Several commercial companies have recently introduced dual-head Anger camera systems capable of both coincidence-photon and single-photon imaging. These systems may have the potential to be an alternative to dedicated positron emission tomography (PET) systems for many community hospitals. However, the coincidence-imaging performance of these systems, in terms of system sensitivity and spatial resolution, currently falls significantly short of that of dedicated state-of-the-art PET systems. To increase the overall detection efficiency for coincidence imaging, thick NaI(Tl) crystals are being used in these multi-purpose systems. Unfortunately, thick crystals enhance parallax errors that degrade reconstructed image resolution when the dual-Anger camera system is operated in coincidence-imaging mode; thick crystals also degrade intrinsic spatial resolution performance, particularly when imaging low- energy single-photon tracers. The effects of parallax errors can be minimized if the depth of interaction (DOI) of detected gamma rays is measured. In addition we believe the same DOI information could be used to improve intrinsic spatial resolution. We propose to study the feasibility of a method to use wavelength-shifting (WLS) scintillating optical fibers, coupled in a linear array to the entrance face of a large-area NaI(Tl) crystal, to measure DOI in Anger cameras. We hypothesize that the light output distribution measured by the scintillation fibers varies with DOI: the width of the distribution broadens as the site of interaction moves farther from the entrance face. In this research, we will develop the hardware and software necessary for performing DOI measurement with WLS scintillating fibers, particularly for thick large- area NaI(Tl) crystals like those used in commercial multi-purpose dual- head Anger camera systems. We will evaluate the ability of our approach to reduce parallax errors in coincidence imaging. We also will investigate the potential use of the derived DOI information in 3-D linearity correction techniques to the improve planar intrinsic spatial resolution of Anger cameras.
