach the X-ray detection element.
However, scattered X-rays incident from an oblique direction pass through a collimator plate having a small X-ray transmittance, and are greatly attenuated before reaching the X-ray detection element. However, in a structure in which many collimator plates are arranged, scattered X-rays from a direction intersecting the collimator plates can be removed, but scattered X-rays incident from a direction parallel to the collimator plates can hardly be removed. In a multi-slice detector in which X-ray detection elements are arranged two-dimensionally, in order to eliminate the influence of scattered X-rays, a structure is adopted in which two types of collimators are stacked so that the orientation of the collimator plate is orthogonal. It is necessary to have a structure in which the plates are combined in a grid. However, in any case, it is inevitable that the structure becomes complicated and it is difficult to ensure the accuracy, and that the transmittance of the main X-rays is reduced and the output of the X-ray detection element is reduced. was there.

SUMMARY OF THE INVENTION The present invention has been made to solve the above problems, and has as its object to reduce the scattered X-rays without lowering the detection efficiency for main X-rays as compared with a conventional one-dimensional array detector. An object of the present invention is to realize an X-ray CT apparatus capable of removing the influence.

[0010]

SUMMARY OF THE INVENTION An object of the present invention is to provide a scintillator for detecting an X-ray beam narrowed by a diaphragm mechanism to convert the X-ray from an X-ray source into an analysis and converting the X-ray into light, and a light converted by the scintillator. A plurality of X-ray detecting elements arranged in a channel direction in the form of a plurality of X-ray detecting elements having a photoelectric conversion element for converting the X-ray into an electric signal in an arc shape or a polygon centering on the focal point of the X-ray source. In the X-ray detector arranged in a row, the X-ray detection element rows at both ends of the slice-direction X-ray detection element row arranged in a plurality of rows are scattered X-rays.
This is achieved by an X-ray detector characterized in that the X-ray detection element array detects only lines.

A scintillator which emits light by detecting an incident X-ray and a photodetector which generates an output signal corresponding to the dose of the X-ray detected by the scintillator by receiving the light emitted from the scintillator are combined. X
An X-ray detector for a multi-slice CT apparatus having a plurality of sets of X-ray detection elements in which a plurality of X-ray detection elements are arranged in an arc shape or a polygon centering on the X-ray tube focal point is provided at an X-ray tube emission port. X outside the slice direction of the detector incident X-ray beam limited by the X-ray diaphragm device
A combination of a channel direction collimator in which thin plates made of a material having a large X-ray absorption coefficient and having a dimension covering the entire incident portion of the plurality of X-ray detection element rows are arranged radially in the channel direction. Achieved by the featured X-ray detector for multi-slice CT apparatus.

Further, the output of the main X-ray measuring X-ray detecting element array to which the X-ray beam from the standard attenuator is incident beforehand by using the detector and the X-ray detecting X-ray placed outside the X-ray beam. Obtain the relationship with the output of the array of X-ray detection elements, create scattered radiation correction data, measure the output of the X-ray detection element array for scattered X-rays placed outside the incident X-ray beam at the time of measurement of the subject, and measure the scattering. The ratio of the scattered radiation included in the output of the main X-ray measurement X-ray detection element array is calculated by referring to the data for X-ray measurement, and the ratio of the scattered X-rays from the subject is calculated by subtracting the ratio from the output of each element. This is achieved by a scattered radiation correction method that removes the influence of radiation and performs highly accurate measurement.

More specifically, for the scattered radiation in the channel direction, a collimator in which a large number of plates are arranged radially as in the prior art is provided, and the scattered radiation in the slice direction reaches outside the irradiation field of the main X-ray beam. The intensity of the scattered radiation is measured, and the scattered dose mixed into the irradiation field of the main X-ray beam is estimated from the measured value to make a correction. According to this method, it is possible to limit the alignment between the X-ray detecting element and the scattered radiation removing collimator only in the channel direction without taking a complicated structure, and the effect of the two-dimensionally distributed scattered X-rays becomes possible. Can be effectively removed.

[0014]

FIG. 1 shows the appearance of an X-ray detecting element array.
Shown in The scintillator 1 is divided by separation layers 21 and 22 in the channel direction and the slice direction. On the lower surface of the scintillator 1, a photoelectric conversion element such as a photodiode is arranged (not shown). The X-ray detecting element array constituted by these components is mounted on the substrate 3, and the output current of each X-ray detecting element can be extracted to the outside by the connector 4 provided at the end of the substrate 3. When an X-ray incident from above in the figure enters the scintillator 1, the scintillator 1 emits light, and the light is received by the photodiode to generate an output current. At this time, the emission intensity of the scintillator 1 changes depending on the intensity of the incident X-ray, and the signal current taken out also corresponds to the intensity of the incident X-ray. X
Since the line detection element array is separated in the channel direction and the slice direction, it is possible to measure the two-dimensional intensity distribution of the incident X-ray.

