JPH11183628A - Method and device for inspecting radiation detector and radiation tomograph - Google Patents

Abstract

PROBLEM TO BE SOLVED: To inspect deterioration of a radiation detector in mounted state, by moving beams to be applied to a radiation detector in its thickness direction, measuring sensitivity profile of a plurality of detection elements, and inspecting the deterioration of the detection elements based on it. SOLUTION: A central processing device 60 starts inspecting deterioration by an instruction through an operation device 70, sets X-ray passage opening of a collimator 22 to a value equivalent to, for example, minimum slice thickness, and applies thin X-ray beams to a detector array 24. While the beam thickness is maintained, a collimator piece is moved in a beam thickness direction in one piece, an X-ray beam application position is swept from an edge in the thickness direction of the detector array 24 to an edge, and an X-ray detection signal at each application position is measured for each X-ray detection element, thus obtaining a sensitivity profile in the thickness direction of the detector array 24 for each X-ray detection element. The central processing device 60 displays a deterioration reporting message or the like on a display 68 when the amount of reduction from the sensitivity profile before deterioration being measured in advance is equal to more than a specific limit.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method and apparatus for inspecting a radiation detector and a radiation tomography apparatus, and more particularly to a method and apparatus for inspecting a radiation detector in which a radiation beam having a width and a thickness is received by a radiation detecting element array. And a radiation tomography apparatus including such an inspection apparatus.

[0002]

2. Description of the Related Art As an example of a radiation tomography apparatus, there is, for example, an X-ray computed tomography (CT) apparatus. In an X-ray CT apparatus, X-rays are used as radiation. An X-ray tube is used for X-ray generation. Then, the radiation irradiation / detection system, that is, the X-ray irradiation / detection system is rotated (scanned) around the subject, and the subject is exposed to X-rays in a plurality of views around the subject. Is measured, and a tomographic image is generated (reconstructed) based on the projection data.

An X-ray irradiator has an X-ray beam (b) having a width encompassing an imaging range and having a predetermined thickness in a direction perpendicular thereto.
eam). The thickness of the X-ray beam is determined by the collimator (colli
meter).

The X-ray detecting apparatus detects X-rays by a multi-channel X-ray detector in which a large number of X-ray detecting elements are arranged in an array in the direction of the width of the X-ray beam. The multi-channel X-ray detector has a length (width) corresponding to the width of the X-ray beam in the direction of the width of the X-ray beam.
The X-ray detecting element has a longer length (thickness) in the direction of the thickness of the X-ray beam than the thickness of the X-ray beam.

[0005] The X-ray detecting element comprises, for example, a scintillator which emits light upon irradiation with X-rays, and a photodiode L (photodiode) which converts the light into an electric signal. The X-ray beam is applied to the central part in the thickness direction of such an X-ray detection element through a collimator. The thickness of the X-ray beam is adjusted by the opening of the X-ray passage opening of the collimator according to the slice thickness of the imaging.

[0006]

The scintillator deteriorates with time due to X-ray irradiation. In particular, the central portion in the thickness direction is a portion to which the X-ray beam is irradiated regardless of the slice thickness, so that the cumulative irradiation amount of the X-ray becomes the highest and the deterioration tends to be particularly large. Due to such partial deterioration of the scintillator, X-rays in the thickness direction of the X-ray detecting element
The sensitivity distribution (sensitivity profile) of the line detection becomes non-uniform. Further, the change in the sensitivity profile due to deterioration differs for each individual X-ray detection element.

In the X-ray tube, the X-ray focal point shifts due to thermal expansion or the like due to a rise in temperature during use, and this changes the irradiation position in the thickness direction of the X-ray detector through the X-ray passage opening of the collimator. Appear. When the irradiation position of the X-ray beam is displaced in the thickness direction, the unevenness of the sensitivity distribution as described above causes the X-ray detection signal to lose its proportionality with respect to the amount of received X-rays, thereby causing an artifact in the reconstructed image. .

For this reason, it is desirable to detect the deterioration as early as possible. However, conventionally, it is not known until artifacts appear, and if the deterioration has occurred enough to cause the artifacts, the X-ray detector is removed and repair or replacement is performed. Therefore, there is a problem that the operation of the X-ray CT apparatus has to be stopped during that time.

