WO1999044077A1

WO1999044077A1 - Combined large area semiconductor radiation detector and imager

Info

Publication number: WO1999044077A1
Application number: PCT/US1999/004038
Authority: WO
Inventors: Jack F. Butler; Clinton L. Lingren; Bo Pi
Original assignee: Digirad Corporation
Priority date: 1998-02-25
Filing date: 1999-02-24
Publication date: 1999-09-02
Also published as: AU3310299A

Abstract

A method and system for operating a radiation detector comprising an array of detector elements (100) as a single large area radiation detector, each detector element generating an output indicative of an electrical pulse amplitude when such detector element is exposed to a radiation event, including processing each output signal indicative of an electrical pulse amplitude from each detector element within a computer (106) to generate a single summing histogram (112) representative of the sum of the output signals received from all detector elements being combined. Related computer program embodiments are also described.

Description

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COMBINED LARGE AREA SEMICONDUCTOR RADIATION DETECTOR AND IMAGER

TECHNICAL FIELD This invention relates to a system and method for operating a radiation detector comprising an array of detector elements as a single large area radiation detector.

BACKGROUND

Energy can be absorbed into a semiconductor material to induce ionization which generates electrons and holes. Since the number of pairs of electrons and holes has a dependence on the absorbed energy, the measurements of the total generated charges can be used to determine the energy. The energy may be originated from different ionizing radiation sources, including but not limited to, particulate radiation, such as alpha or beta particles, and electromagnetic radiation, such as gamma or x rays. This forms the basis of many semiconductor detectors for measuring electromagnetic energy or detecting particles.

High-resistivity semiconductor radiation detectors are widely used for detecting ionizing radiation due to their ability to operate at room temperature, their small size and durability, and other features inherent in semiconductor devices. Such detectors are used in a wide variety of applications, including medical diagnostic imaging, nuclear waste monitoring, industrial process monitoring, and space astronomy.

Many aspects of a semiconductor radiation detector may be explained by a simple planar radiation detector which is constructed by forming two planar electrodes on two opposing facets of a semiconductor crystal. The two electrodes are applied with different electrical potentials to produce an electrical field within the semiconductor crystal. This electrical field forces the radiation-induced electrons and holes to migrate in opposite directions towards the respective electrodes, i.e., the electrons to the more positive electrode and the holes to the more negative electrode. Moving electron and hole "clouds" create charge-pulse signals in external signal conditioning circuitry - 2 -

If all the electrons and holes generated by the ionizing radiation reach their respective electrodes, the output charge signal will exactly equal the charge from the energy deposited within the semiconductor crystal. Because the deposited charge is directly proportional to the energy of the ionizing radiation, the semiconductor radiation detector provides a means for measuring the energy of the ionizing radiation. The ability to measure this energy is an important function of radiation detectors.

U.S. Patent 5,677,539 teaches a radiation detector that uses a novel arrangement of electrodes that share induced charge from ionizing events in the detector, that properly shape the electric field, and that focus charge collection toward a small electrode, resulting in a significant reduction in low-energy tailing in a semiconductor detector. This patent further teaches that such detectors can be configured in a pixel array to form an imaging device that is particularly useful for radiation cameras, such as are used in industrial and medical applications. The teachings of this patent are hereby incorporated by reference. Allowed U.S. Patent Application Serial No. 08/672,831 , entitled "Semiconductor

