Liver fat quantification (MRI-PDFF)

MRI-PDFF (proton density fat fraction) measures the share of your liver's protons that belong to fat triglycerides, versus all its water and fat protons. The result is a percentage that directly reflects how much fat is in your liver. The scan uses a multi-echo, chemical-shift method that separates water and fat signals, while correcting for several confounders (T1 bias, T2* decay, and the spectral complexity of fat). Simpler in-and-out-of-phase scans cannot correct for those. A PDFF above 5% defines liver steatosis, the threshold for metabolic dysfunction-associated steatotic liver disease (MASLD, formerly NAFLD). Permutt et al. (2012) studied 51 biopsy-confirmed patients and found a strong correlation with steatosis grade (r² = 0.56, p < 0.0001). Mean PDFF was 8.9% for grade 1, 16.3% for grade 2, and 25.0% for grade 3. In trials, MRI-PDFF has largely replaced needle biopsy, because it samples the whole liver. A 30% relative drop from baseline is linked to resolution of MASH on biopsy, and is now a common trial endpoint. Excess liver fat speeds insulin resistance and carries its own heart risk, which makes MRI-PDFF a longevity-relevant marker. One limit: it does not directly measure fibrosis, the stage most predictive of liver-related death.

Last reviewed: June 22, 2026

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