SUMMARY OF THE INVENTION The present invention has been made to solve the above problems, and an object of the present invention is to provide an inspection method and apparatus for inspecting deterioration of a radiation detection element while a radiation detector is mounted on a utilization apparatus, and An object of the present invention is to provide a radiation tomography apparatus including such an inspection apparatus.

[0010]

Means for Solving the Problems (1) According to a first aspect of the present invention, one of two directions perpendicular to an irradiation direction and perpendicular to each other has a relatively large width in one of the two directions. Inspection of a radiation detector in which a plurality of radiation detecting elements having a length greater than the thickness of the radiation beam in the direction of the thickness of the radiation beam having a relatively small dimension are arranged in the direction of the width of the radiation beam. A method of measuring the sensitivity profile of the plurality of radiation detection elements by moving the radiation beam to irradiate the radiation detector in the direction of its thickness, based on the measured sensitivity profile of each radiation detection element A method for inspecting a radiation detector, which inspects for deterioration.

(2) A second aspect of the present invention for solving the above-mentioned problem is that one of two directions perpendicular to the irradiation direction and perpendicular to each other has a relatively large width and the other has a relatively small width. A plurality of radiation detection elements having a length greater than the thickness of the radiation beam in the thickness direction of the radiation beam having a thickness, a radiation detector inspection device arranged in the direction of the width of the radiation beam, Measuring means for measuring the sensitivity profiles of the plurality of radiation detection elements by moving the radiation beam to be irradiated on the radiation detector in the direction of its thickness, and determining deterioration of each radiation detection element based on the measured sensitivity profiles And a determination unit that performs the determination.

(3) A third aspect of the present invention for solving the above-mentioned problems is that one of two directions perpendicular to the irradiation direction and perpendicular to each other has a relatively large width and the other has a relatively small width. A radiation irradiating means for irradiating a radiation beam having a thickness, and a plurality of radiation detection elements having a length greater than the thickness of the radiation beam in the direction of the thickness of the radiation beam are arranged in the direction of the width of the radiation beam; A radiation detector to which the radiation beam is irradiated, tomographic image generating means for generating a tomographic image of a passage area of the radiation beam based on radiation detection signals of a plurality of views by the radiation detector, and irradiating the radiation detector Measuring means for moving the radiation beam in the direction of its thickness to measure the sensitivity profiles of the plurality of radiation detection elements, and Determining means for determining deterioration of the radiation detection elements on the basis of the degree profile, a radiation tomography apparatus characterized by comprising a.

In the second invention or the third invention, the measuring means moves the radiation beam in the direction of its thickness by a collimator, and the control system includes the thickness control of the radiation beam. This is preferable because it can be unified into one system and can meet the demand for simplification.

In the second invention or the third invention, the measurement means may move the radiation detector relatively to move the radiation beam in the direction of its thickness by moving the radiation detector. Two systems of a beam thickness adjustment mechanism and a thickness direction irradiation position control mechanism can be separately provided, which is preferable in that multilateral control becomes possible.

(Operation) In the present invention, the irradiation position of the radiation beam on the radiation detector is moved in the thickness direction to measure the sensitivity profile of each radiation detection element, and the deterioration of each radiation detection element is determined based on the change in the sensitivity profile. To inspect.

[0016]

Embodiments of the present invention will be described below in detail with reference to the drawings. Note that the present invention is not limited to the embodiment.

FIG. 1 shows a block diagram of the X-ray CT apparatus. This device is an example of an embodiment of the present invention. The configuration of the present apparatus shows an example of an embodiment relating to the apparatus of the present invention. An example of an embodiment of the method of the present invention is shown by the operation of the present apparatus.

(Configuration) As shown in FIG. 1, the present apparatus includes a scanning gantry 2, a photographing table 4, and an operation console 6.

The scanning gantry 2 has an X-ray tube 20 as a radiation source. An X-ray (not shown) emitted from the X-ray tube 20 is shaped by the collimator 22 into, for example, a fan-shaped X-ray beam, and is applied to the detector array 24. The X-ray tube 20 and the collimator 22 are an example of an embodiment of a radiation irradiation unit in the present invention.

The detector array 24 is an example of an embodiment of the radiation detector according to the present invention. Detector array 24
Has a plurality of X-ray detection elements arranged in an array in the direction of the width of the fan-shaped X-ray beam. The configuration of the detector array 24 will be described later.

