Effects of external shielding on the performance of a 1 mm3 resolution breast PET camera

We are constructing a 1 mm³ resolution, high sensitivity breast-specific PET camera, consisting out of 2 imaging heads each built out of many layers of 8×8 arrays of 1×1×1 mm³ LYSO crystals coupled to Position Sensitive Avalanche Photo-Diodes (PSAPDs). This paper investigates the effects of shielding on out-of-field background event rate from highly vascular organs like heart, liver, spleen and brain. In order to optimize the performance of the breast specific PET camera, we must address this background by proper external shielding, which reduces the single photon event rate. This avoids data acquisition saturation and reduces the amount of scatter and random coincidences, which enhances the image SNR. Shielding should ideally reduce out-of-field activity while maintaining a high sensitivity for photons emitted from the breast. Shielding design is studied using the Monte-Carlo simulation package GRAY, which supports complex mesh based primitives for phantoms and detector shapes. An anatomically accurate model of the female torso based on the realistic NURBS CArdiac Torso (NCAT) phantom is used, which was manipulated to include a slight breast compression to a width of 7.5 cm. In these simulation studies we exploit the unique feature of GRAY allowing a direct import of the camera´s CAD drawings made in Solid Works. As shielding material we use an alloy of 97% tungsten, 2.1% Ni and 0.9% Fe, which has a density of 18.5 g/cm³ and an high atomic number Z. Different shielding thicknesses and locations are investigated and their influence on the system count rate is analyzed. We show that the count rate is reduced by 45% (57 %) when placing 2.54 mm (5.08 mm) of tungsten shielding around the panels. The highest observed event rate per ASIC is 19 kHz (15 kHz), which is well below the 83 kHz maximum estimated count rate of our data acquisition system. The shielding only reduces the counts coming from the breast by 8% (- - 11%).