The scatter background in high-resolution pinhole SPECT

The authors have evaluated the scattered background in high-resolution pinhole SPECT by Monte Carlo modeling of a point source of 140 keV photons with an 8 cm diameter water-equivalent sphere as a scattering object. The detector geometry was 5 cm between center and pinhole, and 15 cm from pinhole to detector. The pinhole diameter ranged from 2 to 4 mm. For comparison, the authors have also modeled the same source and scatter geometry for a high-resolution parallel-hole SPECT detector. The detected scattered/unscattered ratio is about 0.2. This ratio and the shape of the scattered background are almost constant for both pinhole and parallel-hole geometry and over a range of pinhole diameters. The shape, though not the amplitude, of the scattered background is independent of axial motion of the source. The pinhole geometry imposes some distortion from scattering at a distance from the central plane. Some implications of these modelling results for scatter correction in pinhole SPECT are discussed