Small target repeatability of 68Ga and 18F: effects of target concentration and imaging time on SUV measurements in clinically relevant phantoms.

Quantification of tumor uptake using PET imaging is important for the evaluation of therapy response. For ¹⁸F FDG PET scans, a change in uptake of 25% is commonly considered significant. For scans using novel radiopharmaceuticals, the threshold of significance is unclear. Factors including imaging time, tumor size, activity concentration, and radiopharmaceutical may affect the repeatablity of uptake metrics. This work evaluates the effect of these parameters on the repeatablity of maximum SUV (SUV_max) and mean SUV (SUV_mean) in phantoms using ¹⁸F and ⁶⁸Ga. An Esser PET phantom (Data Spectrum, Durham NC) was scanned on a Biograph Horizon PET/CT scanner (Siemens Medical Solutions, Malvern PA) using ¹⁸F and ⁶⁸Ga. Data were acquired for 5 minutes with reconstructions between 0.5-5 minutes. The background activity mimicked clinical scans with target-to-background (T/B) ratios from 1.7-19.8. The SUV_max and SUV_mean were measured for 5 slices. The mean, standard deviation, and coefficient of variation (COV) were calculated. The effects of radionuclide, imaging time, activity concentration, and target size on COV were evaluated using multivariate gamma regressions. COV for ⁶⁸Ga was 40% higher and 54% higher on average than for ¹⁸F for SUV_max and SUV_mean, respectively. Decreased lesion size, imaging time, and activity concentration were significantly associated with increased COV for both metrics (P < 0.001). COV was substantially reduced at high T/B for ⁶⁸Ga. At the highest T/B the COV for SUV_max and SUV_mean was within the typical range seen for ¹⁸F. COV is relatively high for small targets (8 mm) but is dramatically reduced with high radiotracer uptake.