Experimental comparison of mechanical and electronic gamma-ray collimation

Abstract

Electronically collimated gamma cameras based on Compton scattering are gaining increased attention as the associated hardware and physical principles are further developed. To date, however, there has not been a direct, simultaneous experimental comparison between electronic collimation and mechanical collimation. This paper examines the relative performance of these two techniques at medium gamma-ray energies (0.1 – 1 MeV). A mechanically collimated (pinhole) camera was built and its performance was compared to an electronically collimated camera. Planar radioactive sources were imaged simultaneously by both cameras for identical periods of real time. Data are presented for several radioactive sources of various energies and shapes, including a multi-energy source. Results using an iterative image reconstruction techniques are presented. Comparing critical performance measures such as spatial resolution and efficiency for the two cameras may highlight differences between them, but does not provide an unambiguous basis for comparison. A tool from estimation theory, the resolution-variance curve, was applied to analyze their relative performance. Additionally, data from the two cameras were combined, creating a ‘dual-collimated camera’, and its resolution-variance performance was also examined. All data sets were also compared on a ‘per detected photon’ basis by reconstructing an equal number of events. Results showed that the pinhole camera performed better at the lowest energy examined, 279 keV. At 412 keV, the electronically collimated camera performed best on a ‘per detected photon’ basis, but the higher efficiency of the pinhole camera caused both cameras to have nearly identical performance when all the data was included. At 811 keV, the highest energy used, the electronically-collimated camera demonstrated superior performance. At all energies tested, the combined data set performed as well as, or better than, the best camera operating individually.