HK1137515B - Dual-screen digital radiographic imaging detector array - Google Patents

Description

Imaging detector array for dual screen digital radiography

Technical Field

The present invention relates generally to digital radiography, and more particularly to the use of a dual-screen, asymmetric phosphor screen in a digital radiographic flat-panel imaging detector array to improve image quality.

Background

Medical X-ray detectors employing scintillating phosphor screens to absorb X-rays and produce light typically suffer from a loss of spatial resolution due to lateral light diffusion of the phosphor screen. To reduce lateral light diffusion and maintain acceptable spatial resolution, the phosphor screen must be made sufficiently thin. The spatial resolution and X-ray detection capability of an imaging device are often characterized by a Modulation Transfer Function (MTF) and an X-ray absorption efficiency, respectively. Thin phosphor screens produce better MTF at the expense of reduced X-ray absorption. In general, the coating density and thickness of the phosphor screen are designed for a compromise between spatial resolution and X-ray absorption efficiency.

In order to improve X-ray absorption and maintain spatial resolution, it is known to use dual screens in conjunction with digital Computed Radiography (CR) to improve X-ray absorption efficiency. In such CR devices, a reservoir phosphor screen is used in place of the instant luminescent screen used in conventional masking film devices. No film is required for CR. Upon X-ray exposure, the storage phosphor screen stores a latent image in the form of a captured charge, which is then read out, typically by a scanning laser beam, to generate a digital X-ray image.

Recently, digital flat panel imaging detector arrays based on active matrix thin film electronics have promise for applications such as diagnostic radiology and digital mammography. There are two methods of X-ray energy conversion for Digital Radiography (DR), namely the direct method and the indirect method. In the direct method, X-rays absorbed in a photoconductor are directly converted into charge signals, stored in each pixel electrode of an Active Matrix Array (AMA), and read out using a Thin Film Transistor (TFT) to generate a digital image. Amorphous selenium (a-Se) is commonly used as a photoconductor. No phosphor screen is required for the direct method. In the indirect method, a phosphor screen is used to absorb the X-rays, and photons emitted by the phosphor screen are detected by an AMA having a single Photodiode (PD) and TFT switches at each pixel. The photodiode absorbs light emitted by the fluorescent material in proportion to the absorbed X-ray energy. Then, as with the direct method, the stored charge is read out with a TFT switch. Several types of imaging arrays based on thin film transistors are available for image detection. The imaging array includes a hydrogenated amorphous silicon (a-Si: H) photodetector with an amorphous silicon TFT switch, an amorphous silicon photodetector with low temperature poly-silicon (LTPS), and an organic photodetector with an organic TFT (OTFT) switch.

Fig. 1 shows a block diagram of circuitry for a typical type of known flat panel imaging volume 10 including a sensor array 12. The a-Si based sensor array includes m data lines 14 and n row select or gate lines 16. Each pixel includes an a-Si photodiode 18 connected to a TFT 20. Each photodiode 18 is connected to a common bias line 22 and the drain 24 of its associated TFT. The gate line 16 is connected to a gate driver 26. The bias line 22 carries a bias voltage applied to the photodiode 18 and the TFT 20. The TFT20, controlled by its associated gate line 26, transfers stored charge to the data line 14 when addressed. During readout, the gate lines are turned on for a finite time (approximately 10 to 100 microseconds) allowing enough time for the TFTs 20 of the row to transfer their pixel charge to all m data lines. The data lines 14 are connected to charge amplifiers 28 operating in parallel. In general, the charge amplifiers 28 are divided into groups, each group typically having 32, 64, or 128 charge amplifiers. The associated charge amplifiers in each group detect the image signals and clock the signals to multiplexer 30, whereby the signals are multiplexed and then digitized by analog to digital converter 32. The digital image data is then transferred to memory via the coupling. In some designs, a Correlated Double Sampling (CDS) circuit 34 may be disposed between each charge amplifier 28 and multiplexer 30 to reduce electronic noise. The gate lines 16 are sequentially turned on, taking about several seconds for the entire frame to be scanned. Additional image correction and image processing are performed by computer 36 and the resulting image is displayed on display 38 or printed by printer 40.

Fig. 2 shows a cross-section (not to scale) of a single typical kind of known imaging pixel 50, wherein the known imaging pixel 50 is used for example in a conventional a-Si based flat panel imaging volume in which the image detection element is a PIN photodiode. Each imaging pixel 50 has a PIN photodiode 52 and a TFT switch 54 formed on a substrate 56. An X-ray converter (e.g., scintillating phosphor screen 58) is coupled to the photodiode TFT array. The TFT switch 54 includes the following layers: a first metal layer 60 forming TFT gate electrodes and row select lines; an insulating layer 62 forming a gate insulator for the TFT; an intrinsic amorphous silicon layer 64 forming a channel region for the TFT; amorphous silicon constituting the n-type doped layer 66 forming a source and a drain for the TFT; a second metal layer 68 forming TFT source and drain contacts and data lines; and an insulating layer 70. The PIN photodiode 52 includes the following layers: a third metal layer 72 forming the back contact of the PIN photodiode and the interconnection between the TFT and the PIN photodiode; an amorphous silicon film 74 containing p-type doping; intrinsic amorphous silicon film 76; an amorphous silicon film 78 containing p-type doping; a transparent contact electrode 80 such as indium tin oxide; an insulating layer 81, and a fourth metal layer 82 forming the uppermost contact of the PIN photodiode. Also shown in fig. 2 are X-ray photon path 84 and visible photon path 86. When a single X-ray is absorbed by the screen 58, a large number of photons are emitted isotropically. Only a portion of the emitted light reaches the photodiode and is detected. The operation of such an a-Si based pixel with an a-Si PIN electrode will be understood by those skilled in the art.

Fig. 3 shows a cross section of two adjacent pixels 90 of another kind of known image sensor array 92. In this configuration, the photodiode 94 is bonded in a vertical fashion over the TFT switch 96, rather than in the side-by-side configuration shown in fig. 2. The vertically integrated sensor array includes: a substrate 98; a first metal layer 100 forming a TFT gate electrode and a row selection line; an insulating layer 102 forming a gate insulator of the TFT; an intrinsic (i.e., undoped) amorphous silicon layer 104 forming the TFT channel; an n-doped amorphous silicon film 106 forming a source region and a drain region of the TFT; and a second metal layer 108 patterned to form source and drain contacts and a data line. The insulating layer 110 serves to separate the TFT plane 112 from the PIN photodiode plane 114. The PIN photodiode includes: a third metal layer 116 forming a back contact electrode; a sequentially deposited n-doped layer 118 of amorphous silicon; a p-doped layer 122 of amorphous silicon and an intrinsic amorphous silicon layer 120; and then a transparent contact electrode 124. The photodiode layers are patterned to form respective photosensitive elements. The insulating layer 126 and the fifth metal layer 128 forming the bias line complete the pixel. The vertically coupled configuration provides improved photosensibility compared to the side-by-side configuration due to the higher proportion of photosensing area relative to pixel area (known as fill factor).