The X-ray tube 20, collimator 22 and detector array 24 constitute an X-ray irradiation / detection system. The configuration of the X-ray irradiation / detection system will be described later. A data collection unit 26 is connected to the detector array 24. The data collection unit 26 collects detection data of individual X-ray detection elements of the detector array 24.

The irradiation of X-rays from the X-ray tube 20 is controlled by an X-ray controller (controller) 28. The illustration of the connection relationship between the X-ray tube 20 and the X-ray controller 28 is omitted.

The collimator 22 is controlled by a collimator controller 30. The illustration of the connection relationship between the collimator 22 and the collimator controller 30 is omitted.

The part consisting of the detector array 24, the data collection unit 26, the collimator 22, and the collimator controller 30 is an example of the embodiment of the measuring means in the present invention.

The above-mentioned X-ray tube 20 to collimator controller 30 are mounted on the rotating unit 32 of the scanning gantry 2. The rotation of the rotation unit 32 is controlled by a rotation controller 34. The illustration of the connection relationship between the rotation unit 32 and the rotation controller 34 is omitted.

The imaging table 4 carries a subject (not shown) into and out of the X-ray irradiation space of the scanning gantry 2. The relationship between the subject and the X-ray irradiation space will be described later.

The operation console 6 has a central processing unit 60. The central processing unit 60 is an example of an embodiment of a determination unit according to the present invention. Also, it is an example of an embodiment of the tomographic image generating means in the present invention. The central processing unit 60 is constituted by, for example, a computer. The central processing unit 60 has a control interface
(interface) 62 is connected. The scanning gantry 2 and the imaging table 4 are connected to the control interface 62.

The central processing unit 60 includes the control interface 6
2, the scanning gantry 2 and the imaging table 4 are controlled. The data acquisition unit 26, X-ray controller 28, collimator controller 30, and rotation controller 34 in the scanning gantry 2 are controlled through a control interface 62. It should be noted that illustration of individual connections between these units and the control interface 62 is omitted.

A data collection buffer 64 is also connected to the central processing unit 60. Data collection buffer 6
4 is connected to the data collection unit 26 of the scanning gantry 2. The data collected by the data collection unit 26 is input to the data collection buffer 64. Data collection buffer 6
4 temporarily stores the input data. Central processing unit 6
The storage device 66 is also connected to 0. The storage device 66 stores various data, reconstructed images, and programs.
(program) and so on.

The central processing unit 60 also includes a display device 6
8 and the operating device 70 are connected to each other. The display device 68 displays a reconstructed image and other information output from the central processing unit 60. Operation device 7
Numeral 0 is operated by the operator to input various instructions and information to the central processing unit 60.

FIG. 2 shows a schematic configuration of the detector array 24. The detector array 24 has a large number (for example, 1000)
X-ray detectors 24 (i) are arranged in an arc to form a multi-channel X-ray detector. i is a channel number, for example, i = 1 to 1000. X-ray detector 2
FIG. 4 (i) is an example of an embodiment of the radiation detecting element in the present invention.

FIG. 3 shows the relationship among the X-ray tube 20, collimator 22, and detector array 24 in the X-ray irradiation / detection system. 3A is a front view, and FIG. 3B is a side view. As shown in FIG.
The rays are shaped into a fan-shaped X-ray beam 40 by the collimator 22 and are applied to the detector array 24. FIG. 3A shows the spread of the fan-shaped X-ray beam 40, that is, the width of the X-ray beam 40. FIG. 3B shows the thickness of the X-ray beam 40.

With the body axis crossing the fan surface of the X-ray beam 40, for example, as shown in FIG.
Is placed in the X-ray irradiation space. X
A projection image of the subject 8 sliced by the line beam 40 is projected on the detector array 24. The thickness of the X-ray beam 40 at the isocenter of the subject 8 gives the slice thickness th of the subject 8. The slice thickness th is
It is determined by the X-ray aperture of the collimator 22.

