Abstract:
A nuclear imaging apparatus includes a radiation detector comprising an array ( 18 ) of solid state detector elements ( 22 ) responsive to incident gamma radiation by emitting a current spike. A pixel correction processor ( 44 ) detects defective detector elements in the array and a flood correction circuit ( 66 ) corrects detected radiation events ( 70 ) based on sensitivity differences between a plurality of groupings of detector elements in the array. A reconstruction processor ( 76 ) reconstructs an image representation from the corrected radiation events ( 74 ).

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims the priority benefit under 35 U.S.C. §119(e) of the U.S. Provisional Application Serial No. 60/300,357, filed Jun. 23, 2001. The Provisional Application is incorporated herein by reference in its entirety. 
     INCORPORATION BY REFERENCE 
     The present application incorporates herein by reference thereto the following applications and U.S. Patents in their entireties: U.S. Application of Gagnon et al., Ser. No. 09/206,508, filed Dec. 7, 1998, now U.S. Pat. No. 6,359,279 entitled DETECTOR FOR NUCLEAR IMAGING; U.S. Application of Zeng, Ser. No. 09/708,960, filed Nov. 8, 2000, now U.S. Pat. No. 6,603,123 entitled CORRECTION FOR DEPTH-DEPENDENT SENSITIVITY IN ROTATING SLAT-HOLE GAMMA CAMERA; U.S. Application of Griesmer et al., Ser. No. 09/721,817, filed Nov. 24, 2000, now U.S. Pat. No. 6,586,744 entitled METHOD OF COOLING HIGH DENSITY ELECTRONICS; U.S. Application of Griesmer et al., Ser. No. 09/722,124, filed Nov. 24, 2000, now U.S. Pat. No. 6,472,668 entitled HIGH VOLTAGE DISTRIBUTION SYSTEM FOR CZT ARRAYS; U.S. Application of Kline et al., Ser. No. 09/722,131, filed Nov. 24, 2000, now U.S. Pat. No. 6,459,086 entitled DIGITAL PEAK DETECTOR FOR RADIATION DETECTION SYSTEM; U.S. Application of Kline et al., Ser. No. 09/722,132, filed Nov. 24, 2000, now U.S. Pat. No. 6,472,667 entitled DATA REDUCTION ARCHITECTURE FOR NUCLEAR MEDICINE IMAGER; U.S. Application of Natterer et al., Ser. No. 09/808,931, filed Mar. 15, 2001, now U.S. Publication No. 20020177773 entitled FAST TRANSFORM FOR RECONSTRUCTION OF ROTATING SLAT-HOLE DATA; U.S. Application of Zeng et al., Ser. No. 09/809,467, filed Mar. 15, 2001, now U.S. Pat. No. 6,593,576 entitled VARIABLE ANGULAR SAMPLING RATE FOR ROTATING SLAT-HOLE FOR GAMMA CAMERAS; U.S. Application of Zeng et al., Ser. No. 09/808,935, filed Mar. 15, 2001, entitled FOCUSED ROTATING SLAT-HOLE FOR GAMMA CAMERAS. Each of the above-incorporated applications is assigned or subject to assignment to the assignee of the present application. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to the art of nuclear medical imaging. It finds particular application in conjunction with rotating one-dimensional (1D) slat-collimated gamma cameras and single photon emission computed tomography (SPECT), and will be described with particular reference thereto. However, it is to be appreciated that the present invention is also amenable to other like applications and other diagnostic imaging modes, such as positron emission tomography (PET). 
     Nuclear imaging employs a source of radioactivity to image the anatomy of a subject. Typically, a radiopharmaceutical is injected into the patient. Radiopharmaceutical compounds contain a radioisotope that undergoes gamma-ray decay at a predictable rate and characteristic energy. Various scanning techniques exist in which the emitted γ-rays are detected. Based on information such as detected position and energy, the radiopharmaceutical distribution is located in the body and a representation of some feature of the subject, such as an organ, abnormality, etc., is reconstructed. 
