Dependence of the coincidence aperture function of narrow BGO crystals on crystal shape and light encoding schemes.

Although the spatial resolution in positron emission tomography is improved by reducing the crystal width, the resolution becomes increasingly non-uniform towards the edges of the field of view. This is mainly due to penetration of the gamma rays through one or more crystals before detection and the inability of current systems to resolve the depth of interaction within the crystal. This paper compares crystals with straight and pointed tips with and without tungsten septa to determine the effects of pointed tips and septa on spatial resolution and efficiency. Monte Carlo calculations are compared with extensive experimental results obtained from 16 coincidence lines between two four-BGO crystal/dual PMT detector assemblies. The spectral efficiencies for the different combinations are compared. A modest improvement in signal-to-noise ratio at the maximum resolvable frequency is demonstrated when using tungsten septa and pointed crystals. The effects of the optical coding scheme used are discussed and it is shown that most coding schemes cause diminishment of spatial resolution even at the centre of the field of view, resulting from Compton scattering to adjacent crystals.

Performance study of single-slice positron emission tomography scanners by monte carlo techniques.

A Monte Carlo simulation of the gamma ray transport within a single-slice positron emission tomograph has been generated to study the effects of system parameters on performance. Included in the simulation are the radioactive source distribution, collimators, and detectors with intercrystal septa. Data are first presented to show the coincidence and singles sensitivities as a function of ring radius. Then, for a fixed radius of 26 cm, the variation of sensitivities are shown as a function of the following variables: slice thickness, patient port size, intercrystal septum dimensions, lower energy discriminator level, and coincidence fan angle. Simulation-generated sensitivity data are compared with experimental values for several tomographs andgood agreement is obtained. Discrepancies between two definitions used in experimentally determining scatter fractions are discussed. The Monte Carlo simulation shows that small radii rings have an effective count rate (quality factor) that is more than 90 percent of that for larger rings at low and moderate activity levels (=/< 0.25 muCi cm(-3)), contrary to what is predicted from analytical calculations. It is concluded that small radius rings are better suited for low dose-rate static studies, while larger radius rings are preferred for high dose-rate dynamic studies.