Analysis of Tri-PET Background Counts

Tri-PET is a hybrid modality having characteristics of both SPECT and PET, where a multi-pinhole collimator is used with detectors operating in coincidence mode rather than single-photon mode. When using standard clinical PET detectors, Tri-PET is capable of achieving ~1 mm spatial resolution for small animal imaging. A unique feature of Tri-PET is its ability to reject most background counts by comparing the coincidence line of response to the multi-pinhole geometry. However, some background counts remain, and it is necessary to understand the source and magnitude of the background counts. Background counts were analyzed in computer simulations of a 148-pinhole Tri-PET collimator designed for high resolution imaging of small animals in a clinical PET scanner. By modifying parameters of the simulations (tungsten vs non-penetrating collimator, realistic vs ideal detector energy resolution, no scatter medium vs mouse-size phantom), the background counts were classified according to their sources. For this configuration, 74% of Tri-PET counts were classified as true counts, and 26% were classified as background counts. The sources of the background counts, namely penetration through the bulk collimator (2 cm thickness), incorrect pinhole assignment during Tri-PET data processing, and scatter (from the collimator or mouse phantom), were found to be 20%, 5%, and 1%, respectively, of total Tri-PET counts. Background counts are at an acceptable level and may be minimized further through additional modifications in collimator design, by increasing the collimator thickness and by increasing the distance between pinholes.