Liver fat quantification (MRI-PDFF)

MRI-PDFF (magnetic resonance imaging–proton density fat fraction) measures the fraction of liver protons belonging to fat triglycerides relative to all water and fat protons, yielding a percentage that directly reflects hepatic fat content. A multi-echo chemical-shift-encoded acquisition separates water and fat signals while correcting for T1 bias, T2* decay, and spectral fat complexity — confounders that affect simpler in-and-out-of-phase sequences. A PDFF above 5% defines hepatic steatosis, the threshold for metabolic dysfunction-associated steatotic liver disease (MASLD, formerly NAFLD). Permutt et al. (2012) showed in 51 biopsy-confirmed patients a strong correlation with steatosis grade (r² = 0.56, p < 0.0001), with mean PDFF of 8.9% (grade 1), 16.3% (grade 2), and 25.0% (grade 3). In the trial setting, MRI-PDFF has largely replaced needle biopsy because it samples the entire liver; a 30% relative reduction from baseline is associated with histological MASH resolution, widely adopted as a trial endpoint. Excess hepatic fat accelerates insulin resistance and carries independent cardiovascular risk, making MRI-PDFF a longevity-relevant biomarker; the method does not directly quantify fibrosis, the histological stage most predictive of liver-related mortality.