X-ray beam 40 for detector array 24
FIG. 5 shows a more detailed schematic diagram of the irradiation state of FIG. As shown in FIG.
By displacing 0,222 in a direction to narrow the X-ray passing aperture, the slice thickness th of the projected image on the detector array 24 can be reduced. In addition, the collimator piece 2
The slice thickness th of the projected image on the detector array 24 can be increased by moving 20, 20 in the direction that widens the X-ray passage aperture. By adjusting the X-ray passing aperture, the slice thickness is set to, for example, 1, 2, 3, 5, 7,
It is set to a predetermined thickness such as 10 mm. The dimension (thickness) of the detector array 24 in the slice thickness direction is, for example, about 30 mm.

By simultaneously moving the collimator pieces 220 and 222 in which the slice thickness th is set in the direction of the slice thickness while maintaining the relative positional relationship, the irradiation position of the X-ray beam 40 on the detector array 24 is improved. Is controlled in the thickness direction of the detector array 24. The irradiation position in the thickness direction is controlled by the collimator piece 22.
Instead of moving 0, 222, the detector array 24 may be displaced relative to the collimator 22 in the thickness direction of the X-ray beam 40, as indicated by the dashed arrow.

In this way, two separate systems for adjusting the slice thickness and controlling the irradiation position in the thickness direction can be provided, and multilateral control is possible. On the other hand, if all the operations are performed by the collimator 22 as described above, the control system can be unified into one system, and the demand for simplification can be met.

FIGS. 6, 7 and 8 show detector array 2
4 shows a schematic configuration of a detector module (module) constituting No. 4. 6 is a plan view, FIG. 7 is an AA cross-sectional view, and FIG. 8 is an exploded view.

As shown in these figures, the detector module 240 has a plurality of scintillators 242 to 248 arranged in parallel with each other. Multiple scintillators 242
248 each constitute an individual channel in the detection array 24. The scintillators 242 to 248 are integrated by a bonding material 250.

The detector module 240 also has a plurality of photodiodes 272 to 278 formed on the semiconductor substrate 270 in parallel with each other. Semiconductor substrate 270
Is mounted on a base 290 by, for example, bonding.

The arrangement of the photodiodes 272 to 278 corresponds to the arrangement of the scintillators 242 to 248. The photodiodes 272 to 27 shown in FIG.
8, the lower surfaces (light emitting surfaces) of the scintillators 242 to 248 shown in FIG. The adhesive 310 is an optical adhesive.

Such a detector module 240 is represented by X
A plurality of detectors are arranged adjacent to each other in the direction of the width of the line beam 40 to form the detector array 24 shown in FIG. The base 290 is fixed to a predetermined support frame (not shown) or the like by appropriate means.

X-rays enter the scintillators 242 to 248 from above in FIG. Scintillators 242 to 24
8 emits light in accordance with the intensity of the incident X-ray. Light emitted from the scintillators 242 to 248 is detected by the photodiodes 272 to 278, respectively.

Scintillator 242 and photodiode 2
72 pairs, scintillator 244 and photodiode 27
Four pairs, scintillator 246 and photodiode 276
Pair and scintillator 248 and photodiode 27
8 correspond to the X-ray detecting elements 24 shown in FIG.
Form (i).

FIG. 9 shows the configuration of an electric circuit for X-ray detection.
One channel of the detector array 24 is shown. As shown in the figure, the equivalent circuit of the photodiode 24k is
Diode 90, capacitor 92, resistor 9
4 and a DC constant current source 96 in parallel.

A connector (connect) is connected to such a parallel circuit.
or) The integration circuit 100 is connected via 98. The integration circuit 100 exists in the data collection unit 26. The integration circuit 100 includes an operational amplifier (OP amplifier) 10
2, the positive input terminal is connected to common, the negative input terminal is connected to a resistor 104, the negative input terminal is connected to the output terminal via a capacitor 106, and a switch 108 is connected to the capacitor 106 in parallel. Connected and configured.

In such a circuit, the DC constant current source 9
The current 6 changes in proportion to the intensity of the X-ray incident on the scintillator, and as a result, the input current of the integrating circuit 100 changes. Such an input current is integrated and output as an X-ray detection signal. Similar X-ray detection circuits are configured for all the channels of the detector array 24 to obtain X-ray detection signals.

(Operation) The operation of the present apparatus will be described. Based on a command from an operator given through the operation device 70,
The imaging of the subject 8 is performed under the control of the central processing unit 60. That is, the X-ray irradiation / detection system including the X-ray tube 20, the collimator 22, and the detector array 24 rotates (scans) around the body axis of the subject 8 while maintaining their mutual relation, and performs one of the scans. Multiple per rotation (eg 10
The projection data of the subject is collected at the view angle of 00).
The collection of projection data is performed by a system including the detector array 24, the data collection unit 26, and the data collection buffer 62.

