Abstract:
An inverse collimator detector for nuclear medicine imaging applications is provided. The inverse collimator detector includes an inverse collimator wherein high density, high atomic number collimator material is placed in the location where the conventional collimator has no material, and no material is placed where the conventional collimator has high density, high atomic number collimator material. The inverse collimator detector of the present invention allows significantly higher detection efficiency for incident photons while providing distance information and maintaining high resolution for isolated, small sources of radioactivity associated with molecular imaging agents.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     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.  
         [0003]     2. Description of the Background Art  
         [0004]     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.  
         [0005]     Single photon imaging, either planar or SPECT, relies on the use of a collimator placed between the source and 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.  
         [0006]     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 parallel-hole collimator offers greater sensitivity than a pinhole collimator, and its sensitivity does not depend on how closely centered the object is to the detector.  
         [0007]     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. The pinhole collimator offers the benefit of high magnification of a single object, but loses resolution and sensitivity as the field of view (FOV) gets wider and the object is farther away from the pinhole.  
         [0008]     Other known types of collimators include converging and diverging collimators. The converging collimator has holes that are not parallel; rather, the holes are focused toward the organ with the focal point being located in the center of the FOV. The image appears larger at the face of the scintillator using a converging collimator. The converging collimator has a lower sensitivity than the parallel-hole collimator, especially with thick objects.  
         [0009]     The 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 diverging collimator has a lower sensitivity than the parallel-hole collimator, especially with thick objects.  
         [0010]     The ability to image “hot spots” (i.e., small, isolated intense sources of radioactivity) has become an important imaging task in nuclear medicine. Conventional collimated nuclear medicine imaging is not designed to image small, isolated volumes of radioactivity with high resolution or in an efficient manner. It is merely intended to allow CT-like accumulation of planar or projection image data for reconstruction of large body volumes, such as the torso or the pelvis. This imaging task limits the acquisition techniques in nuclear medicine to the parallel-hole and, with corrections for distortions, converging collimation. Consequently, 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. Thus, there exists a need in the art for improvements in collimator technology to enhance the imaging of small, isolated intense sources of radioactivity through improved detection efficiency and spatial information.  
       SUMMARY OF THE INVENTION  
       [0011]     The present invention solves the existing need by providing a new collimator geometry that enhances the imaging of small, isolated intense sources of radioactivity with high resolution or in an efficient manner.  
         [0012]     According to one preferred embodiment of the present invention, an inverse collimator detector for detecting isolated, small sources of radiation is provided. The inverse collimator detector includes a scintillator that interacts with radiation emanating from a target object being imaged, and an inverse collimator having a plurality of collimation holes filled with collimation rods and a plurality of openings formed between the filled collimation holes. The inverse collimator is provided between the target object and the scintillator. Also, one or more photosensors are optically coupled to the scintillator to receive interaction events from the scintillator.  
         [0013]     According to another embodiment of the present intention, an inverse collimator is provided. The inverse collimator includes an array of collimation holes providing a path for perpendicularly incident photons, and a plurality of openings formed between the collimation holes. The collimation holes have a diameter of approximately [broadest range] and a depth of approximately [broadest range]. Also, a plurality of collimation rods are disposed within the collimation holes. The collimation rods have a diameter corresponding to the diameter of the collimation holes and a length corresponding to the depth of the collimation holes. The length of the collimation rods determines the sensitivity of the inverse collimator. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]     The accompanying drawings, which are incorporated herein and form part of the specification, illustrate various embodiments of the present invention and, together with the description, further serve to explain the principles of the invention and to enable a person skilled in the pertinent art to make and use the invention. In the drawings, like reference numbers indicate identical or functionally similar elements. A more complete appreciation of the invention and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein:  
         [0015]      FIG. 1  is a perspective view of an inverse collimator detector according to an exemplary embodiment of the present invention;  
         [0016]      FIG. 2  shows a cross-sectional view of an inverse collimator of the inverse collimator detector of  FIG. 1 ;  
         [0017]      FIG. 3A  is a schematic diagram of the inverse collimator illustrated in  FIG. 1 ;  
         [0018]      FIG. 3B  is a schematic diagram of the inverse collimator illustrated in  FIG. 2 ;  
         [0019]      FIG. 4  shows an inverse collimator in a square array (with no septa);  
         [0020]      FIG. 5  shows an inverse collimator in a hexagonal square array; and  
         [0021]     FIGS.  6 ( a )-( d ) are graphs illustrating the improvement in sensitivity and spatial resolution using the inverse collimator detector according to an exemplary embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0022]      FIG. 1  is a perspective view of an inverse collimator detector  10  according to an exemplary embodiment of the present invention. Referring to  FIG. 1 , the inverse collimator detector  10  includes an inverse collimator  12  spaced apart from a scintillator  14 . The inverse collimator detector  10  uses the inverse collimator  12  and the scintillator  14  to resolve images of small, isolated sources of radioactivity associated with molecular imaging agents.  
         [0023]     The scintillator  14  absorbs the photons that pass through the inverse collimator  12 , and converts the energy into light. The scintillator  14  can be either organic or inorganic. In the preferred embodiment, the scintillator  14  is an inorganic crystal scintillator (Csl(Na)) [is this correct?], as it is capable of detecting low energy gamma-rays. The scintillator  14  can be optically coupled to one or more photosensors (not shown), which convert the incoming light pulses into an amplified electronic signal.  
