Molecular breast imaging-A phantom study on the impact of collimator selection on the detection of sub-10 mm breast lesions

Molecular breast imaging (MBI) with a dedicated breast gamma camera system can detect small breast lesions with a sensitivity of >85%. The objective of this study was to determine the optimal collimation for tumor detection based on count densities obtained from clinical MBI studies. Seven collimators were evaluated using a cadmium zinc telluride detector designed for breast imaging. These included LEUHR, LEHR, LEGP, LEHS, and LEUHS collimators and two system-specific collimators-a long bore (LB) and general-purpose (GP) collimator with square holes matched to the detector elements. Collimators were evaluated using a breast phantom comprising a 20×20×20 cm plastic box containing 16 glass "tumors" with internal diameters ranging from 4-10 mm. Breast thickness was set to 6 cm and tumor depth was varied from 1-5 cm. The phantom and spheres were filled with water and Tc-99m to give a tumor to background (T/B) ratio varying from 3:1 to 35:1. Total counts acquired in each image simulated the range of count densities observed clinically. Counts acquired were adjusted to compensate for differences in collimator sensitivity. Tumor signal-to-noise ratio (SNR) was measured through ROI analysis. Images acquired at clinical count densities contained significant amounts of noise, especially at T/B ratios of 10:1 or less. Highest tumor SNR was obtained with the LEHS collimator for the 6, 8, and 9 mm tumors at depths of 1 and 3 cm. At a tumor depth of 5 cm, the highest SNR was obtained with either the matched GP or LEHS collimators for the 6-9 mm tumors. Low SNR was obtained with all collimators for the 4 mm tumors at 1 and 3 cm and no 4 mm tumors were visible at a depth of 5 cm. High sensitivity collimators may be better than high-resolution collimators for detecting tumors <1 cm in low count images of the breast, especially for tumors located within 1-4 cm of the collimator face, but proper collimator design to eliminate aliasing artifacts is important for pixilated systems.