Fig. 4 shows a cross-section (not to scale) of another type of known imaging pixel 140 in a prior art a-Si based flat panel imager, in which the image sensing element is a metal-insulator-semiconductor (MIS) photosensor 142. Each imaging pixel 140 includes a MIS photosensor 142 and a TFT switch 144 formed on a substrate 146. The TFT switch 144 includes the following layers: a first metal layer 148 forming TFT gate electrodes and row select lines; an insulating layer 150 forming a gate insulator of the TFT; an intrinsic amorphous silicon layer 152 forming a channel region of the TFT; amorphous silicon containing n-type doped layer 154 forming TFT source and drain electrodes; an insulating layer 156; and a second metal layer 158 forming TFT source and drain contacts and data lines. The MIS photodiode 142 includes the following layers: a first metal layer 148 forming a gate electrode of the MIS photosensor; an insulating layer 150 forming a gate insulator; forming an amorphous silicon film 152 of a channel region; an amorphous silicon film 154 forming a drain electrode; a transparent electrode 160 in contact with the n-type layer 154; an insulating layer 156; and a second metal layer 158 forming the uppermost contact. The operation of such a-Si based indirect flat panel imaging volume with MIS photosensors is understood by those skilled in the art.

Those skilled in the art will recognize that other types of photosensors, such as continuous PIN photodiodes, continuous MIS photosensors, phototransistors, and photoconductors, can be implemented with a variety of materials, including amorphous silicon semiconductors, polycrystalline silicon semiconductors, or single crystal silicon semiconductors, as well as non-silicon semiconductors. Those skilled in the art will also recognize that other pixel circuits, such as three-transistor operational pixel circuits, four-transistor operational pixel circuits, and shared-transistor operational pixel circuits, may be used to form a radiographic imaging array.

Those skilled in the art will recognize that many other read array configurations are commonly used. Also, those skilled in the art will recognize that semiconductor materials other than amorphous silicon, such as polysilicon semiconductors, organic semiconductors, and different alloy semiconductors, such as zinc oxide, may be used for the backplane array and the detection array. More recently, thin film transistor arrays have been fabricated on flexible substrates (made of plastic, metal foil, or other suitable organic and inorganic materials) rather than on conventional non-flexible fragile glass substrates. TFT arrays on flexible substrates are combined with liquid crystals for flexible transflective displays, with emissive devices for emissive displays, with photosensors for visible light imaging and radiographic imaging applications.

Reference is made to commonly assigned, co-pending (a) U.S. patent application No.11/951,483 entitled "DUAL energy imaging heartbeat for DUAL-energy imaging DUAL-ENERGY IMAGING," filed on 6.12.2007 by vanmeter et al; (b) U.S. patent application No.60/889,356 entitled "DUAL ENERGY DECOMPOSITION RENORMALIZATION" (DUAL ENERGY DECOMPOSITION RENORMALIZATION) filed by vanmeter on 6.2.2007; and (c) U.S. patent application No.60/896,322 entitled "REGISTRATION METHOD FOR Dual energy projection" filed 3/22 of 2007 by Dhanantwari et al. The invention of which the above application is concerned pertains to another imaging technique, referred to as dual energy subtraction imaging, which can be used to reduce the impact of the anatomical background on disease detection in digital chest radiography and angiography. This technique is based on the fact that bone and soft tissue have different energy dependent absorption characteristics. Generally, two primary digital images are generated. One is a low energy high contrast image and the other is a high energy low contrast image. By taking a non-linear combination of the two images, a pure bone image and a pure soft tissue image can be obtained. Such imaging techniques would improve anatomical illustration and pathological diagnosis using images.

In U.S. patent application No.11/487,539, dual digital radiography arrays are disclosed, each imaging a respective phosphor screen. In one embodiment, the image is formed by transmitting X-rays through the target to a digital radiographic imaging volume. To form an image, a digital radiography imaging volume uses two flat plates (a front plate and a back plate) to capture and process X-rays. Preferably, the thickness of the scintillating phosphor layer of the back plate is greater than or equal to the thickness of the scintillating phosphor layer of the front plate. A filter is placed between the front and back plates to minimize light transmission (cross) from one screen to the other. Each plate has a first array of signal sensing elements and readout devices and a second array of signal sensing elements and readout devices. In addition, a first passivation layer is disposed on the first array of signal sensing elements and readout devices, and a second passivation layer is disposed on the second array of signal sensing elements and readout devices. The front plate and the back plate are simultaneously exposed to an exposure of X-rays. The first scintillating phosphor layer is responsive to X-rays passing through the target to produce light which illuminates the signal sensing elements and readout devices of the first array of signal sensing elements to provide signals representative of a first X-ray image. The second scintillating phosphor layer is responsive to X-rays passing through the target and the front plate to produce light which illuminates the signal sensing elements and readout devices of the second array of signal sensing elements to provide signals representative of a second X-ray image. Combining the signals of the first and second X-ray images produces a composite X-ray image having a higher quality.

In another embodiment disclosed in U.S. patent application No.11/487,539, a respective flat panel imaging volume is fabricated on each of two sides of a substrate to form a digital radiography imaging array. The first imaging member is primarily sensitive to light emitted by a first phosphor screen positioned adjacent the first imaging member. The second imaging volume is primarily sensitive to light emitted by a second phosphor screen positioned adjacent the second imaging volume. Unlike the use of two flat plates, a plate and a back plate, to capture radiographic images, digital radiographic imaging employs a single substrate covered on the front side with a first fluorescent layer and on the back side with a second fluorescent layer. In one aspect of this embodiment, the thickness of the second scintillating phosphor layer can be greater than or equal to the thickness of the first scintillating phosphor layer. NIP photodiodes are used on each side of the substrate. Each side of the substrate is covered with a light blocking layer or penetration reducing layer to minimize light penetration from the phosphor screen on one side of the substrate to the photodiode on the other side of the substrate. The first and second scintillating phosphor layers are simultaneously exposed to X-ray exposure, and photodiodes on the front and back sides of the substrate detect the front and back images, respectively.

It is desirable to extend the application of dual scintillating screens (scintillating phosphor layers) to indirect Digital Radiography (DR) apparatus. Moreover, there is a need to extend the application of dual scintillating screens into indirect DR apparatus for single exposure dual energy subtractive imaging.

Disclosure of Invention

It is an object of the present invention to provide an improved dual screen digital radiography imaging apparatus.

An advantage of the present invention is that it allows DR imaging to be performed such that a first image optimized for Modulation Transfer Function (MTF) is combined with a second image optimized for sensitivity to result in an X-ray image of higher quality in a DR imaging system.

In one embodiment of the present invention, a radiographic imaging apparatus includes: a first scintillating phosphor screen having a first thickness; a second scintillating phosphor screen having a second thickness; a substrate disposed between the first screen and the second screen, the substrate being substantially transparent to X-rays for the apparatus; and an imaging array disposed between the first side of the substrate and one of the panels, wherein the one panel is one of the first panel and the second panel, the imaging array comprising a plurality of pixels, each pixel comprising at least one photosensor and at least one readout element. The readout element may be a thin film transistor formed on one side of the substrate. As used in this specification, "substantially transparent" means that X-rays pass through the substrate in an amount or substantial amount sufficient to be detected by the photosensor to produce a radiographic image.

