Evaluation of Image Based Spatially Variant and Count Rate Dependant Point Spread Functions on the HRRT PET scanner

Spatial resolution is of high importance especially in preclinical and brain PET imaging and characterization of the scanner's resolution properties requires measuring its point spread function (PSF). In our previous work we measured the PSF in 2 PET/eT scanners using a printed point source array. Here we extend the work to accurately measure the spatial variation of the PSF on the High Resolution Research Tomograph (HRRT) and assess the impact of the scanner's depth of interaction (DOI) capability on the spatial resolution. Furthermore we characterize the dependency of the PSF to count rate. An array of 15x11 printed sources was scanned twice, initially on its own (10min scan) and subsequently with an extension line containing similar to 300MBq of carbon-11 at the scan starts (15 x 10min contiguous frames). Data were reconstructed with OP-OSEM and invariant PSF OP-OSEM. The PSF was found to be radially dependent in all directions but importantly radially symmetric and almost axially independent. The FWHM improves by similar to 1.3mm using the PSF-OP-OSEM but is still radially variable with almost 1 mm degradation within the boundaries within which the brain is typically located. When DOI was not taken into account, a degradation up to 0.7mm was seen in the radial FWHM. In terms of count rate dependency, a clear resolution deterioration was seen, for count rates above 5-10 kcps with such count rates observed during many clinical scans.