Comparison of circular and polygonal planar orbits for SPECT scanning

Linogram acquisitions scan along straight lines that can be as close as tangent to the Field of View (FoV); the reduced distance of approach promises to improve both sensitivity and resolution. The orbit provides truncated but complete data, however the sensitivity advantage is not certain since not all points in the object are present at all times within the FoV of the detector and also because the orientation of the plane of the aperture relative to the object is different from the reference case of a circular orbit. Herein, analytic formulas and Monte Carlo simulation are used to calculate and compare the sensitivity afforded by circular and polygonal orbits at different points in the FoV for both pinhole and slit-slat collimation. Results show that, for the same collimator and detector, for slit-slat collimation an TV-sided polygonal orbit generally provides average sensitivity lower than the circumscribed orbit. However, spatial resolution is better and can be traded for sensitivity, generally resulting in a slight advantage for the linear orbit. For a pinhole collimator the polygonal orbit always provides better sensitivity for the same resolution, but reduced axial FoV and increased axial blurring should be expected. These results depend on the polygonal orbits being tangent to the FoV: when a few millimeters of space are allowed to accommodate mechanical constraints, circular orbits are shown to provide best sensitivity at constant resolution.