WO2011116989A2

WO2011116989A2 - Radiation imaging detector with charge transfer readout

Info

Publication number: WO2011116989A2
Application number: PCT/EP2011/001543
Authority: WO
Inventors: Jules Hendrix; Denny L. Lee
Original assignee: Jules Hendrix; Lee Denny L
Priority date: 2010-03-26
Filing date: 2011-03-28
Publication date: 2011-09-29
Also published as: DE102010013115A1; WO2011116989A3

Abstract

The invention provides a radiation imaging detector comprising: a) a first dielectric layer (1), b) a plurality of electric field shaping electrodes (2) deposited on the first dielectric layer (1), c) a charge accumulating electrode (3) deposited on the first dielectric layer (1), d) a second dielectric layer (4) deposited over the electric field shaping electrodes (2) and the charge accumulating electrode (3), e) readout electrodes (5) deposited over the second dielectric layer (4), f) a radiation charge conversion layer (6) deposited over the second dielectric layer (4) and the readout electrodes (5), and g) a top bias electrode (7). The invention further provides a method for detecting radiation comprising the steps of a) providing the radiation imaging detector described above, b) generating a read out image signal, and c) detecting said read out image signal.

Description

Radiation Imaging Detector with Charge Transfer Readout

1. FIELD OF INVENTION

The invention relates to an image capture panel for recording x-ray image information. More particularly, the invention relates to a method and apparatus for capturing the x-ray image and reading out the image signal without using a large area thin film transistor (TFT) array.

2. DESCRIPTION OF RELATED ART

Digital X-ray radiogram can be produced by using layers of radiation sensitive materials to capture incident X-ray as image-wise modulated patterns of light intensity (photons) or as electrical charges. Depending on the intensity of the incident X-ray radiation, electrical charges generated either electrically or optically by the X-ray radiation within a pixel area are quantized using a regularly arranged array of discrete solid state radiation sensors. U.S. Pat. No. 5,319,206, issued to Lee et al. on Jun. 7, 1994 and assigned to E. I. du Pont de Nemours and Company, describes a system employing a layer of photoconductive material to create an image-wise modulated areal distribution of electron-hole pairs which are subsequently converted to corresponding analog pixel (picture element) values by electro-sensitive devices, such as thin-film transistors (TFT). U.S. Pat. No. 5,262,649 (Antonuk et al.) describes a system employing a layer of phosphor or scintillation material to create an image-wise modulated distribution of photons which are subsequently converted to a corresponding image-wise modulated distribution of electrical charges by photosensitive devices, such as two dimensional amorphous silicon photodiodes. These solid state systems have the advantage of being useful for repeated exposures to X-ray radiation without consumption and chemical processing of silver halide films .

In Indirect Conversion systems (e.g. U.S. Pat. No. 5,262,649) that utilize a scintillation material to create an image-wise modulated distribution of photons from the absorbed X-ray energy, photons generated from the absorbed X-ray may undergo multiple scattering or spreading before they are detected by the two dimensional photosensitive device, resulting with degradation of image sharpness or a lower TF (Modulation Trans- fer Function) . The degradation of image sharpness is significant especially for a thicker layer of scintillation material is required to capture sufficient x-ray quanta for image forming . In Direct Conversion systems (Fig. 1) utilizing a photoconduc- tive material, such as selenium described in U.S. Pat. No. 5,319,206, before exposure to image-wise modulated X-ray radiation, an electrical potential is applied to the top electrode to provide an appropriate electric field. During expo- sure to X-ray radiation, electron-hole pairs (indicated as - and +) are generated in the photoconductive layer (referred to in Fig. 1 as "X-ray Semiconductor") in response to the intensity of the image-wise modulated pattern of X-ray radiation, and these electron-hole pairs are separated by the applied bi- asing electric field supplied by a high voltage power supply.

The electron-hole pairs move in opposite directions along the electric field lines toward opposing surfaces of the photoconductive layer. After the X-ray radiation exposure, a charge image is stored in the storage capacitor of the TFT array. This image charge is then readout by an orthogonal array of thin film transistors and charge integrating amplifiers. In Direct Conversion systems, since the electric field is perpendicular to the collecting electrode, the image sharpness or MTF is preserved regardless of the thickness of the photocon- ductive material. Thicker X-ray conversion material can be used to absorb sufficient X-ray energy without compromising the resulted image quality.

