Presentations

Abstract

Introduction:

Hepatic steatosis, characterized by the accumulation of intracellular fat within hepatocytes, is a common liver condition with significant implications for public health. Hepatic steatosis can manifest with varying presentations, ranging from asymptomatic fatty liver detected incidentally to more severe cases, potentially progressing to cirrhosis. Magnetic resonance imaging (MRI) is a powerful non-invasive tool for assessing hepatic steatosis, enabling evaluation of fat content, distribution, and associated findings within the liver. This poster delves into the methodologies employed in MRI-based assessment of hepatic steatosis and highlights the challenges inherent in accurate quantification.

Main Body:

Hepatic steatosis is the result of abnormal accumulation of triglycerides within hepatocytes. There are various underlying causes, including nonalcoholic fatty liver disease, alcoholism, metabolic, toxic, and infectious causes. Patients with steatosis can subsequently develop fibrosis, cirrhosis, or hepatic insufficiency.

Hepatic steatosis can be broadly categorized into three groups:

1. Diffuse hepatic steatosis: This is the most common form where the entire liver demonstrates a homogeneous fatty appearance. "Focal fat sparing" can be seen in diffuse hepatic steatosis and is characterized by reduced or absent fat accumulation in specific areas of the liver.
2. Focal hepatic steatosis: Less common, it involves localized fat deposits in the liver. Focal hepatic steatosis can mimic primary or secondary hepatic masses but should demonstrate absent or minimal mass effect.
3. Multifocal hepatic steatosis: Multifocal hepatic steatosis, also known as multinodular hepatic steatosis, describes multiple nodular or ovoid fat foci that are randomly distributed throughout the liver.

MRI is a reliable non-invasive tool for evaluation of hepatic steatosis. Hepatic steatosis appears hyperintense on T1-weighted images and mildly hyperintense on T2-weighted images. To further characterize hepatic steatosis on MRI, fat suppression techniques are utilized. There are different methods available to suppress fat on MRI; the technique used depends on the purpose of fat suppression and the amount of fat in the tissue of interest and whether the fat is microscopic or macroscopic.

Detecting microscopic fat can be achieved with opposed-phase imaging (OPI). OPI with in-phase (IP) and out-of-phase (OOP) acquisitions is a widely used MRI technique for microscopic fat detection and quantification. The signal from water and fat are additive on IP images, while the signal on OP images is the difference between water and fat signals and results in reduced signal from fatty tissue. Thus, hepatic steatosis shows a signal intensity drop on OOP images in comparison with IP images. OPI is highly sensitive for detecting microscopic fat if the fat fraction (FF) is greater than 10%. FF refers to the proportion of fat within the liver and serves as a quantitative measure to assess the severity of hepatic steatosis.

Quantifying the degree of hepatic steatosis can be achieved on OPI using the relative IP and OOP signal intensity values of the liver and the spleen. The following formula is then used to calculate the percentage of signal intensity loss: [(Liver IP / Spleen IP) - (Liver OOP / Spleen OOP) ] / [(Liver IP / Spleen IP)] x 100. This method becomes unreliable for quantification when the FF is greater than 50% because the OOP signal intensity paradoxically increases due to the presence of relatively fewer water molecules to cancel out the fat signal.

Alternative forms of fat suppression include fat saturation and inversion recovery imaging. These techniques are especially useful for suppressing larger quantities of intracellular lipid or adipose tissue but are not useful in evaluating smaller concentrations of intracellular lipid.

Conclusion:

Hepatic steatosis is a common condition with important implications to public health. It is important to be able to identify hepatic steatosis because it may have serious consequences if left untreated. Knowledge and familiarity of imaging patterns of steatosis may prevent misdiagnosis. MRI with IP and OOP is a proven method for detection and quantification of intracellular fat in hepatic steatosis. MRI is particularly useful for diagnosing atypical forms of hepatic steatosis or when diagnosis is not certain, allowing for non-invasive diagnosis and quantification.