In another embodiment, a radiographic imaging apparatus includes: a substrate substantially transparent to X-rays for use in the apparatus; a first scintillating phosphor screen having a first thickness disposed on the first side of the substrate; a second scintillating phosphor screen having a second thickness disposed on a second side of the substrate such that the substrate is between the first and second scintillating phosphor screens; the substrate being transparent with respect to light emitted by the first and second screens; and an imaging array formed on one side of the substrate, the imaging array comprising: a first set of photosensors primarily sensitive to light emitted by the first screen; and a second set of photosensors primarily sensitive to light emitted by the second screen.

The asymmetric dual-screen digital radiography apparatus of the present invention has various advantages over single-screen digital radiography apparatuses. The higher spatial frequency response or MTF of the inventive device results in sharper images. The higher the X-ray absorption, the higher the detection speed that results. The lower the noise level of the exemplary device of the present invention, the less quantum speckle effect is given. The higher the Detective Quantum Efficiency (DQE) of various embodiments of the invention, the higher the overall image quality provided. Furthermore, the use of a pair of asymmetric screens in an indirect DR apparatus significantly resolves the conflict between maintaining both a good level of X-ray absorption (typically requiring a screen having an increased thickness) and a high spatial resolution (typically requiring a screen having a reduced thickness) in an X-ray phosphor screen design. Furthermore, the use of a flexible substrate (e.g., metal foil, plastic paper, or a combination thereof) for a flat panel imaging device results in improved mechanical strength and physical durability of the device because the X-ray absorption loss of the substrate is reduced.

In all of the above embodiments of the present invention, a single imaging array is used to detect both screens. Within each pixel, one or more photodiodes are used to image the first screen and one or more photodiodes are used to image the second screen. The use of a single readout array ensures accurate registration (registration) of the two screen images and provides a thinner and more robust flat panel assembly requiring less support electronics, especially requiring fewer row drivers and column amplifiers and digitizers, than is the case when multiple panels are used.

Preferably, in the above-described radiographic imaging apparatus, the Modulation Transfer Function (MTF) of the first screen exceeds the MTF of the second screen such that the MTF is 50% of the spatial frequency (f) for the first screen_1/2) A spatial frequency at least 0.5c/mm higher than the second screen; the first screen is disposed on a second side of the substrate, wherein the second side of the substrate is opposite to the first side of the substrate.

Preferably, in the above radiographic imaging apparatus, the X-ray absorption efficiency of the second screen exceeds the X-ray absorption efficiency of the first screen by at least 10%.

Preferably, in the above radiographic imaging apparatus, wherein the first screen is thinner than the second screen, the apparatus is oriented to receive exposure to X-rays incident in a direction of a side of the first screen during use.

Preferably, in the above radiographic imaging apparatus, an opaque material separating the first group and the second group of photosensors is further included.

Preferably, in the above radiographic imaging apparatus, one of the first and second sets of photosensors has a thickness that exceeds a light absorption length of at least a portion of the electromagnetic spectrum.

Preferably, in the above radiographic imaging apparatus, further comprising an array of readout elements in communication with the first and second sets of photosensors.

Preferably, in the above radiographic imaging apparatus, wherein the array of readout elements is fabricated in a first plane, the first set of photosensors is fabricated in the first plane, and the second set of photosensors is disposed in a second plane parallel to and outside the first plane.

Preferably, in the above radiographic imaging apparatus, wherein the first and second sets of photosensors are stacked on top of each other, wherein the readout element is disposed adjacent to the stacked photosensors.

Preferably, in the above radiographic imaging apparatus, wherein the first and second sets of photosensors and the readout elements are formed in the same plane.

Preferably, in the above radiographic imaging apparatus, wherein the first and second sets of photosensors are formed in a first plane, the readout elements are formed in a second plane that is outside the first plane.

Preferably, in the above radiographic imaging apparatus, wherein the substrate is formed on the second scintillating phosphor screen.

Preferably, in the above radiographic imaging apparatus, further comprising a light-absorbing colorant or light-scattering particles that are substantially uniformly dispersed in the substrate to reduce light scattering along the substrate.

Preferably, in the above radiographic imaging apparatus, further comprising a light absorbing colorant layer added to the surface of the substrate to reduce light scattering along the substrate.

Preferably, in the above-mentioned radiographic imaging apparatus, a light-absorbing colorant diffused into a surface layer of the substrate to reduce light scattering along the substrate is further included.

The above and other objects, features, and advantages of the present invention will become apparent to those skilled in the art upon a reading of the following detailed description when taken in conjunction with the accompanying drawings wherein there is shown and described several exemplary embodiments of the invention.

Drawings

While the specification concludes with claims particularly pointing out and distinctly claiming the subject matter of the present invention, it is believed that the invention will be better understood from the following description when taken in conjunction with the accompanying drawings.

Figure 1 is a schematic illustration of a prior art imaging plate used in a flat panel imaging volume.

Figure 2 is a cross-sectional view of one known type of imaging pixel incorporating a PIN photodiode.

Fig. 3 is a cross-sectional view of another type of known imaging pixel in which a PIN photodiode is vertically incorporated over a TFT switch.

Fig. 4 is a sectional view of another type of known imaging pixel in which an MIS photosensor is formed in a planar, side-by-side arrangement with a TFT switch.

FIG. 5 is a schematic cross-sectional view of one embodiment of an imaging pixel in accordance with the present invention.

Fig. 6 shows a detailed cross-sectional view of the inventive imaging pixel of fig. 5.

FIG. 7 shows a schematic cross-sectional view of another embodiment of an imaging pixel in accordance with the invention in which some photosensors are sensitive to light from one side of the assembly and other photosensors are sensitive to light from the other side of the assembly.

FIG. 8 is a schematic cross-sectional view of another embodiment of the present invention similar to FIG. 7, but wherein a light shielding component is provided to direct light to the photosensor.

Fig. 9 shows a detailed cross-sectional view of the inventive imaging pixel of fig. 8.

Fig. 10 shows a schematic cross-sectional view of two juxtaposed pixels according to fig. 8 and illustrates how light scattering may occur in the transparent substrate.

Fig. 11 shows a schematic cross-sectional view as in fig. 10, but including features in the transparent substrate for reducing light scattering.

FIG. 12 shows a schematic cross-sectional view of another embodiment of the present invention in which the photosensors of the imaging device are arranged in two separate plates or layers with a light blocking layer between the plates.

Fig. 13 shows a detailed cross-sectional view of the inventive imaging pixel of fig. 12.

Fig. 14 shows a schematic diagram of a circuit for each imaging device according to the present invention.

FIG. 15 shows a schematic cross-sectional view of the embodiment of FIG. 8 in which light absorbing colorants are diffused through the transparent substrate to reduce light scattering.

FIG. 16 shows a schematic cross-sectional view of the embodiment of FIG. 8 with a light absorbing colorant applied to the transparent substrate surface to reduce light scattering.

FIG. 17 shows a schematic cross-sectional view of the embodiment of FIG. 8 in which a light absorbing colorant is diffused into the layer at the surface of the transparent substrate to reduce light scattering.

