Abstract:
A multi-pinhole collimator nuclear medical imaging detector divides a target object space into many non-overlapping areas and projects a minified image of each area onto a segmented detector, where each segment functions as an independent detector or imaging cell. Septa may be provided between the collimator and the detector, to physically isolate the segments.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention generally relates to nuclear medicine, and systems for obtaining nuclear medical images of a patient&#39;s body organs of interest. In particular, the present invention relates to a novel detector configuration for single photon imaging including single photon emission computed tomography (SPECT) and planar imaging. 
   2. Description of the Background Art 
   Nuclear medicine is a unique medical specialty wherein radiation is used to acquire images that show the function and anatomy of organs, bones or tissues of the body. Radiopharmaceuticals are introduced into the body, either by injection or ingestion, and are attracted to specific organs, bones or tissues of interest. Such radiopharmaceuticals produce gamma photon emissions that emanate from the body. One or more detectors are used to detect the emitted gamma photons, and the information collected from the detector(s) is processed to calculate the position of origin of the emitted photon from the source (i.e., the body organ or tissue under study). The accumulation of a large number of emitted gamma positions allows an image of the organ or tissue under study to be displayed. 
   Single photon imaging, either planar or SPECT, relies on the use of a collimator placed in front of a scintillation crystal or solid state detector, to allow only gamma rays aligned with the holes of the collimator to pass through to the detector, thus inferring the line on which the gamma emission is assumed to have occurred. Single photon imaging techniques require gamma ray detectors that calculate and store both the position of the detected gamma ray and its energy. 
   Two principal types of collimators have been used in nuclear medical imaging. The predominant type of collimation is the parallel-hole collimator. This type of collimator contains hundreds of parallel holes drilled or etched into a very dense material such as lead. The parallel-hole collimator accepts only photons traveling perpendicular to the scintillator surface, and produces a planar image of the same size as the source object. In general, the resolution of the parallel-hole collimator increases as the holes are made smaller in diameter and longer in length. 
   The conventional pinhole collimator typically is cone-shaped and has a single small hole drilled in the center of the collimator material. The pinhole collimator generates a magnified image of an object in accordance with its acceptance angle, and is primarily used in studying small organs such as the thyroid or localized objects such as a joint. The pinhole collimator must be placed at a very small distance from the object being imaged in order to achieve acceptable image quality. Pinhole collimators offer the benefit of high magnification of a single object, but lose resolution and sensitivity as the field of view (FOV) gets wider and the object is farther away from the pinhole. 
   Other known types of collimators include the slant-hole collimator, converging and diverging collimators, and the fan beam collimator. The slant-hole collimator is a variation of the parallel-hole collimator but with all holes slanted at a specific angle. This type of collimator is positioned close to the body and produces an oblique view for better visualization of an organ whose line of sight may be partially blocked by other parts of the body. The converging collimator has holes that are not parallel but instead are focused toward the organ, with the focal point being located in the center of the field of view. The image appears larger at the face of the scintillator using a converging collimator. A diverging collimator results by reversing the direction of the converging collimator. The diverging collimator is typically used to enlarge the FOV, such as would be necessary with a portable camera having a small scintillator. The fan beam collimator is typically used with a rectangular camera head to image smaller organs. The holes are parallel when viewed from one direction and converge when viewed from another direction. The fan beam collimator allows the maximum surface of the crystal to be used to capture imaging data. In most applications, the choice of collimation represents a trade-off between the size of the FOV and the sensitivity and spatial resolution required to properly visualize the target object or organ. 
   Conventional single photon imaging systems with parallel-hole collimation use large area (on the order of 2000 cm 2 ) monolithic scintillation detectors, and typically have an intrinsic spatial resolution of approximately 3.5 mm FWHM (Full Width Half Maximum). Such detectors are made either of sodium iodide crystals doped with thallium (NaI(TI)), or cesium iodide (CsI). Scintillations within the NaI crystal caused by absorption of a gamma photon within the crystal, result in the emission of a number of light photons from the crystal. The scintillations are detected by an array of photomultiplier tubes (PMTs) in close optical coupling to the crystal surface. 
   The intrinsic spatial resolution is primarily determined by the size of the PMTs. The design of the parallel-hole collimator (i.e., the length and diameter of the collimator holes) fixes the system resolution, and represents a trade-off between sensitivity (i.e., the number of detected gamma rays) and spatial resolution (i.e., sharpness of the image) of the imaged target object. The system spatial resolution is a quadrature sum of the geometric resolution of the collimator and the intrinsic resolution of the camera. In most clinical imaging studies, the predominant spatial resolution achieved is determined by the geometric resolution of the collimator, and thus there has not been a strong incentive to increase the intrinsic spatial resolution of the gamma camera. 