     In a traditional Anger-type camera, the detector includes a scintillation crystal that is viewed by an array of photomultiplier tubes. The heads have collimators disposed between the crystal and the subject to limit the trajectory along which radiation can be received. Typically, the collimators are thick lead plates with an array of apertures or bores. Radiation traveling in a trajectory through one of the bores strikes the crystal; whereas radiation traveling in other trajectories hits the collimator and is absorbed. In this manner, each scintillation defines a ray, typically perpendicular to the face of the crystal, although magnifying and minifying collimators are also known. The thicker the collimator, the more accurately the ray trajectory is defined, but count efficiency or sensitivity is reduced since more radiation is absorbed in the collimator without reaching the detector. 
     Rather than using a single, large scintillator and photomultiplier tubes, others have proposed using an array of small scintillators, each associated with a photodiode or other photosensitive device which senses a scintillation in each individual scintillation crystal. Other types of individual solid-state detectors have also been suggested. 
     To improve the amount of radiation that reaches the detector, it has been proposed to use collimator sheets in a single direction across a row of detectors such that detected radiation defines a plane instead of a ray. The detectors are rotated to collect the planes at many angles. For three-dimensional images, the detector was positioned at a plurality of locations around the subject and the rotating data collection process repeated. 
     Solid state radiation detectors, such as cadmium-zinc-telluride (CZT) detectors, cadmium-telluride detectors, and the like, are also known, which utilize the photoelectric effect to detect radiation. That is, received radiation photons liberate electrons from their orbits around atoms of the target material. A high bias voltage is applied across the detector material to aid the photoelectric phenomenon and electron propagation. The electrons are detected as an electrical signal. Although very good performance can generally be expected from room-temperature CZT, sometimes a pixel is defective, for example, due to crystal impurities, crystal boundaries, electrical contacts, and other reasons. 
     In a conventional two-dimensional array, a dead pixel can hardly be tolerated and, techniques are known to avoid “holes” in the image, such as substituting the value of an adjacent pixel, substituting an “average” value of pixels neighboring the dead pixel, etc. However, such techniques degrade spatial resolution and sensitivity. In the case of pixels having poor energy resolution, their presence, too, degrades performance of the two-dimensional array, although an ill-behaved pixel is generally more tolerable than no pixel at all. 
     The process of selecting and testing CZT crystals for two-dimensional arrays adds a significant cost to an already expensive technology and might, in practice, lead to a substantial relaxation of the performance criteria. 
     The present invention provides a new and improved method and apparatus that overcome the above referenced problems and others. 
     SUMMARY OF THE INVENTION 
     In accordance with one aspect of the present invention, a nuclear imaging apparatus comprises a radiation detector including a plurality of rows of detector elements, which generate an output pulse in response to each detected radiation event. A rotor rotates the radiation detector and a plurality of summing circuits, each connected with one of the detector element rows, generate a sum of the output pulses therefrom during a sampling period. A correction circuit adjusts the sums with correction factors, each row having a preselected correction factors. A reconstruction processor reconstructs an image representation from the adjusted sums and rotational position of the detector information corresponding to each sampling period. 
     In accordance with another aspect of the present invention, a nuclear imaging apparatus includes a radiation detector comprising an array of solid state detector elements responsive to incident gamma radiation by emitting a current spike. A pixel correction processor detects defective detector elements in the array and a flood correction circuit corrects detected radiation events based on sensitivity differences between a plurality of groupings of detector elements in the array. A reconstruction processor reconstructs an image representation from the corrected radiation events. 
     In a further aspect, a method of diagnostic imaging includes exposing a solid state radiation detector to a known radiation source. The radiation detector comprises a two-dimensional array of detector elements or pixels generating a detectable signal responsive to incident gamma radiation, and each detector element comprising a distinct channel. Radiation events are detected at each detector element and defective and nondefective pixels are identified and detector element correction values are calculated to normalize the energy spectrum of each nondefective pixel. An energy window is defined based on the normalized spectra to distinguish those photons having the energy characteristic of the radiation source. Also, a weighting factor for each row of pixels is calculated to scaling each row to a nominal value when the detector is exposed to a known radiation source. A radioactive isotope is injected into a subject located in an imaging region and the detector array is rotated while detecting radiation events indicative of nuclear decay. Multiple planar projections are generated of an examination region at a plurality of angular orientations, wherein radiation events from the defective pixels is either not recorded or discarded. The detector array is moved around a longitudinal axis of the subject and the steps of rotating and detecting are repeated. The detected radiation events are collected by row and corrected with the weighting factors to generate corrected data. An image representation of the subject is reconstructed using the corrected data. 
