WO2005103762A2

WO2005103762A2 - Flat panel x-ray imager with avalanche gain layer

Info

Publication number: WO2005103762A2
Application number: PCT/US2005/012950
Authority: WO
Inventors: Denny L. Lee
Original assignee: Direct Radiography Corp.
Priority date: 2004-04-15
Filing date: 2005-04-14
Publication date: 2005-11-03
Also published as: WO2005103762A3

Abstract

A flat panel imager generates an electronic x-ray image though direct conversion of x-ray energy to electrical charges in a selenium-based layer (104) that has first electrical fields (of the order of 10 Volts per micrometer) established therein between an upper electrode (100) and a lower electrode (106). At least one electrically conductive grid (306) extends generally laterally at a level above but close to a lower surface of the selenium-based layer, and is biased to establish higher electrical fields in portions of the layer below the grid (at least 75 Volts per micrometer) to thereby promote an avalanche effect in the high-field portion of the layer.

Description

FLAT PANEL X-RAY IMAGER WITH AVALANCHE GAIN LAYER IN THE PHOTODETECTOR

REFERENCE TO RELATED APPLICATIONS

This patent application claims the benefit of the filing dates of provisional patent application 60/562,747 filed on April 15, 2004 and provisional patent application 60/588,513 filed on July 15, 2004. The contents both provisional applications are hereby incorporated by reference herein.

FIELD This patent specification is in the field of radiography and pertains more specifically to x-ray imaging using a digital flat panel detector.

BACKGROUND

Flat panel x-ray imaging devices that generate electrical signals related to local x-ray exposure have been developed in recent years. An example is discussed in U.S. Pat. No. 5,319,206, the contents of which are hereby incorporated by reference, and a current version is commercially available from the assignee of this patent specification, Hologic, Inc. of Bedford, MA. An improvement involving the use of a gain layer is disclosed in commonly owned U.S. Pat. No. 6,437,339, the contents of which also are incorporated by reference. These examples are direct conversion panels, in which x-ray photons are directly converted to electron-hole pairs and thus into electrical signals, and differ in this respect from indirect conversion panels in which x-ray photons are first converted to light and the light is then converted to electrical signals. It is believed that direct conversion panels have a number of advantages, including better spatial resolution.

Direct conversion flat panel x-ray imaging devices offer good spatial resolution and dynamic range properties and can replace x-ray film in a variety of radiograph ic procedures, such as, without limitation, chest x-ray imaging and mammography. However, there typically is a balance between x-ray dose and image quality. It is desirable in general to limit x-ray dose to the level that would just give the requisite image quality. Such panels typically use an amorphous Selenium (a-Se) based layer in which the incoming x-ray energy is converted to electron-hole pairs. An electric potential across the a-Se layer and a thin-film transistor array are used to derive the electrical signals representing the spatial distribution of the x-ray energy impinging on the panel.

