Helical pinhole SPECT for small-animal imaging: a method for addressing sampling completeness

Pinhole collimators are widely used to image small organs and small animals because sensitivity and resolution improve as the distance between the aperture and the object decreases. Axial blurring is present in reconstruction of SPECT projection data when pinhole apertures follow a circular orbit because the object is incompletely sampled. For an object with constant axial extent, the blurring worsens as the radius of rotation (ROR) decreases. In contrast, helical orbits of pinhole collimators can give complete sampling at small ROR, where sensitivity and resolution are improved. Herein, a metric of sampling completeness is introduced. It is used to evaluate the sampling of an object as a function of ROR, axial position, and radial position for circular orbits. The metric is also used to determine the completely sampled volume for a helical orbit of a pinhole aperture. Experimental and computer-simulated projections of circular orbits and helical orbits are reconstructed, yielding similar results; helical orbits reduce axial blurring because of their sampling properties.