Liver steatosis is a major health problem since 20-30% of the population is affected with non-alcoholic fatty liver disease (NAFLD).
NAFLD comprises a wide range of liver injury, from simple steatosis to steatohepatitis, progressive fibrosis and eventually cirrhosis with its complications.
Liver biopsy is considered the gold standard for steatosis grading. However, it is an invasive procedure with some risks of complications, and cannot be used as a screening procedure. On the other hand, steatosis severity may change within weeks of therapeutic intervention; thus, liver biopsy is unsuitable for monitoring changes over short periods of time.
For the assessment of liver steatosis several imaging techniques are available.
Conventional B-mode ultrasound (US) is the imaging modality most widely used. It gives a subjective estimate of the fat content in the liver and has a low sensitivity for detecting mild steatosis. The presence of underlying chronic liver disease may reduce the accuracy of US in the diagnosis of hepatic steatosis.
The use of computed tomography is limited by the radiation exposure. Moreover, the technique has a low accuracy for mild steatosis, and the presence of other underlying diffuse liver disease may alter liver attenuation values.
Magnetic resonance imagingñderived proton density fat fraction (MRI-PDFF) is a technique that accurately quantifies the hepatic fat content and has been proposed as an alternative to the histological assessment of liver steatosis in patients with NAFLD. MRI-PDFF is not influenced by confounding factors and is operator-independent. However, it may have applicability limitations due to claustrophobia, metal implants or pacemakers, or inability to suspend respiration for image acquisition. Moreover, the availability and the costs should be taken into account when it is used for screening purpose.
Controlled Attenuation Parameter (CAP) is available on the FibroScan device. CAP measures the attenuation, in decibel/meter, of the US beam that traverses the liver tissue. The performance of the CAP has been assessed in cohort of patients and in a meta-analysis with individual patient's data, taking the histologic grading of steatosis as the reference.
Other new ultrasound-based techniques are nowadays available and their performance is under evaluation. These techniques are based on the quantification of the ultrasound attenuation in the tissue or the estimation of the speed of sound within soft tissues, that varies slightly with fat content: an increase in fat content leads to a decrease in the speed of sound.