FIG. 2 shows the structure of a collimator array that attenuates scattered X-rays incident from an oblique direction and efficiently guides only main X-rays incident from a focal point direction to an X-ray detecting element.
The collimator array has a thin collimator plate 5 made of a material (molybdenum, tungsten, lead, or the like) having a large X-ray attenuation rate, with one end facing the X-ray tube focal point and the other end facing the separation layer 21 of the scintillator. Are fixed by supporting members 6 at both ends. Although not explicitly shown in FIG. 2, the respective collimator plates 5 are not arranged in parallel with each other, but are arranged radially with a predetermined opening angle.

FIG. 3 shows a positional relationship in which the X-ray detecting element array and the collimator array described above are combined.
As can be seen from the figure, the collimator plate 5 can effectively remove scattered X-rays obliquely incident in the channel direction, whereas the collimator plate 5 can obstruct scattered X-rays obliquely incident in the slice direction. The removal effect by 5 cannot be expected, and the effect remains. A method of removing the influence of the scattered X-rays incident in the slice direction will be described later.

FIG. 4 shows an overall view in which the X-ray detector 8 in which the X-ray detection element array and the collimator array are combined is incorporated in the rotating plate 12 of the CT scanner. Rotating plate 1
A hole is made in the center of 2 and a subject 10 (patient) is placed in the center, and the whole can be rotated around this. An X-ray tube 9 is placed at one end on the rotating plate, and an X-ray detector 8 is placed so as to oppose the subject 10 with the X-ray tube 9 interposed therebetween. Inside the X-ray detector 8, an X-ray detection element array 7 is arranged in a polygon behind the collimator plate 5 described above. The X-rays emitted from the X-ray tube 9 and having passed through the subject 10 and having reached the X-ray detector 8 pass through the space where the collimator plates 5 are arranged, and then reach the scintillator surface of the X-ray detection element array 7. Therefore, the intensity information of the incident X-ray is converted into an electric signal, sent to the subsequent signal amplifier 11, amplified, and then converted into digital data. While the rotating plate 12 rotates, the amount of X-ray transmission from each direction of the subject 10 is measured, and the information is transmitted to the image processing device, and the tomographic image of the subject 10 is reconstructed.

The relationship between the collimator, the X-ray detecting element, the main X-ray radiated from the focal point of the X-ray tube and transmitted through the subject, and the scattered X-ray generated by the subject will be described with reference to FIGS. FIG. 5 shows a cross section in the channel direction of the X-ray detecting element. Since the collimator plate 5 is arranged radially perpendicular to the channel direction, scattered X-rays generated in a direction obliquely passing through the collimator plate are
Is a material having a large X-ray attenuation rate, most of the material is attenuated, and the effect of scattered X-rays does not appear.

Next, FIG. 6 shows a cross section in the slice direction orthogonal to the channel direction. Although the collimator plate 5 is placed in the direction of separating the channels, but not in the direction of separating the slices, the scattered X-rays incident parallel to the collimator plate 5 may be attenuated by the collimator plate 5. Without being incident on the scintillator 1 and becomes part of the output current. Here, in the row of X-ray detecting elements in the slice direction, the elements at both ends are used as scattered X-ray measuring elements,
It is not used as an element for measuring line transmittance. For this reason, the X-ray radiated from the X-ray tube 9 is prevented from being incident on the outermost slice scintillator 1a by the aperture limiting collimator 14 for limiting the X-ray in the slice direction. Thereby, the object 10 is placed on the outermost slice scintillator 1a.
Only scattered X-rays from

By paying attention to the output of the X-ray detecting element corresponding to the outermost slice scintillator 1a,
Internal slice scintillator 1 for measuring X-ray transmittance of 0
It is possible to assume the amount of scattered X-rays incident on b. FIG. 7 illustrates a method of estimating the scattered X-ray dose. Before measuring the subject, the standard attenuator 15 is placed in place of the subject, and the X-ray shielding block is arranged so as to correspond to a slice position where the main X-ray measuring element is located at a part of the opening of the aperture limiting collimator 14. Place 16. X-ray irradiation is performed in this state, and the output of each detection element is measured. The main X-ray is not incident on the scintillator 1b of the slice corresponding to the position of the X-ray shielding block 16 and the scattered X from the standard attenuation body 14
Only the line will be incident. By repeating the measurement by sequentially shifting the position of the X-ray shielding block 16 to the position of the main X-ray measurement element of each slice, the distribution of the scattered X-ray intensity at all slice positions can be known. In addition, by using the standard attenuator 15 having a different size, the scattering X
It is possible to know in advance the distribution of the scattered X-ray intensity when there are many or few lines.

