WO2019028205A1

WO2019028205A1 - Dual-screen digital radiography with asymmetric reflective screens

Info

Publication number: WO2019028205A1
Application number: PCT/US2018/044924
Authority: WO
Inventors: Anthony LUBINSKY; Wei Zhao; John A. Rowlands
Original assignee: Research Foundation of the State University of New York
Current assignee: Research Foundation of the State University of New York
Priority date: 2017-08-03
Filing date: 2018-08-02
Publication date: 2019-02-07
Anticipated expiration: 2020-02-03
Also published as: US20220404513A1; CA3071647A1; JP2020529607A; WO2021158587A1; JP2025087809A; CN111433864A; US12025757B2; JP2023075117A; KR20200083433A; US11460590B2; US20200174141A1

Abstract

A structure operable to detect radiation is described. The structure may include a first screen of a first thickness and a second screen of a second thickness greater than the first thickness. The structure may further include a photosensor array disposed between the first screen and the second screen. A back side of the first screen may face incident radiation directed towards the structure. The first screen may include a first reflective layer that may reflect the light photons scattered among the first screen towards the photosensor array. The second screen may face the photosensor array such that the first screen and the second screen are oriented in opposite directions. The second screen may include a second reflective layer that may reflect the light photons that passed through the photosensor array towards the photosensor array.

Description

DUAL-SCREEN DIGITAL RADIOGRAPHY WITH ASYMMETRIC REFLECTIVE

SCREENS

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of U.S. Provisional Application No.

62/540,620 filed on August 3, 2017, and U.S. Provisional Application No. 62/711,883 filed on July 30, 2018.

FIELD

[0002] The present application relates generally to radiation detectors and digital radiography.

BACKGROUND

[0003] In digital radiography, an imaging system may include a screen that absorbs radiation and produces light, where the produced light is sensed by an array of photosensors to generate electrical signals. The generated electrical signals may be used by the imaging system to produce a digital image. In some examples, a quality (e.g., sharpness, resolution) of the produced image may be affected by various phenomenons such as light scattering, and/or other phenomenon.

SUMMARY

[0004] In some examples, a structure in digital radiography applications are generally described. The structure may include a first screen of a first thickness and a second screen of a second thickness greater than the first thickness. The structure may further include a

photosensor array disposed between the first screen and the second screen. The first screen may be oriented to face the photosensor array such that a back side of the first screen faces incident radiation directed towards the structure. The first screen may include a first phosphor layer that converts the incident radiation directed at the structure into light photons. The first screen may further include a first reflective layer disposed on the back side of the first screen. The first reflective layer may reflect the light photons scattered among the first phosphor layer towards the photosensor array. The second screen may be oriented to face the photosensor array such that the first screen and the second screen are oriented in opposite directions. The second screen may include a second phosphor layer. The second screen may further include a second reflective layer disposed on a back side of the second screen. The second reflective layer may reflect the light photons that passed through the photosensor array towards the photosensor array. The photosensor array may be operable to capture the light photons and convert the captured light photons into electrical signals.

[0005] In some examples, an imaging system is generally described. The imagine system may include a processor configured to be in communication with a structure. The structure may include a first screen of a first thickness and a second screen of a second thickness greater than the first thickness. The structure may further include a photosensor array disposed between the first screen and the second screen. The first screen may be oriented to face the photosensor array such that a back side of the first screen faces incident radiation directed towards the structure. The first screen may include a first phosphor layer that converts the incident radiation directed at the structure into light photons. The first screen may further include a first reflective layer disposed on the back side of the first screen. The first reflective layer may reflect the light photons scattered among the first phosphor layer towards the photosensor array. The second screen may be oriented to face the photosensor array such that the first screen and the second screen are oriented in opposite directions. The second screen may include a second phosphor layer. The second screen may further include a second reflective layer disposed on a back side of the second screen. The second reflective layer may reflect the light photons that passed through the photosensor array towards the photosensor array. The photosensor array may be operable to capture the light photons and convert the captured light photons into electrical signals. The processor may be configured to receive the electrical signals from the structure and produce an image using the electrical signals.