Based on the projection data collected by the data collection buffer 62, the central processing unit 60 generates a tomographic image, that is, performs image reconstruction. Image reconstruction is performed by, for example, projecting data of 1000 views obtained by one rotation scan,
For example, filtered back projection (filtere
d back-projection) method. The reconstructed image is stored in the storage device 66, and
The image is displayed on the display device 68 as a visible image.

When the integrated value of the operation time of this apparatus is, for example,
Each time a predetermined time, such as 00 hours, is reached, a deterioration test of each X-ray detection element 24 (i) in the detector array 24 is performed. Note that the deterioration inspection may be performed as a part of a periodic inspection of the present apparatus, or may be performed as a part of a start-up inspection before the start of daily operation.

Hereinafter, the deterioration inspection of the X-ray detection element 24 (i) will be described. When the operator issues a deterioration inspection command through the operation device 70, the central processing unit 60 starts the deterioration inspection. The deterioration inspection is performed in a state where the subject 8 does not exist in the X-ray irradiation space and the rotation of the X-ray irradiation / detection system is stopped.

First, the X-ray passage opening of the collimator 22 is set to an opening corresponding to a minimum slice thickness of, for example, 1 mm.
Thereby, for example, as shown in FIG.
A line beam 40 irradiates the detector array 24. In addition,
Since the thinner the X-ray beam 40 for deterioration inspection is, the better, the beam thickness is smaller than the minimum slice thickness (for example, 0.5 m).
m, etc.) may be determined as the beam thickness for deterioration inspection and used.

While maintaining such a beam thickness, the collimator pieces 220 and 222 are integrally moved in the direction of the beam thickness to shift the irradiation position of the X-ray beam 40 from one end of the detector array 24 in the thickness direction to the other end. And X at each irradiation position
A line detection signal is measured for each X-ray detection element 24 (i).
Alternatively, when a mechanism for moving the detector array 24 in the thickness direction is provided, the above-described sweep is performed by the detector array 24.
May be moved.

By such a measurement, for example, as shown in FIG. 11, a sensitivity profile in the thickness direction of the detector array 24 can be obtained for each of the X-ray detection elements 24 (i). The sensitivity profile is the X-ray detection element 24
When there is no deterioration in (i), for example, as shown by a solid line,
It is almost flat except for both ends of the sweep range.

On the other hand, as the deterioration progresses, the sensitivity particularly in the vicinity of the center decreases as shown by a broken line, for example. Therefore, the central processing unit 60 provides a message of deterioration notification when the amount of decrease d from the sensitivity profile (solid line) before deterioration measured in advance exceeds a predetermined limit (for example, several percent).
(message) is displayed on the display device 68.

Alternatively, the tendency of deterioration is predicted from the history of the periodically performed deterioration inspection, and the time when the deterioration exceeds an allowable limit is predicted. At this time, the sensitivity of the X-ray detection element is calibrated (calibra
) may be executed.

On the basis of such notification, the operator or maintenance person recognizes the deterioration of the detector array 24, and prepares in advance and takes measures such as repair and replacement at an appropriate time such as periodic inspection. I do. If the inspection history is sent to a maintenance organization such as a maintenance center through a communication line, more effective prevention or preparation can be performed. This makes it possible to take appropriate measures before artifacts appear in the reconstructed image, thereby avoiding any trouble in operation.

The life expectancy of the detector array 24 can be predicted from the statistics of the actual replacement data, and this can be reflected in the service inventory of the detector array 24 and the production planning.

In the above, an example using X-rays as radiation has been described. However, the radiation is not limited to X-rays, but may be other types of radiation such as γ-rays. However,
At present, X-rays are preferred because practical means for generating, detecting, controlling, and the like are the most substantial. Also,
The radiation detecting element is not limited to one using a scintillator, but may be a solid-state radiation detecting element of another system using, for example, cadmium tellurium (Cd.Te).

[0059]

As described above in detail, according to the present invention, an inspection method and apparatus for inspecting deterioration of a radiation detecting element while a radiation detector is mounted on a utilization apparatus, and such an inspection apparatus Can be realized.

