Experimental evaluation and image reconstruction based optimization of the spatially variant PSF on the ingenuity TF PET/MR scanner

The Ingenuity TF PET/MR is a newly developed hybrid scanner combining the molecular imaging capabilities of PET with the excellent soft tissue contrast of MRI. Due to the magnetic field, spatial resolution in PET could potentially be affected. As such, it is important to characterize the system's point spread function (PSF) and understand its variation under spatial transformations to guide clinical studies. An array of 12×7 printed point sources was scanned for 120 minutes to sample the spatially variant PSF with the MR magnet on and the reconstructed data (blob-based LOR-RAMLA) were fitted in image space with a double 3-D Gaussian mixture model to derive parameters of interest. Based on the analysis, the axial resolution was found to be the least variant (up to 5.7mm) followed by the tangential (up to 6.4mm) and the radial (up to 6.9mm). The radial and tangential components were found to be variant under radial transformations and almost invariant under axial transformation. However, this radial dependency was found to be minimal with the resolution degrading only by 2.5mm towards the edge of the radial FOV. Overall the PET subsystem exhibits excellent resolution characteristics mainly due to the fact that the raw data are not under-sampled/re-binned enabling the spatial resolution to be dictated by the scanner's intrinsic resolution and the image reconstruction parameters. To this extent, blob parameters used during image reconstruction could be further optimized to improve the system's resolution characteristics.