X-ray imaging system and solid state detector therefor

What is claimed is:

1. An x-ray imaging system comprising:

an x-ray source for producing an x-ray field having sufficient energy such that compton scattering and pair production have a higher combined probability of producing free electrons in silicon than the photoelectric effect; and

an x-ray detector comprising a solid state device having a plurality of layers, one of said layers including a semiconductor material with a plurality of charge storage devices, said detector having an extra absorber material for the purpose of enhancing x-ray absorption, said extra absorber material being exposed to said x-ray field to produce free electrons and being positioned sufficiently close to said semiconductor material to permit said free electrons to interact with said charge storage devices.

2. An x-ray imaging system as claimed in claim 1 wherein said detector is placed between said source and an object to be viewed such that said x-ray field passes through said detector, impinges on said object and forms an image of said object in said detector by back scatter from said object.

3. An x-ray system as claimed in claim 2 including means for subtracting an image of said source from the image produced in said detector.

4. An x-ray imaging system as claimed in claim 1 wherein said detector comprises a thin film device and wherein another of said layers comprises preprocessor circuitry connected to said charge storage devices.

5. An x-ray imaging system as claimed in claim 1 including a support housing for said detector, said support housing having means for removably receiving said detector.

6. An x-ray imaging system as claimed in claim 1 wherein said charge storage devices are part of a dynamic random access memory.

7. An x-ray imaging system as claimed in claim 1 including means for normalizing the soak times required for discharging all of the charge storage devices which are intended to discharge simultaneously.

8. An x-ray imaging system as claimed in claim 7 wherein said normalization means comprises means for storing a different normalizing factor for each of said charge storage devices.

9. An x-ray imaging system as claimed in claim 1 wherein each said charge storage device forms a single pixel of an image formed on said detector.

10. An x-ray imaging system as claimed in claim 1 including an interconnection layer for providing external connections to said detector, said interconnection layer being separate from said layer containing said charge storage devices.

11. An x-ray imaging system as claimed in claim 4 including an interconnection layer for providing external connections to said preprocessors, said interconnection layer being between said layer containing said charge storage devices and said layer containing said preprocessors.

12. An x-ray imaging system as claimed in claim 1 including means for reading analog voltages from said charge storage devices.

13. An x-ray imaging system as claimed in claim 1 wherein said extra absorber material is on one side of said layer containing said charge storage devices and a metallization layer is on an opposite side of said layer containing said charge storage devices.

14. An x-ray imaging system as claimed in claim 1 wherein said detector is an integrated circuit and the absorber material is a metallization layer of the integrated circuit.

15. An x-ray imaging system as claimed in claim 1 wherein each of said charge storage devices is at least about 50 microns on a side.

16. An x-ray imaging system as claimed in claim 1 wherein said x-ray detector is about 14 inches by 17 inches.

17. An x-ray imaging system as claimed in claim 16 wherein each charge storage device is arranged in a basic cell about 0.1 mm square.

18. An x-ray imaging system as claimed in claim 1 wherein said extra absorber material comprises an increased thickness gate oxide layer.

19. An x-ray imaging system as claimed in claim 1 wherein said detector is a dynamic random access device in which charges are stored on each of said charge storage devices and dissipated by free electrons resulting from said x-ray field.

20. An x-ray imaging system comprising:

an x-ray source for producing an x-ray field having an energy level sufficiently high that compton scattering and pair production have a higher combined probability of producing free electrons in silicon than the photoelectric effect;

an x-ray detector comprising a solid state integrated random access circuit having a semiconductor substrate, a plurality of charge storage devices, and circuit means including a plurality of conductor lines for randomly accessing each of said charge storage devices, said detector being positioned to be exposed to said x-ray field such that charges on said charge storage devices are affected by free electrons produced by said x-ray field; and

means for sensing a charge on each of said charge storage devices through said circuit means.

21. An x-ray imaging system as claimed in claim 20 wherein said detector is placed between said source and an object to be imaged such that said x-ray field passes through said detector, impinges on said object and forms an image of said object in said detector by back scatter from said object.

22. An x-ray system as claimed in claim 21 including means for subtracting an image of said source from the image produced in said detector.

