DIRECT RADIOGRAPHIC IMAGING PANEL WITH AN IMAGING PROPERTY REVERSIBLY ADJUSTABLE WITH AN EXTERNAL ENERGY SOURCE IN CLINICAL USE OF THE PANEL

Abstract: A panel for radiation detection has an integrated array of x-ray sensors, each of which includes a charge storage capacitor, a radiation sensitive layer over the charge storage capacitor, and a control layer over the radiation sensitive layer. The control layer's electrical conductivity can be changed reversibly in the field, after the panel is manufactured and without disassembly of the panel, to make the time constant of the layer T=(1/y)K epsilon 0 change between high and low values in the range of, e.g., 0.03 and 20 seconds, wherein y is the conductivity and K is the dielectric constant of the control layer, and epsilon 0 is the permittivity of free space. The conductivity, and thus the time constant of the control layer, are controllable via the application of electromagnetic energy, preferably infrared radiation. Preferably, the radiation detection layer is a photoconductor and the control layer is an organic photoconductor (OPC).

Description: DIRECT RADIOGRAPHIC IMAGING PANEL WITH AN IMAGING PROPERTY REVERSIBLY ADJUSTABLE WITH AN EXTERNAL ENERGY SOURCE IN CLINICAL USE OF THE PANEL Priority Claim This application claims priority based on provisional patent application 60/247,640 filed November 10, 2000, and hereby incorporates the contents thereof by reference. Field This patent specification relates to radiation sensors in general and more particularly to a radiation detection panel comprising a plurality of radiation sensors useful for imaging x-ray exposures. More specifically, this patent specification is directed to a panel that has a layer with an imaging characteristic that can be changed as needed in clinical use, after manufacture of the panel. The relevant characteristic can be the electrical conductivity of the layer. Background Radiation sensors that convert incident radiation directly to an electrical charge related to the incident radiation are known. Typically, such sensors comprise a layered structure that includes a bottom and a middle conductive electrode separated by a dielectric to form a charge storing capacitor. A radiation detection layer, which may be a photoconductive layer, is placed over one of the electrodes. Over the photoconductive layer there is a "dielectric" layer, and then a top electrode. Charge blocking layers are often provided between the conductive electrodes and the photoconductive layer, and there can be other elements as well. A charging voltage is applied between the top electrode and the bottom capacitor plate. Upon exposure to radiation, a charge related to the exposure accumulates in the storage capacitor formed by the bottom and middle electrodes. Read-out of the stored charge is usually done by addressing the middle electrode and transferring the capacitor charge to a charge-measuring device such as a charge-integrating amplifier. A plurality of such sensors can be formed as an array of rows and columns of a radiation detection panel. By sequentially reading out the charges accumulated in the individual sensors an image is obtained of the relative exposure at different small areas (pixels) of the panel. This process can image the radiation incident on the panel after it has passed through a subject illuminated by the radiation. When the radiation is X-ray radiation and the subject is a patient the resulting image is a radiogram, captured as a plurality of charges. This radiogram can be displayed on a Cathode ray tube or other device for viewing. The charge stored in the capacitor is read-out using a switch that connects, upon command, the middle electrode to the input of a charge measuring device. In practice such switch is usually an FET transistor created integrally with the sensor, having its source electrode connected directly to the middle electrode of the sensor. Both the drain electrode and the gate are accessible from outside the sensor. The drain is connected to a charge integrator. An electrical signal applied to the gate switches the transistor to a conductive state and permits the charge to be transferred from the capacitor to the integrator for detection. The above described technology is discussed in a number of publications and issued patents, exemplary of which are United States Patent 5,319,206 issued June 7, 1994 to Lee et al. and an article by Denny L. Lee, Lawrence K. Cheung and Lothar S. Jeromin, entitled "The Physics of a new direct digital X-ray detector" appearing in the Proceedings CAR '95, Springer-Verlag, Berlin, pp. 83. Both the patent and the article are incorporated herein by reference. An imaging panel that has a "dielectric" layer between the top electrode and the photoconductive layer, with a time constant adjusted in the course of manufacturing the panel is discussed in commonly owned U.S. Patent Application Serial No. 09/110,549 filed on July 6, 1998, now U.S. Patent No. 6,194,727 granted on February 27, 2001, which is hereby incorporated by reference. While a layer between the top electrode and the radiation sensitive layer conventionally has been referred to as a "dielectric" layer even when it is not designed to prevent current flow, and is so referred to in said 727 patent and in said provisional application, in fact the so-positioned layer disclosed in the provisional application and in this application is not an electrically isolating layer that prevents the flow or electrical current but is designed to deliberately permit current flow at a desired rate. Accordingly, this application uses the term "control layer" that is believed to more accurately describe the nature, purpose and operation of this electrically conductive layer that historical terms such as "isolation" and "dielectric" layer. Summary of the Disclosure Direct x-ray imaging panels using the sensor and transistor structure described above have been found to be vulnerable to overexposure in certain applications. The term "exposure" is used in this specification to designate the product of the intensity of the incident radiation and the time during which the radiation impinges on the sensor. One solution to overexposure is to provide a layer that has a sufficiently high electrical resistance to create a secondary field limiting the excessive build-up of charges and voltages. However, the secondary field needs to be cleared before the next x-ray exposure. In radiography, where the time between exposures is relatively long, this can be a part