Gamma-Ray Camera and Medical Imaging System", the teachings of which are hereby incorporated by reference, teaches an imaging system including an imaging head, a signal processor, a data acquisition system and an image processing computer. The preferred imaging head comprises a plurality of closely-packed detection modules, each detection module including a plurality of detection elements mounted to a circuit carrier. The detection elements produce electrical pulses having amplitudes indicative of the magnitude of radiation absorbed by the detection elements. The detection elements are coupled to a circuit carrier contained within the imaging head. The circuit carrier includes circuitry for conditioning and processing the signals generated by the detection elements and for preparing the processed signals for further processing by the signal processor. Each detection element has a corresponding conditioning and processing channel. Each conditioning and processing channel stores the amplitudes of the detection element electrical pulses which exceed a predetermined threshold. When a detection element absorbs sufficient radiation to produce an electrical pulse having an amplitude which exceeds the threshold, the channel associated with the detection element records - 3 - a valid detection element "event". The detection modules employ a "fall-through" circuit which automatically finds only those detection elements that have recorded a valid event. When prompted by the signal processor, the fall-through circuit searches for the next detection element and associated channel having a valid event. Upon finding the next recorded event, the detection module produces the address of the element and the amplitude of the electrical pulse which produced the valid event. The address of each detection element and the corresponding pulse amplitude are provided to the signal processor for further processing. Thus, the signal processor acquires data from the conditioning and processing channels, normalizes and formats the data, and stores it in memory blocks for access by the data acquisition system.

The data acquisition system includes hardware and software which communicate with the signal processor and the image processing computer system. The data acquisition system controls acquisition and processing of data received from the conditioning and processing channels, produces image data based upon the event data in a format that is compatible with existing imaging cameras, and transmits the data to the image processing computer. The data acquisition system also provides a mechanism for maintaining detection element event histograms and pulse-height distribution data. The data acquisition system can produce images in a standard format to allow images to be displayed using commercially available imaging systems. The image processing computer displays images based upon the signals generated by the detection elements. The image processing computer formulates images based upon the processed signals and displays the formulated images on a display device. The image processing computer provides an interface with an operator, controls data acquisition modes, receives image data from the data acquisition system, displays images in real time on a display device, and communicates with display and other readout devices. The image processing computer also provides a mechanism for adjusting operational parameters used within the imaging system.

U.S. Patent Application Serial No. 08/955,474, entitled "Cross-Strip Semiconductor Detector With Cord- Wood Construction", teaches a radiation detector array that includes cross-strip detectors formed by using a multiple-electrode configuration for each - 4 - pixel to achieve improved detection resolution and collection efficiency over conventional cross-strip detectors. A "cord-wood" construction may be used to implement a high-voltage-signal decoupling circuitry that stacks upon the signal electrode array. This configuration provides all-side buttability with minimized dead area between two adjacently interfaced detectors. The teachings of this patent application are hereby incorporated by reference.

U.S. Patent Application Serial No. 08/950,269, entitled "Semiconductor Detector With Coplanar, Orthogonal Signal Strips", teaches a radiation detector array that includes cross-strip detectors formed by using a multiple-electrode configuration for each pixel to achieve improved detection resolution and collection efficiency over conventional cross-strip detectors. In particular, two sets of mutually orthogonal signal strips are coplanar and formed on the same surface of a semiconductor. The bias electrode is a continuous conducting layer formed on the opposite side of the semiconductor. Preferably, the two sets of signal strips are at approximately the same potential relative to each other and the bias electrode is set to a different potential to effect a bias electric field within the semiconductor. More preferably, the two sets of signal strips are at a near ground potential. This cross-strip configuration eliminates the high-voltage signals present in the conventional cross-strip detectors and a bulky decoupling circuitry is no longer necessary. This significantly simplifies the signal processing and circuitry of a cross-strip detector and true all-side buttability among adjacently interfaced detectors can be achieved in forming a large detector array. The teachings of this patent application are hereby incorporated by reference.

One application of a radiation detector array that comprises a plurality of individual detector elements is to combine the outputs of groups of such detector elements to operate as a single large detector. However, simply coupling the individual signal electrodes of each detector element together to derive a single output from the detector array to amplification circuitry is often unsatisfactory. The noise, and thus the energy resolution, of the array can be degraded somewhat by increased detector capacitance. Further, the maximum count rate that can be achieved is limited by the - 5 - random nature of the signals (asynchronous events) and an inherent "dead time" that is related to the width of the gamma pulse signals.