One way to improve the conversion efficiency and thus the signal-to-noise ratio

(SNR) for a given energy of x-rays impinging in a-Se conversion layers is to increase the electrical field sufficiently to create an avalanche effect, in which an x-ray photon is likely to generate multiple electron-hole pairs. See e.g. G. Pang, "Electronic portal imaging with an avalanche-multiplication-based video camera," Med. Phys. 27 (4), 676-684 (2000). However, it is believed that an electric field of approximately 75 Volts per micrometer (75V/μm) or more in the s-Se layer is required to initiate and maintain the avalanche effect. While this is practical in a thin layer of a-Se, it becomes less so in the thickness typically used for medical imaging, which is 200-500 μm. To create a 75V/μm field in a 200 μm thick a_se layer would require applying 15,000 Volts across the a-Se layer, and for a 500 μm layer would require 37,500 Volts. These voltages are difficult to accommodate in a medical device. Even more important, when the entire 200 μm or 500 μm a-Se layer is operating in the avalanche mode, an additional error is introduced because the number of electron-hole pairs that an x-ray photon would generate depends on the depth in the layer at which the first pair was generated. See e.g. D. Hunt, B. Lui, and J.A. Rowlands, "An Experimentally Validated Theoretical Model of Avalanche Multiplication X-ray Noise in Amorphous Selenium,"in Medical Imaging 2000: Physics of Medical Imaging, Proc. SPIE 3977, 106-1 16 (2000). The earlier-cited commonly owned patent (U.S. 6,437,339) discloses an approach in which an avalanche effect would be achieved in a thin layer of a material different from the a-Se layer that converts the x-ray energy to electron-hole pairs. This thin layer can be a gas of a solid material that has a much smaller x-ray absorption cross- section that the a-Se layer and a negligible depth-dependent gain. However, it is difficult in practice to build a suitable gas chamber or a solid material that has the requisite properties. Other efforts to improving conversion efficiency have focused on using different conversion materials, such as PbO and Hgl, or using indirect conversion relying on Csl on avalanche Se, or attacking the challenge of improved SNR by providing in-pixel amplifiers, or tiled CMOS with Csl or SE. However, these proposals are believed to involve materials and techniques that are less understood for medical imaging that direct conversion a-Se layers, and to involve a number of different challenges and solutions that still have to be proved.

Other background information is discussed in the text and cited references of the paper by D.L.Y. Lee, G.Storti, and K. Golden, New developments in full-field radiography detectors: Direct conversion selenium detector with avalanche gain layer. The paper is unpublished as of the filing date of this provisional patent application, and its content are not admitted prior art. Its first-named co-author is a named inventor hereof. The paper is attached hereto and is a part of the disclosure of this provisional patent application.

Accordingly, it is believed that a need still remains to improve the conversion efficiency and SNR of flat panel x-ray imaging devices, and this disclosure is directed to meeting that need.

SUMMARY hi a preferred example, the x-ray dose for a quality image is significantly reduced by providing an electrical grid that is just above the lower surface of the a-Se conversion layer, and is at a voltage level sufficient to induce avalanche mode operation in the thin layer of a-Se that is between the grid and the bottom of the a-Se layer. Because the a-Se layer below the grid is very thin (e.g. about 10-20 μm), a practical voltage difference across it (e.g. about 750-1500 Volts) creates a sufficiently high electric field (e.g. about 75 V/μm or more) in the thin layer to cause an avalanche effect. The thinness of that layer also takes away the concerns with errors due to depth-dependent gain. The grid can be formed in using practical technology of layering and patterning that is well established in the semiconductor industry.

As a result, lower x-ray dose is required for the same image quality, thus reducing patient exposure. The lower dose also reduces the ghosting effect known to be present in such x-ray imagers. Lower noise and high detector quantum efficiency (DQE) follow, and a high modulation transfer function (MTF). Because the grid potential directs all or nearly all of the holes at the pixel electrodes, the fill factor approaches 100%. The gain from the avalanche effect in this structure is for the desired signal, not for the noise. And, the gain can be conveniently adjusted for different requirements, simply by changing the grid potential and thus the electrical field strength in the avalanche layer and its gain factor.

Two or more grids can be used, at respective voltage levels, thus effectively providing two or more avalanche layers with different characteristics. The same proven readout electronics and readout scheme can be used as in the current flat panel imagers commercially available from the assignee.

A non-limiting example of a direct-conversion flat panel x-ray imager using these advances comprises a photoconductive layer (e.g. a-Se) locally generating electrical charges in response to x-ray exposure. An electrical grid extends laterally between the top and bottom surfaces of this layer to thereby divide it into upper and lower regions. A top electrode near the top of the layer is at a voltage level that creates a moderate electrical field in the upper region of the layer (e.g. of the order of 1 OV/μm). The grid is at another voltage level that creates a high electric field in the lower region of the layer (e.g. >75 V/μm). Collector electrodes under the layer are arranged in a pixel pattern, and readout electronics collect the desired signals in the manner used in the current imager commercially available from the assignee hereof. In operation, the charges collected from the collector electrode are representative of the spatial distribution of the x-ray energy impinging on the layer. The term pixel is used here to denote a portion of the flat panel imager that can generate an electrical signal for a positionally corresponding display pixel on a screen displaying an x-ray image.. BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a known flat-panel x-ray detector.

