JP2002202377A - Radiation detector - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a radiation detector that can eliminate a variation per hour caused from a leak current of a detector which converts an X-ray to an electric charge signal to prevent generation of an artifact. SOLUTION: A voltage is applied from a bias power source 9 to an electrode of an X-ray converting layer 27 mounted on a detector module 4 in advance, and a temperature sensor 13 and a rubber heater 5 made of a rubber are placed on a member that holds the detector module 4. Then temperature of the X-ray converting layer 27 is adjusted within 30±0.5 degrees C by a temperature controller 10. The X-ray enters into the detector module 4 and therewith electric charge generates on the X-ray converting layer 27 controlled at a fixed temperature and current flows. This current is accumulated for a unit time to be converted in current/voltage and thereafter is A/D converted by a DAS(data acquisition system) 8 to be data processed by a CPU in a console 11 and image reconfigured, and then a CT image is displayed on a monitor 12.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a radiation detector, and more particularly to a medical X-ray CT apparatus, a medical X-ray fluoroscopic apparatus, and an X-ray foreign substance inspection apparatus for non-destructively inspecting foreign substances such as food. It relates to the radiation detector used.

[0002]

2. Description of the Related Art In an X-ray CT apparatus, an X-ray radiated from an X-ray tube is narrowed down to a fan-shaped X-ray beam by a collimator of a radiation port, and an X-ray tube and an object facing the X-ray tube are centered on a subject. The detector rotates the arc-shaped collimator and the detector, which are arranged in such a way that the detector captures the X-ray information transmitted through the subject, and the signal is processed by a computer to obtain an X-ray tomographic image of the subject. is there. X-rays emitted from the X-ray tube can be transmitted straight through the subject or scattered by the subject. By taking in only the former information, scattered rays entering obliquely are removed, and the crosstalk occurs. To prevent this, a collimator is provided in front of the detector. In this collimator, an X-ray shielding wall is formed of a material that is difficult to transmit X-rays for each channel in front of detectors arranged one-dimensionally.

[0003] The detector is an X-ray tube comprising an X-ray detecting element comprising a scintillator element for converting X-rays into light and a photodiode for detecting the light converted by the scintillator element and outputting it as an electric signal. And about 500 to 1000 channels arranged in an arc with the center as the center. Due to the mechanical arrangement in manufacturing, a combination of a scintillator and a photodiode, which are optically bonded and combined, and 8 to 30 units are arranged on a substrate is considered as one module. The radiation detector for CT is arranged continuously in a substantially arc shape on the upper side and combined with a collimator.

However, in recent years, radiation detectors using semiconductor single crystals or polycrystals have been proposed. The radiation detector uses an X-ray conversion layer made of a semiconductor material that generates charges (electrons-holes) when irradiated with radiation such as X-rays. It has a wide dynamic range, good S / N, and good photoconductive properties. For example, a CdTe single crystal or a CdZnTe single crystal has been proposed.

FIG. 4 shows the structure of the above radiation detector. The radiation detector is a CdTe single crystal or CdZ
A bias voltage is applied to the X-ray conversion layer 27 made of nTe single crystal from the upper electrode 26, and the switch 30 of the TFT 30 is applied to the pixel electrodes 28 arranged directly below the X-ray conversion layer 27 in a matrix on the active matrix substrate 31. The elements are connected, and at the time of irradiation, the switch elements of each TFT 30 are sequentially turned on from the gate driver circuit 33 via the FPC 35 so that the signal charges accumulated in the storage capacitor 29 of each pixel are amplified via the FPC 34. 32. And an A / D converter (not shown) from the amplifier circuit 32.
The data is sent to an external data processing circuit (not shown) via the CPU, the image is formed by the data processing circuit, and an X-ray image is displayed on a monitor (not shown).

[0006] The CdTe single crystal and CdZnTe single crystal have been found to be useful, having sensitivity several to several tens times higher than the conventional one, depending on the applied bias voltage. In addition, since each channel can be separated by an electrode, there is an advantage that a separator or the like is not required as in the case of forming a scintillator array, and channels can be easily separated.