23. An x-ray imaging system as claimed in claim 20 wherein said charge storage devices are divided into groups to form pixels, each pixel having a plurality of charge storage devices and means for varying the sensitivity of said charge storage devices in a single pixel to provide a gray scale.

24. An x-ray imaging system as claimed in claim 20 including a support housing for said detector, said support housing having means for removably receiving said detector.

25. An x-ray imaging system as claimed in claim 24 wherein said detector has a thickness on the order of 1/2 mm.

26. An x-ray imaging system as claimed in claim 20 wherein each of said charge storage devices is contained within a cell having a greatest dimension of approximately 10 microns.

27. An x-ray imaging system as claimed in claim 20 wherein said integrated circuit comprises a dynamic random access memory circuit including at least one sense amplifier for comparing the charge of said charge storage devices to a threshold value, and including means external to said integrated circuit for supplying said threshold value to said integrated circuit.

28. An x-ray imaging system as claimed in claim 20 wherein said circuit means comprises transistors for connecting said charge storage devices to a voltage source.

29. An x-ray imaging system as claimed in claim 20 including means for moving one of said x-ray source and said detector relative to the other by a distance approximating the distance between charge storage devices in said detector.

30. An x-ray imaging system as claimed in claim 20 including means for normalizing the soak times required for discharging all of the charge storage devices which are intended to discharge simultaneously.

31. An x-ray imaging system as claimed in claim 30 wherein said normalization means comprises means for storing a different normalizing factor for each of said cells.

32. An x-ray imaging system as claimed in claim 20 including a plurality of x-ray detectors positioned over one another such that the cells of each x-ray detector are staggered with the cells of the other x-ray detectors such that any one cell of any one x-ray detector is positioned between two cells of another x-ray detector.

33. An x-ray imaging system as claimed in claim 20 including means for exposing the detector to said source for sequential lengths of time to produce a plurality of images, and adding said images together to provide a gray scale.

34. An x-ray imaging system as claimed in claim 33 wherein the intervals between said sequential exposure times are assigned different gray scale values.

35. An x-ray imaging system as claimed in claim 23 wherein said varying means comprises different thickness oxide layers in said charge storage devices.

36. An x-ray imaging system as claimed in claim 23 wherein said varying means comprises different comparison voltage levels for determining the charge on said charge storage devices.

37. An x-ray imaging system as claimed in claim 20 wherein each said charge storage device forms a single pixel of an image formed on said detector.

38. A method of detecting an x-ray image comprising:

storing charges on a plurality of charge storage devices formed in a random access integrated circuit;

producing an x-ray field having an energy level sufficiently high that compton scattering and pair production have a combined probability of producing free electrons in silicon greater than the photoelectric effect;

exposing an object to be imaged to said x-ray field;

exposing said integrated circuit to the x-ray field from said object such that said x-ray field interacts with said integrated circuit to produce said free electrons to reduce the charge on said charge storage devices; and

sensing the charge on said charge storage devices.

39. A method according to claim 38 including producing an image having a gray scale by sequentially exposing said integrated circuit with different exposure times and digitally adding the images obtained at each of said exposure times.

40. A method according to claim 38 comprising producing a back scattered image by positioning said integrated circuit between said source and an object to be viewed, causing said x-ray field to pass through said integrated circuit and forming an image in said integrated circuit through back scattering from said object.

41. A method according to claim 38 including normalizing the outputs from said charge storage devices to compensate for different sensitivities of said charge storage devices.

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BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to x-ray imaging systems and particularly to an x-ray imaging system which utilizes a solid state x-ray detector.

2. Discussion of Related Art

Presently, x-ray imaging systems are utilized in a variety of applications, both as medical diagnostic tools and for industrial quality control. The most common form of x-ray detection resides in the use of silver halide film. However, the use of such film requires the performance of several wet, control requiring chemical developing steps. In addition, this film is expensive, thus increasing the cost of x-ray images produced in this manner.

It would be highly desirable, therefore, to produce an x-ray imaging system which does not require the use of silver halide film. Several detectors have been proposed for this purpose.

