Correlation between apparent diffusion coefficients and standardized uptake values in hybrid ¹⁸F-FDG PET/MR: Preliminary results in rectal cancer

Fluorine-18-fluorodeoxyglucose (¹⁸F-FDG) positron emission tomography (PET) and diffusion-weighted magnetic resonance imaging (DWI) share the same role in clinical oncology and it is feasible to obtain the standardized uptake value (SUV) and apparent diffusion coefficient (ADC) simultaneously by emerging the hybrid positron emission tomography/magnetic resonance (PET/MR). This study investigated the correlation between the ADCs of rectal cancer lesions and their SUVs derived from hybrid PET/MR. Nine patients with histologically proven rectal adenocarcinoma (5 men, 4 women; mean age, 70 ± 15.91 years) underwent torso ¹⁸F-FDG PET/CT and regional hybrid ¹⁸F-FDG PET/MR sequentially. A fixed threshold value of 40 % of maximum uptake was used to determine tumor volume of interest (VOI) on PET image; SUV_max, SUV_peak, and SUV_mean were calculated automatically. A single freehand region of interest (ROI) was drawn on high b-value (b1000) DWI image and copied to corresponding ADC map to determine the ADCmean of rectal cancer lesion. Spearman'rank correlation coefficient (ρ) was calculated to determine the correlation between SUVs and ADC values. SUV_max, SUV_peak, and SUV_mean derived by hybrid PET/MR were 12.35 ± 4.66 (mean ± standard deviation), 9.66 ± 3.15 and 7.41 ± 2.54, respectively. The ADCmean value of rectal cancer lesions was 1.02 ± 0.08 × 10^-3mm²/s. ADCmean was significantly and inversely correlated with SUV values (SUV_max, ρ = -0.95, p < 0.001; SUV_peak, ρ = -0.93, p < 0.001; SUV_mean, ρ = -0.91, p = 0.001). This preliminary hybrid PET/MR study demonstrates a significant inverse correlation exists between metabolic activity on ¹⁸F-FDG PET and water diffusion on DWI in rectal cancer