Geometric Co-Calibration of an Integrated Small Animal SPECT-CT System for Intrinsically Co-Registered Imaging

We present a method for geometric co-calibration of a single-gantry pinhole SPECT-CT small animal imaging system with axially overlapping fields-of-view. Because spatial resolution and noise level differ greatly between both sub-systems the method incorporates different strategies to obtain more accurate calibration values. Firstly, x-ray tomographic calibration is performed on a steel-sphere-phantom which includes multiple steel spheres with 1 mm in diameter, placed in alignment with a straight line. Geometric displacement for the high-resolution x-ray CT system (five parameters) is then calculated based on a mathematical analysis of those projection data. Secondly, a 360-degree pinhole SPECT-CT imaging of point sources, which are labeled with both radioactivity and x-ray absorber, is applied. With the calculated geometric parameters for the CT system the exact positions of the point sources in the CT volume can be extracted from the CT reconstruction result. Given this positional information as prior the SPECT camera geometric displacement as well as the intrinsic co-alignment between the SPECT and the CT sub-systems (six parameters) are estimated with an iterative optimization strategy (Levenberg-Marquardt). For routine imaging, the calibrated parameters are incorporated into the image reconstruction algorithms applied to the SPECT (MLEM) and CT (FDK) data, respectively, without an additional post-reconstruction registration process. Our results show intrinsically aligned images of both modalities with high spatial resolution and low level misalignment.