X-ray mammography represents the principal tool for breast cancer screening, but has several limitations. Due to the low specificity of X-ray mammography, many more biopsies are performed than are necessary. More than 60% of breast biopsies performed because of suspicious X-ray mammograms yield a diagnosis other than cancer. In women with X-ray dense breast tissue, X-ray mammography can miss as many as 20% of cancers. Several studies suggest that combining 99 mTc Sestamibi Scintimammography (SM) with X-ray mammography can increase the accuracy of breast imaging in selected populations. The principal limitation of prone-position scintimammography (PSM) using a standard Anger gamma camera is low sensitivity for subcentimeter cancers (i.e., stages T1a and T1b). Detecting these small cancers is extremely important clinically, since removal of the cancers at these early stages is thought to represent the best opportunity for cure. Our group constructed a high spatial resolution detector specifically dedicated to SM, the Single Photon Emission Mammography (SPEM) camera. Unlike conventional Anger gamma cameras, the SPEM camera incorporates a high spatial resolution position-sensitive photomultiplier tube, coupled to an array of scintillating crystals. The compactness of the SPEM camera allows breast compression to be implemented in a cranio-caudal view, facilitating comparison to X-ray mammograms taken in the same position. Clinical results so obtained have demonstrated increased diagnostic sensitivity in sub-centimeter tumors (80% for SPEM vs 50% with PSM). Factors contributing to this increased sensitivity include improved signal-to-noise ratio (SNR). For sub-centimeter cancers, SPEM SNR values were consistently much higher than those of PSM. Classic detectability studies have demonstrated that an SNR value >5 is required for reliable detection of cancers. For subcentimeter cancers, only the SPEM attained or exceeded this minimum threshold. The results showed that compression, which optimizes resolution by reducing lesion-to-collimator distance, and increases planar image contrast by reducing signal and attenuation from interposed normal breast tissue, plays a fundamental role in the visualization of subcentimeter breast cancers. © 2002 Elsevier Science B.V. All rights reserved.