Page 100 - Quantitative Imaging of Small Tumours with Positron Emission Tomography

cohorts from the recent multicentre study (12). However, while SUV test-retest variability varied between centres, the authors did not observe a direct relationship between injected dosage and repeatability (12). This might be explained by differences in other factors determining repeatability, such as the observer variability in tumour delineation, PET system specifics, adherence to imaging protocols (i.e. uptake interval), and methods for acquiring the SUV normalization factors. Hence, count statistics did not appear to be the main determinant of [18F] FDHT repeatability, which we confirm in the current study where non-significant increases in test-retest RCs were observed after count reduction. Therefore, a potentially modifiable and important determinant of SUV variability in [18F] 4 FDHT imaging seems to be the choice of normalization factors, which, again, need some trade-off between accuracy and precision to be made. The present study contains several limitations. First, while splitting data on a count-wise basis enables evaluation of Poisson noise induced by count reduction, the 50% count scans do not fully represent a 50% shorter image acquisition. However, [18F]FDHT kinetics commonly reach a plateau after 20-30min, yielding stable SUV during the whole body acquisition (8). Second, the present study contains data acquired on a PET system of a single vendor. As between vendors the overlap between bed positions differs, count reduction might have a different impact on measurement variability for these PET systems. Also, for novel PET systems, which may have higher sensitivities and better time of flight performance, in particular for the new digital systems, the impact of reducing acquisition times on measurement variability will be even smaller. Hence, for these systems acquisition times may be reduced even further, but this remains to be investigated for each type of system. As investigated in the present study for analogue PET, a reduction up to 50% compared with current standard practice seems to be feasible for diagnostic and response assessment purposes, warranted that use of SUVmax is avoided. The current approach for evaluating the sensitivity of whole body PET- CT acquisition to scan statistics can be extended to other tracers currently being investigated and/or implemented in clinical practice, such as PSMA-ligand PET- CT. For adequate evaluation of these tracers, however, test-retest data should be available. [18F]FDHT PET repeatability 99