[0006] In some examples, a X-ray apparatus is generally described. The apparatus may include a radiation detector, a X-ray source, and a processor. The X-ray source may be operable to irradiate X-ray to a subject disposed between the X-ray source and the radiation detector. The radiation detector may include a first screen of a first thickness and a second screen of a second thickness greater than the first thickness. The radiation detector may further include a photosensor array disposed between the first screen and the second screen. The first screen may be oriented to face the photosensor array such that a back side of the first screen faces the X-rays irradiated towards the radiation detector. The first screen may include a first phosphor layer that converts the X-rays into light photons. The first screen may further include a first reflective layer disposed on the back side of the first screen. The first reflective layer may reflect the light photons scattered among the first phosphor layer towards the photosensor array. The second screen may be oriented to face the photosensor array such that the first screen and the second screen are oriented in opposite directions. The second screen may include a second phosphor layer. The second screen may further include a second reflective layer disposed on a back side of the second screen. The second reflective layer may reflect the light photons that passed through the photosensor array towards the photosensor array. The photosensor array may be operable to capture the light photons and convert the captured light photons into electrical signals. The processor may be operable to receive the electrical signals from the radiation detector and produce an image of the subject using the electrical signals.

[0007] Further features as well as the structure and operation of various embodiments are described in detail below with reference to the accompanying drawings. In the drawings, like reference numbers indicate identical or functionally similar elements.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008] Fig. 1 illustrates an example structure that can be utilized to implement dual- screen digital radiography with asymmetric reflective screens in one embodiment.

[0009] Fig. 2 illustrates an example structure that can be utilized to implement dual- screen digital radiography with asymmetric reflective screens in one embodiment.

[0010] Fig. 3 illustrates example results of performance measures relating to dual-screen digital radiography with asymmetric reflective screens in one embodiment.

[0011] Fig. 4 illustrates example results of performance measures relating to dual-screen digital radiography with asymmetric reflective screens in one embodiment.

[0012] Fig. 5 illustrates a difference between a standard configuration of a x-ray detector and a dual-screen configuration as described in the present disclosure. [0013] Fig. 6 illustrates experimental results indicating a difference in MTF between standard or conventional configuration with one screen and a dual-screen configuration as described in the present disclosure.

[0014] Fig. 7 illustrates experimental results indicating a difference in DQE between a standard configuration with one screen and a dual- screen configuration as described in the present disclosure.

DETAILED DESCRIPTION

[0015] The following detailed description of embodiments of the invention will be made in reference to the accompanying drawings. In describing the invention, explanation about related functions or constructions known in the art are omitted for the sake of clearness in understanding the concept of the invention to avoid obscuring the invention with unnecessary detail.

[0016] Active-matrix indirect flat-panel imagers (AMFPI) may be used in applications of digital radiography. In some examples, AMFPI's may include a single intensifying screen, and may be fabricated by placing a sensor array (e.g., thin film transistor array) below the intensifying screen such that the AMFPI may be operated by having x-rays incident from above the intensifying screen. A thickness of the single intensifying screen may be based on a tradeoff between x-ray absorption and spatial resolution. For example, increasing thickness may improve absorption and sensitivity, but may also decrease resolution due to light scattering in a phosphor layer of the intensifying screen.

[0017] In screen-film radiography, a dual- screen system may include a dual-emulsion film disposed between two partitions divided from a single intensifying screen. Such a configuration may reduce light scattering due to the reduced distance between the incident radiation and the film, but may invoke the crossover phenomenon, where light photons may penetrate through the film emulsion and followed by reflection from the opposite partition.

[0018] To be further described below, a structure (e.g., structure 100 shown in Fig. 1) in accordance with the present disclosure may address the some of the shortcomings of various digital radiography systems and film- screen radiography systems. [0019] Fig. 1 illustrates an example structure 100 that can be utilized to implement dual- screen digital radiography with asymmetric reflective screens, arranged in accordance with at least some embodiments described herein. The structure 100 may include a first screen 110, a second screen 120, a photosensor array 105, and substrate 107. The first screen 110 may be oriented such that a back side of the first screen faces incident radiation, such as incident X-rays 102, being directed towards the structure 100. The photosensor array 105 may be disposed between the first screen 110 and the second screen 120. The first screen 110 and the second screen 120 may be oriented in opposite directions, such that the first screen 110 and the second screen 120 faces each other. In the orientation of the structure 100 shown in Fig. 1, the back side of the first screen 110 may be a top surface of the structure 100 and a back side of the second screen 120 may be a bottom surface of the structure 100. A thickness of the first screen 110 may be less than a thickness of the second screen 120. The first screen 110 may be disposed above the photosensor array 105, such that incident x-rays 102 may incident on the first screen 110. In some examples, the first screen 110 may be disposed above the photosensor array 105 due to the thickness of the first screen 110 being less than the thickness of the second screen 120.