Detailed Description

Reference is made to commonly assigned, co-pending U.S. patent application No.11/487,539 entitled "asymmetric DUAL screen digital RADIOGRAPHY APPARATUS (APPARATUS for imaging DUAL SCREEN DIGITAL radial)" filed on 14.7.2006 by Yorkston et al; and (b) U.S. patent application No.12/025,086 entitled "DIGITAL radiography imaging APPARATUS (DIGITAL radiography imaging APPARATUS)" filed by Tredwell on 4.2.2008.

The present description is directed in particular to elements forming part of, or cooperating more directly with, apparatus in accordance with the invention. It is to be understood that elements not specifically shown or described may take various forms well known to those skilled in the art. In the following description, terms and expressions such as "above" or "upper" are used in a broad sense to give an arrangement in which layers are opposite to each other. The X-ray imaging disk can of course be exposed in any azimuthal orientation, with the stacked layers generally extending in a horizontal, vertical, or oblique direction.

Fig. 5 to 17 show schematic views of various digital imaging apparatuses according to the present invention. A schematic cross section of a first exemplary embodiment of the invention is shown in fig. 5. The radiographic imaging apparatus 170 includes: a first scintillating phosphor screen assembly 172, a second scintillating phosphor screen assembly 174, and an imaging array 176. First scintillating phosphor screen assembly 172 includes first scintillator 178 having thickness t1 and light management cladding 180 and is disposed on a first side of imaging array 176. Second scintillating phosphor screen assembly 174 includes second scintillator 182 having a thickness t2 and a light control cladding or layer 180 and is disposed on a second side of imaging array 176. The light control cladding or layer may be light absorbing or light reflecting depending on the screen function optimization. The light absorbing layer optimizes high spatial resolution at the expense of sensitivity by absorbing light that would otherwise be scattered relative to neighboring pixels. In contrast, the light reflecting layer optimizes high sensitivity at the expense of spatial resolution. The imaging array 176 includes a thin transparent substrate 186 having pixels 188 formed thereon, each pixel including a readout element 190 (such as a TFT, for example) and a photosensor 192.

In this embodiment, photosensor 192 is sensitive to light from both scintillating phosphor screen assemblies 172, 174. After the X-rays 194 are absorbed in the second phosphor screen assembly 174 and light 196 is subsequently emitted, a portion of the emitted light 196 is absorbed in the photosensor 192. Similarly, after X-rays 198 absorb in phosphor screen 172 and subsequently emit light 200, a portion of the emitted light 200 is absorbed in photosensor 192. As is well known, absorption of light in a photosensor produces electron-hole pairs, referred to as photogenerated charges, which can be stored on the photosensor and subsequently read out by the readout element 190. The first embodiment does not distinguish between the front panel and the back panel. Sandwiching the detector between two screens allows for higher overall sensitivity and resolution compared to a single thick screen.

The photosensor 192 can be some of several types of devices. For example, in one embodiment, the photosensor 192 is a metal-insulator-semiconductor (MIS) photodiode, photoconductor, or phototransistor. The readout element 190 may also be formed from some of several kinds of devices. For example, the readout element 190 may be formed of any one of an amorphous silicon thin film transistor, a polycrystalline silicon thin film transistor, an organic thin film transistor, or a crystalline silicon thin film transistor. The transparent substrate 186 may alternatively be a plastic, glass, ceramic, or multilayer film comprising organic and/or inorganic layers, such as a plastic coated with a silicon nitride film.

FIG. 6 shows a detailed cross-sectional view of a particular embodiment of the radiographic imaging array of FIG. 5 in which the photosensors are amorphous silicon MIS photodiodes and the readout elements are amorphous silicon TFTs. In FIG. 6, a radiographic imaging device 210 includes a first scintillating phosphor screen assembly 212, a second scintillating phosphor screen assembly 214, and an imaging array 216. First scintillating phosphor screen assembly 212 includes first scintillating phosphor 218 and first light absorbing layer 220. Second scintillating phosphor screen assembly 214 includes second scintillating phosphor 222 and second light-absorbing layer 224. The imaging array 216 includes MIS photosensors 226 and TFT readout elements 228 formed on a relatively thin transparent substrate 230. The TFT 228 includes: a first metal layer 232 forming a gate electrode and forming a gate line for a row address; an insulating layer 234 forming a gate insulator; an intrinsic amorphous silicon layer 236 forming a TFT channel; an amorphous silicon layer 238 with n-type doping forming source and drain regions; an insulating layer 240; and a fourth metal layer 242 forming contacts to the source and drain and interconnects with the photosensors. The photosensor 226 includes: a second metal layer 244 transparent to light emitted by the first luminescent screen assembly 212; an insulating layer 246 forming a gate insulator; an intrinsic amorphous silicon layer 248 forming a channel region; an n-doped amorphous silicon layer 250 forming a drain region; a third metal layer 252 forming a transparent electrode contact to the n-doped region; an insulating layer 254; and a fourth metal layer 256 contacting the transparent electrode 252 and forming a bias line. Examples of the transparent metal include Indium Tin Oxide (ITO), Zinc Oxide (ZO), and Indium Zinc Oxide (IZO). The TFT 28 is normally shielded from light with respect to the phosphor screen by using an opaque metal such as Al, Al: Nd, Cr, Mo, or a multilayer film for the first and second metal layers 232, 234. The light shielding layer may also be implemented by using a metal having a high light reflectivity in order to improve the collection efficiency.

A schematic cross-section of a second exemplary embodiment of the invention is shown in fig. 7. In this embodiment, one portion of the photosensors is sensitive to light emitted by the first scintillating phosphor screen and another portion of the photosensors is sensitive to light emitted by the second scintillating phosphor screen. As shown in FIG. 7, radiographic imaging device 270 includes a first scintillating phosphor screen assembly 272, a second scintillating phosphor screen assembly 274, and an imaging array 276. First phosphor screen assembly 272 includes a scintillating phosphor screen 278 having a first thickness T1 and a first light absorbing or light reflecting layer 280. Second phosphor screen assembly 274 includes a second scintillating phosphor screen 282 having a thickness T2 and a second light-absorbing or light-reflecting layer 284. The light absorbing layer can be used to improve spatial resolution (MTF) at the expense of overall light collection efficiency by absorbing light scattered within the phosphor, thereby preventing light scattering into adjacent imaging elements. The light reflecting layer can be used to increase the overall light collection efficiency at the expense of spatial resolution. If first phosphor screen assembly 272 of FIG. 7 is optimized for high signal-to-noise ratio (SNR) and second phosphor screen assembly 274 is optimized for high spatial resolution, thickness T1 may be greater than thickness T2, first light-absorbing or light-reflecting layer 280 may be light-reflecting only, and second light-absorbing or light-reflecting layer 284 may be light-absorbing only. Those skilled in the art will appreciate that the first and second phosphor screens may also be optimized for specific imaging characteristics such as conversion efficiency and MTF by optimizing non-thickness factors such as material selection and material construction of the screens.

