Analysis of an arcuate gamma camera design for transaxial reconstruction

An analysis of the characteristics of an arcuate-shaped scintillation camera design for the reconstruction of a transaxial section of a radionuclide distribution is presented. The detector consists of a NaI(T1) scintillator (in the form of a segment of a ring) coupled to collimators which converge at the centre of the ring and an array of photomultipliers on its periphery. The photomultiplier signals are analysed digitally to determine the positions of light scintillations within the crystal. Gamma-ray projection data for reconstruction are collected in incremental steps through a 360 degree rotation. The arcuate geometry is particularly suitable for transaxial reconstruction because it yields higher resolution and sensitivity than comparable planar cameras. The use of digital rather than analogue analysis of the position of a detected gamma ray is expected to further improve inherent resolution and to remove spatial distortions. For the human head, the system FWHM is expected to be about 6 mm at the extremities and 9 mm at the centre. Computer simulation techniques are used to examine attenuation phenomena and finite count statistics. Reconstructions of an elliptical phantom, which contains source and material distributions, made to simulate a head, demonstrate the mechanisms of attenuation and noise propagation. Both the spatial reconstructions and the signal-to-noise ratios are degraded by the attenuation field. These effects are more pronounced for positron than for single photon emitters. Comparison with positron systems indicate that for single photons (1) statistical degradation would be less (for the same number of detected events), (2) resolution and depth-of-field can be made comparable, (3) count-rate limitations are comparable, (4) comparative cumulative degradiation due to attenuation and the errors in its correction are probably not much different, (5) the sensitivity is lower, and (6) scatter degradation is a serious problem. (author)