For example, U.S. Pat. No. 4,471,378 to Ng discloses a light and particle image intensifier which includes a scintillator and photocathode unit for converting incident image conveying light or charged particles to photoelectrons and a charge coupled device for detecting the photoelectrons and transmitting to data processing and video equipment information relating to the quantity or energy level as well as the location of the electrons impinging on the sensing areas of the charge couple device.

U.S. Pat. No. 4,413,280 to Adlerstein et al discloses an x-ray imaging apparatus which includes a transducer for converting incident x-radiation to a corresponding pattern of electrical charges. The charges generated by the transducer are accelerated onto an array of charge detecting or charge storing devices which store the charges in the form of an electrical signal corresponding to the charge pattern.

SUMMARY OF THE INVENTION

One object of the present invention is to provide a solid state imaging system and detector which are highly sensitive to x-radiation and can produce highly accurate x-ray images.

Another object of the present invention is to provide an x-ray imaging system and detector which can be produced by conventional solid state fabrication technology.

A further object of the present invention is to provide a solid state imaging detector which can be produced in such small sizes as to enable its use in very confined areas.

Another object of the present invention is to provide an x-ray imaging detector which can be substituted directly for x-ray film used in conventional x-ray imaging systems.

Yet another object of the present invention is provide an x-ray detector which is relatively inexpensive to fabricate so as to enable its use in fixed locations for ease of periodic x-ray analysis of mechanical structures and the like.

In accordance with the above and other objects, the present invention is an x-ray imaging system comprising an x-ray source for producing an x-ray field, and an x-ray detector. The x-ray detector comprises a solid state integrated circuit having a plurality of charge storage devices and a circuit for placing a charge on the charge storage devices. The charge storage devices are disposed in an x-ray permeable material and the detector is positioned in the x-ray field such that the charge is dissipated by secondary radiation produced by interaction of the x-ray field in the silicon substrate of the solid state integrated circuit.

The charge storage devices may be divided into groups to form pixels. Each pixel comprises one or a plurality of charge storage devices and the exposure times for discharging the charge storage devices in a single pixel can be different from one another to provide a gray scale.

In accordance with other aspects of the invention, the integrated circuit may be a dynamic random access memory.

Each charge storage device comprises a single cell of the integrated circuit. The cells are spaced from each other such that dead space exists therebetween. Also, the cells are produced in banks of 32,000 with about 1/4 mm dead space between banks. A plurality of detectors may be stacked with the cells of the detectors staggered such that each cell of one detector is positioned behind the gap between cells of another detector so as to eliminate all dead space.

The imaging system also includes processing circuitry for accessing the cells of a detector. The processing circuitry may include a system for normalizing the outputs of all of the cells to compensate for various inherent differences in radiation sensitivities of the various cells.

One of the most important aspects of the digital radiography technique employed in the present invention compared to conventional systems using silver halide film is the ability to perform quantitative radiography. This is achieved practically through image digitization and makes subtraction of radiographic images an extremely useful enhancement technique.

The x-ray image detection system according to the present invention is based on direct acquisition of digital information, utilizing solid-state silicon and hybrid detectors. An x-ray image of an object is projected directly onto the sensor without any intermediate x-ray-to-light conversion and signal magnification. Secondary electrons produced by x-ray interactions with the silicon substrate are collected and digitized using techniques similar to those employed for visible light detection.

One of the major concerns of direct x-ray sensing is designing a solid-state sensor that can withstand the radiation dose accumulation sufficiently to justify the cost of replacing the degraded detectors. The sensor must have good x-ray sensitivity compared to other systems with typical x-ray spectra (30 kVp to 200 kVp), and should have a capability of sensing a continuous large format image. To solve this problem, the sensor used in the present invention is a convention DRAM device. The cost of producing such a device is orders of magnitude less than producing other types of sensors, such as CCD and CID arrays.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects of the present invention will become more readily apparent as the invention is more clearly understood from the detailed description to follow, reference being had to the accompanying drawings in which like reference numerals represent like parts throughout, and in which:

FIG. 1 is a block diagram of the x-ray imaging system of the present invention;

FIG. 2 is a circuit diagram of an integrated circuit detector used in the imaging system of Figure 1;

FIG. 3 is an enlarged schematic showing one charge storage capacitor of the circuit diagram of FIG. 2;