With continued reference to FIG. 7, imaging array 276 includes a thin transparent substrate 286, a first photodetector element 288 sensitive primarily to light emitted by first phosphor screen 278, and a second photodetector element 290 sensitive primarily to light emitted by second phosphor screen 282. First photodetector element 288 includes a first light sensitive element 292, a first readout element 294, and a first light blocking layer 296 configured to reduce the transmission of light from second phosphor screen 282 to first light sensitive element 292. Likewise, the second photodetection element 290 includes a second light sensitive element 298, a second light blocking layer 300, and a second readout element 302. Examples of the first and second light sensitive elements 292, 298 include PIN photodiodes, MIS photosensors, phototransistors, photoconductors, vertical and lateral p-n junction photodiodes, photo capacitors, PIN structured photodiodes, and avalanche photodiodes. Light-sensitive elements 292, 298 may be implemented in inorganic semiconductors, in amorphous, polycrystalline, or crystalline forms, such as amorphous silicon, polycrystalline silicon, and crystalline silicon, or light-sensitive elements 292, 298 may be implemented in organic semiconductors or organic/inorganic combinations. Examples of readout elements 294, 302 known to those skilled in the art include 1-transistor passive pixel circuits, 2-transistor passive pixel circuits, 3-transistor active pixel circuits, 4-transistor active pixel circuits, shared-transistor active pixel circuits, photon-counting pixel circuits, and charge-coupled devices.

An alternative configuration to the embodiment of fig. 7 is shown in fig. 8. In this embodiment, imaging device 318 further includes: a first light shielding assembly 320, first light shielding assembly 320 having a first light transmitting aperture 322 that allows light to be conducted from first luminescent screen assembly 272 to first light sensitive element 292; and a second light shielding assembly 324, the second light shielding assembly 324 having a second transparent aperture 326 that permits light to be conducted from the second luminescent screen assembly 274 to the second light sensitive element 298. Each light shielding layer may absorb or reflect outside each aperture 322, 326 and may be formed of an inorganic material such as a metal, or an organic material such as an absorbing dye in an organic binder, or a combination such as a pigment or carbon contained in an organic binder.

The first masking component 320 includes a first light blocking layer 328 having a first through hole 322 and an insulating layer 330. Likewise, second masking assembly 324 includes a light blocking layer 332 having second transparent apertures 326 and an insulator layer 334. Those skilled in the art will appreciate that the insulating layers 330,340 may not be needed in certain embodiments of the imaging device 318, such as where the shielding layers 328,332 themselves are insulating, while in other embodiments a second insulating layer may be needed to prevent electrical shorting or to block diffusion of impurities, such as sodium, from the thinner transparent substrate into the imaging array. The shielding components 320, 324 may be absorptive or reflective. The absorptive shielding assembly reduces light scattering from the scintillating phosphor screen, thereby reducing photochromic luminance interference (crosstalk) between nearby photodetector elements at the expense of reduced overall light collection efficiency. Absorptive shielding assemblies thus optimize overall performance for high spatial frequency response at the expense of signal-to-noise ratio. For the first shield assembly 320, the reflective shield assembly reflects light incident on the assembly back to the thinner transparent substrate. Some of the light reflected into the substrate may undergo repeated internal reflections before being transmitted through one of the first apertures 322 in the shield assembly. Another portion may be conducted from the transparent substrate into first phosphor screen assembly 272 where it may be absorbed or scattered into an aperture 322 in the shield assembly. The reflective mode shielding assembly thus optimizes the signal-to-noise ratio at the expense of spatial frequency response.

Fig. 9 illustrates a particular embodiment of imaging device 318 shown in fig. 8 in which the light sensitive element uses a MIS photosensor. As described above, imaging device 318 includes first phosphor screen assembly 272, second phosphor screen assembly 274, and imaging array 276. Imaging array 276 includes a thin transparent substrate 286, a first photodetector element 288 sensitive primarily to light emitted by first phosphor screen 278, and a second photodetector element 290 sensitive primarily to light emitted by second phosphor screen 284. The second photodetection element 290 includes a second MIS photosensor 344, which is sensitive mainly to light from the second phosphor screen 282, and a second TFT readout element 346. The first photodetection element 342 includes a first MIS photosensor 348 and a first TFT readout element 350. The first MIS photosensor is primarily sensitive to light from first phosphor screen 278.

With continued reference to fig. 9, each TFT readout element 346, 350 includes: a first metal layer 352 forming a TFT gate electrode and a gate line; an insulating layer 354 forming a gate insulator; an intrinsic amorphous silicon film 356 forming a TFT channel; an amorphous silicon film 358 containing n-type doping forming TFT source and drain regions; a third metal layer 360 forming source and drain contacts; an insulating layer 362; and a fourth metal layer 364 forming the data lines and interconnections between the TFTs 346, 350 and the photo-detection elements 344, 348.

The first MIS photosensor 348 includes a second metal layer 366 forming a transparent gate electrode, an insulator layer 368 forming a gate insulator, an intrinsic amorphous silicon film 370, an n-doped amorphous silicon film 372, a third metal layer 374 forming a contact to the n-doped amorphous silicon film, an insulator layer 376, and a fourth metal layer 378 forming a bias line. The second metal layer 366 may be formed with a transparent conductor such as ITO or IZO. Fourth metal layer 378 in first MIS photosensor 348 is patterned to leave metal on the photodetection region of the MIS photosensor, thereby blocking light from second phosphor screen 282 from being conducted into the photosensor. First MIS photosensor 348 is thus primarily responsive to light from first phosphor screen 278, which is conducted through relatively thin transparent substrate 286 and transparent gate electrode 366.

The second MIS photosensor 344 is similar in construction to the first MIS photosensor 348, including a second metal layer 382 forming a gate electrode, an insulator layer 384 forming a gate insulator, an intrinsic amorphous silicon film 386, an n-doped amorphous silicon film 388, a third metal layer 390 forming a contact to the n-doped amorphous silicon film 388, an insulator layer 392, and a fourth metal layer 394 forming a bias line. Third metal layer 390 is transparent to allow light from second phosphor screen 282 to pass into second MIS photosensor 344, while second metal layer 382 is opaque, thereby preventing light from first phosphor screen 278 from passing to second MIS photosensor 344. Second MIS photosensor 344 is thus primarily sensitive to second phosphor screen 282.

FIG. 10 shows an expanded embodiment of the imaging device shown in FIG. 8 and illustrates how the emitted light can be scattered along substrate 286. X-rays 400 are absorbed in first phosphor screen 278 and light 402 emitted thereby passes through first aperture 322 to reach first light sensitive element 292 corresponding to the location where X-rays 400 are absorbed. Some of the emitted light 404 undergoes multiple internal reflections and scattering along the substrate 286 before finally passing through an adjacent one of the first apertures 322. This light scattering may cause an erroneous signal to be generated by the adjacent light sensitive element 292 due to the corresponding absorption of X-rays by non-corresponding portions. Fig. 11 illustrates one technique for reducing such light scattering. A pattern of light blocking regions 410 is provided in substrate 286. The light blocking region 410 may be formed by thermal diffusion of light absorbing colorants from one or both surfaces of the substrate surface into a relatively thin transparent substrate. U.S. patent 4,621,271 describes a patterned heat transfer method of colorant from a colorant-containing donor sheet (donor sheet) into a substrate. U.S. Pat. nos. 4,772,582; 4,973,572 and 5,578,416 describe a method for transferring colorant from a donor sheet to a receptor sheet via patterned laser transfer using heat generated by laser exposure in a pattern. Alternatively, as described in U.S. Pat. nos. 4,399,209; 4,416,966, respectively; and 4,440,846, the light absorbing substrate can contain a photovoltaically active ingredient that releases a colorant or bleaches a pre-existing colorant when exposed to heat or light in a pattern. Alternatively, the substrate may be formed of a photo-patternable polymer such as polyimide that can be patterned into a photo-pattern, which is then chemically developed to form channels that can be subsequently filled with a light absorbing or reflecting material (such as a colorant-containing polymer), or with reflective or scattering particles, or with a deposited or plated metal. Alternatively, a metal grid defining light blocking regions may be formed by metal plating from a patterned metal seed layer. The patterned grid layer may then be coated with a substantially transparent material such as polyimide.

