SUV and ADC in lung: Do we really need both?

Objectives To investigate the relationship between the Apparent Diffusion Coefficient (ADC) and Standardized Uptake Value (SUV) of lung tumours derived from simultaneous PET/MRI. We hypothesize that combining SUV results with ADC values derived from Diffusion Weighted Imaging can help differentiate between vital, fibrotic and necrotic tumours.

Methods To test this hypothesis we analysed simultaneous wholebody FDG PET/MRI scans from 18 patients with 22 lung lesions acquired on a Siemens Biograph mMR scanner. Free breathing axial 3-directional EPI diffusion data were acquired at 3 b-values (0, 400, 800s/mm²) to produce ADC maps. Lesions as small as 0.6 cm² were detected. Irregular ROIs were drawn on T2 images and copied onto the attenuation corrected PET images and ADC maps to determine the maximum SUV and mean ADC values. Care was exercised to avoid areas of infection and artefacts across the air-tissue interface. The association between SUVmax and ADCmean was assessed using Spearman's rho correlation coefficient since the scatter plot indicated a non-linear relationship.

Results A strong negative correlation between ADCmean and SUVmax was identified (rho=-0.89, p<0.001) indicating that the more cellular the tissue (restricted diffusion, low ADC) the higher its metabolic rate (high SUV). However, the Diffusion - Metabolism relationship appears to be non-linear. Specifically, we observed that dense cellular tumours with ADC values lower than 1×10^-3mm²/s demonstrate a wide range of metabolic uptakes (4<SUVmax<10). On the other hand, the diffusion patterns for tumours with lower FDG avidity (1<SUVmax<4) ranged from restricted motion indicating dense and even fibrous tissue all the way up to unrestricted water motion which can be representing necrotic tissue (1×10^-3mm²/s<ADCmean<3×10^-3mm²/s). This is showcased here by two examples of heterogeneous tumours with necrotic and fibrotic centres.

Conclusions These preliminary results yielded a strong association between ADCmean and SUVmax. Our findings indicate that combining SUV and ADC results can help distinguish necrotic tissue from fibrotic tissue and from vital tissue in lung tumours. To further validate these findings a larger prospective study is required, and we are currently recruiting patients with lung and colorectal tumours post-radiotherapy treatment.