Conventional large area thin film transistor arrays used for both Direct Conversion systems and Indirect Conversion systems consist of a large number of image data lines and control gates lines orthogonal to each other. For example, for an imaging detector with 7.8 Mega pixels, there are 3072 TFT data lines and 2560 TFT gate lines. During the image readout, gate lines are turned on one at a time, allowing the image information from all the transistors from a column with the common gate line to turn on and transfer the image charge information to the corresponding rows of TFT data lines orthogonal to the TFT gate lines. The image information from each data line is then digitized by operational amplifiers and analog-to-digital converters (ADC) connected to each data line. Due to the orthogonal addressing scheme of these TFT arrays, each data line in the TFT will cross over a large number of gate lines inside the TFT panel. Each data line is separated from each gate lines in the TFT array by a thin layer of insulator at each cross over point resulting in a small capacitance between the data line and the gate line. Due to the large number of gate lines that each data line has to cross over, the accumulated capacitance is not negligible. For a panel of 7.8 Mega Pixels or 3072X2560 lines, the accumulative capacitance of each data line is typically in the order of 50 pico-farards (pf ) . Furthermore, the ground line connecting the ground return current from each transistor in the TFT is usually running parallel to the data lines in order to minimize the data line capacitance. Each gate line in the TFT will therefore need to cross over both the data lines and the ground lines and resulting in a gate line capacitance of about two times the data line capacitance. During image readout, the stored image charge from each pixel is transferred from the data line to a charge amplifier connected to each line. A charge amplifier is an operational amplifier with a charge integrating capacitor configured in the high gain amplifier feedback circuit as shown in Fig. 3. Fig. 3 also includes the frequency-independent "thermal noise gain" term:

The data line capacitance ( Ci ) in the input node of the operational amplifier and the feedback capacitor (C₂) in the operational amplifier configured as a charge-to-voltage converter will therefore function as a noise amplifier magnifying the thermal noise of the charge amplifier and the chain of components in the data lines by a gain factor equal to the ratio of the data line capacitance to the feedback capacitance. As an example, if the capacitance of the data line is 50pf, and the feedback capacitance of the operational amplifier is 2pf, the thermal noise gain will be 26. Low dose x-ray information with signal strength less than the magnified noise level will be buried in the noise and not be detected. The high level of background noise is therefore a significant disadvantage of systems known in the art. It is therefore very desirable to design a TFT array with data line capacitance much less than this conventional type of orthogonal gate line crossing. Furthermore, during the readout process, when each gate line is turned on, the gate control voltage is normally switched from a negative voltage of about -5 volts for maximum TFT "off" resistance to a positive voltage of +7 volts or higher to allow a low resistive "on" state for the transistor. This swing of 12 volts or more gate control voltage will inject charges Q_L equal to AV times C to the TFT storage capacitor containing the image information as well as to the data line connected to the charge amplifier, where AV is the change of control gate voltage from an off state to an on state and where C is the parasitic capacitance between the gate terminal and the drain terminal or the source terminal of the field effect transistor (FET) in the TFT. After the image information from each data line is integrated or collected by the charge amplifier, the gate line voltage will switch from positive to negative to turn off all transistors on the same line. Negative charge -Q₀ equals to AV times C will then extract from the image storage capacitor and the data line. In the ideal situation, the injected Qi during the gate turn on process should equal to the -Q₀ during the gate turn off process.

However, because of the hysteresis of the FET transistor, Qi may not be exactly the same as Q₀ and a small amount of net charge is therefore included in the image information as additional noise. This kind of noise is commonly known as TFT switching noise and is again undesirable for low signal or low radiation dose x-ray images. Such TFT switching noise is therefore a significant disadvantage of systems known in the art .

Still furthermore, the speed of image readout depends on how fast each gate line can be turned on and off for the process of image data integration. Large gate line capacitance will limit the speed of the gate switching operation, which is a significant disadvantage of systems known in the art. It is therefore desirable to minimize the gate line capacitance for high readout speed operation required by high frame rate imaging or dynamic imaging.