of an extra, post-exposure step in which the electrical charge pattern creating the secondary field is allowed to dissipate. However, in certain other types of x-ray imaging, such as real time fluoroscopy, the rapid succession of x-ray exposures may not allow time for such discharge. It is desirable and commercially important to be able to use the same direct x-ray imaging panel for different types of imaging, such as radiography and fluoroscopy, and this patent specification describes a way of achieving this that still provides overvoltage protection as and when needed. It is believed that the following theory can be used in describing the operation of direct x-ray imaging panels: When a sensor forming a part of such a panel is exposed to radiation, electron-hole pairs are generated in the radiation detection layer which, under the influence of the electric field produced by the applied charging voltage, travel toward the top and middle electrodes respectively. If such charges are allowed to flow freely, the charge stored in the capacitor formed between the middle and bottom electrodes can keep increasing. The result of such continuous charge increase is an associated increase in the voltage at the middle electrode, which can eventually result in a catastrophic failure of the associated transistor switching element connected to the charge storage capacitor. The presence of a protective layer between the radiation detection layer and the top electrode that can reduce this risk by presenting a barrier to the migrating charges, which begin accumulating at the interface between the radiation detection layer and such a protective layer. These accumulated charges set up a secondary field opposing the applied charging field, thus inhibiting further charge migration and providing a limit to the rising voltage at the middle electrode. While this can help prevent overvoltage risks, the accumulated charges at the interface of such a protective layer and the photoconductor can interfere with subsequent x-ray exposures of the sensor. In order to reduce the effect of such residual charges, there is usually required an additional step in which the trapped charges are eliminated. This extra step is not only time consuming but, for reasons discussed later, can inhibit the use of this type of sensor for imaging modes such as fluoroscopy. Thus, it is believed that a need exists to provide a flat panel detector that has the advantage or overvoltage protection and still can be used in imaging such as fluoroscopy. In one preferred embodiment, this need is met with a flat panel x-ray detector having a control layer that can have its electrical conductivity (the reciprocal of resistivity) changed significantly in response to an externally supplied control such as selective exposure to energy such as light, e.g., infrared light. Such a detector can comprise: a) a charge storage capacitor; b) a first photoconductive radiation-sensitive layer over said charge storage capacitor; c) a control layer over the radiation sensitive layer, where the control has an electrical conductivity v controllable over a range, e.g., by the application of energy from an external source. This control layer can have a time constant T = (1/γ)κε 0 = p ε 0 selectable between a low value and a high value, for example and without limitation, between about 0.10 and 20 seconds, with the lower value preferably as low as 0.03 seconds and most preferably as low as 0.01 seconds, by adjustment of the control layer's electrical conductivity, wherein y is the control layer's electrical conductivity, p is the layer's electrical resistivity, K is the layer's dielectric constant, and ε 0 is the permittivity of free space; and d) a top conductive layer over said control layer. The parameters of the energy (e.g., light) that controls the electrical conductivity of the control layer can depend upon the expected x-ray exposure and the imaging mode of the panel (e.g, static radiography vs. fluoroscopy). When an external source of light is used to control electrical conductivity of the control layer, the light frequency preferably is in the range from visible to infrared. The control layer may be an organic photoconductor (OPC) in which the control properties, e.g., the electrical conductivity y, can be modified using light modulation. In a preferred embodiment, the detector panel uses an array of a multiplicity of radiation sensors, each of said sensors including: a) a charge storage capacitor; b) a radiation sensitive layer over said charge storage capacitor; c) an externally controlled control layer over said radiation sensitive layer, and d) a top conductive layer over said control layer. A system including a preferred embodiment of a flat panel detector can also include a source of light radiation used to modulate the electrical properties of the control layer, so that the system includes the detector panel described above and, also: e) a source of light, with wavelengths that preferably can be in the range of 750- 1000 nanometers, used to illuminate the control layer, such as in a uniform manner across the panel. The source of the energy that controls the electrical conductivity of the control layer can be controlled by the system computer that also controls the readout of the flat panel detector, in order to suitably time the clearing of the secondary field to the readout of the image. In a broader sense, this patent specification describes a direct x-ray imaging panel that has an imaging property (e.g., the electrical conductivity of a layer) that can be controlled by the application of energy from an external source (e.g., a light source) in a manner that makes the panel suitable for different types of x-ray imaging. Brief Description of the Drawing Figure 1 is a schematic elevation of internal structure of a radiation sensor in accordance with a preferred embodiment. Figure 2 is a plan view illustrating a portion of a detector panel comprising a plurality of sensors of the type shown in Fig. 1. Figure 3 illustrates a simplified electrical equivalent circuit of the sensor of Fig. 1. Figure 4 shows relative absorbance of light in the 400-800 nm range of materials that can be used for the photoconductor layer and for the variable conductivity control layer in a preferred embodiment. Figure 5 is a vertical section similar to Fig. 1 but also illustrating an applied external electromagnetic field. Figure 6 illustrates an operation of the sensor of Figs. 1 and 5 in the presence of radiation and an applied electric