The Spatial Relationship between Apparent Diffusion Coefficient and Standardized Uptake Value of (18)F-Fluorodeoxyglucose Has a Crucial Influence on the Numeric Correlation of Both Parameters in PET/MRI of Lung Tumors

The minimum apparent diffusion coefficient (ADC(min)) derived from diffusion-weighted MRI (DW-MRI) and the maximum standardized uptake value (SUV(max)) of FDG-PET are markers of aggressiveness in lung cancer. The numeric correlation of the two parameters has been extensively studied, but their spatial interplay is not well understood. After FDG-PET and DW-MRI coregistration, values and location of ADC(min)- and SUV(max)-voxels were analyzed. The upper limit of the 95% confidence interval for registration accuracy of sequential PET/MRI was 12 mm, and the mean distance (D) between ADC(min)- and SUV(max)-voxels was 14.0 mm (average of two readers). Spatial mismatch (D > 12 mm) between ADC(min) and SUV(max) was found in 9/25 patients. A considerable number of mismatch cases (65%) was also seen in a control group that underwent simultaneous PET/MRI. In the entire patient cohort, no statistically significant correlation between SUV(max) and ADC(min) was seen, while a moderate negative linear relationship (r = −0.5) between SUV(max) and ADC(min) was observed in tumors with a spatial match (D ≤ 12 mm). In conclusion, spatial mismatch between ADC(min) and SUV(max) is found in a considerable percentage of patients. The spatial connection of the two parameters SUV(max) and ADC(min) has a crucial influence on their numeric correlation.