Liver fat quantification using CT attenuation or MRI chemical shift imaging
1 Select Imaging Modality
Non-Contrast CT
Direct liver attenuation measurement. Most specific when <40 HU.
Mean ROI in right lobe, avoid vessels
Portal Venous Phase CT
Liver-spleen comparison. Use when non-contrast unavailable.
Measure liver and spleen at same level, similar-sized ROIs
Liver Signal Intensity
Chemical shift imaging (in-phase and out-of-phase GRE)
Spleen Reference (Optional)
Required if echoes acquired separately (different TR/calibration)
Leave empty for single-acquisition dual-echo sequences
About Hepatic Steatosis Imaging
Hepatic steatosis (fatty liver disease) affects approximately 25% of the global population and is the most common chronic liver disease. Accurate quantification is essential for diagnosis, monitoring treatment response, and assessing transplant donor eligibility.
CT Assessment of Hepatic Steatosis
Non-Contrast CT (Gold Standard for CT)
Unenhanced CT provides the most reliable CT-based assessment of hepatic fat content through direct measurement of liver parenchymal attenuation.
Liver Attenuation
Interpretation
Estimated Fat Content
>60 HU
Normal
<5%
48-60 HU
Borderline
5-10%
40-48 HU
Mild steatosis likely
10-25%
<40 HU
Moderate-severe steatosis
>25%
Specificity: Liver attenuation <40 HU has >95% specificity for moderate-severe steatosis (Pickhardt et al., 2012).
Portal Venous Phase CT (Liver-Spleen Comparison)
When non-contrast images are unavailable, liver-spleen attenuation difference provides useful assessment:
Liver-Spleen Difference
Interpretation
>5 HU (Liver > Spleen)
Normal (no steatosis)
-5 to +5 HU
Borderline / Mild steatosis possible
<-10 HU (Liver < Spleen)
Steatosis likely
<-20 HU
Moderate-severe steatosis
Liver-Spleen Ratio: A ratio <0.9 suggests steatosis. This method is less affected by contrast timing variability.
MRI Assessment of Hepatic Steatosis
Chemical Shift Imaging (In-Phase/Out-of-Phase)
MRI exploits the different resonance frequencies of water and fat protons. At 1.5T, water and fat signals are:
In-phase: Water and fat signals add together
Out-of-phase: Water and fat signals cancel (subtract)
Signal loss on out-of-phase images indicates intracellular fat (steatosis). This technique is widely available but provides a fat signal fraction, not absolute fat content.
If out-of-phase signal is higher than in-phase signal, this suggests hepatic iron deposition rather than fat. Iron causes faster T2* decay, affecting the in-phase acquisition (acquired at a longer TE at 1.5T). In this case:
Fat quantification is unreliable
Consider dedicated iron quantification (R2* mapping)
MR spectroscopy or PDFF with T2* correction may be needed
MRI-PDFF (Proton Density Fat Fraction)
For accurate fat quantification, MRI-PDFF (multi-echo Dixon with T2* correction) is the reference standard. It corrects for:
T1 bias
T2* decay (including iron effects)
Multi-peak fat spectral modeling
MRI-PDFF correlates directly with histological fat fraction and is increasingly used in clinical trials. This calculator uses the simpler dual-echo method, which is adequate for clinical grading but less precise than PDFF.
Clinical Applications
NAFLD/MASLD screening: Initial assessment of suspected fatty liver
Treatment monitoring: Response to lifestyle changes, weight loss, or medications
Liver transplant donors: >30% steatosis is a relative contraindication
Metabolic syndrome: Hepatic fat is a marker of cardiometabolic risk
Limitations
CT: Radiation exposure; affected by glycogen, iron, copper, amiodarone
Dual-echo MRI: Confounded by iron; underestimates fat at high fractions
Regional heterogeneity: Focal fat or focal sparing may not represent whole liver
Neither method assesses: Inflammation (NASH) or fibrosis
References
Starekova J, Reeder SB. Quantification of Liver Fat Content with CT and MRI: State of the Art. Radiology. 2021;301(2):250-262. doi:10.1148/radiol.2021204288
Pickhardt PJ, Park SH, Hahn L, et al. Specificity of unenhanced CT for non-invasive diagnosis of hepatic steatosis. AJR Am J Roentgenol. 2012;199(2):W197-W202. doi:10.2214/AJR.11.8175
Ma X, Holalkere NS, Kambadakone RA, et al. Imaging-based quantification of hepatic fat: methods and clinical applications. Radiographics. 2009;29(5):1253-1277. doi:10.1148/rg.295085186
Reeder SB, Cruite I, Hamilton G, Sirlin CB. Quantitative Assessment of Liver Fat with Magnetic Resonance Imaging and Spectroscopy. J Magn Reson Imaging. 2011;34(4):729-749. doi:10.1002/jmri.22580
Idilman IS, Aniktar H, Idilman R, et al. Hepatic steatosis: quantification by proton density fat fraction with MR imaging versus liver biopsy. Radiology. 2013;267(3):767-775. doi:10.1148/radiol.13121360
European Association for the Study of the Liver (EASL). EASL Clinical Practice Guidelines on non-invasive tests for evaluation of liver disease severity and prognosis. J Hepatol. 2021;75(3):659-689. doi:10.1016/j.jhep.2021.05.025