Evaluation of the influence of truncation artifacts using in-vivo based quantification accuracy methods in combined PET/MRI

Aim: Quantitative accuracy of standardized uptake values (SUV) and tracer kinetic uptake parameters in patient investigations requires determination of regional activity concentrations in PET data. This determination rests on the assumption that the scanner calibration is valid in-vivo. In a recent study we introduced a method to test this assumption. For 3 different PET and PET/CT systems the activity concentration of urine samples measured in a well-counter were compared to those derived from PET images of the bladder. The study demonstrated a low but systematic underestimation of 7-12% of PET relative to a cross-calibrated well-counter for 56 subjects. In the present study we have applied this method to the Philips Ingenuity-TF PET/MR to evalute the impact of MR-based truncation artifcats on the overall quantitative accuracy of this system. Methods: 21 clinical whole-body F18-FDG scans were included in this study. The bladder region was imaged as the last bed position and urine samples were collected afterwards. PET images were reconstructed including MR-based attenuation correction with and without truncation compensation and 3D region-of-interests (ROI) of the bladder were delineated by 3 observers. Activity concentrations were determined in the PET images for the bladder as well as for the urine by measuring the samples in a well-counter. Results: The in-vivo activity concentrations of the bladder were significantly lower in PET/MR than in the well-counter with a ratio of the former to the latter of 0.756?0.060 (mean?std.dev.) and a range of [0.604-0.858]. Linearity scans revealed a systematic error of 8-11 % (avg. 9 %). After correcting for this systematic bias caused by shortcomings of the manufacturer?s calibration procedure the PET to well-counter ratio increased to 0.832?0.064 [0.668-0.941]. After applying compensation for truncation of the upper extremities in the MR-based attenuation maps the ratio further increased to 0.871?0.069 [0.693-0.992]. Conclusions: Our results show, that the Ingenuity-TF PET/MR underestimates activity concentrations in the bladder by 17% which is 7 percentage points larger than in the previously investigated PET/CT systems. This difference in behavior can be attributed to remaining limitations of MR-based attenuation correction as our results on truncation compensation related influences suggest - leaving only a 2 pp. larger underestimation of activity concentrations if corrected. Thus, quantification accuracy of the Ingenuity-TF PET/MR can be considered acceptable for clinical purposes. The comparison of PET images from the bladder region with urine samples has proven a useful method to evaluate quantification accuracy of different PET systems in-vivo.