Liver fat quantification: where do we stand?

Abstract

Excessive intracellular accumulation of triglycerides in the liver, or hepatic steatosis, is a highly prevalent condition affecting approximately one billion people worldwide. In the absence of secondary cause, the term nonalcoholic fatty liver disease (NAFLD) is used. Hepatic steatosis may progress into nonalcoholic steatohepatitis, the more aggressive form of NAFLD, associated with hepatic complications such as fibrosis, liver failure and hepatocellular carcinoma. Hepatic steatosis is associated with metabolic syndrome, cardiovascular disease and represents an independent risk factor for type 2 diabetes, cardiovascular disease and malignancy.Percutaneous liver biopsy is the current reference standard for NAFLD assessment; however, it is an invasive procedure associated with complications and suffers from high sampling variability, impractical for clinical routine and drug efficiency studies. Therefore, noninvasive imaging methods are increasingly used for the diagnosis and monitoring of NAFLD. Among the methods quantifying liver fat, chemical shift encoded MRI (CSE-MRI) based proton density fat-fraction (PDFF) has shown to the most promise. MRI-PDFF is increasingly accepted as quantitative imaging biomarker of liver fat that is transforming daily clinical practice and influencing the development of new treatments for NAFLD. Furthermore, CT is an important imaging method for detection of incidental steatosis, and the practical advantages of quantitative ultrasound hold great promise for the future. Understanding the disease burden of NAFLD and the role of imaging may initiate important interventions aimed at avoiding the hepatic and extrahepatic complications of

Magnetic Resonance Spectroscopy of Hepatic Fat from Fundamental to Clinical Applications

“…It is estimated that up to 30% of NAFLD patients may have NASH and have a greater risk of progression to end-stage liver disease [ 10 ]. Notably, NAFLD is the second most common etiology for liver transplantation and is projected to become the most common indication within the next decade [ 11 , 12 , 13 ]. Several studies have shown that increased liver fat content is also associated with non-liver-related mortality and morbidity.…”

“…US is also not specific for liver fat. US estimates the liver fat content through the attenuation of soundwaves, which are also attenuated by many other liver diseases, such as fibrosis, hepatitis, and hemochromatosis [ 11 , 27 , 28 ]. The method is also potentially difficult to perform in individuals with high body mass index (BMI) and those at high risk for liver fat accumulation [ 29 , 30 ].…”

“…In addition, single-energy CT also shows a high correlation with MRS (r 2 = 0.86) and is reported to be more accurate than dual-energy CT (r 2 = 0.423) [ 44 ]. However, unenhanced CT still has a limited sensitivity of 50% and specificity of 77.2% for mild steatosis, and a sensitivity of 72.7% and specificity of 91.3% for moderate-to-severe steatosis [ 11 , 45 ]. Another limitation is that the presence of fibrosis, edema, or iron deposition within the liver potentially affects the reliability of steatosis assessment [ 46 ].…”

“…Another limitation is that the presence of fibrosis, edema, or iron deposition within the liver potentially affects the reliability of steatosis assessment [ 46 ]. While unenhanced CT provides a more quantitative assessment than US or TE, it has lower accuracy than MRI and MRS [ 11 ]. Moreover, the utilization of ionizing radiation in CT therefore renders this method unsuitable for repeated measurements in sensitive populations, such as children or pregnant women.…”

“…The relative chemical shift between water and fat is approximately 3.5 parts per million (ppm) [ 47 ]. The relative signal difference between IP and OP images could then be used for the subjective assessment of liver fat [ 11 ]. However, this method requires the correction of multiple confounding issues, especially for mild steatosis, and is limited by the upper limit for liver steatosis at 50% [ 48 ].…”