field across the device. Figure 7 is a view similar to Figs. 1 , 5 and 6 but also shows provisions for reducing the sensitivity of a switching transistor to infrared light. Detailed Description of Preferred Embodiments Throughout the following detailed description, similar reference characters refer to similar elements in all figures of the drawings. Referring to Fig. 1 , a sensor 10 is built on a substrate 30 that may be glass, ceramic, or other suitable insulating material providing enough mechanical strength to support the layers and associated electrical circuitry. An electrically conductive element 32 forms a first or bottom conductive microplate 32, and an electrically conductive element 36 forms a second conductive microplate. A dielectric layer 34 is placed between the two microplates to form a charge-accumulating capacitor 14. Conductive elements 32 and 36 can be thin layers of a conductive material such as indium-tin-oxide, or a thin layer of an electrically conductive material such as a metallic material, e.g., between 5θA and 100A thick. An FET transistor 40 is also built on substrate 30. Such transistor preferably comprises a gate electrode 42, a semiconductor 43 which is typically αSi, a contact layer 44, typically a layer of αSi doped to n+ conductivity, a source electrode 46 and a drain electrode 48. Source electrode 46 is connected to the second conductive microplate 36, and the drain electrode 48 is connected to a conductor leading to a contact for connecting the FET 40 to a charge detector. The technology for making arrays of FET transistors connected to arrays of microplates is known. (United States Patent 5,641 ,974 issued to den Boer et. al. discusses one way of making such a transistor but there are many other ways to make such arrays that are suitable for the purpose of the detector panels discussed here.) A radiation detection layer which is typically a photoconductive layer 50, and which preferably exhibits very high dark resistivity (low conductivity), is over the previously deposited layers. In radiography, particularly in medical applications, the incident radiation is X-ray radiation, and the radiation detection layer is an X-ray photoconductor. The photoconductive layer can comprise amorphous selenium, lead iodide, lead oxide, thallium bromide, cadmium telluride, cadmium sulfide, mercuric iodide or any other suitably photoconductive material. It can comprise organic materials such as polymers, loaded with X-ray absorbing compounds exhibiting photoconductivity when the captured radiation is X-ray radiation. In a preferred embodiment, this layer is a continuous amorphous selenium layer with graded arsenic content, 200 to 1000 microns thick. This patent specification refers to "selenium" as the material for layer 50, but includes in that term the preferred photoconductor that comprises amorphous Se with graded As content and possibly dopants such as chlorine, as used in the current commercial product of the assignee, as well as possible other doping. See U.S. Patent 5,880,472. A control layer 52, in this example a control layer whose electrical conductivity can be controlled with an external source of energy, is placed over the photoconductive layer 50, and a conductive top electrode 20 is placed over the control layer 52. Top electrode 20 is preferably a layer of chromium; other conductive material such as indium-tin-oxide, aluminum, etc. may be used. The top electrode properties are selected so that it is substantially transparent to the radiation one wishes to detect. When such radiation is X-ray radiation the top electrode is preferably a conductive layer which is highly penetrable by such radiation. Except for control layer 52, the radiation sensor 10 preferably is substantially the same as in the flat panel detector commercially available from the assignee hereof. However, the presence and use of control layer 52 is believed to significantly improve performance and expand the fields of use of the sensor. Control layer 52 is made of a material that normally exhibits relatively low dark conductivity (high dark resistivity), preferably dark conductivity much lower than that of the photoconductive layer 50, but is sensitive to and responds to incoming energy such as light to increase its conductivity to a value higher than that of the photoconductive layer 50. In a preferred embodiment, control layer 52 is an organic photoconductor (OPC) layer that can be continuous and, for example and without limitation, 10 to 50 microns thick, and has a high photosensitivity to electromagnetic radiation such as light having wavelengths in the range of, for example and without limitation, 750-1000 nanometers. Various types OPC materials are commonly used to coat the photoconductor drums of office laser printers, for example in Hewlett Packard Laser Jet 5 printers. A thin layer 54, that can be either a unidirectional charge blocking layer permitting passage of one type of charge and not another, i.e., either negative charges or positive, between the second microplate and the photoconductor, or an insulating layer which permits no charge flow between the microplate and the photoconductor, is placed between the second microplate 36 and the photoconductive layer 50. Such unidirectional charge blocking layers are known in the art, and typically comprise a non-conductive oxide formed on the microplate surface facing the photoconductor. Commonly assigned United States Patent 6,025,599, hereby incorporated by reference, teaches the use of such a layer. Another unidirectional charge blocking layer 60 is optionally placed between the control layer 52 and the conductive top electrode 20 The technology for forming the sensors preferably involves vacuum deposition of alternating layers of conductive and insulating materials, and is known in the art. See for instance "Modular Series on Solid State Devices" Volume 5 of Introduction to Microelectronics Fabrication by R.C. Jaeger, published by Addison-Wesley in 1988. Control layer 52 preferably is spin-coated. Illustrated in Figure 6 is a programmable power supply 90 for applying a charging voltage to the sensor. The power supply 90 is connected to the top electrode 20 and the bottom microplate 32 of the storage capacitor. Referring now to Fig. 2, a plurality of sensors 10 can be arrayed on a supporting structure 12 to provide an imaging panel capable of capturing radiation produced images. Such a panel can be made as a single unit or can be a composite of a plurality of smaller panels assembled