     In yet another aspect, a method of calibrating a nuclear imaging device includes exposing a solid state radiation detector to a known radiation source. The radiation detector comprises a two-dimensional array of detector elements or pixels generating a detectable signal responsive to incident gamma radiation, and each detector element comprising a distinct channel. Radiation events are detected at each detector element and defective and nondefective pixels are identified and detector element correction values are calculated to normalize the energy spectrum of each nondefective pixel. An energy window is defined based on the normalized spectra to distinguish those photons having the energy characteristic of the radiation source. Also, a weighting factor for each row of pixels is calculated to scaling each row to a nominal value when the detector is exposed to a known radiation source. 
     Another advantage of the present invention is that a gamma camera with improved performance is provided, especially in terms of energy resolution. 
     Another advantage is that the cost of the array is substantially decreased by allowing the use of detector crystals that would ordinarily be rejected due to the presence of one or more bad pixels. 
     Another advantage of the present invention is that dead and defective pixels can be turned off to improve energy resolution without any loss of spatial resolution and only a minimal effect on sensitivity. 
     Still further benefits and advantages of the present invention will become apparent to those skilled in the art upon a reading and understanding of the preferred embodiments. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention may take form in various components and arrangements of components, and in various steps and arrangements of steps. The drawings are only for purposes of illustrating preferred embodiments and are not to be construed as limiting the invention. 
     FIG. 1 is a diagrammatic illustration of a nuclear imaging device in accordance with the present invention. 
     FIG. 2 is a block diagram illustrating the pixel correction processor in greater detail. 
     FIG. 3 is a perspective view of a detector array and collimator arrangement in accordance with the present invention. 
     FIG. 4 a flow diagram illustrating an exemplary method of the present invention. 
     FIG. 5 is an energy spectrum of a CZT detector element or pixel, which exhibits good energy resolution. 
     FIGS. 6-9 are energy spectra of CZT pixels exhibiting poor energy resolution and which are typical of pixel energy responses of pixels which are turned off in accordance with the subject invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     With reference to FIGS. 1-3, a region of interest  12  of a subject  10  is disposed in an imaging region. In the preferred embodiment, a radiopharmaceutical  14  is injected into the subject, near the region to be imaged. For example, if a physician wanted to view a blockage in an artery, the isotope would be injected into the bloodstream upstream from the blockage. As another example, the radiopharmaceutical  14  is injected into the circulatory system and its selective absorption by tissue of interest is monitored. 
     Atomic nuclei of the radioactive isotope decay over time. Energy is released at the time of decay in the form of a radiation photon, more specifically, a γ-ray of characteristic energy. 
     Many of the γ-rays produced during an imaging process are lost, propagating in useless directions. However, some of the γ-rays pass through collimators  16 , thin tungsten, lead, or other high-z vanes or septa in the preferred embodiment, and strike a detector array  18 . In the preferred embodiment, the detector array  18  includes a linear array of solid state detector elements, such as cadmium telluride crystals, cadmium zinc telluride (CZT) crystals, or other semiconductor detectors. When a γ-ray strikes the detector, it frees many electrons from their bonds to the detector material. These electrons are propelled by an applied bias voltage across the thickness of the crystal and form an electrical signal. 
     In a preferred embodiment, the linear detector array is defined by multiple two-dimensional detector arrays or tiles  20 . The collimators  16  extend in the y-direction. For imaging purposes, the detector array  18  is treated as a one-dimensional array in the direction transverse to the collimator vanes, i.e., the x-direction. That is, for acquiring image data, the detectors  22  of a single row are all sampled together as if they were a single elongated crystal for higher photon counts. As such, each row between the collimator vanes defines a plane of activity and the reconstruction is performed using plane integral reconstruction techniques as are known to persons skilled in the art. 
