Measuring Transverse Shift Parameters for Pinhole SPECT Using Point Sources at Multiple Radii of Rotation

Accurate determination of the electronic shift and mechanical shift is very important for pinhole SPECT imaging. SPECT reconstruction can yield high-resolution images, but the electronic shift and mechanical shift must be well known. A previously discussed method for the calibration of the shifts is extended to compensate for an angular-dependent radius of rotation (ROR) and to allow for the fitting of multiple point-source scans with different source positions or different RORs. The proposed method allows for the simultaneous determination of electronic and mechanical shifts. A second method is also presented that uses averaging of point source centroid data to find the shifts. It is shown theoretically that there is a linear relationship between the average centroid in the transverse direction and the electronic shift and mechanical shift in the transverse direction. The slope and intercept of the line depend on the ROR and distance (R) of the point source from the axis of rotation. Thus, the shifts can be determined if there are multiple intersecting lines, generated by changing ROR and R. This linear picture is useful for conceptual understanding; however, its usefulness is reduced when the ROR varies with gantry angle. The theoretical formulas for slope and intercept suggest that the shifts should be more sensitive to changes in the ROR than in R. The sensitivities to changes in ROR and R were tested experimentally using both fitting methods. Results showed that both methods are more sensitive when ROR is changed instead of R, but the angular-dependent method is more robust when there are experimental imperfections.