   Conventional commercial gamma cameras are designed to minimize cost by using the largest possible size PMTs, and thus achieve an intrinsic spatial resolution of about 3.5 mm FWHM. However, recent detector technology has enabled the design of small gamma cameras with intrinsic spatial resolution of less than 1 mm FWHM. Thus, there exists a need in the art for improvements in collimator technology to take advantage of such increased intrinsic spatial resolution in the development of new commercial gamma cameras. 
   SUMMARY OF THE INVENTION 
   The present invention solves the existing need by providing a new collimator geometry that produces increased sensitivity without sacrificing spatial resolution. 
   According to one preferred embodiment of the invention, a multi-pinhole collimator detector is provided, which divides the target object space into many non-overlapping areas and projects a minified image of each area onto a segmented detector, where each segment functions as an independent detector. 
   More specifically, according to one aspect of the invention, a nuclear medical imaging detector is provided, which includes a scintillator that interacts with radiation emanating from a target object being imaged, a pinhole collimator, having one or more pinhole apertures formed therein, provided between the target object and the scintillator, and wherein the distance between the pinhole collimator and the scintillator is selected as a function of the number of pinhole apertures provided in the collimator, such that the one or more pinhole apertures collectively project a unitary minified radiation image of the target object onto said scintillator, and one or more photosensors optically coupled to the scintillator to receive interaction events from the scintillator. 
   According to another aspect of the invention, a multi-pinhole collimator for a nuclear medical imaging device is provided, wherein the multi-pinhole collimator includes a plurality of pinhole apertures having aperture acceptance angles selected such that each pinhole aperture projects a non-overlapping area of a field of view of a target object being imaged onto a scintillator of the imaging device, so as to collectively project a unitary minified radiation image of the target object onto the scintillator. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will become more clearly understood from the following detailed description in connection with the accompanying drawings, in which: 
       FIG. 1(   a ) is a perspective view of a multi-pinhole collimator detector according to one preferred embodiment of the invention; 
       FIG. 1(   b ) is a cross-sectional view of  FIG. 1(   a ); 
       FIG. 2  shows examples of multi-pinhole collimation of gamma photons from a target organ, using varying numbers of pinholes; 
       FIG. 3(   a ) is a graph illustrating parallax error as a function of incident impinging angle, scintillator thickness and acceptance angle; and 
       FIG. 3(   b ) is a graph illustrating the mean parallax error as a function of acceptance angle. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Referring to  FIG. 1(   a ), according to one preferred embodiment of the invention, a multi-pinhole collimator detector  10  includes a pinhole plate  12  spaced apart from a scintillator  14 . The pinhole plate  12  contains a number of pinholes  12   a  formed therein. The pinhole parameters such as pinhole diameter and shape, pinhole collimator material, pinhole arrangement, number of pinholes, focal length and acceptance angle are not fixed, but are determined subject to optimization based on required system performance specifications for the particular system being designed, as will be understood by those skilled in the art. 
     FIG. 1(   b ) is a cross-sectional view of  FIG. 1(   a ) and illustrates multiple conical projections on segmented detector surfaces in segmented detector cells  18 , where the conical projections do not overlap each other. Detector segmentation can be achieved by placing septa  20  made of suitable dense material such as lead, etc. between the pinhole plate  12  and the scintillator  14 , by appropriately adjusting the pinhole acceptance angle and focal length without septa, or by adjusting the acceptance angle and focal length in addition to providing the septa. The segmented detector cell  18  acts as an independent detector and receives gamma ray projections from the partial volume of a target object. Scintillation interactions with absorbed gamma photons are then detected by photosensors  16 , which are provided for each segmented detector cell  18 . 
   The multi-pinhole collimator of the invention allows imaging of large FOVs with relatively small but high-resolution detectors, by minifying images from non-overlapping segments of the target object space and projecting them onto the segmented detector cells  18 . This is a significant feature of the invention, which is not present in previously known multi-pinhole or coded aperture solutions. 
     FIG. 2  shows examples of the imaging of a 20×20 cm 2  FOV (i.e., a total viewing area of            ·(10 cm) 2 ≈314.15 cm 2 ) by using a 100 cm 2  detector (i.e., a detector with 10 cm length and width) with 1 mm FWHM intrinsic spatial resolution, and a collimator having an array of multiple pinholes. The distance b between the center of the target object and the collimator is fixed at 10 cm, and the pinhole diameter is 1 mm. The number of pinholes determines the important tradeoff between system resolution and sensitivity as summarized in table 1.
   
