Due to the limitations of conventional gamma cameras for breast imaging, many researchers are developing dedicated imagers for use in scintimammography that can be positioned closer to the breast, thereby improving spatial resolution. The purpose of this paper was to compare the performance characteristics of several dedicated gamma cameras with two different types of position sensitive photomultiplier tubes (PSPMTs), and four different NaI crystal arrays. Quantities evaluated include intrinsic spatial resolution, spatial resolution versus source-to-crystal distance, energy resolution, intrinsic nonuniformity, and system sensitivity. In order to assess the impact of changing crystal size on lesion detectability, a contrast-detail study was also performed. Our studies demonstrate that the camera with the newer PSPMTs (Hamamatsu model H8500) shows superior performance in terms of uniformity and energy resolution. The contrast-detail performance of the camera with the highest spatial resolution (1.4-mm crystal pitch with a high resolution collimator) was poor, even with relatively high input fluence. However, the use of a high-efficiency collimator significantly improves object detectability. The camera with the largest (3.2 mm) crystal pitch performed better than the others tested, with both high resolution and high efficiency collimators. However, its performance for small lesion size was better with the high-resolution collimator. Hence, based on the results of the contrast-detail study, of the cameras evaluated here, the camera with the 3.2-mm crystal pitch and the high-resolution collimator is best suited for imaging breast tumors.Index Terms-Breast imaging, scintimammography, small field of view gamma cameras.
Breast scintigraphy is a technique by which the biological properties of breast lesions can be assessed using an injected radiopharmaceutical. It may be particularly useful for women with radiographically dense breasts, in whose mammograms, lesions are often obscured by breast tissue. We are evaluating a dual modality breast scanner developed at the University of Virginia for its ability to distinguish between benign and malignant lesions. The scanner obtains a digital mammogram and a gamma ray emission image in quick succession with the breast held under mild compression, resulting in a fused image in which structures in the digital mammogram can be directly correlated with those in the scintigram. Our experience has shown that radiopharmaceutical uptake by normal breast tissue can sometimes obscure uptake by small lesions. It would therefore be advantageous to correct for this background uptake if possible. One potential way of accomplishing this is to use the information from the digital mammogram to help predict the background radiopharmaceutical distribution. With this in mind, we retrospectively investigated the degree of spatial correlation between the distribution of background activity and the distribution of radiodense breast tissue in normal breasts. Using a histogram-based analysis, we have quantified the degree of correlation in 16 images obtained from a total of 8 patients. We also used the mammographic images to quantify the radiographic density of each breast. Our results suggest that spatial correlation between areas of high radiopharmaceutical uptake and parenchymal density exists in the most dense regions of the breast for either extremely dense or heterogeneously dense breasts. High correlation was also observed for some homogeneously fatty breasts. In the latter case however, variation in breast thickness appeared to be the cause of the increased correlation. Correlation properties are approximately equal in both right and left breasts for a particular patient, except in cases exhibiting focal radiotracer uptake in a lesion. Although our preliminary results suggest that correlation between radiopharmaceutical uptake and parenchymal density exists, the number of cases thus far is too small for definitive conclusions. In addition, the planar nature of the dual modality scans imposes an inherent limitation on our ability to take into account attenuation of the emitted gamma radiation, which thus constitutes an uncontrolled variable in the correlation analysis. In principle, this problem can be eliminated by 3-dimensional imaging.
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