Motion-corrected planar projection imaging for awake and freely moving small animals

Awake and/or freely moving small animal single photon emission imaging allows the pseudo-continuous study of ¹²⁵I-labelled macromolecule kinetics, which could not otherwise be performed with systems involving restraint or anaesthesia. Estimating motion free projections in freely moving small animal planar imaging can be considered as a limited angle tomography problem (except that we wish to estimate the projections rather than the 3D volume), where the angular sampling depends on the observed motion. In this study, we hypothesise that the motion corrected planar projections estimated by reconstructing the 3D volume using an iterative reconstruction algorithm and integrating it along the projection path, will closely match the true planar distribution, regardless of the observed motion. We tested this hypothesis using 3D simulations based on a dual opposed detector system, where motion was modelled with 6 degrees of freedom (i.e. rigid body). We also investigated the quantitative accuracy of regional activity extracted from the geometric mean of opposing motion corrected planar projections. Results showed that it is feasible to accurately estimate motion-corrected projections for a wide range of motions around all 3 axes. Errors were dependent on the observed motion, as well as the surrounding activity of overlapping organs, but geometric mean estimates of regional activity were within 10% of expected values. We conclude that quantitatively accurate motion-free projections of the tracer distribution in a freely moving animal can be estimated from dual opposed detectors using MLEM-based motion correction.