Characterising Extravasated PET Doses; Evaluation of Metrics Associated with Topical Sensor Monitoring

Introduction: Partial extravasation of the radiopharmaceutical dose in positron emission tomography (PET) can undermine image quality and quantitation. The LARA topical sensor has been validated using a number of metrics to characterise injection quality following manual injection using 18F-FDG. An assessment of the performance of these metrics for autoinjector administration across a variety of radiopharmaceuticals has been undertaken.

Methods: A single site, single PET/CT scanner was used to characterise injections using a KARL100 autoinjector with standardised apparatus, flush volume and infusion rate using Rad-inject pump (1 min infusion followed by 2 syringe flushes) for 18F-FDG, 68Ga-PSMA and 68Ga-DOTATATE. 296 patients with topical application of LARA sensors were retrospectively analysed using conventional statistical analysis and in parallel, an artificial neural network. Three new metrics were developed (CEnd ratio, QIM1 and QIM2) to complement proprietary metrics in characterising extravasated injections (figure 1).

Results: The proprietary extravasation score greater than 200 demonstrated 33% sensitivity and 96.8% specificity in 18F-FDG studies (not validated in 68Ga studies). Extravasation amongst all radiopharmaceuticals was independently predicted by a tc50 greater than 1200 (100% sensitivity and 100% specificity), ndAvgN greater than 25 (100% sensitivity and 99.7% specificity), CEnd ratio greater than 2 (100% sensitivity and 100% specificity), QIM1 greater than 1600 (100% sensitivity and 100% specificity) or QIM2 greater than 1500 (100% sensitivity and 100% specificity). Furthermore, a ndAvgN cut-off of zero (0) classified abnormal TACs (venous retention but excluding extravasation) from normal TACs with a 95.5% sensitivity and 41.4% specificity.

Conclusions: Partial extravasation of PET doses are readily detected and differentiated using TAC metrics and these metrics could provide deeper insight into the impact of partial extravasation on image quality or quantitation. Further validation of key metrics and QIM are recommended in a larger and more diverse cohort. Funding: This project was funded by an ANSTO/ANZSNM Research Grant. 1 Figure 1: Annotated TACs for normal (A), normal with autoinjector multiple peaks and wider bolus (B), venous retention with resolution (C), and extravasation without resolution (D). 2

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