Tumor SNR analysis in scintimammography by dedicated high contrast imager

A new gamma camera dedicated to scintimammography (single photon emission mammography-SPEM) now has a full-breast field of view. One can clinically examine a mildly compressed breast with a cranio-caudal-like projection as one would in X-ray mammography. This camera is based on pixelated scintillation arrays and position sensitive photomultiplier tubes. By reducing the collimator-tumor distance, we enhanced the geometric spatial resolution and the contrast. Unfortunately, due to the low counting rates in scintimammography, low contrast images are usually seen, particularly with small tumors. The aim of this paper is to evaluate how a camera, based on a pixelated detector, can improve the SNR values for small tumors by effectively correcting the spatial response. The procedure is based on good pixel identification. We used a small gamma camera with a metal channel dynode position sensitive photomultiplier (Hamamatsu R7600-C8) coupled to different CsI(Tl) scintillator arrays with a general purpose collimator. This type of photomultiplier drastically reduces the charge spread and improves the intrinsic characteristics of the imager. The dimensions of the CsI (Tl) matched the photomultiplier´s active area (22 × 22 mm²). Utilizing its very high intrinsic spatial resolution, we created a look up table to correct gain and spatial nonuniformities. We used a breast and torso phantom to characterize the SNR as a function of pixel size, thickness of the breast, tumor size, and depth. The data showed that the SNR depends principally on the match between the tumor and pixel size. For instance, for a 6 mm diameter tumor, the best SNRs were obtained by a 2 × 2 mm² array. For larger tumors, up to 10 mm diameter, a larger pixel 3 × 3 mm² or 4 × 4 mm², optimizes the SNR value. We compared the results of this camera with those from both a SPEM gamma camera and a standard Anger camera.