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Aim To investigate the ability of apparent diffusion coefficient (ADC) heterogeneity index to discriminate liver metastases (LM) from normal-appearing liver (NAL) tissue as compared to common magnetic resonance imaging (MRI) metrics, and to investigate its correlation with 2-[18F]-fluoro-2-deoxy- d- glucose (18F-FDG) positron-emission tomography (PET) standardised uptake value (SUV). Materials and methods Thirty-nine liver metastases in 24 oncology patients (13 women, 11 men; mean age 56±13 years) with proven LM from heterogeneous sources were evaluated on a PET/MRI system. Abdominal sequences included Dixon and diffusion-weighted imaging (DWI) protocols with simultaneous PET. Tissue heterogeneity was calculated using the coefficient of variance (CV) of the ADC, and compared in LM and in NAL tissue of the same volume in an adjacent portion of the liver. The correlations between various ADC measures and PET SUV in distinguishing LM from NAL were evaluated. Results A good correlation was found between ADCcv and SUVpeak (r=0.712). Moderate inverse correlation was found between ADCmin and SUVpeak (r=–0.536), and a weak inverse correlation between ADCmean and SUVpeak (r=–0.273). There was a significant difference between LM and NAL when ADCcv (p Conclusions The ADCcv index is a potential biomarker of LM with better correlation to 18F-FDG PET SUVpeak than conventional MRI metrics, and may serve to quantitatively discriminate between LM and NAL.