Abstract
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.
Highlights
Semiquantitative assessments of 18F-FDG uptake on PET/CT with metrics such as the SUV are valuable tools for oncologic response assessment [1]
MRI-based attenuation correction maps are often affected by artifacts that can vary between imaging sessions, potentially reducing the repeatability of PET quantitation [5]
The CT-based attenuation correction approach used by PET/CT entails the direct measurement of photon attenuation by tissues, a method that is uncommonly affected by serious artifacts
Summary
Semiquantitative assessments of 18F-FDG uptake on PET/CT with metrics such as the SUV are valuable tools for oncologic response assessment [1]. Simultaneous PET/MRI systems can provide whole-body staging and treatment response assessment in a single examination [3]. These hybrid systems have necessitated the development of MRI-based methods for attenuation correction [4]. MRI-based attenuation correction maps are often affected by artifacts that can vary between imaging sessions, potentially reducing the repeatability of PET quantitation [5]. Prior studies have suggested that the addition of PET hardware to MRI systems can worsen DWI artifacts related to eddy currents, creating the potential for greater variability in ADC values between imaging sessions [8]. The primary aim of this study was to determine the test– retest repeatability of several commonly used PET/MRI-based quantitative imaging metrics in patients with solid malignancies
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