FIG. 2 illustrates a section through the panel of FIG. 1.

FIG. 3 illustrates a new technology using a grid to increase the electrical field in a lower region of a charge generating layer.

FIG. 4 is a plan view of a structure using a grid.

FIG. 5 illustrates a section showing electrical field effects.

FIG. 6 illustrates a section using multiple grids.

FIG. 7 illustrates an embodiment in which grids divide a collector plate in four portions.

FIGS. 8-13 illustrate electrical field distributions.

Fig. 14 illustrates a double-grid structure electric field simulation.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Fig. 1 illustrates an overall structure of a flat panel x-ray imager currently available commercially from the assignee hereof, and is used to explain certain principles of operation. The terms x-ray receptor, x-ray detector, flat panel detector, and other terms may be used to refer to the same structure. The illustration is schematic and not to scale, and may not show all of the elements of the panel that is currently available commercially from the common assignee. The x-rays come from above in this illustration, penetrate a top or upper electrode 100 and a separation layer 102 and enter a-Se layer 104. Electrical charges resulting from the interaction in layer 104 are collected at charge collection electrodes or plates 106 which are in a pixel array and are under an electron blocking layer 108. A respective set of a thin-film transistor 1 10 and a signal storage capacitor 1 12 is connected to each charge collection (or pixel) electrode 106. A respective source line is connected to each transistor source of a row of transistors, and a respective gate line 1 16 is connected to the gate of each transistor in a column of transistors. Source lines 1 14 are connected to respective charge amplifiers 1 18. A programmable high-voltage (H.V.) power supply 120 sets the desired voltage level at top electrode 100 (e.g. of the order of 3,000 Volts). The structure is built on a glass substrate 122.

Fig. 2 illustrates a partial cross-section of the device of Fig. 1 , and also is schematic and not to scale and may not show all elements of the panel. As illustrated, and as currently understood, x-ray energy entering photoconductor layer 104 generates electron-hole pairs, and the voltage difference between top electrode 100 and charge collection electrodes 106 directs the electrons up and the holes down. As a result, charges related to the holes collected at respective electrodes 106 are stored in signal storage capacitors 112, and are read out through charge amplifiers 108.

Fig. 3 illustrates a section through one of the pixel positions of a flat panel imager using a grid in accordance with the principles disclosed in this patent specification. Except for the grid and its support, the structure can be the same as in the current imagers commercially available from the assignee. In Fig. 3, charge collector electrode 106 has a portion that connects to a drain electrode 1 10a of transistor 1 10, and a portion that is over the channel 1 10b of the transistor and partly over a source electrode 1 10c that connects to a source pad 302 leading to signal processing circuitry (not shown). Photoconductor 104, such as a-Se, has a bottom surface immediately above electrode 106 (and may be separated therefrom by an electron blocking layer not visible in Fig. 3), and extends down into a via 300. A vertically extending, tapered wall or ridge 304 of a material such as BCP (commercially available Benzocyclobutane used in the semiconductor industry) extends up into layer 104, to a selected height above the bottom surface of the layer (e.g. to a height of the order of about 10-20 μm, or about 10-30 μm, above the major portion of electrode 106) and incorporates a gain grid 306 made of an electrically conductive material that is at or near the top of ridge or wall 304. Fig. 3 thus illustrates an example of the new technology, as seen in a vertical section through a collector plate 106 that is at a lateral end of the array of plates 106 forming the lower electrode. Collector plate 106 is not entirely flat but includes a downward via 300 to connect electrically to a drain electrode 1 10a of a thin film transistor 1 10 whose source electrode 1 10c connects to a source line 1 14 leading to a source pad 302 that in turn leads to signal processing circuitry (not shown). BCP ridge or wall 304 extends up from electrode 106 and insulating material structure that is over the thin film transistor 1 10 and includes the illustrated SiNx and BCB layers. Electrically conductive grid electrode 306 is surrounded by the BCB material of ridge 304. Ridges such as 304, with grid electrodes 306 therein, form a waffle-shaped structure. When the height of the photoconductor layer 104 is of the order of 500 micrometers (only a part of the vertical extent of layer 106 is visible in Fig. 3), the height of ridges 304 can be of the order of 10-30 micrometers, with the grid 306 being at or near the top of ridges 3-4, e.g. within a micrometer or or a few micrometers down from the top of ridge 304. The grid can be less than a micrometer thick.