The problem of the invention has therefore been to provide a detector system that is able to perform low noise image capture and fast readout operation, and that therefore avoids the significant disadvantages of Thin Film Transistor (TFT) arrays that are known in the art. The invention has solved this problem by providing the radiation imaging detector and a method of using such detector for detecting radiation as described herein and according to the claims. The invention further provides a method of constructing an imaging array without using TFT for low noise operations.

All references cited herein are incorporated herein by reference in their entireties.

3. FIGURES

The invention is described in conjunction with the following drawings in which like reference numerals designate like elements and wherein:

Fig. 1 shows a prior art flat panel x-ray detector using thin-film transistors (TFTs);

Fig. 2 shows the arrangement of gate lines and data lines in a conventional TFT array;

Fig. 3 shows the equivalent noise gain circuit of a charge amplifier;

Fig. 4 is an exemplary horizontal layout drawing of a

section of 4 by 4 pixel of the present invention showing an arrangement of Charge Accumulation Electrodes, column pixel lines, field shaping electrodes, and output data lines;

Fig. 5 is an exemplary vertical cross section of the

detector layer structure;

Fig. 6 is an enlarged view of the bottom middle section of

Figure 5 above;

Fig. 7 shows the electrical field lines of the detector in x-ray image accumulating mode;

Fig. 8 is an enlarged view of the electrical field lines terminating on the insulating interface above the charge accumulation electrode;

Fig. 9 shows an enlarged view of Fig. 4 and shows that the electric field lines are terminating above the charge accumulation electrode;

Fig. 10 shows the electrical field lines of the detector in image charge transfer and readout mode;

Fig. 11 shows the enlarged view of Fig.10, the change of

electric field lines and the change of equipotent lines in the readout mode;

Fig. 12 shows the connection between the data lines and the charge amplifiers.

4. DESCRIPTION

The invention provides a flat panel consisting of a layer of photoconductive material (e.g. Selenium) . The absorbed X-rays produce electric charges in this layer. The charges are col^¬ lected on one storage capacitor per pixel, the so-called pixel-capacitor. The read-out of the image information is car^¬ ried out by transferring these charges from the storage capacitor onto read-lines.

The charges are read-out from the read-lines by means of one charge-sensitive-amplifier per read-out line. The advantage of this system is that the TFT transistor is, thus, obsolete and is eliminated. The TFT is the major source of the noise in the present flat-panel read-out system.

The invention provides a radiation imaging detector compris^¬ ing: a) a first dielectric layer (1), b) a plurality of electric field shaping electrodes (2) deposited on the first dielectric layer (1), c) a charge accumulating electrode (3) deposited on the first dielectric layer (1), d) a second dielectric layer (4) deposited over the electric field shaping electrodes (2) and the charge accumulating electrode (3), e) readout electrodes (5) deposited over the second dielectric layer (4) , f) a radiation charge conversion layer (6) deposited over the second dielectric layer (4) and the readout electrodes

(5) , and g) a top bias electrode (7). The radiation imaging detector of the invention can function without the need for a thin film transistor (TFT) . Avoiding the need for a TFT transistor can be one technical advantage of the detector according to the invention in that a major source of noise that is a disadvantage of the systems known in the art is eliminated.

The radiation imaging detector of the invention can further comprise: h) a first bias potential applied to the top bias electrode (7) to direct the radiation generated charges to move toward the charge accumulating electrode (3), i) a second bias potential applied to the field shaping electrodes (2) adjacent to the readout electrodes (5) to direct the charges away from the readout electrodes (5), and j) a third bias potential applied to the charge accumulating electrode (3) to direct the charges to the interface between the second dielectric layer (4) and the radiation charge conversion layer (6). Figure 8 shows the shape of the equal potential lines (9a) and the electric field lines (10) in this charge accumulation mode .

In one embodiment of the invention, the radiation imaging detector can further comprise: h) a first bias potential applied to the top bias electrode (7), i) a second bias potential applied to the electric field shaping electrodes (2) adjacent to the readout electrodes (5), and j) a third bias potential applied to one line of the charge accumulating electrodes (3) is changed to direct the charges accumulated on the interface between the second dielectric layer (4) and the radiation charge conversion layer (6) of the said line to move to the adjacent readout electrodes (5) . In this embodiment of the invention, one line of information can be read out. This step can be repeated for each column of lines connecting one column of charge accumulating electrodes (3) until the entire panel is read out and a complete x-ray image is formed. Figure 10 shows the equal potential lines (9a) and the electrical field lines (10) in this charge transfer mode.