[Brief description of the drawings]

FIG. 1 is a block diagram of a device according to an example of an embodiment of the present invention.

FIG. 2 is a schematic configuration diagram of a detector array in an apparatus according to an embodiment of the present invention.

FIG. 3 is a schematic configuration diagram of an X-ray irradiation / detection system in an apparatus according to an embodiment of the present invention.

FIG. 4 is a schematic configuration diagram of an X-ray irradiation / detection system in an apparatus according to an embodiment of the present invention.

FIG. 5 is a schematic configuration diagram of an X-ray irradiation / detection system in an apparatus according to an embodiment of the present invention.

FIG. 6 is a schematic configuration diagram of a detector module in an apparatus according to an example of an embodiment of the present invention.

FIG. 7 is a schematic configuration diagram of a detector module in an apparatus according to an embodiment of the present invention.

FIG. 8 is a schematic configuration diagram of a detector module in an apparatus according to an embodiment of the present invention.

FIG. 9 is an electrical connection diagram of an X-ray detection system in an apparatus according to an embodiment of the present invention.

FIG. 10 shows an example of the X in the apparatus according to the embodiment of the present invention.
FIG. 2 is a schematic configuration diagram of a line irradiation / detection system.

FIG. 11 shows X in the apparatus according to the embodiment of the present invention.
It is a graph which shows an example of the sensitivity profile of a line detection element.

[Explanation of symbols]

 2 Scanning gantry 20 X-ray tube 22 Collimator 24 Detector array 24 (i) X-ray detecting element 26 Data collection unit 28 X-ray controller 30 Collimator controller 32 Rotating unit 34 Rotation controller 4 Imaging table 6 Operation console 60 Central processing unit 62 Control Interface 64 Data collection buffer 66 Storage device 68 Display device 70 Operating device 40 X-ray beam 8 Subject 220, 222 Collimator piece 240 Detector module 242 to 248 Scintillator 250 Coupling material 270 Semiconductor substrate 272 to 278 Photodiode 290 Base 310 Adhesive 96 DC constant current source 98 Connector 100 Integration circuit

Claims (3)

[Claims] 1. The radiation beam in a thickness direction of the radiation beam, which has a relatively large dimension width in one of two directions perpendicular to the irradiation direction and perpendicular to each other, and has a relatively small dimension thickness in the other direction. A method for inspecting a radiation detector, in which a plurality of radiation detection elements having a length greater than the thickness of the radiation beam are arranged in the direction of the width of the radiation beam, wherein the radiation beam applied to the radiation detector has a thickness of A method of measuring sensitivity profiles of the plurality of radiation detection elements by moving the radiation detection elements in a direction, and inspecting deterioration of each radiation detection element based on the measured sensitivity profiles. 2. The radiation beam in the direction of the thickness of the radiation beam, which has a relatively large dimension width in one of two directions perpendicular to the irradiation direction and perpendicular to each other, and a relatively small dimension thickness in the other direction. A radiation detector inspection device in which a plurality of radiation detection elements having a length larger than the thickness of the radiation beam are arranged in the direction of the width of the radiation beam, wherein the radiation beam irradiating the radiation detector has a thickness of the radiation beam. Measuring means for measuring the sensitivity profiles of the plurality of radiation detecting elements by moving the radiation detecting elements in a direction, and determining means for judging deterioration of each radiation detecting element based on the measured sensitivity profiles. Inspection equipment for radiation detectors. 3. Irradiation means for irradiating a radiation beam having a relatively large dimension width in one of two directions perpendicular to the irradiation direction and perpendicular to each other, and a relatively small thickness in the other direction; A radiation detector in which a plurality of radiation detecting elements having a length greater than the thickness of the radiation beam in the direction of the thickness of the radiation beam are arranged in the direction of the width of the radiation beam, and the radiation beam is irradiated; A tomographic image generating means for generating a tomographic image for a passing region of the radiation beam based on a radiation detection signal of a plurality of views by a detector, and moving the radiation beam to irradiate the radiation detector in a direction of its thickness. Measuring means for measuring the sensitivity profiles of the plurality of radiation detection elements; and each radiation detector based on the measured sensitivity profiles. Radiation tomography apparatus characterized by comprising a determination means for determining degradation of the device.