In the example of Fig. 3, the bottom of the photoconductor layer 104 is considered to be at the approximate level of the tops of collector electrodes 106. Thus, the bottom of layer 104 is not continuous, because layer 104 is penetrated from below by ridges 104. In addition, small portions of layer 104 also extend further down, into vias 300, but the effect of these portions on the avalanche gain layer that is between the level of grid 306 and the top of collector plates 106 is disregarded in the discussion below for the sake of conciseness and clarity.

As illustrated in the simplified plan view of Fig. 4, walls or ridges 304 form a wafflelike structure and, in this example, surround each pixel electrode 106 on all 4 sides. Fig 4 illustrates a partial top view of a structure of the type illustrated in section in Fig. 3, and is not to scale. The view of Fig. 4 is centered on a collector plate 106 surrounded laterally by other collector plates of which only parts are visible. Ridges 304 surround the central collector plate 106 on all four sides, and extend laterally to form similar structures around the other collector plates 106, thereby forming a waffle-like structure with holes vertically aligned, but not necessarily concentric, with the collector plates 106. Grid conductors 306 thus also surround each collector plate 106 on all four sides. Fig. 5 illustrates how the grid is believed to affect the electric field in the phtoconductor layer 104. It is a section through an x-ray imager panel using the new technology, but is simplified in comparison to Fig. 3 in order to focus on a particular aspect of the new design. As earlier noted, the voltage at the upper electrode 100 is different from the voltage at the grid 306 in order to make the electric field in a lower region of the selenium-based layer 104 (e.g. the portion below grid 306) significantly higher than the electric field in an upper region of layer 104 (e.g., the region above grid 306). As a non-limiting example, the upper electrode 100 can be at 3000 Volts relative to collector plates 106, while the voltage at the grid relative to collector plates 106 can be in the range of 1000 Volts to 2000 Volts. The resulting much higher electric field in the lower region of layer 104 has two effects that are particularly significant: it directs the charges (generated in layer 104 from interaction with x-rays) toward central areas of collector plates 106, and it provides conditions favoring electrical charge avalanching at the lower region of the layer 104, and consequent increase in the signal-to-noise ratio (SNR), i.e., the electrical signal from a collector plate 106 generated in response to a given amount of x-ray energy impinging on the imager panel portion that is vertically aligned with that collector plate 106. Lines 502, created by a computer simulation of pertinent parameters, illustrate a possible distribution of the electric field in layer 104.

Fig. 6 illustrates an example in which the grid structure comprises several individual grids 304 each of which can be at a respective voltage level. In this example, four grids are used, labeled Gi, G₂, G₃ and G₄, but a greater or a lesser number of grids can be used. The example illustrated in Fig. 6 is schematic, so it may not show the actual shapes of components, and is not to scale, but is simplified to highlight relevant principles. The dimensions and materials identified in Fig. 6 pertain to a particular example and are not to be taken as a limitation on the scope of the technology disclosed here. Fig. 6 illustrates examples of the heights of the grids 304, in micrometers, above the zero level of the (main portion of) collector plates 106. The thickness (in the vertical direction) of the grids is less than the figure may suggest. The actual thickness of a grid typically is less than a micrometer. The height of the ridges 304 in the example of Fig. 6 is 25-30 micrometers, and preferably but not necessarily is in the range of 10-30 micrometers. Separate power supplies (not shown) or a resistor ladder supplied from a single power supply (not shown), can supply the appropriate voltages to the grids. The voltages shown in Fig. 6 are only an example. Different voltages can be used to accomplish the effect of controlling the electric field in the desired manner for directing the electrical charges resulting from the interaction of x-rays with layer 104 to the collector plates 106 and for creating conditions favorable for achieving avalanching and thus improving SNR. Lines 602 and 604, from a computer simulation, illustrate a possible configuration of the electric field in layer 104.