[0020] Screen 110 may include a scintillating phosphor layer 114 and a reflective layer

112, where the reflective layer 114 may be made of a highly reflective material. Screen 120 may include a scintillating phosphor layer 122 and a reflective layer 124, where the reflective layer 124 may made of a highly reflective material. For example, the reflective layers 114, 124 may be coated with a layer of white material, such as titanium dioxide. The reflective layers 114, 124 may be of same or different size, and may be coated with same or different materials. Each of the phosphor layers 114, 124 may include phosphor crystals that may capture the incident x-rays 102 and convert the captured x-rays into light photons. In some examples, a thickness of the phosphor layer 114 may be less than a thickness of the phosphor layer 124, such that the screen 110 may be thinner than the screen 120. In some examples, the screens 110, 120 may each be granular type (e.g. Gd02S2:Tb), or columnar (e.g. CsLTI) type, or a combination of both. In some examples, an additional support for the thicker screen (e.g., screen 120) may optionally disposed below the reflective layer 122 for increased structural stability.

[0021] The photosensor array 105 may include a photosensitive storage element 108 that may include a plurality of switching elements 106. The substrate 107 may be of small optical thickness may be disposed between the photosensor array 105 and the phosphor layer 124. The photosensitive storage element 108 and the switching elements 106 may be disposed on top of the substrate 107. The photosensor array 105 may be a-Si:H n-i-p photodiodes, MIS-type, or other types. The photosensor array 105 may be sensitive to light incident from either side, and may have a low transmittance at the wavelengths emitted by the screens 110, 120. For example, the photosensor array 105 may have high optical absorption (above 90%) at the wavelength of the light emitted by the screens 110, 120, such that pixel crosstalk and crossover effects may be reduced. In an example, the substrate 107 may be of thin glass, plastic, or cellulose with thickness less than 30 microns, and preferably less than 10 microns. The photosensor array 105 may capture the light photons and may convert the captured light photons into electrical signals, where the electrical signals may be used by a device (separate from the structure 100) to produce a digital image. For example, each switching element 106 may correspond to a pixel of an image, such that toggling particular columns, rows, groups of pixels may cause a read out of a group of pixel values to produce an image.

[0022] In an example, the structure 100 may be a component of an imaging system that produces images. In operation, the phosphor layer 114 may receive the incident x-rays 102 and convert the incident x-rays 102 into light. As the converted light reaches the photosensor array 105, the photosensor array 105 may capture the light photons from the converted light, and may convert the light photons to electrical signals. In an example shown in Fig. 1, when the incident X-rays 102 reaches the phosphor layer 114, the crystals in the phosphor layer 114 may convert the X-rays into photons 140. The photons 140 may scatter among the phosphor layer 114. Some of the scattered photons may be directed towards the photosensor array 105, while other scattered photons may be directed away from the photosensor array 105. The reflective layer 112 may reflect the scattered photons toward the photosensor array 105 in order for the photosensor array 105 to capture the scattered photons.

[0023] In some examples, the incident x-rays 102 may not be fully captured by the phosphor layer 114 (e.g., phosphor layer 114 may not have enough crystals to convert all incident x-rays). The uncaptured x-rays may pass through the photosensor array 105, and the crystals among the phosphor layer 124 of the second screen 120 may convert the captured x-rays into light photons 150. The photons 150 may scatter among the phosphor layer 124. Some of the scattered photons may be directed towards the photosensor array 105, while other scattered photons may be directed away from the photosensor array 105. The reflective layer 122 may reflect the scattered photons toward the photosensor array 105 in order for the photosensor array 105 to capture the scattered photons. Thus, the second screen 120 facilitates the photosensor array 105 to recapture photons that was not absorbed by the photosensor array 105 from the reflections of the screen 110.

[0024] In some examples, the light converted from the top screen 110 (facing the incident x-rays) may be weighted by adjusting the optical properties of the photosensor array 105. The light from the screen 110 may include more information from the low-energy part of the incident x-ray spectrum due to beam hardening effects, and emphasizing this may improve the visibility of low contrast objects in images generated by the imaging system utilizing the structure 100.

[0025] In an example, a process may be implemented by a computer device or hardware processor to construct the structure 100 may begin with executing a radiographic examination to determine a beam quality, or the half-value layer (HVL) of the phosphor layers 114, 124. Then, mathematical models may be used to determine performance measures such as signal-to-noise ratio (SNR), modulation transfer function (MTF), as a function of a ratio of coating weights or thicknesses of the two screens 110, 120. Then, based on the results from the radiographic examination and performance measures, a thickness ratio of the phosphor layers 114, 124 is selected that may provide an optimum performance in a desired implementation of the structure 100.

