Accuracy of auto-calibration for pinhole micro-SPECT

We have developed a pinhole SPECT device for small animal imaging which "auto-calibrates" by acquiring calibration data from point sources simultaneously with the emission data. The entire device has integrated pinholes and is placed on the patient table of an uncollimated clinical gamma camera system with no special alignment performed and no mechanical fixtures. Four point sources are placed in grooves machined on the surface of a plastic cylinder, such that the sources are at equal radius and located at the vertices of a trapezoid. The cylinder is otherwise hollow and contains the object to be imaged. The projected point sources are tracked and their coordinates are determined as a function of projection angle. A global least squares fit is performed to estimate the sixteen geometric parameters associated with the dual detector/pin hole configuration, along with the axial locations of the sources and the wobble parameters of the cylinder. Angular sampling over 180 degrees provides highly accurate calibration. Through phantom experiments and simulations, we find that ⁵⁷Co point sources with activities as low as 2μCi per source allow for accurate calibration and robust localization of the sources in the presence of scatter from 2mCi ^99mTc. The corresponding error in image reconstruction is 0.1 mm or better. This error exhibits a statistical component depending on counts acquired and a non-statistical component of magnitude 0.03 mm.