Solid-state radiation detector and medical examination and/or treatment device

ABSTRACT

In a solid-state radiation detector and a medical examination and/or treatment device having such a solid-state radiation detector, the detector has a pixel matrix, with each pixel supplying an output signal dependent on the incident radiation. The pixel matrix has a conversion layer that converts the incident radiation into charge, a storage capacitor for storing the charge and a transistor for reading out the charge. The capacitance of the storage capacitor is set to be so small that because of the voltage drop across the storage capacitor, the output signal of the pixel exhibits, starting from a specific value of the incident radiation dose, a sublinear response with reference to the radiation dose.

INFORMATION

DETAILED DESCRIPTION OF THE INVENTION

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a solid-state radiation detector according to the invention, with a pixel being specifically illustrated. Located below an upper electrode is a direct conversion layer in the form of a selenium layer of amorphous selenium. The pixel further has a substrate from which a thin-film transistor is produced, preferably in the form of a high-voltage transistor (HVTFT). Also provided is a storage capacitor that serves for storing charge that can be read out by the transistor via a pixel electrode . The transistor is connected to the storage capacitor via its drain terminal D. Also shown are the gate terminal G and the source terminal S of the transistor .

Applied to the conversion layer is a voltage VSe that serves to separate and move the holes or electrons produced in the conversion layer upon exposure to radiation, as is illustrated graphically in FIG. . The charge collected at the pixel electrode is stored in the storage capacitor and read out as required via the transistor . The design of such a pixel is well known, as is its mode of operation (see J. Rowlands, S. Kasap, Amorphous Semiconductors Usher in Digital X-ray Imaging, in Physics Today, November 1997, pages 24 ff.).

FIG. 2 shows the equivalent circuit diagram of a pixel, including the selenium layer capacitance CSe, the capacitance of the storage capacitor Cp and the thin-film transistor.

If the voltage VSe is now applied to the conversion layer , this leads to a charging current ISe and thus to a voltage drop across the conversion layer.

In the solid-state radiation detector according to the invention, the capacitance of the storage capacitor Cp is now selected to be so small as to set up across the storage capacitor Cp a substantially high voltage drop Vp or a sufficient change in voltage dU=qp/Cp that is caused by the signal charge qp at the pixel P. This change in voltage is required to be of the order of magnitude of the externally applied voltage VSe. Because of this voltage drop and the voltage divider property of the combination of the two capacitances CSe and Cp, a reduction is now obtained in the effective voltage across the selenium layer in accordance with the relationship Veff=VSe−qp/Cp. Thus, dependent on the voltage divider properties and the high voltage drop across the storage capacitor, instead of the entire voltage VSe being present, only a substantially reduced voltage fraction is present that is dependent on the magnitude of the voltage reduction which, in turn, depends on the capacitance of the storage capacitor, which was selected to be small.

Assuming, for example, an applied voltage of 2000 V in the case of a selenium layer 200 μm thick, this corresponds to a field strength of 10 V/μm. Approximately 400 electron/hole pairs are generated and collected under these conditions by an X-ray quantum with an energy of 20 keV. If, after a sufficiently high radiation dose has been reached, the effective, applied voltage is lowered to, for example, 1000 V or 500 V due to the voltage reduction provided according to the invention, only approximately 200 or approximately 100 electron/hole pairs are still produced in conjunction with a constant energy of the incident X-ray quanta, since a lowering of the field strength present over the selenium layer accompanies the voltage reduction. Consequently, substantially fewer charge carriers are produced and this leads to the output signal becoming nonlinear or sublinear with respect to the radiation dose.

These relationships are described graphically in FIGS. 3 and 4. FIG. 3 shows a logarithmic plot of the formation energy in eV required to produce an electron/hole pair against the field strength present over the selenium layer in V/μm. It can be seen that the formation energy for producing an electron/hole pair increases substantially as the effective field strength is lowered. Consequently, fewer electron/hole pairs are produced in conjunction with a declining field strength, and this lowers the signal. This is shown graphically in FIG. 4, where the output signal is plotted against the radiation dose. In the range of low doses, there is clearly a linear relationship between the signal and the radiation dose which continues linearly up to a specific maximum dose in the case of known solid-state radiation detectors according to the prior art, having a very large capacitance of the storage capacitor. Once this dose has been reached, the output signal suddenly experiences the aforementioned limitation, as shown by the dashed continuation of the linear curve.

