A modified orthogonal-distance ray-tracer method applied to dual rotation PET systems

Objective.a new projector, orthogonal-distance ray-tracer varying-full width at half maximum (OD-RT-VF), was developed to model a shift-variant elliptical point-spread function (PSF) response to improve the image quality (IQ) of a preclinical dual-rotation PET system.Approach.the OD-RT-VF projector models different FWHM values of the PSF in multiple directions, using half-height and half-width tube-of-response (ToR) values. The OD-RT-VF method's performance was evaluated against the original OD-RT method and a ToR model with constant response. The evaluation involved simulations of NEMA NU 4-2008 IQ and Derenzo phantoms, as well as a real mouse injected with [¹⁸F]-NaF scanned with the easyPET.3D system.Main results.the OD-RT-VF method demonstrated superior image resolution and uniformity (11.9% vs 15.9%) compared to the OD-RT model. In micro-derenzo phantom simulations, it resolved rods down to 1.0 mm, outperforming the other methods. For IQ phantom simulations, the OD-RT-VF projector at convergency achieved hot rods recovery coefficients ranging from 22.4% to 93.3% and lower spillover ratios in cold regions of 0.22 and 0.33 for air and water, respectively. For bone radiotracer imaging, OD-RT-VF produced clearer images of major skeletal parts, with less noise compared to OD-RT and better resolution compared to ToR projectors.Significance.the study shows that the OD-RT-VF projector method enhances PET imaging by providing better resolution, uniformity, and IQ. This model, in addition to a list-mode and GPU-based reconstruction addressing the data sparsity of dual-rotation PET geometries, unlocks their imaging potential for small animal imaging.