to achieve a desired size. U.S. Patent 5,381 ,014 issued to Lee et al. on October 8, 1996 discusses making larger panels using smaller units. As illustrated in Figure 3, an electrical equivalent of the multilayer structure of sensor 10 can be represented as a number of capacitors connected in series. In the case where the detector includes a control layer 52, there are four primary capacitors connected in series, Cd, Cse, Cin and Cst. Cd is the capacitor formed by the top electrode 20 and the radiation detection layer top surface 51 and includes the control layer 52 separating the top electrode from the radiation detection layer. This capacitor dissipates its charge in a manner that depends to a significant degree on the electrical conductance of layer 52. The fact that layer 52 is designed to conduct electricity is an important characteristic for the operation of sensor 10. Cse is the capacitor formed by the top surface 51 of the radiation detection layer 50, and the insulating layer 54. Cin is the capacitor formed by the interface between radiation detection layer 50 and insulating layer 54. Cst is the storage capacitor formed by the middle and bottom microplates 36 and 32. As disclosed in the aforementioned article by Lee et al. the capacitor values can be optimized such that, as a non-limiting example, a 10 V/micron electric field is initially applied across the radiation detection layer 50. Figure 3 also shows another electrical equivalent of a sensor, in which the capacitors Cd, Cse, Cin and Cst are represented each as a pure capacitor Cd', Cse', Cin' and Cst' each connected in parallel with a resistance Rd, Rse, Rin and Rst. For purposes of this patent specification we are interested primarily in capacitor Cd. Capacitor Cd can be represented by an equivalent circuit of a capacitor in parallel with a resistor, and has associated with it a time constant T. (1) τ = (1/Gd)Cd' = RdCd'. Where: (a) Gd = γL/A and Rd = p L/A, where Gd represents the electrical conductance of layer 52, Rd represents the electrical resistance of layer 52, y represents the electrical conductivity of layer 52, p represents the electrical resistivity of layer 52, A represents the area of layer 52, and L is the thickness of layer 52; and (b) Cd' = κε 0 A L, and K is the control constant of the material of layer 52, and ε 0 = 8.85 x 10 "14 farads/cm and is the permittivity of free space. (K = 1 for vacuum). By substitution of (a) and (b) in (1): (2) τ = (1/γ)κε 0 = pκε 0 . This shows that the time constant associated with the layer 52 is dependent on the conductivity y (or resistivity p) of the material, and that this time constant can be adjusted by varying the conductivity of the layer. The conductivity of the control layer 52 can be adjusted via the application of electromagnetic radiation such as light, preferably in the infrared region, from an external source. Figure 4 illustrates the relative absorbance of an organic photoconductor (TiOPc) and amorphous selenium (a-Selenium) as a function of the wavelength of applied light. It can be seen that for light of wavelength of 750 nanometers, for example, selenium has much less relative absorbance than an organic photoconductor. By judicious application of the proper intensity and wavelength of light in the infrared region in this example, it should be possible to effect a large conductivity change in layer 52, while not substantially affecting the conductivity of the primary x-ray-sensitive photoconductor layer 50. A typical construction of layer 50 is between 200 and 1000 microns thick. Selenium is essentially transparent to infrared light, and has an absorption coefficient of about 10/cm for light of wavelength around 750 nanometers, resulting in about 40% absorption in a 500- micron thick layer. Selenium is even less absorbing for longer wavelength light. See, for example, Hartke and Regensburger, Electronic States in Vitreous Selenium, Phys. Rev. A, 139, 3A, A970 (1965). The relatively high transparency and low photosensitivity of selenium to infrared light means that the OPC layer 52 should be affected much more significantly than the selenium layer 50. A method of controlling the characteristics of the control layer 52 is illustrated in Fig. 5. An external source of electromagnetic radiation 100, preferentially infrared light uniformly illuminating the sensor, is applied to the layer 52. The responses of layers 52 and 50 to this radiation 100 are shown in Figure 4, for suitable layer materials of OPC for layer 52 and selenium (with As content) for layer 50. A proper selection of the control layer 52 should allow controlling the time constant of this layer using the external light 100. The time constant for the layer 52 in the dark, τ dark , is given by equation (2), and the dark conductivity γ dark of the layer 52 ideally is relatively low so that the τ dark can be several seconds or longer. The time constant for layer 52 in the presence of the external light 100, T |ight , ideally is shorter, for example and without limitation, less than about 0.5 seconds, preferably less than 0.05 seconds, and most preferably less than 0.03 and even 0.01 seconds. The layer 52 is thin relative to the thickness of the radiation-sensitive layer 50. For example, layer 52 is 10-50 microns thick (about 20 microns is currently preferred) while layer 50 is 200-1000 microns thick.. Because of the thinness of layer 52 and because it is comprised of low-Z materials, layer 52 is substantially non-responsive to x-rays. Thus its behavior can be modulated using the external light source 100. In operation, upon exposure to radiation, electron-hole pairs are generated in the radiation detection layer. This is illustrated in Fig. 6. When the charging voltage 90 applied to the top electrode is positive, electrons drift toward the interface between control layer 52 and photoconductor 50 and are collected there, establishing a reverse field in the photoconductor. This reverse field opposes the applied field and eventually grows to the point where further charge migration is stopped. The charges trapped at the interface between the control layer 52 and radiation detecting layer 50 are not permanently trapped there, but dissipate over a time period, flowing through the RdCd' circuit at a rate determined by the circuit time constant T. By changing the electrical conductivity Y of the control layer material 52, T may be adjusted and consequently the time for the charges accumulated on the interface between the radiation detector 50 and the control