A second approach to processing the signal from such an array is to use multiple analog amplifiers - for example, one per column or row, or one per element - in such a detector array, anc combine the outputs of the amplifiers. This approach has several drawbacks. First, any variance in gamma-ray conversion gain in the detector or amplifiers from one element (or column/row) to another will cause a broadening in the energy peak and, therefore, reduced signal resolution. Second, the broadband noise in the amplifiers will add in quadrature when the signals are combined, thus reducing energy resolution. Third, the maximum count rate that can be achieved is limited by the random nature of the signals and an inherent "dead time" that is related to the width of the gamma pulse signals.

Accordingly, the inventors have determined that a better approach is required for processing the output of a radiation detector array that combines the outputs of a plurality of detector elements so as to operate as a single large detector. The present invention provides a system and method for operating a multi-detector element gamma camera as a single large area radiation detector and/or imager.

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SUMMARY

A system and method for operating a radiation detector comprising an array of detector elements as a single large area radiation detector by processing the output signal from each detector element within a computer to generate a single histogram representa- five of the sum of the signals received from all detector elements being combined. In the preferred embodiment, the output signal from each detector element is amplified and conditioned, converted to digital form, and gain corrected before processing into a summing histogram.

More particularly, in one aspect the invention includes a method for operating a radiation detector comprising an array of detector elements as a single large area radiation detector, each detector element generating an output signal indicative of an electrical pulse amplitude when such detector element is exposed to a radiation event, including the step of processing each output signal indicative of an electrical pulse amplitude from each detector element within a computer to generate a single summing histogram representative of the sum of the output signals received from all detector elements being combined. Related system and computer program embodiments are also described.

Advantages of the invention include the following:

• The detector capacitance seen by each amplifier is not increased. • The amplifier noise does not add in quadrature.

• If gain normalization is essentially perfect, the energy resolution of the combination of detector elements should be approximately the average of the energy resolutions of the individual detector elements.

• The maximum count rate is not limited by the dead time of the amplification circuitry.

• The invention may be used in conjunction with detector element modules of the type described in allowed U.S. Patent Application Serial No. 08/672,831, which store the amplitude of each pulse until the pulse is transmitted to the computer processing system. This has the effect of completely de-randomiz- ing the pulse data, allowing synchronization with the computer system. Thus, the computer obtains data as quickly as its interface to the modules allows without concern for data loss. Data rates can be increased by increasing the clock rate of the computer or by using parallel ports or data buses.

The details of the preferred embodiment of the invention are set forth in the accompanying drawings and the description below. Once the details of the invention are known, numerous additional innovations and changes will become obvious to one skilled in the art.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a radiation detector array configured in accordance with the present invention as a large area imager.

FIG. 2 is a flowchart showing a preferred implementation of the histogram combining function of the present invention.

Like reference numbers and designations in the various drawings indicate like elements.

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DETAILED DESCRIPTION

Throughout this description, the preferred embodiment and examples shown should be considered as exemplars, rather than as limitations on the invention.

FIG. 1 is a block diagram of a radiation detector array configured in accordance with the present invention as a large area radiation detector and/or imager. An array of detector elements 100 includes at least one module that includes a plurality of individual detector elements. Several suitable detector arrays are taught in the patents and applications incorporated by reference above. The output signals from the detector elements 100 are coupled to a signal conditioner 102. The analog output of the signal conditioner is converted by an analog-to-digital converter 104 to a digital signal which is coupled to a computer processing system 106 for further processing.

The array of detector elements 100 and signal conditioner 102 may be combined in detection modules of the type described in allowed U.S. Patent Application Serial No. 08/672,831. As noted above, such detection modules store the amplitude of each detected pulse until the pulse is transmitted to the computer processing system 106. This has the effect of completely de-randomizing the pulse data, allowing synchronization with the computer processing system 106. Thus, the computer processing system 106 obtains data as quickly as its interface to such modules allows without concern for data loss. Data rates can be increased by increasing the clock rate of the computer or by using parallel ports or data buses.