     
       
             
             
             
             
             
             
             
           
             
             
             
             
             
             
             
           
         
             
               TABLE 1 
             
             
                 
             
             
               # PHS 
               θ (degree) 
               a (cm) 
               area (cm2) 
               Mag 
               Rs (mm) 
               efficiency 
             
             
                 
             
           
           
             
                 
             
           
        
         
             
               1 
               90 
               5.0 
               100 
               0.5 
               3.6 
               1.0 
             
             
               4 
               73 
               3.4 
               25 
               0.34 
               4.9 
               4.6 
             
             
               9 
               67 
               2.5 
               11.1 
               0.25 
               6.4 
               8.6 
             
             
               16 
               64 
               2.0 
               6.25 
               0.2 
               7.8 
               12.1 
             
             
               25 
               62 
               1.7 
               4 
               0.17 
               9.1 
               14.4 
             
             
                 
             
           
        
       
     
   
   As shown, for a collimator with one pinhole, the acceptance angle θ is 90 degrees and the focal length a is set to 5.0 cm, providing a magnification m =a/b=0.5. While this configuration provides a spatial resolution of 3.6 mm, the sensitivity or efficiency is only 1.0. As the number of pinholes is increased, the efficiency goes up, but the resolution becomes worse. For each configuration, the entire area of the detector is used, being segmented into separate imaging cells each receiving a non-overlapping partial volume of the target object FOV. 
   The acceptance angle of the pinhole aperture plays a key role in determining several system performance factors such as spatial resolution, sensitivity, penetration and FOV coverage. 
   There is significant loss of resolution when a gamma ray impinges on a pinhole aperture at an oblique angle as a result of the depth of interaction (DOI) effect.  FIG. 3  shows the theoretical analysis of parallax error for a CsI(TI) scintillator interacting with 150 KeV gamma photons based on equations (1) and (2) below.  FIG. 3(   a ) shows parallax error as a function of gamma ray incident angle/crystal thickness. The mean parallax error as a function of acceptance angle is shown in  FIG. 3(   b ), where a 6 mm thick CsI(TI) scintillator interacting with 150 KeV gamma photons was considered. 
   
     
       
         
           
             
               
                 
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   Where, θ is the angle of incidence relative to the normal, T is the thickness of the scintillator, μ is the mass attenuation coefficient for a given scintillator with a given gamma photon energy, and 2α is the acceptance angle of the pinhole. 
   For pinhole collimation systems using conventional gamma cameras, the DOI effects are negligible because they are overwhelmed by the relatively poor spatial resolution. However, for high resolution detectors, i.e., intrinsic spatial resolution of 2 mm or better (lower), including solid state detectors, position sensitive PMTs (PSPMTs) or photo diode array-coupled scintillator detectors, the parallax error effect on image quality becomes critical. 
   A second factor that affects spatial resolution is the penetration component of gamma rays that pass through the collimator material near the pinhole aperture. As acceptance angle increases, the path length for penetration through the collimator material lengthens, thereby decreasing the probability of transmission. In addition, as demonstrated by  Investigation of Spatial Resolution and Efficiency Using Pinholes with Small Pinhole Angle , M. B. Williams, A. V. Stolin and B. K. Kundu, IEEE TNS/MIC 2002, incorporated herein by reference in its entirety, for the small acceptance angle pinhole, the non-penetration component predominates over the penetration component (i.e., the sum of those gamma rays passing through the pinhole aperture) over the entire FOV, whereas for the large acceptance angle pinhole, the penetration component predominates over the non-penetration component. 
   The novel multi-pinhole collimator according to the present invention, with the features of segmentation of the detection surface area of the photodetector and non-overlapping partial volume projection limits the acceptance angle by design and thereby minimizes DOI error and penetration effects. 
   Alternatively, a curved scintillator plate can be used to eliminate DOI issues if appropriate methods of coupling photo sensors to the curved plate are utilized. 
   In summary, the novel multi-pinhole collimator detector according to the invention provides a high-resolution detector that is segmented into a number of small, independent imaging cells, each associated with a separate photodetector. Each segmented cell receives a conical projection of a non-overlapping partial FOV volume of a target object. The non-overlapping areas are provided by using dense material septa, and/or appropriately adjusting the acceptance angle and focal length of the pinhole apertures. Specific design parameters such as, e.g., pinhole diameter, length, shape, arrangement, number, focal length, acceptance angle and collimator material, will be determined by the design engineer based on the specifics of the system performance parameters of the system being designed. 
   The invention having been described, it will be apparent to those skilled in the art that the same may be varied in many ways without departing from the spirit and scope of the invention. Any and all such modifications are intended to be included within the scope of the following claims.