Design, modeling and performance of a hybrid portable gamma camera.

Abstract

The combination of a mechanically-collimated gamma-ray camera with an electronically-collimated gamma camera offers both the high efficiency and good angular resolution typical in a mechanically-collimated camera for lower photon energies and the uncoupling of spatial resolution and efficiency provided by an electronically-collimated camera at higher energies. The design, construction, performance modeling and measured performance of the Hybrid Portable Gamma Camera (HPGC) are presented here. Intended for industrial use, the HPGC offers good angular resolution and efficiency over a broad energy range (50 keV to 2 MeV) by combining a MURA coded aperture camera with a Compton scatter camera in a single system. The HPGC consists of two detector modules: (1) a NaI(Tl) scintillator with Anger logic readout and (2) a CsI(Na) pixellated crystal viewed by a position-sensitive photomultiplier tube. Analytical calculations of angular resolution components and efficiency for the HPGC were compared to Monte Carlo calculations of the same quantities. The predicted angular resolution performance for on-axis point sources, a central scattering angle of 45$\sp\circ$ and a detector separation distance of 35 cm ranges from 3.5-6$\sp\circ$ FWHM over the sensitive energy range. The mechanical collimation intrinsic efficiency for energies up to 800 keV varies from 0.50 to 0.05 while the electronic collimation intrinsic efficiency for energies above 400 keV is $7.0\times10\sp{-4}$ to $5\times10\sp{-5}.$ The experimentally measured angular resolution and efficiency values show good agreement with the modeling predictions for incident energies of 412 keV and 662 keV. Although work has been done on mechanical collimation cameras and electronic collimation cameras operating independently, no truly hybrid imaging system has been constructed that uses the same gamma ray for both mechanical collimation and electronic collimation information. This dissertation compares the relative information per photon for three imaging modalities: mechanical collimation, electronic collimation and hybrid collimation. The analysis is done for point sources at two incident energies (412 keV and 662 keV) in the medium energy range of operation for the HPGC (400 keV to 800 keV) where neither mechanical collimation nor electronic collimation performs particularly well acting independently. A tool from estimation theory called resolution-variance analysis is used to compare the three modalities. Results show that hybrid collimation is superior to mechanical and electronic collimation at both 412 keV and 662 keV over the resolution range likely to be used for such a camera.