Effects of pinhole material and aperture size on lesion contrast and SNR in breast SPECT
Pinhole collimators made of depleted uranium-238 (DU) and tungsten alloy (VV) with aperture diameters from 1 - 4 mm were used to image <0.6 ml breast lesions in a pendulous breast reference frame, centered on the axis-of-rotation. with incomplete circular orbit (ICO <180 deg) SPECT. Breast, body and myocardial activity were included in the phantoms to simulate clinical imaging conditions. The background from the DU aperture was <12% contribution to the emission photopeak, and was inversely proportional to aperture size. Scans performed for clinical counting times included lesion:background ratios of 12:1 and 7:1. Contrast and signal-to-noise ratios (SNR) were better with smaller apertures, and measurements indicate that 2 - 3 mm apertures may have the best performance down to 0.11 ml lesions with near clinical uptake ratios of Tc-99m compounds. Contrast generally decreased for increasing aperture diameter, indicating that the enhanced spatial resolution may outweigh the loss in sensitivity in small lesion identification with ICO breast SPECT. Dedicated pinhole ICO SPECT of the breast appears to be a dramatically improved technique over conventional planar breast scintimammography, providing enhanced contrast and SNR for imaging smaller lesions with the dedicated high resolution pinholes.