As in the case of FIG. 10, X-rays 412 are absorbed in first phosphor screen 278, and light 414 emitted thereby passes through one of first apertures 322 to first light sensitive element 292 corresponding to the location at which X-rays 400 are absorbed. However, if any emitted light 416 undergoes multiple internal reflections and scattering along the substrate 286, such scattered light encounters the light blocking region 410, which prevents the scattered light from reaching adjacent ones of the first apertures 322. Accuracy is improved due to the reduced light scattering.

An alternative imaging device 420 is shown in fig. 12. Features common to the embodiment of figure 5 are identified by the same reference numerals. In this embodiment, the imaging array 22 has photosensors arranged in separate, substantially parallel planes. Specifically, first plane 428 has a first light sensitive element (photosensor) 424 that is primarily sensitive to light emitted by first phosphor screen 178, and second plane 432 has a second light sensitive element (photosensor) 426 that is disposed above first plane 428 and is primarily sensitive to light emitted by second phosphor screen 182. The imaging array 422 includes: a thin transparent substrate 186, a first plane 428 on the opposite side of the substrate 186 from the first phosphor screen 178 provided with a first light sensitive element 424, a light blocking layer 430 disposed on the first plane 428, a second plane 432 on the opposite side of the light blocking layer 430 from the light sensitive element 424 provided with a second light sensitive element 426, and first and second readout elements 434, 436 for the first and second light sensitive elements 424, 426, respectively. In the imaging device 420, the readout elements 434, 436 are disposed in the first plane 428 along with the first light sensitive element 424. The readout elements may alternatively be in the second plane 432 or a third plane not shown. First light sensitive element 424 is primarily sensitive to light from first phosphor screen 178 and second light sensitive element 426 is primarily sensitive to light from second phosphor screen 182. Light blocking layer 430 may not be needed if light sensitive elements 424, 426 are sufficient to absorb light from their respective phosphor screens, so that only a small portion of light from first phosphor screen 178 is conducted through first light sensitive element 424 to second light sensitive element 426, and only a small portion of light from second phosphor screen 182 is conducted through second light sensitive element 426 to first light sensitive element 424.

Figure 13 shows a partially exploded cross-sectional view of a particular embodiment of the embodiment shown in figure 12. The imaging array 422 includes: a transparent substrate 186; a first plane 428, the first plane 428 including a first light sensitive element 424 (referenced below in parentheses in the drawings) disposed on an opposite side of substrate 186 from first phosphor screen 178; a light blocking layer 430; a second plane 432, the second plane 432 including a second light sensitive element 426 (indicated by parentheses above the figure) disposed on the opposite side of the light blocking layer 430 from the first light sensitive element 424; and readout elements 434, 436 (referenced below in parentheses in the drawings) for light sensitive elements 424, 426, respectively.

With continued reference to fig. 13, the readout elements 434, 436 are preferably TFTs and the light sensitive elements 424, 426 are MIS photosensors. Each TFT includes: a first metal layer 438 forming a gate electrode and a gate line; an insulating layer 440 forming a gate insulator; an intrinsic amorphous silicon film 442 forming a channel; an amorphous silicon layer 444 containing n-type doping forming source and drain regions; a third metal layer 446 forming source and drain contacts; an insulating layer 448; and a fourth metal layer 450 that forms the interconnection of the TFTs to their respective MIS photosensors 424, 426 and forms the data lines. First MIS photosensor 424, which is primarily sensitive to light from first phosphor screen 178, includes: a second metal layer 452 transparent to light emitted by first screen 178; an insulating layer 454 forming a gate insulator; an intrinsic amorphous silicon layer 456 forming a channel region; an n-type doped amorphous silicon layer 458 forming a drain region; a third metal layer 460 forming a drain contact; a portion of insulating layer 448; a fourth metal layer 450 that forms the interconnection of the TFT 436 to the MIS photosensor 424 and forms the data line. An insulating layer 462 is formed over the readout elements and MIS photosensors. The second MIS photosensor 426 includes: a fifth metal layer 464 forming a gate electrode; an intrinsic amorphous silicon film 466 which forms a channel region; an n-type doped amorphous silicon film 468; a sixth metal layer 470 forming a transparent contact to the second MIS photosensor 426; and an insulating layer 472. In the embodiment shown in fig. 13, layers 464-472 extend over the entire imaging surface of the imaging array, thereby allowing nearly the entire surface to be photo-sensitive.

Fig. 14 illustrates an example of an imaging array circuit 480 suitable for reading out the imaging arrays illustrated in fig. 7-13. Each imaging array includes: pixels 482 (shown by dashed boxes) arranged in rows 484, 486 and columns 488, 490; voltage sources 492, 494 for sequentially scanning the rows; and a readout circuit 496 for detecting the charge. Each pixel 482 includes: a first photo sensor 498 and a corresponding TFT switch 500 for connecting the photo sensor to the data line 502 according to the control of the first gate line 504; and a second photo sensor 506 and a corresponding TFT switch 508 for connecting the photo sensor to the data line 502 according to the control of the second gate line 510. The first and second gate lines of each row are biased with voltage sources 492, 494, respectively. The charge generated by the exposure of the photosensitive element is sensed by a readout circuit 496 on each data line. Sensing circuit 496 preferably includes an operational amplifier 514, a feedback capacitor 516, and a switch 518. During X-ray exposure, the bias voltage supplied to the gate lines is held at a negative voltage to turn off all the TFT switches. After exposure, the gate lines are sequentially addressed by switching the bias to a positive value, creating a conductive path between the source and drain of the TFT, thereby connecting the charge amplifier at the end of each column to the selected image sensing element. The charge amplifier detects the charge on the light sensitive element and then the gate line returns to a negative value, thereby turning it off. The output of the charge amplifier may be digitized and stored. After scanning all the gate lines, the image planes representing the first and second light sensitive elements may be combined to generate an image.

