Count-rate analysis from clinical scans in PET with LSO detectors

The purpose of optimising the acquisition parameters in positron emission tomography is to improve the quality of the diagnostic images. Optimisation can be done by maximising the noise equivalent count rate (NECR) that in turn depends on the coincidence rate. For each bed position the scanner records coincidences and singles rates. For each patient, the true, random and scattered coincidences as functions of the single count rate(s) are determined by fitting the NEMA (National Electrical Manufacturers Association) 70 cm phantom count rate curves to measured clinical points. This enables analytical calculation of the personalised PNECR [pseudo NECR(s)] curve, linked to the NECR curve. For central bed positions, missing activity of ∼70% is estimated to get maximum PNECR (PNECRmax), but the improvement in terms of signal-toz-noise ratio would be ∼15%. The correlation between patient weight and PNECRmax is also estimated to determine the optimal scan duration of a single bed position as a function of patient weight at the same PNEC. Normalising the counts at PNECRmax for the 70 kg patient, the bed duration for a 90 kg patient should be 230 s, which is ∼30% longer. Although the analysis indicates that the fast scanner electronics allow using higher administered activities, this would involve poor improvement in terms of NECR. Instead, attending to higher bed duration for heavier patients may be more useful.