Fig. 7 illustrates a currently preferred embodiment in which ridges 304, with one or more grids 306 therein, surround each collector plate 106 and divide into four areas. As seen in Fig. 7, ridges 304 (with one or more grid electrodes 306 therein) intersect at a central area of the collector electrode 106 that is shown in the center of the figure, and other ridges 306 with grid electrode(s) therein surround the same collector electrode 106. In the example of Fig. 7, the approximate dimensions of a collector electrode 106 are 70 by 70 micrometers.

Figs. 8-13 result from computer simulation of geometry illustrated in Fig. 7, and pertain to electric field parameters related to a quadrant of a collector plate or electrode 106, when the upper electrode 100 is at 3000 Volts and the grid electrode 306 is at 1250 volts (in a configuration such as illustrated in Figs. 3 and 7.. Figs. 8-12 illustrate the electric field at a plane that is parallel to the major surface of a collector electrode 106 and is at the indicated height Z in micrometers above that major surface. The directions X and Y are in the plane of the quadrant of the plate 106, and are in micrometers. The electric field at the contours labeled A through X is in units of electric field strength. Fig. 13 pertains to the same conditions, but illustrates the field lines in a vertical section through the geometry of Figs. 8-12. While not expressly illustrated in Fig. 13, it should be clear from earlier illustrations that the grid electrode 306 is embedded and surrounded on all sides by material 304.

Fig. 14 is similar to a portion of Fig. 6 but illustrates the electrical field lines in a computer simulation of a structure that uses two grids 306a and 306b embedded in the material of ridges 304. Only a part of the vertical extent of photoconductor 104 is illustrated. The heights of grids 306a and 306b above the major surface of charge collection electrode 106 can be, for example, 16 and 21 micrometers, respectively. Top electrode 100 (not visible in Fig. 14) can be at, e.g., 3,000 Volts, grid 306 can be at, e.g., 0-2000 Volts, and grid 306b can be at, e.g., 2000 Volts. The grid voltages can be adjusted such that the electric field lines are just inside the lateral walls of ridges 304.

The new structure disclosed by way of examples above can be made using processes known in the semiconductor industry. For example, the process used to make the x- ray imager panels currently available commercially from the common assignee can be modified by introducing several processing steps just before the deposition of layer 104 over the thin film transistor panel. In particular, just before depositing layer 104, a thin layer of BCB is formed over the already completed thin film structure, in the manner known in the semiconductor industry, e.g. a BCB layer 10-30 micrometers thick. A thin layer of an electrical conductor, e.g. Cu, Al, or a metal alloy, e.g. less than a micrometer thick, if formed over the BCB layer, and a thinner (e.g. less than a couple of microns) layer of BCB is formed over the conductor layer. The

BCB/conductor structure is them patterned into the desired structure of ridges 304, with imbedded grid 306, using patterning technology from the semiconductor industry (e.g., photoresist masking and etching). If more than one grid 306 should be built into the ridges 304, the process is modified by adding another electrically conductive layer over the thin, top layer of BCB, and repeating the process if additional grids are desired, before patterning into a waffle-like structure.