The scintillating phosphor screen in the embodiment of fig. 7-13 may be a conventional X-ray image-enhancing screen. The intensifying screen has a luminescent layer in which the instant-emitting phosphors are dispersed as particles in a polymer matrix, and has additional layers such as a support layer, a protective outer layer, and a retainer. Suitable prompt-emitting phosphors are well known, for example rare earth oxysulfides doped with rare earth activators, such as Gd₂O₂Tb, calcium tungstate, yttrium oxide, barium fluorohalide (bariumfluorohalide), HfO₂:Ti、HfGeO₄:Ti、LuTaO₄、Gd₂O₃:Eu、La₂O₂S、LaOBr、CsI:Tl、YTaO₄、Y₂O₂S:Tb、CaWO₄Eu, LaOBr Tm, or a combination thereof. Mixtures of different phosphors may also be used. The median particle size (median particle size) used is typically between about 0.5 microns and about 40 microns. A median particle size between 1 micron and about 20 microns is desirable to facilitate optimization of formulation and characteristics such as speed, sharpness, and noise. The scintillating phosphor screens used in embodiments of the present invention may be prepared using conventional coating techniques in which the phosphor is mixed with a solution of a resin binder material and coated onto a substrate using methods such as doctor blade coating. The binder may be selected from a well-known variety of organic polymers that are transparent to X-rays, excitation, and luminescence. Binders commonly used in the art include sodium o-sulfobenzoate acetal of polyvinyl alcohol; chlorosulfonated polyethylene (ethylene); macromolecular bisphenol polycarbonates (blends of macromolecular bisphenol polycarbonates) and copolymers comprising bisphenol carbonates and polyalkylene oxides (copolymers comprising bisphenol carbonates and polyalkylene oxides); aqueous ethanol soluble nylons (aquousenethanol soluble nylons); poly (alkyl acrylates and methacrylates) and poly (alkyl acrylates and methacrylates)Copolymers of esters with acrylic acid and methacrylic acid) and copolymers of poly (alkyl acrylates and methacrylates with acrylic and methacrylic acid); poly (vinyl butyral); and polyurethane elastomers (poly (urethane) elastomers). Any conventional phosphor to binder ratio may be used. In general, thinner phosphor layers and sharper images are achieved with higher phosphor to binder weight ratios. The ratio of phosphor to binder is desirably in the range of about 7: 1 to 25: 1. The enhanced screen is not limited to the use of crystalline phosphors for X-ray to light conversion. For example, a scintillating glass or an organic scintillator may be used.

Several embodiments of the present invention employ multiple scintillator layers in a DR imaging apparatus to maximize the somewhat conflicting requirements of increased signal-to-noise ratio (SNR) and increased Modulation Transfer Function (MTF). For example, in the embodiment of FIG. 12, phosphor screen 182 has a thickness t2 that is thinner than thickness t1 of phosphor screen 178. Phosphor screen 182 is optimized for resolution and MTF due to its inherently lower scattering, while thicker phosphor screen 178 is optimized for SNR. For example, the phosphor screen 182 may have a thickness of 97 microns (with Gd2O2S: Tb up to 45.3 mg/cm)²Cover weight) and phosphor screen 178 may be 186 microns thick (with Gd2O2S: Tb up to 82.7 mg/cm)²The cover weight of). Phosphor screen 182 may have a light control layer 184 of black, absorbing material and phosphor screen 178 may have a light control coating 180 of black absorbing material. With a typical X-ray beam as used in conventional radiographic techniques, the MTF will equal the spatial frequency (f) of 50%_1/2) 3.8c/mm and 2.4c/mm for phosphor screen 182 and phosphor screen 178, respectively. Meanwhile, phosphor screen 178 has an X-ray absorption efficiency of 47% and phosphor screen 182 of 29%. In actual design, the MTF of phosphor screen 178 exceeds that of phosphor screen 182 such that phosphor screen 178 has a spatial frequency (f) with an MTF of 50%_1/2) At least 0.5c/mm higher than the case of the phosphor screen 182. In addition, the X-ray absorption efficiency of phosphor screen 182 may exceed that of phosphor screen 178 by at least 10%. The imaging array 422 is capable of reading out individual fluorescenceThe images produced by the screens 178, 182, and thus the composite image, can provide higher quality than can be provided by prior DR systems having a single phosphor screen.

Phosphor screens useful in embodiments of the present invention may include Gd as a material component₂O₂S:Tb、Gd₂O₂S:Eu、Gd₂O₃:Eu、La₂O₂S:Tb、La₂O₂S、Y₂O₂S:Tb、CsI:Tl、CsI:Na、CsBr:Tl、NaI:Tl、CaWO₄、CaWO₄:Tb、BaFBr:Eu、BaFCl:Eu、BaSO₄:Eu、BaSrSO₄、BaPbSO₄、BaAl₁₂O₁₉:Mn、BaMgAl₁₀O₁₇:Eu、Zn₂SiO₄:Mn、(Zn，Cd)S:Ag、LaOBr、LaOBr:Tm、Lu₂O₂S:Eu、Lu₂O₂S:Tb、LuTaO₄、HfO₂:Ti、HfGeO₄:Ti、YTaO₄、YTaO₄:Gd、YTaO₄:Nb、Y₂O₃:Eu、YBO₃:Eu、YBO₃Tb or (Y, Gd) BO₃Eu, one or more of them or their combination. For example, phosphor screens 178 and 182 may have the same or different material compositions. For example, phosphor screens 178 and 182 may have the same phosphor material but different particle size distributions. The median particle size of the phosphor material on phosphor screen 182 may range from about 1 micron to about 5 microns, and the median particle size of the phosphor material on phosphor screen 178 may range from about 6 microns to about 15 microns. For example, the number of atoms of the heavy elements of the phosphor screen used in the embodiments of the present invention may vary. For example, for higher X-ray energy absorption, phosphor screen 182 may have a composition with a higher atomic number of elements than the composition of phosphor screen 178. For example, phosphor screen 182 may comprise Gd2O2S: Tb and phosphor screen 178 may comprise Y2O2S: Tb. The number of gadolinium (Gd) atoms is 64 and the number of yttrium (Y) atoms is 39.

Moreover, the spatial frequency of the phosphor screen used in the embodiments of the present invention can be matchedThe situation is different for different structures using different fluorescent materials. For example, phosphor screen 182 may include a columnar structure of phosphor such as CsI: Tl, and phosphor screen 178 may include a columnar structure of phosphor such as Gd₂O₂Tb as a powder phosphor. When evaporated under appropriate conditions, a layer of CsI will agglomerate in the form of needle-like, tightly packed crystals at a higher packing density. Such columnar or needle-like phosphors are well known in the art. For example, with reference to ALN steps et al, "vapor deposited CsI Na layer: screen for Application to X-Ray Imaging Devices (Vapor depositedCsI: Na Layers: Screens for Application in X-Ray Imaging Devices) "Philips Research report (Philips Research Reports) 29: 353-362 (1974); and T.J. et al, "enhanced columnar structures in CsI Layers formed using a Substrate pattern" IEEE Nuclear science Command (IEEE trans. Nucl. Sci.) 39: 1195-1198 (1992.) in this form, the spatial frequency response (or resolution) is improved relative to a powder phosphor screen of the same thickness, similar to powder screens, reflective backings are used to maximize the light collection capability of the layer by re-conducting photons to the exit face. Tl layer and phosphor screen 178 may have a thickness of 93 microns of Gd.₂O₂And S is a Tb layer. The spatial frequency response of phosphor screen 182 may be higher than the spatial frequency response of phosphor screen 178. MTF equal to 50% of the spatial frequency value (f)_1/2) 4.7c/mm and 3.3c/mm for phosphor screens 182 and 178, respectively. Generally, in accordance with the present invention, X-ray radiation is incident on the side of the imaging device 170, 210, 270, 318, 420 or the other, which has a thinner phosphor screen, which is closer to the X-ray source so that the MTF of the thinner screen is optimized. The X-ray radiation may also be selectively incident on the thicker side of the phosphor screen so that the SNR of the thicker screen is optimized.

