Geometric calibration of a triple-head gamma-camera PET system

Rebinning or direct reconstruction of gamma-camera PET data requires that the position of each detector is known accurately for every gantry position. The position of each detector head with respect to the rotation axis is determined by three parameters: radius, tangent, and orientation. Mismeasurement in these parameters can result in degraded image resolution. It is found that a 1 mm degradation in resolution can be caused by a single head being mispositioned by 9 mm radially, or 3.5 mm tangentially, or 0.5 degrees in orientation. To compensate for inevitable misalignment, we present a software geometric calibration method to determine the detector head position for a triple-head coincidence imaging system. The technique uses a point source acquisition to find an optimal set of detector head position parameters such that the summed squared distance from the point source to all line-of-responses is minimized. Experiments show that this method works very well for simulated data where depth-of-interaction is compensated, and reasonable well for real gamma-camera PET data where depth-of-interaction is not compensated. For the later case, the technique can detect down to 1 mm radial shift and 2 mm tangential shift.