In one embodiment of the radiation imaging detector of the in- vention, the plurality of electric field shaping electrodes (2) can comprise the electric field shaping electrodes (2a), (2b) and (2c) . In further embodiments, any number of additional electric field shaping electrodes (2d), (2e) , (2f) etc. can be comprised in said plurality of electric field shaping electrodes (2).

In a further embodiment of the invention, the radiation imaging detector can further comprise: k) a plurality of column pixel lines (8) . (column pixel lines (8) are necessary element of this invention)

In a further embodiment of the invention, the radiation imaging detector can further comprise: 1) a plurality of charge amplifiers (9) connected to each of the readout electrodes (5) to form a radiation image.

In some embodiments of the radiation imaging detector of the invention, the second dielectric layer (4) can comprise silicon dioxide (Si0₂) .

The invention further provides the use of the radiation imaging detector of the invention as described herein for detecting radiation. The invention further provides a method for detecting radiation comprising the steps of: a) providing the radiation imaging detector of the invention , b) generating a read out image signal, and c) detecting said read out image signal.

In one embodiment of the method of the invention, step b) can comprise: i) applying a first bias potential to the top bias electrode (7) to direct the radiation generated charges to move toward the charge accumulating electrode (3), ii) apply- ing a second bias potential to the field shaping electrodes (2) adjacent to the readout electrodes (5) to direct the charges away from the readout electrodes (5), and iii) applying a third bias potential to the charge accumulating electrode (3) to direct the charges to the interface between the second dielectric layer (4) and the radiation charge conversion layer (6) . Figure 8 shows the shape of the equal potential lines (9a) and the electric field lines (10) in this charge accumulation method. In one embodiment of the method of the invention, step b) can comprise: i) applying a first bias potential to the top bias electrode (7), ii) applying a second bias potential to the field shaping electrodes (2) adjacent to the readout electrodes (5), and iii) changing a third bias potential to one column of the charge accumulating electrodes (3) to direct the charges to the interface between the second dielectric layer (4) and the radiation charge conversion layer (6) of the said column to move to the adjacent readout electrodes. Figure 10 shows the equal potential lines (9a) and the electric field lines (10) in this charge transfer method.

In one embodiment, the invention provides a detector structure comprising a dielectric substrate or first dielectric layer (1), with a rectangular array of Charge Accumulation Elec- trodes, P(x,y)'s (3), Field Shaping Electrodes GX's, (2a) and Field Shaping Electrodes GY' s (2b, 2c) , as shown in Figure 1, deposited on one surface of said dielectric substrate or first dielectric layer (1). All Charge Accumulating Electrodes (3) with the same Y direction are connected by a select line SX