[0026] For example, the thicknesses of the two scintillating phosphor layers 114, 124 may be chosen to maximize a detective quantum efficiency (DQE) of an imaging system utilizing the structure 100. The DQE is the output signal-to-noise ratio (SNR) per input quantum, and the DQE depends on spatial frequency and x-ray exposure levels. A fundamental limit on DQE performance is given by the product of the x-ray absorption efficiency and two noise factors, one of which quantifies the variation in the magnitude of response to an absorption event (the Swank factor) and one quantifying the variation in spatial response to an event (the Lubberts factor). The Lubberts Factor describes the dropoff in DQE due to the variation in the spatial spreading of light arising from x-ray absorption events occurring at various distances from the photosensor array. In an example to maximize the detective quantum efficiency, the thinner (less thickness) of the two scintillating phosphor layers 114, 124 may be chosen to be between 30% and 45% of a sum of thicknesses of the two scintillating phosphor layers 114, 124.

[0027] In some examples, the thicknesses of the two scintillating phosphor layers 114,

124 may be chosen to maximize the MTF of an imaging system utilizing the structure 100. To maximize the MTF, the thinner of the two scintillating screens is chosen to be between 20% and 40% of the total scintillating layer thickness.

[0028] In an example, the structure 100 may be a component of an imaging system. The imaging system may include the structure 100, a processor, and a memory configured to be in communication with each other. The first screen 110 of the structure 100 may receive the incident X-rays 102, and may convert the incident X-rays into light photons. The reflective layer 114 may reflect the light photons scattered among the first phosphor layer 112 towards the photosensor array 105. The reflective layer 124 of the second screen 120 may reflect the light photons that passed through the photosensor array 105 back towards the photosensor array 105. The photosensor array 105 may convert capture photons into electrical signals, and may output the electrical signals to the processor. The processor may store the electrical signals in the memory, and may produce an image using the electrical signals.

[0029] In an example, the structure 100 may be a radiation detector among an apparatus comprising a X-ray source and a processor. The X-ray source may be a X-ray tube that produces X-rays, or other devices that may produce X-rays. A subject, such as an object, may be disposed between the X-ray source and the structure 100. The X-ray source may irradiate X-ray onto the subject, where the subject may absorb a portion of the X-rays, causing an attenuation of the X- rays. The attenuated X-rays may be directed towards the structure 100 as incident X-rays 102. The first screen 110 of the structure 100 may receive the incident X-rays 102, and may convert the incident X-rays into light photons. The reflective layer 114 may reflect the light photons scattered among the first phosphor layer 112 towards the photosensor array 105. The reflective layer 124 of the second screen 120 may reflect the light photons that passed through the photosensor array 105 back towards the photosensor array 105. The photosensor array 105 may capture the light photons and convert the captured light photons into electrical signals. The processor may be operable to receive the electrical signals from the radiation detector and produce an image of the subject using the electrical signals. [0030] Fig. 2 illustrates an example structure 200 that can be utilized to implement dual- screen digital radiography with asymmetric reflective screens, arranged in accordance with at least some embodiments described herein. Fig. 2 may be described below with references to the above descriptions of Fig. 1.

[0031] The structure 200 may include the first screen 110, the second screen 120, a photosensor array 205, and a fiber optic plate 202. The photosensor array 205 may include the photosensitive storage element 108, which may include the plurality of switching elements 106. The fiber optic plate 202 may be of essentially zero optical thickness, such as a negligible optical thickness and physical thickness of 1 to 3 mm. In some examples, the fiber numerical aperture of the fiber optic plate 202 may be relatively large.

[0032] Fig. 3 illustrates example results of performance measures relating to dual-screen digital radiography with asymmetric reflective screens, arranged in accordance with at least some embodiments described herein. Fig. 3 may be described below with references to the above descriptions of Figs. 1-2.

