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

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 SUV_max, 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 SUV_peak, 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 ADC_median of 3.5% appeared lower than the wCVs for SUV_max (6.6%-8.7%) and SUL_peak (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.

Keywords: PET/MRI; apparent diffusion coefficient; cervical cancer; diffusion-weighted imaging; quantitative imaging; repeatability.

FIGURE 1.

Schematic of study imaging protocol. All subjects received 370 MBq of ¹⁸F-FDG at 0 min. PET/CT imaging began at 60 min, lasting for 15 min. Immediately after PET/CT, PET/MRI began, lasting for 30 min. DWI was performed at beginning (DWI 1) and at end (DWI 2) of PET/MRI session. Occasionally, because of various subject-related factors, session 2 imaging began at a different time from session 1 imaging (note horizontal offset in bars representing PET/CT sessions 1 and 2 and PET/MRI sessions 1 and 2). To control for these differences, static PET images were reconstructed using overlapping intervals of 1, 3, and 5 min in PET data (brackets), thereby achieving identical effective uptake times. To minimize differences in uptake times between PET/MRI and PET/CT, these reconstruction intervals were selected from latest overlapping portion for PET/CT and earliest overlapping portion for PET/MRI.

FIGURE 2.

Bland–Altman plots for SUV_max. For SUV_max from PSF 3-min reconstructions on PET/CT (A) and PET/MRI (B), Bland–Altman plots are shown. For each subject (ο), percentage change (%∆) between measurements (y-axis) is plotted against mean of 2 measurements. Orange line (μ) indicates mean %∆ across all subjects; dotted lines indicate 1 SD from mean %∆; solid black lines indicate 1.96 SDs from mean %∆, constituting 95% limits of repeatability (range of %∆ within which 95% of observations are expected to fall). Distance along y-axis between μ and either 1.96 SD line is repeatability coefficient. Comparing plots in A and B (identical y-axis ranges), 95% limits of repeatability appear substantially narrower for PET/MRI; correspondingly, repeatability coefficient and wCV were larger for PET/CT than PET/MRI. LOR = 95% limits of repeatability; RC = repeatability coefficient.

FIGURE 3.

Bland–Altman plots for SUL_peak. For SUL_peak from PSF 3 min reconstructions on PET/CT (A) and PET/MRI (B), Bland–Altman plots are shown. For each subject (ο), percentage change (%∆) between measurements (y-axis) is plotted against mean of 2 measurements. Orange line (μ) indicates mean %∆ across all subjects; dotted lines indicate 1 SD from mean %∆; solid black lines indicate 1.96 SDs from mean %∆, constituting 95% limits of repeatability (range of %∆ within which 95% of observations are expected to fall). Distance along y-axis between μ and either 1.96 SD line is repeatability coefficient. Comparing plots in A and B (identical y-axis ranges), 95% limits of repeatability appear slightly narrower for PET/MRI; correspondingly, repeatability coefficient and wCV were slightly larger for PET/CT than PET/MRI. LOR = 95% limits of repeatability; RC = repeatability coefficient.

FIGURE 4.

SUL_peak repeatability from PET image analysis. A 65-y-old woman with biopsy-proven squamous cell carcinoma of cervix was imaged with 24 h between session 1 (A) and session 2 (B). Transaxial PET images with manual whole-tumor contours (pink), T2-weighted MR images, and fused PET/MR images are shown from top to bottom for each imaging session. Note intersession contour similarities. SUL_peak (small pink sphere) was located in same tumor region for both sessions, with similar values.

FIGURE 5.

Bland–Altman plots for ADC_median. For ADC_median on PET/MRI, Bland–Altman plots are shown. For each subject (ο), percentage change (%∆) between measurements (y-axis) is plotted against mean of 2 measurements. Orange line (μ) indicates mean %∆ across all subjects; dotted lines indicate 1 SD from mean %∆; solid black lines indicate 1.96 SDs from mean %∆, constituting 95% limits of repeatability (range of %∆ within which 95% of observations are expected to fall). Distance along y-axis between μ and either 1.96 SD line is repeatability coefficient. The 95% limits of repeatability were substantially narrower than for SUV_max and SUL_peak, with numerically lower repeatability coefficient and wCV values. LOR = 95% limits of repeatability; RC = repeatability coefficient.

FIGURE 6.

ADC_median repeatability from ADC map analysis. A 33-y-old woman with biopsy-proven squamous cell carcinoma of cervix was imaged with 48 h between session 1 (A) and session 2 (B). Transaxial ADC maps with manual contours (pink), T2-weighted MR images, and fused PET/MR images are shown from top to bottom for each imaging session. ADC_median values for sessions 1 and 2 were similar.