Measuring the variation in radius of rotation as a function of gantry angle for ultra-high-resolution pinhole SPECT

Pinhole SPECT is sensitive to alignment and calibration. For circular orbits, it is typically assumed that the radius of rotation (ROR) is constant. Uncompensated deviation in the ROR from a constant has the potential to worsen resolution. Herein, we describe a method for determining the ROR as a function of gantry angle. This method uses a laser to serve as a reference line and a laser target mounted to the pinhole collimator to measure position. The target rotates with the collimator and is read-out at multiple angles. The two-dimensional read-out values are used to calculate the ROR as a function of angle. Experimental radionuclide SPECT projection data were acquired. The projection data were reconstructed using the angular-dependent correction and using an average ROR. The results indicate that it is difficult to distinguish the effect of this correction when the amplitude is less than about 0.5 mm. However, the correction is very pronounced for an amplitude of about 1 mm.