Design and characterization of a low profile NaI(Tl) gamma camera for dedicated molecular breast tomosynthesis

A new low profile gamma camera is being developed for use in a dual modality (x-ray transmission and gamma-ray emission) tomosynthesis system. Compared to the system's current gamma camera, the new camera has a larger field of view (~20×25 cm) to better match the system's x-ray detector (~23×29 cm), and is thinner (7.3 cm instead of 10.3 cm) permitting easier camera positioning near the top surface of the breast. It contains a pixelated NaI(Tl) array with a crystal pitch of 2.2 mm, which is optically coupled to a 4×5 array of Hamamatsu H8500C position sensitive photomultiplier tubes (PSPMTs). The manufacturer-provided connector board of each PSPMT was replaced with a custom designed board that a) reduces the 64 channel readout of the 8×8 electrode anode of the H8500C to 16 channels (8X and 8Y), b) performs gain non-uniformity correction, and c) reduces the height of the PSPMT-base assembly, 37.7 mm to 27.87 mm. The X and Y outputs of each module are connected in a lattice framework, and at two edges of this lattice, the X and Y outputs (32Y by 40X) are coupled to an amplifier/output board whose signals are fed via shielded ribbon cables to external ADCs. The camera uses parallel hole collimation. We describe the measured camera imaging performance, including intrinsic and extrinsic spatial resolution, detection sensitivity, uniformity of response, energy resolution for 140 keV gamma rays, and geometric linearity.