[0033] A graph 302 indicating the Lubberts factor and DQE of an imaging system utilizing a dual-screen structure (e.g., structure 100, and/or 200) with white backings (reflective layers 112, 122), and a resolution of 51p/mm (line pairs per millimeter), is shown in Fig. 3. In the graph 302, a total thickness of the two screens in the dual-screen structure is 160 microns (0.160 mm), where the back side (the side including the reflective layer 112) of the thinner screen (first screen 110) is located at 0 microns, and the back side (the side including the reflective layer 114) of the thicker screen is located at 160 microns. As shown by graph 302, the optimal DQE point is at 0.06 mm, which means the optimal position of the photosensor array (e.g., photosensor arrays 105, 205 described above) relative to the total thickness is 0.06 mm (60 microns) away from the 0 micron point, or the back side of the thinner screen where the incident x-rays are being received. The ratio of the thicknesses of the two screens to maximize DQE, by having the photosensor array at 0.06 mm, is approximately 37%.

[0034] A graph 304 indicating the Lubberts factor, the DQE, and the swank factor of an imaging system utilizing a dual-screen structure (e.g., structure 100, 200, and/or 300) without reflective layers, is shown in Fig. 3. As shown by graph 304, the DQE is lower than the DQE indicated in graph 302, which means the inclusion of reflective layers would increase the DQE of the imaging system. The Lubberts Factor describes the drop-off in DQE due to the variation in the spatial spreading of light arising from x-ray absorption events occurring at various distances from the photosensor array.

[0035] Fig. 4 illustrates example results of performance measures relating to dual-screen digital radiography with asymmetric reflective screens, arranged in accordance with at least some embodiments described herein. Fig. 4 may be described below with references to the above descriptions of Figs. 1-3.

[0036] A graph 402 shows the results of a number of calculations in which a single intensifying screen is subdivided into two parts of different relative thicknesses and sandwiched around the photosensor array at different positions shown on the x-axis of the graph 402. Similar to the example in Fig. 3, the dual-screen structure relating to graph 402 includes (e.g., structure 100 and/or 200) white backings (reflective layers 112, 122), and a resolution of 51p/mm (line pairs per millimeter). The total thickness of the two screens is 160 microns. A 70 kVp RQA5 incident x-ray beam is incident from the left. The MTF and normalized noise power spectrum (NNPS) are shown in graph 402 for each configuration. The optimal MTF point is at 0.04 mm, which means the optimal position of the photosensor array (e.g., photosensor arrays 105, 205 described above) relative to the total thickness is 0.04 mm (40 microns) away from the 0 micron point, or the back side of the thinner screen where the incident x-rays are being received. The ratio of the thicknesses of the two screens to maximize MTF, by having the photosensor array at 0.06 mm, is approximately 25%.

[0037] Fig. 5 illustrates a difference between a standard configuration of a x-ray detector and a dual-screen configuration as described in the present disclosure. As shown in Fig. 5, the standard configuration includes one scintillator, and the glass substrate is the bottom-most layer of the detector. The dual-screen configuration adds another screen ("Screen 2") that is thicker than the top screen ("Screen 1") underneath the glass substrate, and both top and bottom screens have respective reflective backing.

[0038] Fig. 6 illustrates experimental results indicating a difference in MTF between standard or conventional configuration with one screen and a dual-screen configuration as described in the present disclosure. As shown in Fig. 6, the modeled MTF of the dual-screen configuration, under both a high sensitivity configuration and a high resolution configuration, is greater than the modeled MTF of a conventional configuration. Also shown in Fig. 6, the measured MTF of the dual-screen configuration, under both a high sensitivity configuration and a high resolution configuration, is greater than the measured MTF of a conventional

configuration.

[0039] Fig. 7 illustrates experimental results indicating a difference in DQE between a standard configuration with one screen and a dual- screen configuration as described in the present disclosure. The experimental results shown in Fig. 7 are based on an experiment using RQA9 incident x-ray beam. As shown in Fig. 7, the measured DQE of the dual- screen configuration is greater than the conventional configuration.

[0040] The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms "a", "an" and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms "comprises" and/or "comprising," when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof.

[0041] The corresponding structures, materials, acts, and equivalents of all means or step plus function elements, if any, in the claims below are intended to include any structure, material, or act for performing the function in combination with other claimed elements as specifically claimed. The description of the present invention has been presented for purposes of illustration and description, but is not intended to be exhaustive or limited to the invention in the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the invention. The embodiment was chosen and described in order to best explain the principles of the invention and the practical application, and to enable others of ordinary skill in the art to understand the invention for various embodiments with various modifications as are suited to the particular use contemplated.