As noted above, the detection elements 100 produce electrical pulses having amplitudes indicative of the magnitude of radiation absorbed therein. Preferably, each detection element has a corresponding conditioning and processing channel which stores the amplitudes of the detection element electrical pulses which exceed a predetermined threshold. When one of the detection elements 100 absorbs sufficient radiation to produce an electrical pulse having an amplitude which exceeds the threshold, the channel associated with the detection element records a valid detection element "event". The detection modules employ a "fall-through" circuit which automatically finds only those detection elements that have recorded a valid hit. When prompted by the signal conditioner 102, the fall-through circuit searches for the next detection element and associated channel having a valid event. Upon finding the next recorded event, an output signal is generated that includes the address of the detection element that generated the pulse and the amplitude of the electrical pulse which produced the valid event.

The digitized amplitude signals are preferably processed through a gain correction function 108 within the computer processing system 106, which may be a look-up table for normalizing the data to a calibration curve. Thereafter, the computer processing system 106 produces image data based upon the event data in a format that is compatible with existing imaging cameras, and preferably generates an output image or transmits the image data to an image processing computer. The computer processing system 106 also provides a mechanism for maintaining detection element event histograms and pulse-height distribution data. In accordance with the invention, a specialized histogram function is applied to the digitized amplitude signals so as to combine the outputs from a plurality of detector elements 100 and operate the system as a large area radiation detector and/or imager.

FIG. 2 is a flowchart showing a preferred implementation of the histogram combining function of the present invention. A pulse amplitude is received from a detector element having address n (STEP 200). Thereafter, in conventional fashion, an array histogram of pulse amplitudes is created for each such detector element n by adding a constant (e.g., "1") to the contents of one "bin" of a histogram associated with detector element n (STEP 202). The "bins" of each histogram represent a pulse amplitude range. The bin that gets incremented corresponds to a particular pulse energy amplitude range. For example, in a 100-element detector array, detector element #35 may generate a pulse amplitude of "5" on a scale of one to ten for a detection event. Thus, bin #5 of an array histogram associated with detector element #35 will be incremented. The array histograms are stored in conventional fashion in an array histogram memory 110 within the computer processing system 106 (FIG. 1).

A second, summing histogram is also created for the same pulse amplitude events. However, the summing histogram is generated by adding a constant (e.g., "1") to the contents of the bins of a single histogram, such that each bin is the sum of the - 10 - signals of like amplitude from all detector elements that are being combined (STEP 204). Using the example from above, detector element #35 may generate a pulse amplitude of "5" for a detection event. Thus, bin #5 of the summing histogram will be incremented. Thereafter, detector element #65 may generate a pulse amplitude of "5" for a detection event. Thus, bin #5 of the summing histogram again will be incremented. If detector element #35 again generates a pulse amplitude for a detection event, the bin of the summing histogram corresponding to that amplitude range will be incremented. The summing histogram is stored in conventional fashion in a summing histogram memory 112 within the computer processing system 106 (FIG. 1). The summing histogram data may be displayed in any desired format on a display device (STEP 206). Optionally, the array histograms for the n detector elements 100 may also be displayed as an image. In particular, an image may be formed within an image memory 114 within the computer processing system 106 (FIG. 1) by selecting a subset of histogram bins defining an "energy window" from each array histogram, adding the values of such bins, and treating the total for each detector element as a pixel value of an image. The contents of the image memory 114 may then be displayed in known fashion.

By combining the outputs from groups of detector elements 100 within a detector module, or from groups of detector modules, such as an entire gamma camera, such groups can operate as single large area detectors with high sensitivity. (Note that one camera may comprise more than one large area detector by appropriate grouping of subsets of constituent detector elements, and thus may serve as an imager with "big" pixels.) In the process of combining the detector elements, the resultant energy resolution of the detector system may be somewhat reduced, but, if properly combined in accordance with the invention, may be maintained very near the resolution of the individual detector elements.