     The detector array  18  is mounted on a head  30  that is mounted to a gantry  32 , which is indexed around the region of interest. In one embodiment, a motor spins the detector array about a center axis  34  passing through the center of the array  18  and extending in the z-direction. In alternative embodiments, the detector array is rotated about an axis offset from the center point of the detector array radiation receiving surface. In still further embodiments, more complex spin orbits, such as a Reuleaux orbit, are used. More complex rotational movements about a longitudinal axis of the subject are also contemplated. 
     While spinning, the detectors move through parallel to a longitudinal axis  36  of the subject  10 , and 90° from the longitudinal axis  36 . A motor control  40  selects a range of motion of the detector array  18 , if any, within the rotatable gantry and the rotation of the gantry  32 . These two motions of the detector array  18 , that is, the spin orbit about its own axis, and rotation of the head  30  about a long axis of the subject  10 , give the detector array  18  a sufficient variety of views of the subject  10  to reconstruct an accurate three-dimensional image representation. 
     During detection of radiation events, the spin motion of the detector array can be continuous or performed in stepwise (step and shoot) fashion, or any combination thereof. Any translation of the detector head  30 , e.g., rotation of the head about a long axis of the subject and/or translation of the head parallel to a long axis of the subject can likewise be performed in continuous or stepwise fashion, or any combination thereof. 
     In one embodiment, the detector array  18  spins about its own center, while the head  30  remains stationary. With parallel collimators perpendicular to the array  18 , the array  18  spins 180°. It is to be understood that this value is a minimum range of rotation, enough to obtain a full set of views. The array  18  could also perform more spin rotations to increase photon counts in that position and integrate the counts over a longer duration. For parallel collimators  16  oriented in a manner other than perpendicular to the array  18 , 360° of spin rotation is performed to obtain a full view. 
     Various other spin and rotational trajectories are also contemplated. Also, the detector array can be stationarily mounted to the movable gantry, which is indexed around to region of interest. It is also not necessary that the detectors be moved or rotated with respect to the patient; relative motion may be provided by moving the patient with respect to the detectors. 
     Exemplary slat-hole scanners of the type for use in conjunction with the present invention may have characteristics as described in the incorporated copending U.S. applications referenced above. 
     In operation, the array is first calibrated on a pixel-by-pixel basis, and is again calibrated on a row-by-row basis for row uniformity or sensitivity correction. For pixel correction, the array  18  is exposed to a known radioactive flood source emitting at the desired energy level. 
     In the pixel calibration mode, the response of each pixel is individually recorded and examined. The radiation events are acquired by readout electronics  42 , which includes analog-to-digital converters, multiplexers, and so forth, as necessary so that each pixel is treated as a separate channel. The events within a wide energy window (e.g., 200% of the photopeak) are detected and analyzed by pixel correction circuit or processor  44 , the functional components of which are illustrated in FIG.  2 . 
     The pixel correction processor  44  includes a counter  46 , which sums the number of radiation events for each channel. A sufficient number of counts is to provide an accurate or reproducible depiction of each pixels energy spectrum. In an exemplary embodiment, the detector array is made up of tiles having 4×8 pixels each in the y- and x-directions, respectively. The array consists of tiled rows of 4 of the chips extending end-to-end in the y-direction, thus forming a 128-pixel array arranged in 8 rows of 16 pixels each. The array is extended in the x-direction with additional tiles in like manner, and a collimator is placed between each of the rows. In a preferred embodiment, a tiled array consists of 4 chips by 24 chips to provide a 16×192 pixel array, i.e., 192 rows of 16 pixels each. In this exemplary embodiment, which has about 3,072 pixels, about 33,000,000 counts are used for pixel correction, corresponding to about 11,000 counts per pixel. 