[0007]

The conventional radiation detector is constructed as described above, but is composed of CdTe single crystal or CdTe.
The X-ray conversion layer 27 made of dZnTe single crystal needs to apply a bias voltage of several hundred volts per 1 mm in thickness to the upper electrode 26 in order to take out the charge generated by the radiation. Even when no radiation is incident, a leak current flows, and the leak current increases or decreases due to a temperature change of the X-ray conversion layer 27.
The higher the temperature is, the more the leak current increases. However, the leak current is a DC component. For example, when the incident γ-ray is measured in the pulse count mode, the signal is processed by the AC coupling and measured. FIG. 5 shows an X-ray photon 1 emitted from the X-ray tube.
DAS (Data Aquisition S) corresponding to the pulse count mode when the light enters the detector.
(system: data collection system) shows a signal waveform at a preamplifier inside. (A) shows a signal waveform from the X-ray conversion layer 27, and a leak current component (LC) is added as an offset. When the waveform of this signal is shaped by AC coupling, the offset component disappears as shown in FIG. Then, usually, a slightly lower threshold (threshold level) LL
(C) Only signals higher than that are regarded as valid signals and scattered components are removed. (D) When this signal is counted during the sampling time ST for one view, (e) data Co is obtained. Therefore, if there is no change in the leakage current in a very short time, there is almost no effect on the image even in a severe environmental temperature range. On the other hand, when the X-ray conversion layer 27 is used in the current mode, as shown in FIG. 6, (a) all the incident X-ray photons are converted into a current output, and the leak current component (LC) is a signal component. Is added to For this reason, the leak current is measured under conditions where normal radiation does not enter, and this is used as an offset value, and correction is performed by subtracting (offset correction) after data collection. Then, (b) the charge is integrated during the sampling time ST for one view, and (c) A / D conversion is performed to obtain data Vo. Therefore, the offset correction for subtracting the offset after the acquisition is effective only when the leak current does not change and is constant. For this reason, there is a problem that data collection in the current mode must be performed while minimizing a temperature change of the X-ray conversion layer 27 that causes a change in the leak current.

[0008] Since the leak current is sensitive to temperature, a slight change in temperature causes noise. For example, in the case of an X-ray CT apparatus, a virtual image called an artifact appears and affects diagnosis. To reduce the leakage current itself, X
It is most effective to lower the temperature of the line conversion layer 27 and keep it constant. However, lowering the temperature is not realistic due to the difficulty of dew condensation countermeasures and control.

The present invention has been made in view of such circumstances, and it is possible to keep the temperature of an X-ray conversion layer constant, eliminate a change in leak current with time, and prevent the occurrence of artifacts. It is an object to provide a radiation detector.

[0010]

In order to achieve the above object, a radiation detector according to the present invention comprises a flat X-ray conversion layer made of a semiconductor having electrodes on upper and lower surfaces and converting X-rays into a charge signal. In a radiation detector in which a collimator formed of a collimator plate for removing scattered X-rays is arranged in a one-dimensional or two-dimensional manner, a heater and a temperature sensor are provided on a member holding the X-ray conversion layer, It is provided with a temperature adjusting means for adjusting the X-ray conversion layer to a predetermined temperature.

Further, in the radiation detector according to the present invention, the flat X-ray detector made of a semiconductor for converting an X-ray into a charge signal is provided.
The X-ray conversion layer is made of a CdTe single crystal or a CdZnTe single crystal, and the temperature of the X-ray conversion layer is adjusted to 30 ° C. or more.

The radiation detector of the present invention is constituted as described above, and is provided with a temperature control controller in the apparatus, and a member holding an X-ray conversion layer made of a single crystal of CdTe or single crystal of CdZnTe. , A heater and a temperature sensor are provided,
When the temperature of the X-ray conversion layer is slightly higher than room temperature,
The temperature is adjusted to 0 ° C or higher. As a result, although the absolute value of the leakage current of the X-ray conversion layer increases, the leakage current can be kept constant without a change over time, and the occurrence of artifacts can be prevented by a signal having less noise.