The curve profile differs, however, in the case of the solid-state variation detector according to the invention. Here, the curve profile shows a sublinear profile starting from a specific pixel-referred radiation dose Dp. The signal no longer increases linearly in direct proportion to the incident radiation dose, but a substantially higher dose is required to increase the signal because of the sublinear relationship. The result of this is that no sudden limiting effect occurs as previously, even in the case of high radiation doses due to unattenuated radiation. This offers the possibility of driving the solid-state radiation detector at a substantially higher level and not only, as previously customary, in the range from approximately 5% to 10%. Conventional operation has been conducted primarily at the lower range near the origin of the linear curve shown, in order to ensure adequate protection against the risk of overdriving. In the case of the solid-state radiation detector according to the invention, the actual drive-level range can be substantially shifted to higher values on the curve.

FIG. 5 shows a basic sketch of a medical examination and/or treatment device according to the invention. This device has a radiation source , a solid-state radiation detector according to the invention, and apparatus supporting the two, which can be designed in the form of a C-arm, for example. A processing device is used to control the operation of the radiation source and the solid-state radiation detector which is read out by suitable driving by the processing device . The processing device has a compensation unit for compensating the sublinearity that occurs because of the inventive extreme reduction in the capacitance of the storage capacitor Cp, so that after the compensation the sublinear output signals can be appropriately compensated and be further processed as a quasi-linear signal using the signals recorded in the linear range. The resulting image is presented at a display .

The examination and/or treatment device shown in FIG. 5 is merely of an exemplary nature. Of course, the solid-state radiation detector according to the invention can be used in any radiation device. It is conceivable, for example, to use the detector in mammography, the detector being provided in this case with a selenium layer approximately 200 μm thick, to which a voltage of approximately 2000 V is applied in order to achieve the regulating field strength of 10 V/μm. If the detector is used in radiography, where substantially higher doses are used, the selenium layer thickness can be, for example, 600 μm, and the applied voltage 6000 V. Here, as well, a field strength of 10 V/μm is then set up over the selenium layer. Field strengths other than 10 V/μm also can be set.

Although modifications and changes may be suggested by those skilled in the art, it is the intention of the inventor to embody within the patent warranted hereon all changes and modifications as reasonably and properly come within the scope of the inventor's contribution to the art.

DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the basic design of a pixel of a solid-state radiation detector according to the invention.

FIG. 2 shows an equivalent circuit of the pixel.

FIG. 3 is a diagram illustrating the required absorbed energy for forming an electron/hole pair in a selenium conversion layer, as a function of the field strength across the selenium layer.

FIG. 4 shows the profile of the pixel-side output signal in relation to the irradiated radiation dose.

FIG. 5 schematically illustrates an examination and/or treatment device according to the invention.

CLAIMS

1. A solid-state radiation detector comprising: a pixel matrix comprised of a plurality of pixels; each pixel in said plurality of pixels supplying an output signal dependent on radiation incident thereon, and having a conversion layer which converts said incident radiation into charge, a storage capacitor for storing said charge, and a transistor for reading out said charge; and said storage capacitor having a sufficiently small capacitance so that, due to a voltage drop across said storage capacitor, the output signal of the pixel exhibits, starting from a specific value of an incident radiation dose, a sublinear response relative to said incident radiation dose.

2. A sold-state state radiation detector as claimed in claim 1 wherein said capacitance of said storage capacitor has a value so that a ratio of a maximum of said voltage drop across the storage capacitor to a maximum of a voltage drop across said conversion layer is 1:10 or greater.

3. A sold-state state radiation detector as claimed in claim 2 wherein said capacitance of said storage capacitor has a value so that said ratio is 1:5 or greater.

4. A sold-state state radiation detector as claimed in claim 1 wherein said transistor is a high-voltage transistor.

5. A sold-state state radiation detector as claimed in claim 1 wherein said conversion layer is a selenium layer, and wherein said transistor is a thin-film transistor.

6. A sold-state state radiation detector as claimed in claim 1 wherein said conversion layer is a selenium layer wherein said transistor is a high-voltage thin-film transistor.

7. A medical device for examination or treatment of a subject, comprising: a radiation source which emits radiation; a solid-state radiation detector for detecting said radiation from said radiation source, said solid-state radiation detector comprising a pixel matrix comprised of a plurality of pixels, each pixel in said plurality of pixels supplying an output signal dependent on radiation incident thereon, and having a conversion layer which converts said incident radiation into charge, a storage capacitor for storing said charge, and a transistor for reading out said charge, and said storage capacitor having a sufficiently small capacitance so that, due to a voltage drop across said storage capacitor, the output signal of the pixel exhibits, starting from a specific value of an incident radiation dose, a sublinear response relative to said incident radiation dose; and a processing device supplied with the respective output signals of said pixels, said processing device including a unit for compensating for said sublinear response.

8. A device as claimed in claim 7 wherein said unit is a unit for analytical compensation for said sublinear response.

9. A device as claimed in claim 7 wherein said unit is a look-up table containing a plurality of compensation values.

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