FIG. 4 is a cross section of a chip showing the structure depicted schematically in FIG. 3;

FIG. 5 is a view of a portion of the detector of the present invention stacked over additional detectors to fill up the dead space between cells;

FIG. 6 is an end elevational view of the stacked detectors of FIG. 5;

FIG. 7 is a diagrammatic representation showing the system of the present invention used in place of x-ray film;

FIG. 8 is a flow diagram depicting a method of normalizing the cells of the present invention;

FIG. 9 shows the orientation of the detector and the detector leads;

FIG. 10 is a graph showing pixel logic hold time as a function of accumulated radiation exposure based on data taken at 120 kVp filtered through 0.25 mm Al with 50% of total detecting pixels discharged beyond the threshold point;

FIG. 11 is a graph showing pixel integration time as a function of accumulated exposure based on data taken at 120 kVp filtered through 0.25 mm Al;

FIG. 12 shows a thin film detector embodiment of the present invention;

FIG. 13 is a cross section of one detector of the embodiment of FIG. 12;

FIG. 14 is a circuit diagram of the detector of FIG. 12; and

FIG. 15 is a cross section showing the interconnection between the sensing layer and the preprocessor portion of the detector of FIG. 12.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 shows the x-ray system 10 to comprise a high energy x-ray source 12 and a detector 14 positioned to receive the radiation from source 12. Source 12 can be any standard high energy x-radiation source having an output in the range of 8 Kev or higher. Sources such as this are well known and manufactured by, for example, G.E. or Siemens. Alternatively, source 12 be an ultra small focal spot source such as manufactured by Ridge or Magnaflux, also having an output in the range of 8 Kev or higher. Any size focal spot source can be used. Currently, the smallest focal spot available is one micron. Also, the source 12 and detector can be placed as close to the object 0 to be x-rayed as desired due to the configuration of the detector 14, as will become readily apparent.

Detector 14 may be a dynamic random access memory such as the IS32 OpticRAM sold by Micron Technology, Inc. of Boise, Idaho. This device is an integrated circuit DRAM having 65,536 elements and is used as a solid state light sensitive detector. The Micron DRAM is specifically adapted to sense light inasmuch as there is no opaque surface covering the integrated circuit. However, any type of dynamic random access memory may be used for detector 14 as long as the covering is transparent to x-radiation. In fact, as will become apparent, any type of dynamic memory element may be used as detector 14. The memory element does not have to be a random access memory, although the use of a random access memory facilitates preprocessing and image processing routines.

The IS32 OpticRAM image sensor is a solidstate device capable of sensing an image and translating it to digital computer-compatible signals. The chip contains two arrays each of which contains 32,768 sensors arranged as 128 rows by 256 columns of sensors (4,420 microns.times.876.8 microns). Each pixel, 6.4 microns on a side, consists of two elements, a MOS capacitor and a MOS switch. The fill factor is 50 percent. The sensor is a random access device and thus, pixels may be individually accessed.

The detector 14 operates by the projection of radiation penetrating the object onto the 65,536 radiation-sensitive elements of the array-pair. Radiation striking a particular element will cause the capacitor, which is initially charged to five volts, to discharge toward zero volts. The capacitor will discharge at a rate proportional to the intensity of the radiation field to which it is exposed.

To determine whether a particular element is black or white, one can read the appropriate row and column address associated with the physical location of the element. The sensor reads the voltage value of the capacitor and performs a digital comparison between the voltage of the capacitor and a fixed externally applied threshold voltage bias. A white pixel indicates the capacitor is exposed to a radiation field sufficient to discharge the MOS capacitor below the threshold point, whereas a black pixel has not received sufficient exposure.

The output of detector 14 is passed to preprocessor 16 which serves the function of normalizing the outputs of all of the cells of detector 14. That is, the sensitivity of the cells of detector 14 will inherently vary. A normalization value can be stored in preprocessor 16 so as to normalize the output of each of the cells to ensure a field describing reading.

The normalized output of preprocessor 16 is passed to image processor 18 which manipulates the data using conventional image processing programs as well as new image processing programs which will be made possible by the present invention, such as "zoom" programs which are not currently in existence. This image can be displayed on a high resolution monitor 20, can be stored on a laser disc recorder 22, can be printed using a dry silver printer 24, or can be sent via satellite to remote image processors (not shown). A menu driven program is displayed on a computer monitor 26 prompting appropriate instructions and data which can be entered into the image processor 18 using a keyboard 28.