     In determining which pixels are “dead,” the number of counts recorded by each pixel is counted. If the number of recorded radiation events does not reach some preselected threshold value, that pixel is considered to be defective or “dead.” The threshold can be, for example, some percentage of the expected number of counts, i.e., in the range of from about 10-50% of the expected number of counts. In the described example in which 11,000 counts are expected per pixel, a preferred screening threshold, below which a pixel is considered to be dead, is about 2,000 recorded events. The addresses of those pixels that fail to reach the threshold number of counts are logged or stored in a memory  48 , and the dead pixels are turned off. This can be done by a number of methods. In the preferred embodiment, the bad pixels are eliminated by storing a gain correction factor of zero, e.g., in the gain correction table  58 . Other methods of eliminating the bad pixels are also contemplated. For example, the pixel can be turned off electronically, e.g., by disabling or disconnecting the bad pixels from the front end electronics. Also, the identified bad pixels can be eliminated by disregarding these pixels&#39; contributions at any later stage during data acquisition and processing, e.g., when culminating events in an image. 
     For each pixel that records at least the threshold number of counts, the counts are sorted according to pixel and energy, e.g., using a multichannel analyzer and a pulse height analyzer, to generate a spectrum. The spectra are examined by a spectrum analyzing circuit  50  for a prespecified degree of energy resolution. The majority of pixels will exhibit good energy resolution, as illustrated in FIG.  5 . Some pixels, however, do not exhibit a single or well-defined peak and these pixels, exhibiting poor energy resolution ability, are also turned off or discarded in accordance with the present invention. Exemplary spectra of pixels having poor energy resolution and which would be discarded in accordance with the present invention are shown in FIGS. 6-9. The pixel addresses corresponding to pixels lacking the selected energy resolution are likewise logged in the table or database  48  and disabled as described above. 
     Since the dead and poor energy pixels (herein referred to collectively as “bad” or “defective” pixels) are not used, they are not subject to any further correction. The good pixels are normalized using a pixel-by-pixel correction scheme. A baseline offset correction value is determined for each pixel when no events are occurring by an offset correction circuit  52  and pixel specific values are stored in a look-up table or other storage device  54 . Also, a gain correction factor is also calculated for each pixel using a gain correction circuit  56  and pixel specific gain correction factors are stored in a look-up table or other storage device  58 . In operation, the corresponding offset correction value from table  54  is subtracted from the channel output signal and the resultant difference is multiplied by the pixel-appropriate gain correction factor from the table  58 . 
     After the spectra are normalized, an energy window is defined. An energy window is defined based on the normalized spectra to discriminate photons having the energy characteristic of the radiation source, e.g., to eliminate scatter or noise, and so forth. Preferably, the window is defined in accordance with a prespecified energy criteria relative to the photopeak. In an especially preferred embodiment, the energy window is defined by specifying a energy width of from about 10-15% of the photopeak energy, approximately centered about the photopeak, most preferably about 10%. 
     Preferably, the pixel correction calibration is performed for multiple radioactive sources, and multiple pixel correction values and energy window definitions are stored, i.e., for each radioactive source to be used for imaging. The appropriate values are then recalled in later studies as appropriate for the radioactive source used. Alternatively, due to the linearity of the solid state system, one calibration can be used for a range of energy windows. The present invention is particularly well-suited for studies in which plural radioactive tracers and plural energy windows are used. The energy resolution that is achievable with the present invention makes it possible to use radionuclides together having energies which cannot be discriminated by conventional nuclear cameras. 
     After the pixel correction calibration is complete, a flood calibration acquisition for row sensitivity or uniformity is performed. This row-by-row calibration is performed after the pixel correction. That is, the defective pixels are turned off or their contributions otherwise disregarded, the pixel outputs are normalized using the offset and gain correction factors, and the radiation events are screened using the defined energy window. 
     The detector array  18  is exposed to a statistically significant number of photons from the radioactive flood source, the number of photons being a sufficient number to ensure consistency from one calibration to the next. Preferably, the number of photons is that which provides a variation of ±1% or better. For example, in the above described exemplary embodiment having about 3,072 pixels, the number of photons in the row uniformity calibration acquisition is about 23,000,000, that is, about 7,500 photons per pixel corresponding to about 120,000 photons per 16-segment row. 