(8) running in the Y direction. The select line SX (8) can be the column pixel line (8) . The cross over area of the select lines SX' s is electrically isolated from the Field Shaping Electrodes (2b, 2c) running in the Y direction by insulating materials. A second layer of dielectric material (4), the second dielectric layer (4), such as Si02, with a thickness of 0,1 to 5 um is then deposited on top of the P(x,y)'s (3), GX's (2a) and GY' s (2b, 2c). On the top side of the said dielectric material (4), a set of data lines DY' s (5) are deposited between the P(x,y) electrode and the GY (2c), Field Shaping Electrode lines. The set of data lines DY' s (5) can be the readout electrodes (5) . The radiation charge conversion layer (6), such as amorphous selenium with sufficient thickness for radiation absorption is deposited on top of the said second dielectric layer (4) and data lines (5), wherein the latter can be the readout electrodes (5) . A top bias electrode (7) is then deposited on the top surface of the radiation charge conversion layer (6). During x-ray image acquisition, a high voltage bias, the first bias potential, is applied to the top bias electrode (7), developing an electric field between the top electrode and the bottom electrodes, wherein the latter are the charge accumulating electrodes (3) . With the exposure of radiation, electron-hole pairs will be generated in the radiation charge conversion layer (6). Depending on the polarity of the high voltage bias, either holes (positively charged) or electrons (negatively charged) will be driven toward the bottom layer of the detector. In this example, to illustrate the principle of the detector of the invention, a positive high voltage bias is used. Holes generated by the radiation such as x-ray will be driven by the bias field toward the bottom of the detector, to the second dielectric layer (4). A second positive bias potential is also applied to the Field Shaping Electrodes (2) adjacent to the data lines (5), or readout electrode (5), and between the data readout electrode (5) and the charge accumulating electrode (3). Since each of the data lines (5) or readout electrode (5) is connected to a charge integrating amplifier (9), or charge amplifier (9), the data line potential is at zero volt, or near zero volts. The Charge Accumulation Electrode (3) is biased with a third bias potential in the negative range. With appropriate voltages, the all the electric field lines starting from the top high voltage electrode (7) will be directed to the bottom of the detector and all ending on the dielectric layer (4) between the radiation charge conversion layer (6) and the charge accumulating electrode (3). Holes generated by the radiation within one pixel area will be accumulated at this pixel dielectric interface. At the end of x-ray exposure, the potential of one selected line of the charge accumulating electrode (3) will be changed from negative to positive. The direction of the electric field on the dielectric on top of the charge accumulation electrode (3) connected to this said selected line (8) or column pixel line (8) will be reversed and the charges accumulated on the dielectric interface (4) of this line will move along the reverse field to the adjacent data line orthogonal to the charge accumulating electrode line. Charges accumulated on each of the charge accumulation electrode will be transferred to the data lines adjacent to the respective pixel on the said selected line (8), or column pixel line (8) . Charges of each pixel along the charge accumulating electrode (3) will be integrated by the plurality of charge amplifiers (9) connected to each data line (5) or readout electrode (5) and the resulting charge value will be digitized and stored in the computer memories. After one line of charges are integrated and stored, the potential of the charge accumulating electrode (3) will be returned to negative and the potential of the next charge accumulating electrode line will be changed to positive value, reversing the electric field on the dielectric interface of this next line. Pixel charges previously accumulated on this line will then be transferred to the orthogonal data lines adjacent to the pixel. This action will be repeated until the image charges of the whole panel is read out.

Conventional TFT panels in the Prior Arts consist of orthogonal arrays of pixels addressed by orthogonal gate lines and data lines. The thinkness of the insulating material between the gate lines and the data lines is typically 200nm to 400nm normally limited by the TFT manufacturing process. The parasitic capacitance from the crossover of these gate lines and data lines inside the TFT structure result in a sizable data line capacitance. When charge amplifiers are used for the readout of image information, the thermal noise is greatly am- plified by the ratio of the data line capacitance and the feedback capacitor of the charge amplifier. The switching of the gate voltage that is typically 12 volts or higher also contributes to the switching noise in the readout image. However, in the present invention, conventional TFT manufacturing process is not used. The insulating spacing in the crossing of data lines to the orthogonal Field Shaping Electrode lines and charge accumulating electrode lines can be greatly increased.

The parasitic capacitance of the data line therefore can be greatly decreased resulting with much less noise amplification in the charge integrating amplifiers. Furthermore, a shielding electrode can also be inserted at the crossing of data lines and the charge accumulation control lines so that the switching of the potential of charge accumulation control lines will not be coupled to the data lines. As shown in Fig. 4, each pixel consists of one or more charge accumulation electrodes (3) separated from the radiation absorption and charge conversion layer (6) by a thin layer of the second dielectric material (4) such as one micron of silicon dioxide as shown in Figures 5 and 6. All the charge accumulation electrodes (3) in the same column are connected by column pixel lines (8) and are connected to a first variable voltage power supply. Each of the charge accumulation elec- trode (3) is surrounded by lines of field shaping electrodes (2a, 2b, 2c, or more...) also covered by a layer of the second dielectric material (4) such as one micron of silicon dioxide as shown in Figure 5 and 6. All the field shaping electrodes (2's), i.e. the plurality of electric field shaping electrodes (2), are connected to one or more switchable voltage power supplies. Running orthogonal to the column pixel lines (8) are the output data lines (5), or readout electrodes (5), which are also sandwiched by two field shaping electrodes (2's) . During the x-ray image acquisition mode, an appropriate high voltage bias is applied to the top electrode (7) producing a mostly uniform electric field throughout the bulk of the x-ray absorption and charge generation layer (6) such as selenium.

