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.