layer 52 to dissipate, can also be adjusted. As stated earlier, a plurality of radiation detection sensors arrayed on a support is used to create a panel to capture an image. Each of the sensors forms a PIXEL, or picture element. In operation, the panel is charged by applying a charging voltage between the top electrode and the bottom microplates. It is then exposed to radiation, which carries image information as a modulated intensity. The radiation impinges on the panel for a preset duration, and charges related to the radiation intensity are generated and stored in the storage capacitors. Appropriate signal processing, (preferably of the type discussed in United States Patent 5,648,660 issued to Lee et al.) is used to recover the accumulated charges in all of the storage capacitors in the detectors in the panel and to reconstruct a visible image. Two of the x-ray imaging modes in medical radiology are (a) single shot imaging to produce still pictures, and (b) real-time continuous exposure imaging for real-time continuous image observation. The first kind is generally referred to as radiography, and involves the taking of single exposure still pictures known as radiograms. Exposures are short, typically a few milliseconds, often as few as 0.002 sec. and the intensity of the radiation is relatively high. The second method requires a continuing image capture and display, usually in real time, generally known as fluoroscopy. In this instance the radiation exposure is relatively long and the radiation levels low. Exposures of a few minutes are common, and the display can be a real-time display of a plurality of sequentially obtained images from the panel. Images may be displayed as rapidly as 30 images per second to create the visual impression of motion, or can be displayed at a lower frame/image rate. In such applications the individual charge storage capacitors can be read out and discharged as frequently as, for example and without limitation, every 0.020 to 0.100 sec. As mentioned before, in the case of radiograms, because the capacitors are not discharged until the exposure is over, it is possible that high exposure levels will raise the voltage in the middle electrode to a point where the switching element can be damaged. Thus, it is desirable to encourage charge build up at the interface of the photoconductor 50 and control layer 52. As discussed above such a charge produces a counter biasing electric field and thus limits the charge accumulation in the storage capacitor and prevents excessive voltages from developing and damaging the switching element. Therefore, in a panel for still picture capture, the control layer desirably has a longer time constant T associated with it. On the other hand, in fluoroscopic applications the radiation intensity is low and the storage capacitors are repeatedly discharged during readout. This readout repeats frequently, e.g., every few milliseconds or 10-30 times per second, which tends to prevent excessive charge accumulation and resulting switching element damage. In both applications, it is desirable to obtain an image free of artifacts due to charge build-up at the control layer. This requirement means that between successive imaging exposures, any secondary, biasing field created by the charges trapped in the interface between the photoconductor and the control layers should become insignificant or non-existent or at least substantially uniform and constant over the imaging area of the panel. This, in turn, also means that between exposures, the charge in the interface between the photoconductor and the control layers should dissipate or rapidly change to a uniform level and remain substantially constant. One benefit of such dissipation or uniformity is removal of undesirable consequences of ghosting, i.e., remnants of latent images formed by charges remaining from a previous image. For a panel intended to be used in radiographic applications where single shot images are obtained every few minutes or so, a control layer 52 with a time constant of the order of seconds and possibly as long as, e.g., 20 seconds can be used in order to provide the needed overexposure protection. See U.S. Patent 5,319,206. Given sufficient time between exposures, the trapped charges dissipate simply by the passage of time, or one can use other methods to assure the dissipation of charges and elimination of prior exposure artifacts, such as flooding the panel with light, as disclosed in U.S. Patent 5,563,421 issued to Lee et al. on October 8, 1996. For a panel useful for fluoroscopic applications time constants as low as a few milliseconds are desirable to assure sufficient dissipation of the accumulated charges at the interface between readouts. Time constants of 100 milliseconds or shorter may thus be called for. In accordance with the preferred embodiments described in this patent specification, a dual-purpose panel is provided that is useful in both types of medical radiography. This can be achieved by selectively and reversibly adjusting the time constant of layer 52, without disassembly of or remaking the panel. When the panel is used for radiographic applications, layer 52 has lower conductivity and, consequently, a longer time constant, and the sensor can behave, during the short time that the panel is being exposed to x-rays and is then read out, similarly to the sensor described in U.S. Patent 5,319,206. The time constant of layer 52 can be long enough to provide the needed charge build-up that prevents damage to the transistor due to voltage build-up in the charge storage capacitor. When the panel is to be used in fluoroscopic applications, the action of energy such electromagnetic radiation that can be infrared light, can be selectively applied to the panel. The top electrode 20 is transparent to such light, and so the light penetrates to layer 52 and causes its conductivity to increase sufficiently so that the time constant of this layer is reduced, for example and without limitation, to approximately 50-100 milliseconds or less. This time constant permits the charges trapped at layer 52 to dissipate rapidly so that any remnants of such charges from previously read-out x-ray images would not have a significant effect on x-ray image read-out later during long, multi-frame fluoroscopic type imaging. Because the exposure level per frame (image) in fluoroscopic imaging tends to be significantly less than in radiography, the lower time constant of layer 52 can still be enough to protect against overvoltage. The control