Measurement Repeatability of 18F-FDG PET/CT Versus 18F-FDG PET/MRI in Solid Tumors of the Pelvis

Knowledge of the within-subject variability of 18F-FDG PET/MRI measurements is necessary for proper interpretation of quantitative PET or MRI metrics in the context of therapeutic efficacy assessments with integrated PET/MRI scanners. The goal of this study was to determine the test–retest repeatability of these metrics on PET/MRI, with comparison to similar metrics acquired by PET/CT. Methods: This prospective study enrolled subjects with pathology-proven pelvic malignancies. Baseline imaging consisted of PET/CT immediately followed by PET/MRI, using a single 370-MBq 18F-FDG dose. Repeat imaging was performed within 7 d using an identical imaging protocol, with no oncologic therapy between sessions. PET imaging on both scanners consisted of a list-mode acquisition at a single pelvic station. The MRI consisted of 2-point Dixon imaging for attenuation correction, standard sequences for anatomic correlation, and diffusion-weighted imaging. PET data were statically reconstructed using various frame durations and minimizing uptake time differences between sessions. SUV metrics were extracted for both PET/CT and PET/MRI in each imaging session. Apparent diffusion coefficient (ADC) metrics were extracted for both PET/MRI sessions. Results: The study cohort consisted of 14 subjects (13 female, 1 male) with various pelvic cancers (11 cervical, 2 rectal, 1 endometrial). For SUVmax, the within-subject coefficient of variation (wCV) appeared higher for PET/CT (8.5%–12.8%) than PET/MRI (6.6%–8.7%) across all PET reconstructions, though with no significant repeatability differences (all P values ≥ 0.08) between modalities. For lean body mass-adjusted SUVpeak, the wCVs appeared similar for PET/CT (9.9%–11.5%) and PET/MRI (9.2%–11.3%) across all PET reconstructions, again with no significant repeatability differences (all P values ≥ 0.14) between modalities. For PET/MRI, the wCV for ADCmedian of 3.5% appeared lower than the wCVs for SUVmax (6.6%–8.7%) and SULpeak (9.2%–11.3%), though without significant repeatability differences (all P values ≥ 0.23). Conclusion: For solid tumors of the pelvis, the repeatability of the evaluated SUV and ADC metrics on 18F-FDG PET/MRI is both acceptably high and similar to previously published values for 18F-FDG PET/CT and MRI, supporting the use of 18F-FDG PET/MRI for quantitative oncologic treatment response assessments.

Note: For the OSEM reconstructions, parameters were selected to achieve identical recovery coefficients for PET/CT and PET/MRI. Similarly, for the PSF reconstructions, manufacturerprovided resolution recovery parameters were first applied, and then the filters were adjusted to achieve identical recovery coefficients for PET/CT and PET/MRI. Time-of-flight imaging was not available for either scanner used in this study.
Abbreviations: OSEMordered-subset expectation maximization; PSFpoint-spread function Supplemental Note: Numbers provided are means ± standard deviations across all patients and reflect values obtained for both imaging sessions averaged on a per-patient basis. SUVpeak and SULmax were calculated only for the WT contour, as some of the 40% isocontours were not large enough for calculation of a peak value and because maximum values for the WT contour and the 40% isocontour are identical.
Abbreviations: 40% -40% isocontour; MTVmetabolic tumor volume; OSEMordered-subset expectation maximization; PSFpoint spread function; SULlean body mass-adjusted standardized uptake value; TLGtotal lesion glycolysis; WT -whole tumor contour Supplemental Note: Numbers provided in the session 1 and session 2 columns are means ± standard deviations across all patients. For the majority of patients, two usable DWI acquisitions were available for each imaging session. For these patients, the two data points from a given session were averaged to obtain a single value per imaging session. The p values are based on results of the Wilcoxon signed-rank test.
Abbreviations: ADCapparent diffusion coefficient; ADC20mean value obtained from the intra-lesion voxels with the lowest 20% of ADC values; ADCtroughlowest mean ADC value obtainable for a 1 cm 3 sphere placed within the confines of the lesion (analogous to the inverse of peak for PET imaging) ; DTVdiffusional tumor volume Supplemental Abbreviations: %Δpercent change between sessions; OSEMordered-subset expectation maximization; PSFpoint-spread function; wCVwithin-subject coefficient of variation; SULlean body mass-adjusted standardized uptake value Supplemental   Note: For patients with more than one usable set of ADC data for a given session (as two DWI acquisitions were performed per session), one set was selected at random from each session for this analysis.
Abbreviations: %Δpercent change between sessions; ADCapparent diffusion coefficient; wCVwithinsubject coefficient of variation Supplemental Note: For patients with more than one usable set of ADC data for a given session (as two DWI acquisitions were performed per session), one set was selected at random from each session for this analysis.
Abbreviations: ADCapparent diffusion coefficient; ADC20mean value obtained from the intra-lesion voxels with the lowest 20% of ADC values; ADCtroughlowest mean ADC value obtainable for a 1 cm 3 sphere placed within the confines of the lesion (analogous to the inverse of peak for PET imaging) ; DTVdiffusional tumor volume; wCVwithin-subject coefficient of variation Supplemental  Note: Data are based on whole tumor contours except for metrics preceded by a '40%' designation; these metrics are based on the 40% isocontour. Numbers provided in the mean │%Δ│ column are means (across all subjects) of the absolute values of relative differences (i.e., percentage changes) between imaging sessions 1 and 2. The p values are based on results of the Wilcoxon signedrank test for comparisons 1-17 & 22-23 (paired data) and the Mann-Whitney U test for comparisons 18-21 (unpaired data).