Abstract
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Introduction: We assessed count rates in clinical FDG patient studies using different PET/CT scanners including digital and photomultiplier (PMT) -based systems. The aim was to work towards greater harmonization of data collection, despite very different scanner sensitivities and performance characteristics.
Methods: 30 patients were studied using clinical FDG whole-body protocols on three different Biograph PET/CT scanners: mCT 3 ring (mCT3R, 16.2 cm axial field-of-view, 5.6 cps/kBq sensitivity, n=10 patients), mCT 4 ring (mCT4R, 21.8 cm, 9.8 cps/kBq, n=10) and Vision 600 (Vision, 26.1 cm, 16 cps/kBq, n=10). Patients were scanned from base-of-skull to mid-thigh, arms-up, following an FDG administration of 5.55 MBq/kg. Scan duration per bed position was fixed for each scanner, based on NEMA sensitivity measurements: 20 minutes / (sensitivity / 9.8 cps/kBq) / estimated number of bed positions. Total trues (actually trues + scatter) were summed over all bed positions. The randoms fraction is the total randoms / total trues over all bed positions.
Results: The actual injected activity across all patients corresponded to 5.55 MBq/kg, exactly matching the intended weight-based formula. Mean activity was 427 ± 100 MBq. The average time between tracer injection and the start of PET data acquisition was 62 ± 4 minutes. Average scan durations for base-of-skull to mid-thigh were 27 ± 2 (mCT3R), 19.5 ± 1.6 (mCT4R) and 11.8 ± 1.1 (Vision) minutes. Total true counts were comparable between scanners: 241 ± 27 ×106 (mCT3R), 283 ± 26 ×106 (mCT4R) and 271 ± 35 ×106 (Vision). The weight-based activity formula meant that total trues did not decrease with increasing patient weight. The mean total trues for all patients below and above the median weight was 253 ± 30 ×106 and 277 ± 34 ×106, p=0.03. The randoms fractions were comparable between different scanners. Average randoms fractions were 1.53 ± 0.32 (mCT3R), 1.51 ± 0.42 (mCT4R) and 1.56 ± 0.37 (Vision). There was no significant difference between the digital (Vision, 4.7 nsec coincidence time window) and PMT-based scanners (mCT, 4.1 nsec coincidence time window), p = 0.39. The randoms fraction increased with increasing patient weight (and injected activity). The mean randoms fraction for all patients below and above the median weight was 1.29 ± 0.12 and 1.78 ± 0.35, p<0.01. Randoms fractions in this range suggest all data may have been acquired close to or slightly above the (unknown) noise equivalent count rate (NECR) peaks for each patient, irrespective of the scanner.
Conclusions: Selecting acquisition durations according to NEMA sensitivity allowed for approximately matched total true counts across different scanners. A weight-based activity formula helped maintain true counts in large patients, albeit with a higher randoms fraction. With this activity formula and a one hour uptake period, randoms exceeded trues in all cases, suggesting these data were acquired close to or above the NECR peak. These routine oncology FDG studies involving around 427 MBq (11.5 mCi) FDG were likely operating towards the upper end of each scanner’s NECR range, even with modern digital PET technology.
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