over the timing and other parameters of the electromagnetic radiation (e.g., infrared light) can be carried out by the same computer that controls functions such as the timing of flat panel detector readout, schematically illustrated at 56 in Fig. 5. Alternatively, a manual control can be provided, such as a manual switch to turn on an infrared light source secured to or integrated within the x-ray imaging panel. The externally applied energy such as infrared light can be applied continuously during fluoroscopic exposure or it can be applied intermittently, in a sequence and/or at energies calculated to achieve the desired reduction in the time constant of layer 52. When the same x-ray imaging panel is used for radiography, the energy that reduces the time constant of layer 52 is kept off or at such a level as to maintain the layer 52 at lower conductivity and, therefore, with a longer time constant. A material useful in control layer 52 in this embodiment is an organic photoconductor such as that manufactured by AEG Elektrofotografie Gmbh, Warstein-Belecke, Germany, and discussed in the article by Deiter Adam, Hans- Josef Humpert, et. al., entitled Photoreceptor Technologies for High Volume Printing: The Potential of As2Se3, a-Si, and OPC, published in IS&T's NIP 13: International Conference on Digital Printing Technologies, Seattle, Washington November 1997, Volume 13. The contents of this article are incorporated herein by reference. The electrical properties of the OPC according to the paper were determined experimentally. The control constant ε 0 of the OPC is typically 3. Samples of the OPC were charged to 300 volts, with subsequent observation of the time constant of the decay of the voltage. For a typical OPC sample, which has a charge generation layer thickness of 0.2-0.3 microns and a charge transport layer thickness of ~25 microns, the dark decay constant, τ dark (decay in the absence of light stimulation), is 58 seconds. The light decay constant, τ πght (decay during stimulation with light), is 0.067 seconds. The dark resistivity p dark is calculated to be about 2.4e14 Ω-cm, and the light resistivity p, ight about 2.7e11 Ω-cm. The observed light decay constant is relatively independent of light wavelengths in the range 750-1000 nanometers. Illumination with the proper wavelength of light allows modulation of the time constant of the layer, with suitable faster and slower time constants for fluoroscopic and radiographic imaging. Referring to Fig. 7, a protective shield 210 can be formed, if needed, over the amorphous semiconductor layer 43 in the sensor's FET switch to prevent damage due to light. The selenium photoconductor layer 50 is relatively transparent to infrared radiation, so such radiation can reach the aSi layer 43. This layer is sensitive to such radiation and could be damaged if left unprotected. For example, the damage could change the amount of leakage current, or cause other changes. To prevent this, layer 43 can be shielded with a light-blocking or at least significantly attenuating shield 210 extending over all or most of the material of layer 43. This shield can comprise a metallic material such as Al or an alloy, or some other opaque material that is sufficiently thick to essentially block the light 100. In one embodiment of the detector, the second conductive microplate 36 comprises ITO, which is relatively transparent to infrared radiation, so the sensor can benefit from a light shield such as 210. However, if the second microplate 36 is not transparent to light, or if there is another layer used for other purposes but also serving to block light from layer 43, the shield 210 may not be needed. Example: A panel useful for both radiographic and fluoroscopic examinations, comprising an array of a plurality of detectors can be constructed by depositing an array of a plurality of first microplates on a glass substrate and building a TFT switching transistor in a space adjacent to each of the microplates. Connecting leads, as needed, are placed between the microplates connecting the drain and gates of the TFT to connection points along the panel sides. Additional leads are placed to provide electrical access to the first microplates. A dielectric layer is placed over the plurality of first microplates, leads and TFTs, and a second plurality of microplates is deposited thereover to produce a TFT module. A passivation layer is created over the second microplates to act as a unidirectional charge-blocking layer and prevent direct ohmic contact between the second microplates and the photoconductive layer to be coated thereon. Finally the TFT source electrode is connected to the middle microplate. In practice, the TFT modules are fabricated using technology for microfabrication of the transistors and capacitors, which is known in the art. See, for example, the aforementioned United States Patent 5,641 ,974 and the references referred to therein. A radiation detection layer of a selenium photoconductor is next applied to the TFT module using conventional vacuum deposition techniques. Apparatus and techniques for vacuum deposition are known to those skilled in the art. Vacuum deposition techniques are discussed, for example, in the Handbook of Deposition Technologies for Films and Coatings, 2nd. Ed., R. F. Bunshah, Ed., Noyes Publications, Park Ridge, NJ, 1994. Physical vacuum deposition of selenium is described, for example, in U.S. Patent 2,753,278 issued to Bixby, et al. Over the selenium photoconductor is next coated an organic photoconductor layer. The OPC material comprises a sequence of layers, deposited using spin coating (preferred), or blade coating, dip coating, or another suitable coating technique. The OPC can include a blocking layer (optional for layer 52 and preferably not used), a charge-generation layer (CGL) and a charge-transport layer (CTL). While in applications such as electrophotographic drums for copiers and printers it is believed that the order of the layers on the Al drum is Al, blocking layer, CGL, and CTL, in the preferred embodiment disclosed here the order is layer 50, then CTL, then CGL, then electrode 20. A blocking layer 60 can be interposed between the selenium 50 and the CTL or GTL, depending on the preferred sequence. Blocking layers are known to be used to improve the uniformity of subsequent layers of the OPC coated over Al. Many organic compounds have been used for the charge transporter in a CTL. It is believed that most are hole transporters. The electron transporters generally