FIG. 2 shows a schematic diagram of a portion of a typical DRAM used in detector 14. The circuit 30 comprises a plurality of cells 32, each of which contains a memory capacitor 34 and an access transistor 36. The individual cells are accessed through left and right digit lines 38 and 40, respectively, as well as word lines 42 and 44. A sense amplifier 46 is provided in the form of a cross coupled MOSFET detector circuit. The sense amplifier 46 has nodes A and B which are coupled to the left digit line 38 and to the right digit line 40, respectively. The cells 32 are divided into a left array 50 and a right array 52. The left array 50 is accessed by the left digit line 38 and the right array 52 is accessed by the right digit line 40. The word lines 42 access the individual cells of array 50 and the word lines 44 access cells of the array 52.

A pair of equilibrate transistors 56 and 58 couple the digit lines together to allow equalization of the digit lines at the end of a refresh cycle and during the recharge state of the next cycle.

The common drains of the cross coupled sense amplifier transistors at node C are connected through an isolation transistor 60 to a pad 62 on the periphery of the integrated circuit chip. The pad 62 is bonded to one of the leads of the circuit chip package.

A pair of pull up circuits 66, 68 are coupled, respectively, to the nodes A and B. The pull up circuits 66, 68 are voltage divider circuits operable to control the voltage level of the digit lines 38 and 40.

FIG. 3 shows one cell of the circuit 30. For convenience, the cell is shown to be one of the array 52 but it could be any of the cells. As shown, the capacitor 34 has two plates 70 and 72 between which a charge is stored. Initially, the capacitor is charged by applying a high potential on word line 44 and a high potential on right digit line 40. This corresponds to a "1" state of the cell. In the presence of incident x-radiation, the charge on capacitor 34 is dissipated as will be discussed below.

With reference to FIG. 4, the portion of the integrated circuit containing the cell shown in FIG. 3 is set forth in cross section. The circuit comprises a p-type silicon substrate 80 onto which an n+ region 82 has been added. A silicon dioxide layer 84 is deposited over the substrate and n+region 82 to form an insulating layer. The lead 40 is connected to the n+ region to form the drain of transistor 34. A metal plate 86 is formed on the oxide layer 84 to form an insulated gate of transistor 36. The capacitor 34 is formed by a metal plate 72 and the interface 70 between p-type substrate 80 and oxide layer 84, which forms the other capacitor plate.

When the cell of FIGS. 3 and 4 is set to the "1" state, charge is built up on the interface 70 to charge the capacitor 34. The gate voltage is then lowered so as to discontinue communication between the drain voltage at line 40 and the capacitor 34. This charge is dissipated due to the absorbtion of x-ray photons in the substrate 80. In FIG. 4, the direction of incident x-radiation is shown by the arrow 88.

The x-radiation can produce free electrons in the substrate 80 either by photoelectric effect, Compton scattering, or pair production. However, because of the high energy of the source used in the present system, the number of electrons produced through photoelectric effect is negligible. The x-ray energy is in the range where compton scattering and pair production have the highest probabilities of producing free electrons.

It is noted that interconnections between components of a cell and between cells are provided on the oxide layer of the semi-conductor. This is indicated in FIG. 4 by showing the leads 40, 44 a 45 extending out of the oxide layer. Such leads represent the interconnections produced by the metalization layer of an integrated circuit.

Irradiation of the cell 30 from either side results in virtually all of the x-radiation being received in the substrate 80 so that the total electron production for any given energy level of radiation is achieved. The free electrons produced by interaction between the substrate and the x-radiation decrease the charge at junction 70 and thus decrease the charge on capacitor 34.

In conventional x-ray systems, compton scattering and the photoelectric effect are the relevant interactions causing the appearance of free electrons. Rayleigh scattering, a type of coherent scattering may also be responsible for the production of some free electrons. The relative occurrences of the different reactions depends on the energy of the x-ray. As discussed above, the present invention uses a high energy source. Rayleigh scattering and photoelect