Design of an Ultra-Near-Field System for Planar Coded Aperture Nuclear Medicine Imaging

Collimators are typically used for the acquisition of nuclear medicine images. Coded apertures provide an alternative means of acquisition, and under specific conditions are associated with a signal-to-noise-ratio advantage. However, under the near-field conditions of nuclear medicine, the point spread function is no longer ideal, and near-field artifacts result. Our previous work has highlighted three potential advances, but in each case, application results in the loss of count statistics. An array of limited-field-of-view coded apertures reduces near-field artifacts, but decreases the number of counts of radioactivity that are acquired. A thin and highly transparent coded aperture allows for ease of manufacture, a potential increase in resolution, and a reduction of thickness artifacts. A greater number of counts must be acquired if the signal-to-noise-ratio is to be maintained. Coded aperture resolution can be enhanced without modification of either the open fraction of the material or the field-of-view of the system. The size of each element in the array is decreased, and the total number of elements in the coded aperture pattern is increased. However, a smaller pattern element collects fewer counts of radioactivity. The design of an ultra-near-field system is proposed, which increases counting efficiency, provided that gamma camera sensitivity is able to follow suit. The design is tested by means of a ray-tracing computer simulator, which utilizes a two-dimensional digital Shepp-Logan phantom as a distributed source. Simulation results are presented for the prior state-of-the-art, and for the proposed design, under both near-field and ultra-near-field geometries. Severe artifacts arise under ultra-near-field conditions. Nevertheless, results show that the proposed design leads to both artifact reduction and an enhancement of resolution, relative to the state-of-the-art and without adjustment of either dose or acquisition time.