Geometrical calibration and aperture configuration design in multi-pinhole SPECT

A clinical gamma camera can be converted into a high resolution SPECT system for small animal imaging, by replacing the clinical collimator(s) with pinhole collimators. However, for optimal performance, an accurate geometrical calibration is required. If it is assumed that the detector orbit is a true circle, the calibration requires the determination of seven parameters. It has been shown that these can be uniquely determined from a SPECT scan of a phantom consisting of three point sources, if two of the distances between these point sources are known. For multi-pinhole SPECT, two point sources should be sufficient, knowledge of the distance between the point sources is not required. The calibration method has been extended for cases where the orbit deviates from the ideal circle. A second interesting problem in multi- pinhole SPECT is the optimisation of the collimator design. This requires a measure of image quality, enabling objective comparison between different designs. An efficient analytical method has been developed for that purpose. With this method, it has been shown that the aperture diameter should be slightly smaller than the desired system resolution. We have also found that increased multiplexing (which comes with an apparent increase in system sensitivity) does not lead to reduced variance for a particular target resolution. In practice, avoiding all overlap seems to yield better performance.