When multiple groups of detector elements 100 are combined, a summing histogram may be generated for each group, and the collection of such summing histograms may then be output as an image in the same fashion as for array histograms. Such an output image will depict the incidence and energy of gamma ray events within - 11 - each group of the detector elements 100 as if the combined elements within each group were a single, large area detector.

Computer Implementation

Implementation of other aspects of the invention follow readily from knowledge of the basic invention. The invention may be implemented in hardware or software, or a combination of both. If the invention is implemented in software, it may be configured as one or more computer programs executing on a processor comprising at least one processor element, a data storage system (including volatile and non-volatile memory and/or storage elements), at least one input port, and at least one output port. Program code is applied to input data to perform the functions described herein and generate output information. The output information is applied to one or more output ports or devices, in known fashion.

Each such computer program is preferably stored on a storage media or device (e.g., ROM or magnetic diskette) readable by a general or special purpose programmable processor, for configuring and operating the processor when the storage media or device is read by the processor to perform the procedures described herein. The inventive system may also be considered to be implemented as a computer-readable storage medium, configured with a computer program, where the storage medium so configured causes a computer or processor to operate in a specific and predefined manner to perform the functions described herein.

A number of embodiments of the present invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. For example, a number of the steps and tests described above can be performed in different sequences and still obtain the same result. In particular, STEPS 202 and 204 in FIG. 2 can be done in any order. As another example, while a preferred form of histogram uses constants to increment histogram bins, any other manner of creating a suitable histogram of pulse amplitudes may be used in conjunction with the invention. As yet another example, the summing histogram may be used without using array histograms or without forming any type of - 12 - imaging. Accordingly, it is to be understood that the invention is not to be limited by the specific illustrated embodiment, but only by the scope of the appended claims.

Claims

- 13 - WHAT IS CLAIMED IS:

1. A method for operating a radiation detector comprising an array of detector elements as a single large area radiation detector, each detector element generating an output signal indicative of an electrical pulse amplitude when such detector element is exposed to a radiation event, including the step of:

(a) processing each output signal indicative of an electrical pulse amplitude from each detector element within a computer to generate a single summing histogram representative of the sum of the output signals received from all detector elements being combined.

2. The method of claim 1 , further including the steps of:

(a) amplifying the output signal from each detector element; and

(b) converting each amplified output signal into a digital output signal before processing into a single summing histogram.

3. The method of claim 2, further including the step of gain correcting the digital output signal before processing into a single summing histogram.

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4. A large area radiation detector comprising:

(a) an array of detector elements, each detector element generating an output signal indicative of an electrical pulse amplitude when such detector element is exposed to a radiation event; (b) a signal processor, coupled to the array of detector elements, for processing each output signal indicative of an electrical pulse amplitude from each detector element to generate a single summing histogram representative of the sum of the output signals received from all detector elements being combined.

5. The large area radiation detector of claim 4, further including: (a) at least one amplifier, coupled to the array of detector elements, for amplifying the output signal from each detector element; and

(b) at least one analog-to-digital converter, each coupled to one of such amplifiers, for converting each amplified output signal into a digital output signal before processing into a single summing histogram.

6. The large area radiation detector of claim 5, further including means for gain correcting the digital output signal before processing into a single summing histogram.

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7. A computer program, residing on a computer-readable medium, for operating a radiation detector comprising an array of detector elements as a single large area radiation detector, each detector element generating an output signal indicative of an electrical pulse amplitude when such detector element is exposed to a radiation event, comprising instructions for causing a computer to:

(a) process each output signal indicative of an electrical pulse amplitude from each detector element to generate a single summing histogram representative of the sum of the output signals received from all detector elements being combined.

8. The computer program of claim 7, further including instructions for causing the computer to output selected histogram bins from a plurality of summing histograms as an image.