A potential problem with the embodiments of fig. 5-10, 12, and 13 relates to light scattering within the thinner transparent substrates 186, 230, 286. Luminescent screens tend to have an optical refractive index of 1.6 or more, while transparent substrates such as plastics have a lower refractive index, typically in the range of 1.46 to 1.59, and amorphous silicon has a refractive index, typically in the range of 2.9 to 3.7. The result is that light entering the thin transparent substrate from the phosphor screen undergoes multiple internal reflections within the substrate, causing optical crosstalk between adjacent pixels, as briefly described with respect to fig. 10 and 11. Since phosphor screens are typically diffuse reflectors, the angle of reflection may not be equal to the angle of incidence at the screen-substrate interface.

One solution to this problem is disclosed in the embodiment of fig. 11. Another solution is shown in the embodiment of fig. 15, where the thinner transparent substrate 286 is formed of a material containing a fairly uniform density of colorants or light scattering particles. For example, the base material may be polyimide, the colorant material may be carbon particles, or the light scattering particles may be titanium dioxide. The colorant may be added, for example, during the substrate manufacturing process. The dye density is preferably selected to obtain low absorption for light propagating directly across the substrate between the scintillator and the photosensor, while obtaining a desired degree of cross-talk suppression of light that undergoes total internal reflection, thereby traveling from one pixel to an adjacent pixel. This is at least partially possible because the substrate is typically thinner than the pixel pitch. Alternatively, a light scattering material (such as glass or polymer beads having a refractive index different from that of the transparent substrate) may be used in place of the colorant to reduce lateral light passage in the substrate. Alternatively, the colorant-containing microbeads may be dispersed in or on the substrate without a uniform colorant concentration. As shown in fig. 15, there is a reduction in light scatterers 404 as compared to the scatterers illustrated and described with respect to fig. 10.

In another solution, shown in fig. 16, light absorbing surface boundaries 530,532 are formed on substrate 286 by applying, for example, a dye, in a pattern to one or both sides of the substrate. The dye concentration is preferably adjusted to inhibit lateral transmission of light through the boundary region, thereby reducing the light scatterers 404. Thermal dye transfer from a donor sheet to a receptor sheet is one example of a process that can be used to obtain absorbing regions in the form of a pattern. For example, laser thermal dye transfer, which is widely used in digital printing proofs and digital printing plate applications, can achieve line widths of 10 microns or less. Examples of receiving layers for colorants include polycarbonate, polyurethane, polyester, polyvinyl chloride, or mixtures thereof. Alternatively, the colorant-receiving material described above may be coated on one or both sides of a substrate such as poly (ether sulfone) or polyimide. The colorant in the form of a pattern transferred from the donor sheet by thermal energy may be a sublimable dye or an inorganic colorant. Examples of the sublimable dye include anthraquinone dyes such as KTB black 146 (product of Nippon Kayaku co., ltd.), azo dyes such as sumikaron diaza black 5G (product of Sumitomo chemical ltd.). An example of an inorganic colorant would be carbon particles.

In another solution, shown in FIG. 17, the colorant is dispersed into one or both surfaces of substrate 286 to form dispersed layers 534, 536. In this embodiment, light undergoing multiple internal reflections forms multiple paths through each of the dispersing layers 534, 536, while light transmitted from phosphor screen 278 directly through substrate 286 to photosensor 292 forms only one path. In this method, light scattering between pixels can be reduced without adversely affecting sensitivity. For example, a colorant pattern in a substrate can be formed by spatially uniform transfer of a colorant from a colorant-containing liquid in contact with a colorant-receiving layer or by thermal transfer from a donor sheet to a receiving layer. Examples of receiving layers, colorants, and dyes are as described in the preceding paragraph. Alternatively, a substantially light transparent substrate material such as polyimide may be coated on one or both surfaces with a binder such as polyimide containing a colorant such as carbon particles or a dye such as those described above.

The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the scope of the invention as described above and as indicated in the appended claims by a person of ordinary skill in the art without departing from the scope of the invention. Thus, provided are flat panel digital imaging apparatus and methods using dual scintillating phosphor screens.

Claims (9)

1. A radiographic imaging apparatus comprising:

a first continuous scintillating phosphor screen having a first thickness;

a second continuous scintillating phosphor screen having a second thickness;

a single substrate disposed between the first screen and the second screen, the substrate being substantially transparent to X-rays for the apparatus; and

a single imaging array disposed in a single plane between the first side of the substrate and one of the first and second screens, the single imaging array comprising a plurality of pixels, wherein each pixel comprises at least a first photosensor, a second photosensor, a first readout element coupled to the first photosensor, and a second readout element coupled to the second photosensor, the first photosensor being coplanar with the second photosensor, the first photosensor configured to receive light from the first screen and the second photosensor configured to receive light from the second screen.

2. The apparatus of claim 1, wherein the first thickness and the second thickness are the same.

3. The apparatus of claim 1, wherein each pixel includes a first shielding component and a second shielding component, the first shielding component including a light blocking layer between the second screen and the first photosensor, the second shielding component including a light blocking layer between the first screen and the second photosensor, the first photosensor sensitive to light from the first screen, the second photosensor sensitive to light from the second screen.

4. The apparatus of claim 1, wherein the substrate is transparent to light from one or both of the first screen and the second screen.

5. The apparatus of claim 1, wherein the at least one readout element comprises a thin film transistor formed on one side of the substrate.

6. The apparatus of claim 1, wherein:

at least some of the photosensors of the plurality of pixels being sensitive to light from the first screen and others being sensitive to light from the second screen;

the first screen includes a first fluorescent material having an element with a first atomic number;

the second screen includes a second fluorescent material having an element with a second atomic number;

the first atomic number exceeds the second atomic number such that the first fluorescent material absorbs a higher energy component of the X-ray radiation.

7. A radiographic imaging apparatus comprising:

a single substrate transparent to X-rays for use with the apparatus;

a first scintillating phosphor screen having a first thickness disposed on the first side of the substrate;

a second scintillating phosphor screen having a second thickness disposed on a second side of the substrate;

the substrate is substantially transparent to light from the first screen and the second screen; and

a single imaging array of pixels formed in a single planar layer on one side of the substrate, the substrate having a thickness less than the two pixel pitch of the imaging array to reduce radiation scattering and light pipe transmission in the substrate, the single imaging array comprising:

a first set of photosensors primarily sensitive to light emitted by the first screen, each of the first set of photosensors including a first light sensitive element, a first readout element, and a first light blocking layer; and

a second set of photosensors primarily sensitive to light emitted by the second screen, each of the second set of photosensors including a second light sensitive element, a second readout element, and a second light blocking layer, wherein the second set of photosensors is separate and distinct from the first set of photosensors,

the first readout element provides a first X-ray image and the second readout element provides a second X-ray image.

8. The apparatus of claim 7, wherein the second set of photosensors and the first set of photosensors are formed in separate planes.

9. The apparatus of claim 7, wherein the first and second sets of photosensors are formed in the same plane.