Investigation of efficiency and spatial resolution using pinholes with small pinhole angle

Pinhole collimators are well suited for small animal SPECT, in which high spatial resolution is required over a small field of view (FOV). Typical pinhole collimators have pinhole angles of 90° or more. If their full FOV is used, the trajectories of imaged gamma-rays can be up to 45° away from the pinhole axis, and form angles of incidence at the surface of a planar detector as large as 45°. The large incident angle produces a depth-of-interaction (DOI) blur thus reducing spatial resolution for these off-axis locations. This loss of the detector resolution is masked by the accompanying improvement of the pinhole point spread function (PSF) due to the reduced effective aperture size. The magnitude of these effects as a function of the angle between the gamma-ray trajectory and the pinhole axis depends on detector absorption efficiency, and on the physical parameters of the pinhole collimator, including pinhole type, composition, and pinhole angle. In addition, the reduced effective pinhole size results in reduced efficiency for off-axis source points. In this paper we investigate the angular dependence of these effects for pinhole collimators with different compositions and pinhole angles. Results are compared to theoretical expressions that take into account gamma-ray penetration of the pinhole material.