Many detection systems for acquiring two-dimensional projections in emission tomography (like SPECT, micro-SPECT, micro-XRF) are based on pinhole optics and a photon counting array detector. Practically all such detection systems use a standard round pinhole with cone-shaped openings on both sides to improve efficiency for inclined beams. Theoretical analysis shows that a square pinhole with pyramidal openings can improve sensitivity and efficiency of photon detection
of such detectors. Replacing a round/cone pinhole by a square/pyramidal one increases the number of counted photons in
the central part of the image by 25% and even more in the sides and in the corners of the imaging area. In the case of multi-pinhole optics, square pinhole shape allows better filling of used detector area without overlaps of partial images. Experimental comparison between round and square pinholes has been done on a micro-XRF setup. The imaging geometry parameters are identical in both cases. Performance is evaluated on efficiency and spatial resolution. The tests show significant efficiency improvement in the case of using a camera with a square pinhole shape.
Alexander Sasov, Alexander Sasov,
"Optimization of pinhole cameras for emission tomographic systems", Proc. SPIE 7078, Developments in X-Ray Tomography VI, 70781R (18 September 2008); doi: 10.1117/12.793211; https://doi.org/10.1117/12.793211