The new technology disclosed by way of examples in this patent specification pertains in general to a flat panel x-ray imager for generating an electronic x-ray image, comprising: a selenium-based layer configured to locally generate electrical charges in response to x-ray exposure, said layer having laterally extending top and bottom surfaces (which need not be continuous); at least one electrically conductive grid laterally extending between an upper region and a lower region of the selenium-based layer; upper and lower electrodes at or near an upper surface and at a lower surface, respectively, of the selenium-based layer; a main power supply connected to establish a selected voltage difference between the upper and lower electrodes; and a grid power supply connected to establish a selected voltage difference between the at least one grid and at least one of the electrodes; wherein in operation of the panel to form an electronic x-ray image, the voltage differences establish a higher electrical field in the lower region than in the upper region of the selenium-based layer. The electrical field in the lower region may be sufficient to cause and avalanche gain in the lower region. The lower electrode can comprise a laterally extending array of individual collector plates, and the at least one grid can have a laterally extending array of individual openings generally aligned vertically with respective collector plates. The grid can comprise a plurality of grids vertically spaced from each other, and the grid power supply can be configured to selectively apply respective different voltages to at least two of said grids. The grid voltages can increase with distance from the bottom surface of the selenium-based layer. The at least one grid can be formed on or in a waffle-shaped support structure which extends upwardly, above the bottom of the selenium-based layer. The waffle-shaped structure can be formed by a laterally extending layer of an electrically insulating material patterned and etched to form a laterally expending array of individual openings into the insulating material layer. The at least one grid can be formed by at least one layer of a conductive material laterally extending on or in the layer of an electrically insulating material and etched together with the electrically insulating material to form an array of individual openings through the layer of electrically conductive material, thereby defining said at least one grid. The lower electrode can comprise a laterally extending array of individual collector plates substantially aligned vertically with said openings in the layer of insulating material and in the at least one grid. The new technology also comprises a method of making and operating the structure disclosed above.

The paper by Lee, et al. that is cited above is a part of this patent specification.

Claims

Claims:

1 . A flat panel x-ray imager for generating an electronic x-ray image, comprising: a selenium-based layer configured to locally generate electrical charges in response to x-ray exposure, said layer having generally laterally extending upper and lower surfaces; upper and lower electrodes at or near the upper surface and the lower surface, respectively, of the selenium-based layer; at least one electrically conductive grid laterally extending between an upper region and a lower region of the selenium-based layer; wherein a first selected voltage difference is established between the upper and lower electrodes, and a second potential difference is established between the at least one grid and at least one of the upper and lower electrodes; and wherein in operation of the panel to form an electronic x-ray image, said voltage differences establish higher electrical fields in regions of the selenium- based layer near the lower surface thereof than in regions of the selenium- based layer near the upper surface thereof.

2. A flat panel imager as in claim 1 including an array of ridges extending up into the selenium-based layer to a height above said lower surface thereof, said ridges having upper portions supporting said at least one grid

3. A flat panel imager as in claim 2 in which said ridges are formed of a material different from that of the selenium-based layer.

4. A flat panel imager as in claim 2 in which said ridges are arranged in a waffle-like array formed of a first set ridges extending in first direction and a second set of ridges extending a second direction transverse to the first direction and intersecting the first set of ridges.

5. A flat panel imager as in claim 4 in which said lower electrode is divided into an array of pixel electrodes vertically aligned with said array of ridges.

6. A flat panel imager as in claim 5 including a thin film transistor (TFT) array comprising transistors each having a source, drain and gate electrodes, said TFT array being located below said lower surface of the selenium-based layer and said pixel electrodes comprising vias extending down to make electrical contact with respective drain electrodes of said TFT array.

7. A flat panel imager as in claim 6 in which said ridges suround each of said pixel electrodes in plan view of the imager.

8. A flat panel imager as in claim 7 in which at least one of said ridges intersects each of said pixel electrodes in plan view of the imager.

9. A flat panel imager as in claim 8 in which at least two of said ridges intersect over each of said pixel electrodes in a plan view of the imager.

10. A flat panel imager as in claim 9 in which said at least one grid comprises two or more grids vertically spaced from each other, each grid being at a respective voltage level.

1 1. A flat panel imager as in claim 10 in which at least two of said grids are at respective voltage levels different from each other.

