Non-invasive means of measuring hepatic fat content

Abstract

Hepatic steatosis affects 20% to 30% of the general adult population in the western world. Currently, the technique of choice for determining hepatic fat deposition and the stage of fibrosis is liver biopsy. However, it is an invasive procedure and its use is limited, particularly in children. It may also be subject to sampling error. Non-invasive techniques such as ultrasound, computerised tomography (CT), magnetic resonance imaging (MRI) and proton magnetic resonance spectroscopy ((1)H MRS) can detect hepatic steatosis, but currently cannot distinguish between simple steatosis and steatohepatitis, or stage the degree of fibrosis accurately. Ultrasound is widely used to detect hepatic steatosis, but its sensitivity is reduced in the morbidly obese and also in those with small amounts of fatty infiltration. It has been used to grade hepatic fat content, but this is subjective. CT can detect hepatic steatosis, but exposes subjects to ionising radiation, thus limiting its use in longitudinal studies and in children. Recently, magnetic resonance (MR) techniques using chemical shift imaging have provided a quantitative assessment of the degree of hepatic fatty infiltration, which correlates well with liver biopsy results in the same patients. Similarly, in vivo (1)H MRS is a fast, safe, non-invasive method for the quantification of intrahepatocellular lipid (IHCL) levels. Both techniques will be useful tools in future longitudinal clinical studies, either in examining the natural history of conditions causing hepatic steatosis (e.g. non-alcoholic fatty liver disease), or in testing new treatments for these conditions.

Figure 1

Ultrasound findings in hepatic steatosis. The steatotic liver is hyper-echoic, compared to the cortex of the right kidney. In addition, there is posterior attenuation of the ultrasound beam and reduced definition of the portal vein walls.

Figure 2

CT findings in hepatic steatosis. In this contrast unenhanced scan, the liver appears darker than the spleen and the hepatic vessels appear bright. The increased brightness of the vessels relative to the liver parenchyma may erroneously suggest the use of contrast.

Figure 3

Principles of magnetic resonance. The magnetic resonance (MR) signal or free induction decay (FID) may be converted by the mathematical process of Fourier transformation to form anatomical information (MR imaging) or localised biochemical information (MR spectroscopy). Modified from Taylor-Robinson SD Applications of magnetic resonance spectroscopy to chronic liver disease. Clin Med 2001; 1: 54-60 Copyright © 2001 Royal College of Physicians. Adapted by permission.

Figure 4

In and opposed phase MR images of a liver illustrating the signal drop-off from an in-phase (A) to an opposed-phase image (B) in a patient with marked steatosis. Reproduced from Rinella et al. Liver Transplantation 2003; 9: 851-856. Copyright (2003) American Association for the Study of Liver Diseases. Reprinted with permission of Wiley-Liss, Inc., a subsidiary of John Wiley & Sons, Inc.

Figure 5

Proton magnetic resonance spectra from three volunteers showing progressive degrees of hepatic fatty infiltration. Resonances from water and lipid (IHCL CH₂) can be clearly seen. For each individual hepatic fatty infiltration was quantified using the equation: -Percentage fat = IHCL CH₂ peak area/Water peak area × 100. Shown are spectra from a liver with very mild fatty infiltration (1.0%), a liver with moderate fatty infiltration (10.2%), and a liver with severe fatty infiltration (74.9%). Adapted from Thomas et al. Gut 2005; 54: 122-127, with permission from the BMJ Publishing Group.