FIG. 8 shows the relationship between each slice position thus measured and the output of the detection element. In the figure, eight X-ray detectors are arranged in the slice direction, both outermost slices are used as scattered X-ray detectors, and six internal X-ray detectors are used for measuring the transmitted X-ray intensity of the subject. Element. The outputs of the main X-ray measurement detecting elements in the second slice to the seventh slice include the main X-ray transmitted through the subject and the scattered X-ray scattered from other portions of the subject. What is important when performing image reconstruction is the main X-rays that have passed through the subject and have information on the amount of attenuation of the subject. Scattered X-rays scattered from other parts are factors that deteriorate the measurement accuracy. It is only. In order to obtain an output using only the main X-rays, the output is obtained by subtracting the output of the scattered X-rays from the output that is normally measured (the output of the main X-rays and the scattered X-rays).
From the measurement result of the scattered X-ray intensity distribution using the standard attenuator 15 and the X-ray shielding block 16 described above, the scattered X-rays incident on the scattered X-ray detection element of the outermost slice and the X-ray detection element of the inner slice are determined. By obtaining the ratio in advance, the scattered X mixed into the X-ray detection element of the inner slice from the output of the scattered X-ray detection element of the outermost slice at the time of measurement of the subject is measured.
It is possible to assume (simulate) the amount of lines.
By subtracting the scattered X-ray dose assumed in this way from the actually measured output, the correct X-ray attenuation amount of the subject from which the influence of the scattered X-rays has been removed can be obtained. The distribution of the scattered X-rays in the slice direction is a gentle distribution with a slightly raised center, but this shape varies depending on the dimensions in the slice direction of the X-ray detection element row, and is constant for all slices when the dimensions in the slice direction are not very large. The scattered X-ray correction may be performed on the assumption that an amount of scattered X-rays is incident. When the dimension of the X-ray detecting element row in the slice direction is large, there is a method of approximating the distribution of the scattered X-ray intensity by a function such as a quadratic curve. In addition, the value of the actually measured scattered X-ray intensity itself is stored in a table, and the scattered X-ray dose incident on the X-ray detection element array of the inner slice from the output of the scattered X-ray detection element of the outermost slice during the measurement of the subject is measured. And a method of correcting the measured output value.

Further, it is possible to perform correction by a method of directly subtracting the output value of the scattered radiation detecting element at the time of measuring the subject from the output of the X-ray detecting element of the internal slice without performing the standard attenuation measurement in advance. . When the X-ray beam is narrowed down in the slice direction by the X-ray tube-side aperture device, the outermost detection element row is not the outermost detection element row (the main X-ray beam is Is not used in place of the scattered radiation detection element array, it is possible to perform highly accurate scattered radiation correction even when the X-ray beam width is variable.

According to the present embodiment, the influence of the scattered X-rays incident from the channel direction and the slice direction can be removed without complicating the detector structure, and the correct X-ray transmission amount of the subject can be measured. Multi-slice CT
An X-ray detector for an apparatus can be realized.

[0024]

As described above, according to the present invention, the scattered X-rays are detected without lowering the detection efficiency for the main X-rays as compared with the conventional one-dimensional array detector.
This has the effect of realizing an X-ray CT apparatus capable of removing the influence of radiation.

[Brief description of the drawings]

FIG. 1 is a view showing the appearance of an X-ray detection element array according to an embodiment of the present invention.

FIG. 2 is a diagram showing an appearance of a collimator array according to the embodiment of the present invention.

FIG. 3 is an external view showing a positional relationship between an X-ray detection element array and a collimator array according to the embodiment of the present invention.

FIG. 4 is a diagram showing a state in which the multi-slice detector according to the embodiment of the present invention is mounted on a rotating plate of a CT scanner.

FIG. 5 is a diagram showing a cross section in the channel direction of the detector according to the embodiment of the present invention.

FIG. 6 is a diagram showing a cross section in the slice direction of the detector according to the embodiment of the present invention.

FIG. 7 is an explanatory diagram of a technique for measuring scattered X-rays in the slice direction using FIG.

FIG. 8 is a graph showing the relationship between the output of main X-rays and the output of scattered X-rays of the detection element at each slice position of the multi-slice detector according to the embodiment of the present invention.

FIG. 9 is a conceptual diagram illustrating the principle of a multi-slice CT apparatus.

[Explanation of symbols]

 1a Scintillator for scattered X-ray measurement slice

Claims (1)

[Claims] 1. A scintillator for detecting an X-ray beam narrowed by a diaphragm mechanism to convert X-rays from an X-ray source into an analysis and converting the light into light, and a photoelectric conversion for converting the light converted by the scintillator into an electric signal. An X-ray detector in which a plurality of X-ray detection elements having a plurality of X-ray detection elements each having a plurality of X-ray detection elements arranged in a channel direction in an arc shape or a polygon centering on the focal point of the X-ray source is arranged in a slice direction. An X-ray detector, wherein the X-ray detection element rows at both ends of the plurality of rows of slice-direction X-ray detection element rows are X-ray detection element rows for detecting only scattered X-rays.