12. A flat panel imager as in claim 1 1 in which the voltage level of at least one of the grids is higher than the voltage level of a grid positioned closer to the lower surface of said selenium-based layer.

13. A flat panel imager as in claim 12 in which each of the grids is at a voltage level higher than a grid positioned closer to the lower surface of the selenium-based layer.

14. A flat panel imager as in claim 1 in which said higher electrical fields in regions of the selenium-based layer near the lower surface thereof are sufficiently high to promote an avalanche effect in portions of the selenium-based layer that are between the at least one grid and the lower electrode.

15. A flat panel imager as in claim 14 in which the electrical fields in the selenium-based layer between the at least one grid and the upper electrode are sufficiently low to discourage an avalanche effect.

16. A flat panel imager as in claim 1 in which the electric fields in portions of the selenium-based layer that are between the at least one grid and the lower electrode are at least 75 Volts per micrometer.

17. A flat panel imager as in claim 16 in which the electrical fields in portions of the selenium-based layer that are between the at least one grid and the upper electrode are of the order of 10 Volts per micrometer.

18. A flat panel imager as in claim 1 in which the at least one grid has laterally spaced openings, and the first and second potential differences are selected to force holes generated due to interactions between x-ray energy and the selenium-based material toward centers of said openings.

19. A flat panel imager as in claim 18 in which said lower electrode is divided into an array of pixel electrodes, and openings in the at least one grid are vertically aligned with said pixel electrodes.

20. A flat panel imager as in claim 19 in which more than two of said openings in the at least one grid are vertically aligned with each of said pixel electrodes.

21. A flat panel imager as in claim 20 in which at least four of said openings are vertically aligned with each of said pixel electrodes.

22. A method of imaging a distribution of x-rays comprising: providing a layer of a material which, when selectively biased electrically, directly converts locally x-ray energy incident thereon to electrical charges, said layer having generally laterally expending upper and lower surfaces; biasing the layer between an upper electrode and a lower electrode to force toward a selected one of the upper and lower electrode electrical charges of a selected polarity generated in the layer due to incident x-ray energy; and using at least one electrically conductive and selectively biased grid that is between the upper and lower surfaces of the layer to establish higher electrical fields in portions of the layer that are between the at least one grid and one of the upper and lower surfaces of the layer than in portions of the layer that are between the at least one grid and the other of said upper and lower surfaces of the layer.

23. A method as in claim 22 in which the at least one grid is much closer to one of the upper and lower surfaces of the layer than to the other, and the higher electrical fields are in portions between the at least one grid and the surface of the layer closer thereto.

24. A method as in claim 23 in which the higher electrical fields are sufficiently high to promote an avalanche effect in portions of the layer that are between the at least one grid and the surface of the layer closer thereto.

25. A method as in claim 22 in which the lower electrode is divided into pixel electrodes, the at least one grid has openings vertically aligned with the pixel electrodes, and the electrical biasing is selected to force toward centers of said openings electrical charges of a selected polarity generated due to interactions between the layer and x-rays incident thereon.

26. A method as in claim 25 in which the electrical biasing is selected to force positive charges generated in the layer toward the lower electrode, and the at least one grid is much closer to the lower electrode than to the upper electrode.

27. A method as in claim 26 in which the higher electrical fields are at least 75 Volts per micrometer.

28. A method as in claim 26 in which the at least one grid is no more than 30 micrometers above the lower electrode.

29. A method as in claim 28 in which the at least one grid comprises at least two grids vertically spaced from each other.

30. A method of imaging an object with x-rays comprising: positioning a layer of a material to receive x-rays that have passed through an object, said layer having established therein first electrical fields in a larger portion of the layer and second, higher electrical fields in a smaller portion of the layer, and said layer responding to incident x-ray energy to directly convert x-ray energy to electrical charges; said layer being biased by the first electrical field to urge charges of one polarity in one direction and charges of an opposite polarity in another direction; and said layer being conditioned by the second electrical fields to promote an avalanche effect in said smaller portion of the layer.