In one embodiment of the invention, a positive voltage of 1KV over ΙΟΟμη of selenium can be used resulting with an electric field of 5 volts per micron throughout most of the bulk of selenium layer. A potential of 1.1KV can be applied to the field shaping electrodes adjacent to the output data electrode and the charge accumulation pixel electrodes. As shown in Figure 12, with the output data electrodes (5) connected to the charge amplifiers (9), the potential of the data electrodes can bear near zero volts range. A negative voltage of -200 volts can be applied to all the charge accumulation pixel electrodes (3) . With this distribution of potentials, all the electric field lines (10) starting from the top high voltage bias electrode can terminate on the top surface of the second dielectric material (4) above the charge accumulation pixel electrodes (3). Upon exposure of x-ray radiation, electron- hole pairs can be generated in the selenium layer proportional to the intensity of the radiation absorbed. Electrons can be driven by the electric field to the top high voltage electrode (7) and holes can be driven by the electric field (10) to the bottom of the detector. Since all the electric field lines (10) with the distribution of shaping potentials can be terminated above the pixel electrodes (3), all the x-ray radiation generated holes can be driven by the electric field (10) and can be accumulated on the dielectric interface (4) separation the selenium layer (6) and the pixel electrode (3) as shown in Figure 9. At the end of the x-ray exposure, the x-ray image can be represented by the amount of charges accumulated over the detector on the interface area above each pixel electrode (3) . When the panel is ready for readout, one column of the charge accumulation pixel electrode potential can be changed from negative to positive, such as from -200 volts to positive +1000 volts. With this change, the electric field above the pixel electrode can be reversed and the electric field lines (10) can initiate from the dielectric interface and terminate at the output data electrode which was still at zero volts. Positively charged holes accumulated on the column of the pixel dielectric interface during the x-ray exposure can now driven by this new distribution of electric field (10) as shown in Figure 10 and Figure 11 and can move from the dielectric interface (4) to the adjacent output data lines (5) and be integrated by the charge amplifiers connected to each line as shown in Figure 12. Image information of one column is therefore acquired. At the end of data acquisition of one column, the potential of the said column will return to the negative value and a next pixel column (8) is changed from nega- tive to positive pushing the image charges of this next column (8) to the rows of data lines (5). This action can be repeated until all the charges on the imaging panel is readout. For large area pixel, more than 2 charge accumulation electrodes can be used in each pixels surrounded by more field shaping electrodes, so that the charges accumulated by each pixel during x-ray exposure can be distributed over more than 2 charge accumulation electrodes.

EXAMPLE

In this example, a positive voltage of 1KV over ΙΟΟμπι of selenium was used resulting with an electric field of 5 volts per micron throughout most of the bulk of selenium layer. A potential of 1.1KV was also applied to the field shaping electrodes adjacent to the output data electrode and the charge accumulation pixel electrodes. As shown in Figure 12, with the output data electrodes (5) connected to the charge amplifiers (9), the potential of the data electrodes was near zero volts range. A negative voltage of -200 volts was applied to all the charge accumulation pixel electrodes (3) . With this distribution of potentials, all the electric field lines (10) starting from the top high voltage bias electrode terminated on the top surface of the second dielectric material (4) above the charge accumulation pixel electrodes (3) . Upon exposure of x-ray radiation, electron-hole pairs were generated in the selenium layer proportional to the intensity of the radiation absorbed. Electrons were driven by the electric field to the top high voltage electrode (7) and holes were driven by the electric field (10) to the bottom of the detector. Since all the electric field lines (10) with the distribution of shaping potentials (9a) were terminated above the pixel electrodes (3), all the x-ray radiation generated holes were driven by the elec- trie field (10) and were accumulated on the dielectric interface (4) separation the selenium layer (6) and the pixel electrode (3) as shown in Figure 9. At the end of the x-ray exposure, the x-ray image was represented by the amount of charges accumulated over the detector on the interface area above each pixel electrode (3) . When the panel was ready for readout, one column of the charge accumulation pixel electrode potential was changed from negative to positive, such as from -200 volts to positive +1000 volts. With this change, the electric field above the pixel electrode were reversed and the electric field lines (10) initiated from the dielectric interface and terminated at the output data electrode which was still at zero volts. Positively charged holes accumulated on the column of the pixel dielectric interface during the x-ray exposure were now driven by this new distribution of electric field (10) as shown in Figure 10 and Figure 11 and moved from the dielectric interface (4) to the adjacent output data lines (5) and were integrated by the charge amplifiers connected to each line as shown in Figure 12. Image information of one column was there- fore acguired. At the end of data acquisition of one column, the potential of the said column returned to the negative value and a next pixel column (8) was changed from negative to positive pushing the image charges of this next column (8) to the rows of data lines (5). This action was repeated until all the charges on the imaging panel were readout. For large area pixel, more than 2 charge accumulation electrodes were used in each pixels surrounded by more field shaping electrodes, so that the charges accumulated by each pixel during x-ray exposure was distributed over more than 2 charge accumulation electrodes.