     During the row-sensitivity calibration acquisition, pixel events are read out by the electronics  42  and corrected by the pixel correction processor  44 , i.e., using the stored offset and gain correction values. An event analyzer  62  detects radiation events falling within the defined energy window and a counter  64  sums the valid photon events by row. The row counts are stored in a buffer, archive, or other memory  70 . 
     A flood correction circuit  66  compares the number with a nominal number of counts per row, i.e., the number of counts expected based on the number of photons used in the calibration, and calculates a weighting factor for each row. The weighting factors scale each row to the nominal, and are stored in a memory  68 . For example, if the number of counts in a row has 10% fewer counts than the nominal, a weighting factor to increase the number of counts for that row by 10% is used. Conversely, a greater than the nominal number of counts might be recorded for a row, such as a row having no defective pixels and/or one or more extra-sensitive pixels. Thus, a scaling factor for each row, which reduces the number of counts to nominal, is stored in the memory  68 . In this manner, the uniformity correction compensates for the eliminated defective pixels, in addition to nonuniformity caused by sensitivity differences between the good pixels. 
     In a preferred embodiment, the number of counts from each row is further examined to ensure that it is within some preselected deviation of the nominal number of counts. A preferred maximum row sensitivity deviation is about 12% of the nominal number of counts, although values other than 12% of the nominal can be used, the image quality improving or deteriorating with count deviations which are lesser or greater, respectively. Advantageously, because the array is made up of 4×8 pixel tiles which can be rearranged, the tiles are rearranged to more uniformly distribute the dead pixels among the rows when a row having a sensitivity outside the permitted range is encountered. For example, if a particular row has so many dead pixels that it decreases the count sensitivity to a point beyond the preselected sensitivity deviation range (e.g., 12% of nominal), the tiles can be repositioned within the array to reduce the number of dead pixels in that row. The rearrangement can be performed by a number of methods. For example, a rearrangement to more uniformly distribute the eliminated pixels throughout the rows can be determined based on the locations of the bad pixels, e.g., using a computer process to effect the redistribution, or, through a manual inspection of the bad pixel locations. In a further embodiment, the flood calibration events are sorted according to pixel, and a pixel tile rearrangement that optimizes the sensitivity uniformity among the various rows is computed. 
     Although the present invention is described in reference to the preferred embodiment in which separate pixel-correction and row-sensitivity acquisitions are performed, in alternative embodiment, only a single flood acquisition is performed to generate the pixel correction factors and to define the energy window as described above. The row-sensitivity correction factors are then determined by reanalyzing the pixel correction data. The data is corrected to normalize the pixels and the data is screened using the defined energy window. 
     After the pixel correction factors, energy window definition, and the row sensitivity correction factors are determined, they are stored for later use when imaging a subject. A flow chart summarizing the above-described calibration technique is shown in FIG.  4 . 
     In an imaging mode of operation, the subject  10  to be imaged is injected with the radionuclide and multiple plane integral views from around the subject  10  are acquired to reconstruct a three-dimensional view of the imaging region  12 . The pixel signals are acquired by the readout electronics  42  and corrected for offset and gain by pixel correction function  44 . The event analyzer  62  detects valid events, i.e., events within the defined energy window. The event analyzer also sorts the events by energy in a dual- or multiple-energy study. The processor  64  sorts the valid radiation events by row and the collected row data are stored in a memory  70 . As the rotatable gantry  32  is rotated to different angular positions around the subject, a plurality of projection images from different angular orientations are collected. A multiplication function  72  multiplies the row counts by the row correction factors  68 , and the resultant products are stored in a memory  74  as corrected row counts. A reconstruction processor  76  backprojects or otherwise reconstructs the corrected data from the memory  74  into a volumetric image representation for storage in a volumetric image memory  78 . An image processor  80  under operator control withdraws selected portions of the volumetric image representation and converts them into appropriate form for display on a video monitor or other human-readable display device  82 . 
     The invention has been described with reference to the preferred embodiment. Modifications and alterations will occur to others upon a reading and understanding of the preceding detailed description. It is intended that the invention be construed as including all such modifications and alterations insofar as they come within the scope of the appended claims or the equivalents thereof.