         [0024]     The inverse collimator  12  can be a plastic member or the like having an array of collimation holes  12   a  with openings  12   b  formed between the collimation holes  12   a . The inverse collimator  12  is approximately 120 mm in diameter having a thickness of approximately 5 mm [are these measurements correct? please provide the broadest range(s)].  
         [0025]     The collimation holes  12   a  can have a circular, square, hexagonal, oval or other cross-sectional shape. In the preferred embodiment, the collimation holes  12   a  have a circular cross-sectional shape. The collimation holes  12   a  are approximately 0.2 to 1.0 mm, [is this correct? what is the broadest range?] and can be arranged in a square array ( FIG. 4 ), hexagonal array ( FIG. 5 ), or the like.  
         [0026]     As illustrated in  FIG. 2 , rods or poles  12   c  can be inserted into the collimation holes  12   a  of the inverse collimator  12 . In the preferred embodiment, lead rods or poles are inserted into the collimation holes  12   a . It will be appreciated by those skilled in the art that the collimation holes  12   a  can be filled with other suitable dense material such as tungsten, copper-beryllium, [list additional material, e.g., brass (maybe?)], etc. The rods  12   c  can have a circular, square, hexagonal, oval or other cross-sectional shape that is compatible with the collimation holes  12   a , in addition to varying lengths.  
         [0027]     Septal penetration star artifact is produced when a source of radioactivity is particularly intense and the energy of the radiation is high. Generally, the “star” consists of a center and six legs (e.g., a hexagonal array collimator) corresponding to septal penetration. The legs have a significantly lower intensity than the center since they are formed through the attenuating lead. Data is used from the legs to enhance the raw acquired image.  
         [0028]     In the present invention, photons create intense star artifacts rather than faint ones. The high count sensitivity allows for sufficient statistics to be accumulated such that shape-dependent deconvolution of the star artifact can be performed. For example, a wide star artifact implies that the source is very close to the collimator surface, and a very narrow star artifact implies that the source is farther away from the collimator surface. The additional counting statistics provide an accurate determination of the star centroid, thereby giving a high degree of spatial resolution in a manner similar to, for example, Anger logic in a gamma camera.  
         [0029]     The length of the rods  12   c  determine the sensitivity of the inverse collimator  12 . For example, the longer the rods  12   c , the lower the sensitivity and the narrower the star response. Accordingly, there will be less overlap of data. The shorter the rods  12   c , the wider the star response, and there will be more overlap of data. The length of the rods  12  range from approximately [what are the broadest measurements?]. In the preferred embodiment, the length of the rods  12   c  is [what are the measurements (diameter and length)?]. The rods  12   c  do not have to be in perfect alignment, thereby limiting the size of the star artifact by the offset of the pattern.  
         [0030]     The pitch  12   b  of the rods  12   c  can be in the order of the intrinsic resolution of the camera. For example, if the pitch of the rods  12   c  is too big, then there will be too many pixels involved to give pixel-sized resolution. If the pitch of the rods  12   c  is too small, then there will be no sensitivity advantage or, alternatively, there will be penetration through the rods  12   c.    
         [0031]      FIG. 3A  is a schematic diagram of the inverse collimator  12  illustrated in  FIG. 1 , and  FIG. 3B  is a schematic diagram of the inverse collimator rods  12   c  illustrated in  FIG. 2 . Referring to  FIG. 3A , the solid circles  32  represent the rods  12   c , which are inserted into holes  12   a  as shown in  FIG. 1 ; the open circle  34  represents a path for the perpendicular photons to enter the inverse collimator detector  10 , and the lines  36  represent the 6-pointed star artifact. The slope of the star arms determines the distance from the inverse collimator  12  to the organs, bones or tissues of interest. The openings or spaces  12   b  between rods  12   c  of the inverse collimator  12  provide a path for photons (except for those that are perpendicular and hit the collimator septa) only moving perpendicular to the scintillator  14 , as illustrated in  FIG. 3B . In other words, photons traveling in all directions except those almost perpendicular to the surface of the detector are eliminated. The energy of the emitted photons as well as their location of origin are accumulated until a satisfactory image is obtained.  
         [0032]     FIGS.  6 ( a )-( d ) are graphs illustrating the improvement in the sensitivity and spatial resolution using the inverse collimator detector  10  of the present invention. [please provide a detailed explanation of the “study” including the results illustrated in FIGS.  6 ( a )-( d ).] Referring to  FIG. 6 ( d ), there is an improvement by a factor of 4.8 in the sensitivity and spatial resolution.  
         [0033]     The inverse collimator of the present invention improves sensitivity over conventional collimation in nuclear medicine by allowing more photons to be detected by the detector, and allowing more of the functioning pixels (detection elements) of the detector to contribute their imaging formation capability. Spatial resolution is maintained and enhanced by computer algorithms that deconvolve the characteristic response of the inverse collimator from raw images. Further, source-to-collimator distance information is available through image processing.  
         [0034]     The foregoing has described the principles, embodiments, and modes of operation of the present invention. However, the invention should not be construed as being limited to the particular embodiments described above, as they should be regarded as being illustrative and not as restrictive. It should be appreciated that variations may be made in those embodiments by those skilled in the art without departing from the scope of the present invention.  
         [0035]     While a preferred embodiment of the present invention has been described above, it should be understood that it has been presented by way of example only, and not limitation. Thus, the breadth and scope of the present invention should not be limited by the above described exemplary embodiment.  
         [0036]     Obviously, numerous modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that the invention may be practiced otherwise than as specifically described herein.