are not chemically stable. A preferred hole transporter is N,N'-bis-(3- methylphenyl)-N,N'-bis-(phenyl)-benzidine (MPPB) (CAS 65181-78-4). This is mixed in equal amounts with a solvent soluble polymer. A preferred polymer is poly (bisphenol A carbonate). A preferred solvent is dioxolane. Equal parts by weight of MPPB and polymer are mixed with solvent with gentle heating (fire hazard!) to give a 10 to 20% solids content. The cooled mixture is spin coated onto selenium to give a CTL approximately 5 to 50 microns thick. A preferred thickness is 20 microns. A well studied charge generation layer that has minimal X-ray cross section yet good response to infrared is titanyl phthalocyanine (TiOPc)(CAS 26201-32-1). It is a pigment and a preferred particle size is 1 micron or less. TiOPc is suspended in a solvent soluble polymer to bind the pigment to the substrate when dried (solvent evaporated). A preferred polymer is polyvinylbutral. A more preferred polymer is poly (bisphenol A carbonate). A preferred solvent is tetrahydrofuran (THF). A more preferred solvent is dioxolane. Equal parts by weight of TiOPc and polymer are mixed with solvent with gentle heating (fire hazard!) to give a 1 to 2% solids content. The cooled mixture is spin coated onto the CTL to give a CGL 0.1 to 5 microns thick. A preferred thickness is 1 micron. Current experiments use a thin copper foil tape with a conductive adhesive for preliminary testing (Sony CU7635R or Chromerics CCH-36-101) rather than the preferred vacuum deposited chrome electrode. After the OPC layer is coated over the selenium layer to a thickness of, for example and without limitation, between 10 and 50 microns, and preferably about 20 microns, a top conductive electrode is placed over the OPC layer by deposition of a thin layer of metal, to complete the panel. As earlier noted, a blocking layer can be used as well. The layers of the sensor can be arranged in different order. For example, the order can be essentially reversed relative to that in Fig.1. Those skilled in the art having the benefit of the teachings of this patent specification can effect numerous modifications thereto. For example, while the use of an OPC control layer such as 52 has been discussed in detail, it should be clear that other materials can be used so long as their electrical properties can be reversibly adjusted in order to provide sufficient overvoltage protection in modes such as radiography and also to allow for sufficiently fast removal of residual effects of frames (images) in other modes, such as fluoroscopy. Further, while the use of infrared light has been discussed, it should be clear that other forms of energy serving the purpose of reversing the appropriate properties of layer 52 can be used. These modifications are to be construed as being encompassed within the scope of the invention set forth in the appended claims.

Claims: CLAIMS 1. A radiation detection sensor comprising a) a charge storage capacitor; b) a radiation sensitive layer over said charge storage capacitor; c) a control layer over said radiation sensitive layer, said control layer having a time constant T = (1/γ)κε 0 that is adjustable without disassembly of the sensor to a value in the range between 0.03 and 20 seconds, wherein y is the electrical conductivity and is the dielectric constant of the control layer, and ε 0 is the permittivity of free space; and d) a top conductive layer over said control layer. 2. The radiation detector sensor according to claim 1 , including a source of electromagnetic radiation selectively energized to illuminate the control layer and thereby adjust the time constant thereof to a value within said range. 3. The radiation detector sensor according to claim 1 , wherein said source of electromagnetic radiation comprises a source of infrared light. 4. The radiation detection sensor according to claim 1 wherein the radiation detection layer is a photoconductor. 5. The radiation detection sensor according to claim 1 wherein the radiation detection layer is an X-ray photoconductor. 6. The radiation detection sensor according to claim 1 wherein the control layer is an organic photoconductor. 7. The radiation detection sensor according to claim 1 wherein the radiation detection layer comprises selenium. 8. A radiation detection sensor comprising: a) a charge storage capacitor; b) a control layer over said radiation sensitive layer, said control layer having a time constant T = (1/γ)κε 0 that is adjustable without disassembly of the sensor to a value in the range between 0.03 and 20 seconds, wherein y is the electrical conductivity and K is the dielectric constant of the control layer, and ε 0 is the permittivity of free space; and c) a radiation sensitive layer over said control layer; and d) a top conductive layer over said radiation sensitive layer. 9. A radiation detection panel comprising an integrated plurality of radiation detector sensors each of said sensors including: a) a charge storage capacitor; b) a radiation sensitive layer over said charge storage capacitor; c) a control layer over said radiation sensitive layer, said control layer having a time constant T = (1/γ)κε 0 that is adjustable without disassembly of the sensor to a value in the range between 0.03 and 20 seconds, wherein y is the electrical conductivity and K is the dielectric constant of the control layer, and ε 0 is the permittivity of free space; and d) a top conductive layer over said control layer. 10. The radiation detection panel according to claim 9 wherein the radiation sensitive layer comprises a continuous layer extending over more than one of said sensors. 11. The radiation detection panel according to claim 10 wherein the control layer comprises a continuous layer extending over more than one of said sensors. 12. A method for forming a radiation detection sensor of the type comprising: a) a charge storage capacitor; b) a radiation sensitive layer over said charge storage capacitor; c) a control layer over said radiation sensitive layer; d) a top conductive layer over said control layer; e) a switch connected to the charge storage capacitor; the method comprising: selecting the material composition and physical characteristics of the control layer to cause the time constant T = (1/γ)κε 0 of the control layer, wherein y is the electrical conductivity and K is the control constant of the control layer, and ε 0 is the permittivity of free space, to exceed about 10 seconds when the control layer is in the dark, and to be less than about 0.2 seconds when the control layer is exposed to electromagnetic radiation having wavelengths in the range of 750- 1000 nanometers; and forming said sensor using said control layer. 