Hepatic Fat MRI Calculator

Fat Signal Fraction: 100 x (liver SIIP-liver SIOP)/(2 x liver SIIP). Used if both echoes are obtained after a single excitation.

Fat Signal Percentage: 100 x (liver SIIP/spleen SIIP) - (liver SIOP/spleen SIOP)/ 2 x (liver SIIP/spleen SIIP). Used if the echoes are acquired separately.

This calculation only gives a rough estimate of hepatic steatosis and likely underestimates hepatic steatosis. Accurate hepatic steatosis estimation can be performed on dedicated MRI studies. This calculator is ideal for estimating fat on a routine MRI abdomen using two point Dixon technique for approximate quantification of steatosis.

When acquiring in-phase and out-of-phase echoes after a single excitation with identical calibration for both, the use of the spleen as an internal reference standard is deemed unnecessary. However, if the echoes are acquired separately, it is advisable to adjust the hepatic signal amplitude values against the splenic values for accuracy and consistency. It is commonly assumed that the liver and spleen have the same T2* relaxation values, which is often incorrect. Since no internal reference is flawless, it is better to acquire both echoes following a single excitation.

Although the accuracy of out-of-phase and in-phase MR imaging exceeds that of both US and CT for quantification of liver fat, it is not without flaws. The main limitation with use of the standard technique is that the signal fat fraction can be calculated, but the content fat fraction cannot. Studies show that three critical factors, T1 relaxation, T2* relaxation, and signal interference between the multiple spectral components of fat, confound the intensity of the fat and water signals and cause the signal fat fraction to be an imperfect estimate of fat content.

References:

• Elfaal M, Supersad A, Ferguson C, et al. Two-point Dixon and six-point Dixon magnetic resonance techniques in the detection, quantification and grading of hepatic steatosis. World J Radiol. 2023;15(10):293-303. doi:10.4329/wjr.v15.i10.293
• Starekova J, Hernando D, Pickhardt PJ, 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
• Sirlin CB. Invited commentary. Radiographics. 2009;29(5):1277-1280. doi:10.1148/027153330290051277

Disclaimer: The author makes no claims of the accuracy of the information contained herein; this information is for educational purposes only and is not a substitute for clinical judgment.