[0013]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS One embodiment of the radiation detector of the present invention will be described with reference to FIGS. FIG. 1 is a view showing a principle diagram in which the radiation detector of the present invention is set in an X-ray CT apparatus. The radiation detector includes a collimator 3 for removing X-ray scattered radiation, a plurality of detector modules 4 disposed behind the X-ray conversion layer 27 for converting incident X-rays into a charge signal, and an X-ray conversion layer. A bias power supply 9 for applying a bias voltage to 27, a rubber heater 5 attached to a member holding the detector module 4 for heating the X-ray conversion layer 27, and detecting the temperature of the X-ray conversion layer 27. The X-ray conversion layer 27 is controlled by adjusting the current flowing through the rubber heater 5 based on the temperature sensor 13 and the temperature information detected by the temperature sensor 13.
And a temperature control controller 10 for controlling the temperature.

The X-ray CT apparatus rotates the X-ray tube 1 and the radiation detector arranged opposite to the X-ray tube 1 around the subject 2 through the gantry controller 7 by operating the console 11, and the X-ray controller. X-rays are radiated from the X-ray tube 1 controlled by 6, transmitted through the subject 2, and scattered by the subject 2 are removed by the collimator 3. CdTe single crystal or CdZn previously provided in the detector module 4
The upper electrode 26 of the X-ray conversion layer 27 made of Te single crystal
A bias voltage is applied from a bias power supply 9 and the temperature sensor 1 attached to a member holding the detector module 4
The temperature of the X-ray conversion layer 27 is controlled by the temperature control controller 10 by the temperature controller 3 and the rubber heater 5, the temperature is adjusted to a temperature slightly higher than room temperature, 30 ° C. or more, and the temperature adjustment accuracy is ± 0.5. It is controlled within ℃. And subject 2
The X-ray image transmitted through is incident on each channel of the X-ray conversion layer 27 provided in the plurality of detector modules 4 and controlled at a constant temperature, and charges are generated in each channel and current flows. This is stored for a unit time, and after current-voltage conversion, DAS
(Data collection system) 8 performs A / D conversion. The A / D values of all the channels are subjected to data processing by the CPU in the console 11, image reconstruction is performed, and the monitor 1
2 displays a CT image.

FIG. 2 is a sectional view of a single slice of the radiation detector in the slice direction. The two ends of the collimator plate 15 are fixed to a main support plate 16 and a support plate 17 each having an arc shape, and are arranged facing the focal point of the X-ray tube.
Below that, a temperature sensor 13 is attached to a detector attachment plate 18 attached to the support plate 17 and the main support plate 16.
The detector module 4 mounted on the substrate 14 having
It is fixed with mounting screws 19. These members are protected by a bottom plate 20, a side plate 22, and a protection plate 23, and a rubber heater 5 made of rubber for heating the detector module 4 is provided on a side surface of the main support plate 16. Then, the whole is fixed to a mounting frame 25 of a gantry base 24 of the X-ray CT apparatus.

It is known that the sensitivity and the leak current of the CdTe single crystal or CdZnTe single crystal mounted on the detector module 4 change depending on the temperature. Sensitivity change is 1
Although it is about 1.5 times at 0 ° C., the leakage current increases as the temperature rises, and is very large at room temperature, about 2.3 times at 10 ° C. As a countermeasure, it is conceivable to lower the absolute value of the leak current by cooling.
It is not practical because the e-crystal may be damaged and the cooling device becomes large-scale. Conversely, if the temperature is kept at a constant temperature slightly higher than room temperature, the leakage current will increase. However, if the temperature is constant, the current is DC, so the previously measured leak component is subtracted from the obtained signal component (offset). Correction), a true signal component is obtained. Therefore, in the present embodiment, the rubber heater 5 is attached to the main support plate 16 of the detector section, the temperature sensor 13 is provided on the substrate on which the detector module 4 is mounted, and the temperature controller 10 controls the detector section. The whole is adjusted to a constant temperature. Ideally, the temperature control setting temperature is 30 °, which is slightly higher than room temperature. However, since the inside of the gantry is mixed with heating elements such as the high-temperature X-ray tube 1, a heat exchanger for cooling it, and a power supply,
Here, the temperature is set to 40 ° C. The smaller the control range, the better, but practically it is about ± 0.3 ° C. If the element area of one channel is small, the leakage current is proportionally reduced, so that ±
It may be 0.5 ° C.