Claims

CLAIMS What is claimed is:

1. A structure comprising:

a first screen of a first thickness;

a second screen of a second thickness greater than the first thickness;

a photosensor array disposed between the first screen and the second screen;

wherein the first screen is oriented to face the photosensor array such that a back side of the first screen faces incident radiation directed towards the structure, and the first screen comprises:

a first phosphor layer that converts the incident radiation directed at the structure into light photons;

a first reflective layer disposed on the back side of the first screen, wherein the first reflective layer reflects the light photons scattered among the first phosphor layer towards the photosensor array;

wherein the second screen is oriented to face the photosensor array such that the first screen and the second screen are oriented in opposite directions, the second screen comprises: a second phosphor layer;

a second reflective layer disposed on a back side of the second screen, wherein the second reflective layer reflects the light photons that passed through the photosensor array towards the photosensor array; and

wherein the photosensor array is operable to capture the light photons and convert the captured light photons into electrical signals.

2. The structure of claim 1, wherein the photosensor array comprises a

photosensitive storage element comprising a plurality of switching elements.

3. The structure of claim 1, further comprising a substrate disposed between the photosensitive array and the second phosphor layer.

4. The structure of claim 3, wherein the substrate is one of glass, plastic, and cellulose.

5. The structure of claim 1, further comprising a fiber optic plate disposed between the photosensitive array and the second phosphor layer.

6. The structure of claim 1, wherein a ratio of the first thickness to the second thickness maximizes a detective quantum efficiency of an imaging system utilizing the structure.

7. The structure of claim 1, wherein a ratio of the first thickness to the second thickness maximizes a modulation transfer function of an imaging system utilizing the structure.

8. An imaging system comprising:

a processor configured to be in communication with a structure comprising:

a first screen of a first thickness;

a second screen of a second thickness greater than the first thickness; a photosensor array disposed between the first screen and the second screen; wherein the first screen is oriented to face the photosensor array such that a back side of the first screen faces incident radiation directed towards the structure, and the first screen comprises:

wherein the second screen is oriented to face the photosensor array such that the first screen and the second screen are oriented in opposite directions, the second screen comprises:

a second phosphor layer; a second reflective layer disposed on a back side of the second screen, wherein the second reflective layer reflects the light photons that passed through the photosensor array towards the photosensor array;

wherein the photosensor array is operable to capture the light photons and convert the captured light photons into electrical signals;

the processor is configured to:

receive the electrical signals from the structure; and

produce an image using the electrical signals.

9. The imaging system of claim 8, wherein the photosensor array comprises a photosensitive storage element comprising a plurality of switching elements.

10. The imaging system of claim 8, wherein the structure further comprises a substrate disposed between the photosensitive array and the second phosphor layer.

11. The imaging system of claim 10, wherein the substrate is one of glass, plastic, and cellulose.

12. The imaging system of claim 8, wherein the structure further comprises a fiber optic plate disposed between the photosensitive array and the second phosphor layer.

13. The imaging system of claim 8, wherein a ratio of the first thickness to the second thickness maximizes a detective quantum efficiency of the imaging system.

14. The imaging system of claim 8, wherein a ratio of the first thickness to the second thickness maximizes a modulation transfer function of the imaging system.

15. An apparatus comprising:

a radiation detector; a X-ray source operable to irradiate X-ray to a subject disposed between the X-ray source and the radiation detector, the radiation detector comprising:

a first screen of a first thickness;

a second screen of a second thickness greater than the first thickness;

a photosensor array disposed between the first screen and the second screen;

wherein the first screen is oriented to face the photosensor array such that a back side of the first screen faces the X-rays irradiated towards the radiation detector, and the first screen comprises:

a first phosphor layer that converts the X-rays into light photons;

a second phosphor layer;

a second reflective layer disposed on a back side of the second screen, wherein the second reflective layer reflects the light photons that passed through the photosensor array towards the photosensor array;

wherein the photosensor array is operable to capture the light photons and convert the captured light photons into electrical signals

a processor operable to:

receive the electrical signals from the radiation detector;

produce an image of the subject using the electrical signals.

16. The apparatus of claim 15, wherein the photosensor array comprises a

photosensitive storage element comprising a plurality of switching elements.

17. The apparatus of claim 15, wherein the structure further comprises a substrate disposed between the photosensitive array and the second phosphor layer.

18. The apparatus of claim 15, wherein the structure further comprises a fiber optic plate disposed between the photosensitive array and the second phosphor layer.

19. The apparatus of claim 15, wherein a ratio of the first thickness to the second thickness maximizes a detective quantum efficiency of the imaging system.

20. The apparatus of claim 15, wherein a ratio of the first thickness to the second thickness maximizes a modulation transfer function of the imaging system.