Design of multipinhole collimators for small animal SPECT

High resolution single photon emission computed tomography (SPECT) of small animals with pinhole collimators is hampered by poor sensitivity. For the target application of imaging the brain of an unanesthetized, unrestrained mouse, the design of multipinhole collimators for small, compact gamma cameras was studied using numerical simulations. The collimators were designed to image the mouse at different axial positions inside a tube aligned with the axis of rotation (AOR) of the SPECT system. The proposed system consisted of a 10 cm×20 cm pixelated detector with 2 cm from the AOR to the collimator mask and a focal length of 7 cm for a magnification of 3.5 on the AOR. A digital mouse phantom was created with a brain:background activity concentration ratio of 8:1. The brain region contained small disks and hot and cool cylinders for contrast and signal-to-noise ratio (SNR) quantitation. SPECT projection data were simulated at 120 angles for four pinhole masks having 1, 5 and two arrangements of 13 pinholes. Projections were scaled to model a 30 min study with doses of 0.3, 1 and 3 mCi in the mouse. Images were reconstructed with an iterative ordered subset expectation maximization (OSEM) algorithm. The peak SNR improved with multipinhole collimation compared with single pinhole imaging when there was no increased activity in the liver or bladder. With such activity, peak SNR was about the same for all collimators. For all cases peak SNR occurred at higher contrast values with multipinhole collimation. The extended axial range of multipinhole arrays enabled the mouse brain to be imaged with decreased axial blurring when the mouse was translated axially. These results suggest that mouse brains can be successfully imaged at realistic activity levels with multipinhole arrays and projection data multiplexing.