FIG. 3 shows a single crystal of CdTe or CdZnT.
The afterglow characteristic of the X-ray conversion layer 27 made of e-single crystal (the detector output when the X-ray excitation is suddenly cut off in a state where the X-ray excitation generates charges and the detector has a constant output). Curve showing time drop). The vertical axis is l of the detector output.
The og scale and the horizontal axis represent the time axis, and show the case where the temperature of the crystal is 20 ° C. and the case where it is 40 ° C. When the temperature of the crystal was 40 ° C. as compared with the case where the temperature of the crystal was 20 ° C., it was confirmed that the afterglow dropped sharply in 10 seconds, and it was found that the afterglow had less influence on the image.

In the above embodiment, the position of the temperature sensor 13 is set on the back side of the substrate 14 on which the detector module 4 is mounted. However, the temperature sensor 13 may be located anywhere as long as the temperature of the detector module 4 can be detected. In addition, the position where the rubber heater 5 is attached is not limited to the position on the side of the main support plate 16 but may be any place where the temperature of the detector unit can be uniformly controlled.

[0019]

According to the radiation detector of the present invention, a flat X-ray conversion layer made of a CdTe single crystal or a CdZnTe single crystal for converting X-rays into a charge signal is used as a detector module. A rubber heater and a temperature sensor for detecting the temperature of an X-ray conversion layer are provided on a member holding the detector module, and a temperature control controller is provided on a console. Is provided, the temperature of the X-ray conversion layer is adjusted to a temperature slightly higher than room temperature, that is, 30 ° C. or more and ± 0.5 ° C. or less. Thereby, although the absolute value of the leakage current of the X-ray conversion layer increases, the leakage current can be kept constant without a temporal change of the leakage current, and the afterglow characteristic is sharply reduced in about 10 seconds, and the noise is reduced. The occurrence of artifacts can be prevented by a signal having a small number of signals.

[Brief description of the drawings]

FIG. 1 is a diagram showing one embodiment of a radiation detector of the present invention.

FIG. 2 is a diagram showing a cross-sectional structure in a slice direction of the radiation detector of the present invention.

FIG. 3 is a diagram showing an afterglow characteristic of the radiation detector of the present invention.

FIG. 4 is a diagram showing a structure of a detector having an X-ray conversion layer.

FIG. 5 is a diagram illustrating signal processing in a pulse count mode.

FIG. 6 is a diagram showing signal processing in a current mode.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 X-ray tube 2 Subject 3 Collimator 4 Detector module 5 Rubber heater 6 X-ray controller 7 Gantry controller 8 DAS 9 Bias power supply 10 Temperature controller 11 Console 12 Monitor 13 Temperature sensor 14 Substrate 15 Collimator plate 16 Main support plate 17 Support plate 18 Detector mounting plate 19 Mounting screw 20 Bottom plate 21 Flexible cable 22 Side plate 23 Protective plate 24 Gantry base 25 ... Mounting frame 26 ... Upper electrode 27 ... X-ray conversion layer 28 ... Pixel electrode 29 ... Storage capacitor 30 ... TFT 31 ... Active matrix substrate 32 ... Amplifier circuit 33 ... Gate driver circuit 34,35 ... FPC

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) H04N 5/32 H01L 31/00 A F term (Reference) 2G088 EE01 EE02 EE30 FF02 GG21 JJ04 JJ05 JJ09 JJ11 LL11 LL12 LL12 LL21 4C093 AA22 CA13 EB13 FA32 FA58 4M118 AA08 AA10 AB01 BA05 CA14 CB05 FB08 FB09 FB13 GA10 HA36 5C024 AX11 CX32 CY01 CY47 EX15 5F088 AA11 AB09 BA13 BB03 BB07 EA04 EA08 EA20 JA20 KA10 LA08

Claims (2)

[Claims] A flat X-ray conversion layer made of a semiconductor having electrodes on upper and lower surfaces for converting X-rays into a charge signal, and a collimator formed of a collimator plate for removing scattered X-rays, are one-dimensional or one-dimensional. In the two-dimensionally arranged radiation detector, a heater and a temperature sensor are provided on a member holding the X-ray conversion layer, and a temperature adjusting unit for adjusting the X-ray conversion layer to a predetermined temperature is provided. Radiation detector. 2. A flat X-ray conversion layer comprising a semiconductor for converting X-rays into a charge signal is formed of a single crystal of CdTe or CdZn.
The radiation detector according to claim 1, wherein the radiation detector is made of a Te single crystal, and the temperature of the X-ray conversion layer is adjusted to 30 ° C or higher.