The above examples show that the radiation imaging detector as well as the method for detecting radiation as described above are particularly useful for detecting radiation, whilst avoid- ing the problems associated with radiation detectors of the prior art, such as high background noise.

While the invention has been described in detail and with ref- erence to specific examples thereof, it will be apparent to one skilled in the art that various changes and modifications can be made therein without departing from the spirit and scope thereof.

Claims

1. A radiation imaging detector comprising: a) a first dielectric layer (1), b) a plurality of electric field shaping electrodes (2) deposited on the first dielectric layer (1), c) a charge accumulating electrode (3) deposited on the first dielectric layer (1), d) a second dielectric layer (4) deposited over the electric field shaping electrodes (2) and the charge accumulating electrode (3), e) readout electrodes (5) deposited over the second dielectric layer (4), f) a radiation charge conversion layer (6) deposited over the second dielectric layer (4) and the readout electrodes (5), and g) a top bias electrode (7).

2. The radiation imaging detector of Claim 1, further comprising : h) a first bias potential applied to the top bias electrode (7) to direct the radiation generated charges to move toward the charge accumulating elec- trode (3), i) a second bias potential applied to the field shaping electrodes (2) adjacent to the readout electrodes (5) to direct the charges away from the readout electrodes ( 5 ) , and j) a third bias potential applied to the charge accumulating electrode (3) to direct the charges to the interface between the second dielectric layer (4) and the radiation charge conversion layer (6).

3. The radiation imaging detector of Claims 1, further comprising : h) a first bias potential applied to the top bias electrode (7 ) , i) a second bias potential applied to the electric field shaping electrodes (2) adjacent to the readout electrodes (5), and j) a third bias potential applied to one line of the charge accumulating electrodes (3) to direct the charges accumulated on the interface between the second dielectric layer

(4) and the radiation charge con version layer (6) to move to the readout electrodes (5) . . The radiation imaging detector of one or more of Claims 1 to 3, wherein the plurality of electric field shaping electrodes (2) comprises the electric field shaping electrodes (2a) , (2b) and (2c) .

5. The radiation imaging detector of one or more of Claims 1 to 4, further comprising: k) a plurality of column pixel lines (8)

6. The radiation imaging detector of one or more of Claims 1 to 5, further comprising:

1) a plurality of charge amplifiers (9) connected to each of the readout electrodes (5) to form a radiation image .

7. The radiation imaging detector of one or more of Claims 1 to 6, wherein the second dielectric layer (4) comprises silicon dioxide (Si0₂) .

8. Use of the radiation imaging detector of one or more of Claims 1 to 7 for detecting radiation.

9. A method for detecting radiation comprising the steps of a) providing the radiation imaging detector of one or more of the Claims 1 to 7, b) generating a read out image signal, and

c) detecting said read out image signal. The method of Claim 9, wherein step b) comprises: i) applying a first bias potential to the top bias electrode (7) to direct the radiation generated charges to move toward the charge accumulating electrode ( 3 ) , ii) applying a second bias potential to the field shaping electrodes (2) adjacent to the readout electrodes (5) to direct the charges away from the readout electrodes (5), and iii) applying a third bias potential to the charge accumulating electrode (3) to direct the charges to the interface between the second dielectric layer (4) and the radiation charge conversion layer (6).