13. The method according to claim 12 wherein the control layer comprises an organic photoconductor. 14. The radiation detection sensor according to claim 12 wherein the top conductive layer is substantially transparent to radiation having wavelengths in the range of 750-1000 nanometers. 15. A method of operating a flat panel x-ray detector to convert incident x- radiation to electronic images, comprising: providing a flat panel x-ray detector comprising a layer of a material having a property controlled by a source of energy to change reversibly from a first state in which said property is at a relatively high level and a second state in which said property is at a relatively low level; and controlling said source of energy to cause said layer to maintain said property at a selected one of said levels for x-ray imaging in one type of radiology and to maintain said property at the other one of said levels for x-ray imaging in another type of radiology. 16. A method as in claim 15 in which said property is a time constant of the control layer. 17. A method as in claim 15 in which said layer comprises an organic photoconductor having a relatively high time constant in the dark but changing to a relatively low time constant when selectively illuminated with selected light. 18. A method as in claim 15 in which said layer comprises an organic photoconductor having a relatively low electrical conductivity in the dark but changing to a relatively high electrical conductivity when selectively illuminated. 19. A method as in claim 15 in which said source of energy is a source of light that is essentially infrared. 20. A flat panel detector comprising: a layered structure including a photoconductive layer and control layer, one of said layers substantially overlaying the other; said photoconductive layer having an electrical property and being responsive to x-rays to locally change said electrical property; and said control layer being responsive to electromagnetic radiation other than x- rays to change reversibly between relatively high and relatively low electrical conductivity. 21. A flat panel detector as in claim 20 in which said layered structure further comprises an array of switches electrically coupled with respective locations at the photoconductive layer, and shield members substantially shielding at least portions of said switches from said electromagnetic radiation other than x- rays. 22. A flat panel x-ray detector as in claim 20 in which said electromagnetic radiation other than x-rays is light. 23. A flat panel detector as in claim 21 in which said light comprises essentially infrared light. 24. An x-ray system for fluoroscopic x-ray examination comprising: a source of x-rays and a source of light; a detector comprising an array of sensors responsive to x-ray exposure to produce respective electrical charges; a control layer at each of said sensors, said control layer being responsive to light to switch from low to high electrical conductivity; and a control causing the x-ray source to expose the detector to x-rays and selectively cause the light source to expose said control layer to light to thereby significantly increase the electrical conductivity of the control layer. 25. A system as in claim 24 in which said light comprises essentially infrared light. 26. A system as in claim 25 in which said control layer consists essentially of an organic photoconductor. 27. An x-ray system having both radiographic and fluoroscopic x-ray examination modes comprising: a source of x-rays and a source of light; a detector comprising an array of sensors responsive to x-ray exposure to produce respective electrical charges; a control layer at each of said sensors, said control layer being responsive to light to change reversibly from low to high electrical conductivity; and a control operative in a fluoroscopic mode to cause the x-ray source to expose the detector to x-rays and to cause the light source to illuminate said control layer with light causing the control layer change to and maintain said high electrical conductivity, and operative in a radiographic mode to cause the x-ray source to expose the detector to x-rays and to keep the light source from illuminating the control layer and thereby maintain said control layer at said low electrical conductivity. 28. A system as in claim 27 in which said light comprises essentially infrared light. 29. A system as in claim 28 in which said control layer comprises an organic photoconductor. 30. A system as in claim 27 in which said control layer consists essentially of an organic photoconductor. 31. A system as in claim 27 in which said control layer comprises an organic photoconductor having a charge transport layer and a charge generating layer. 32. A system as in claim 31 in which said charge generation layer comprises a tytanil phthalocyanine (TiOPc). 33. A system as in claim 32 in which said TiOPc is in a polymer binder. 34. A system as in claim 33 in which said TiOPc is in particles of one micron or less in size. 35. A system as in claim 33 in which said polymer binder comprises polyvinylbutral. 36. A system as in claim 33 in which said polymer is poly (bisphenol A carbonate). 37. A system as in claim 31 in which said charge transport layer comprises a methylphenyl phenyl benzidine (MPTB). 38. A system as in claim 37 in which said MPTB is in a polymer binder. 39. A system as in claim 38 in which said polymer is poly (bisphenol A carbonate). 40. An x-ray system comprising: means for forming an x-ray image comprising first materials responsive to incident x-rays to form a latent image thereof and second materials having an electrical conductivity and a time constant related thereto and responsive to selected radiation other than x-rays to change said electrical conductivity and time constant from a range corresponding to a first radiographic mode to a range corresponding to a second radiographic mode; and means for selectively applying said selected radiation to the layer responsive thereto to change said electrical conductivity and time constant from one of said ranges to the other.

Inventors: DAVIS JAMES E, LEE DENNY L Y, DAVIS, JAMES, E., LEE, DENNY, L., Y.
Applicants: DAVIS JAMES E, HOLOGIC INC, LEE DENNY L Y, DAVIS, JAMES, E., HOLOGIC, INC., LEE, DENNY, L., Y.
Application year: 2001
Main classification: G01T
IPC codes: H01L27, H01L31