Quantitative assessment of liver fat with magnetic resonance imaging and spectroscopy

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Abstract

Hepatic steatosis is characterized by abnormal and excessive accumulation of lipids within hepatocytes. It is an important feature of diffuse liver disease, and the histological hallmark of nonalcoholic fatty liver disease (NAFLD). Other conditions associated with steatosis include alcoholic liver disease, viral hepatitis, human immunodeficiency virus (HIV) and genetic lipodystrophies, cystic fibrosis liver disease, and hepatotoxicity from various therapeutic agents. Liver biopsy, the current clinical gold standard for assessment of liver fat, is invasive and has sampling errors, and is not optimal for screening, monitoring, clinical decision-making, or well suited for many types of research studies. Noninvasive methods that accurately and objectively quantify liver fat are needed. Ultrasound (US) and computed tomography (CT) can be used to assess liver fat but have limited accuracy as well as other limitations. Magnetic resonance (MR) techniques can decompose the liver signal into its fat and water signal components and therefore assess liver fat more directly than CT or US. Most MR techniques measure the signal fat-fraction (the fraction of the liver MR signal attributable to liver fat), which may be confounded by numerous technical and biological factors and may not reliably reflect fat content. By addressing the factors that confound the signal fat-fraction, advanced MR techniques measure the proton density fat-fraction (the fraction of the liver proton density attributable to liver fat), which is a fundamental tissue property and a direct measure of liver fat content. These advanced techniques show promise for accurate fat quantification and are likely to be commercially available soon. Copyright © 2011 Wiley-Liss, Inc.

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Independent predictors of liver fibrosis in patients with nonalcoholic steatohepatitis.

P Angulo, J Keach, K P Batts … (1999)

Nonalcoholic steatohepatitis (NASH) may present with increased hepatic fibrosis progressing to end-stage liver disease. No factors that determine increasing fibrosis and histologically advanced disease have been recognized, thus, liver biopsy is recommended in all patients for diagnosis and prognosis. Our aim was to identify independent predictors of severe hepatic fibrosis in patients with NASH. One hundred and forty-four patients were studied. All patients underwent liver biopsy. Clinical and biochemical variables were examined with univariate and multivariate analysis. Thirty-seven (26%) patients had no abnormal fibrosis, 53 (37%) had mild fibrosis, 15 (10%) had moderate fibrosis, 14 (10%) had bridging fibrosis, and 25 (17%) had cirrhosis. In multivariate analysis, older age (P =. 001), obesity (P =.002), diabetes mellitus (P =.009), and aspartate transaminase/alanine transaminase (AST/ALT) ratio greater than 1 (P =.03) were significant predictors of severe liver fibrosis (bridging/cirrhosis). Body mass index (P =.003) was the only independent predictor of the degree of fat infiltration. Increased transferrin saturation correlated positively with the severity of fibrosis (P =.02) in univariate analysis, and there was a trend for more female patients among those with more advanced fibrosis (P =. 09). However, iron studies or gender were not significant when controlled for age, obesity, diabetes, and AST/ALT ratio. In conclusion, older age, obesity, and presence of diabetes mellitus help identify those NASH patients who might have severe liver fibrosis. This is the subgroup of patients with NASH who would be expected to derive the most benefit from having a liver biopsy and considering investigational therapies.

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Exposure to low-dose ionizing radiation from medical imaging procedures.

Reza Fazel, Harlan M. Krumholz, Yongfei Wang … (2009)

The growing use of imaging procedures in the United States has raised concerns about exposure to low-dose ionizing radiation in the general population. We identified 952,420 nonelderly adults (between 18 and 64 years of age) in five health care markets across the United States between January 1, 2005, and December 31, 2007. Utilization data were used to estimate cumulative effective doses of radiation from imaging procedures and to calculate population-based rates of exposure, with annual effective doses defined as low ( 3 to 20 mSv), high (> 20 to 50 mSv), or very high (> 50 mSv). During the study period, 655,613 enrollees (68.8%) underwent at least one imaging procedure associated with radiation exposure. The mean (+/-SD) cumulative effective dose from imaging procedures was 2.4+/-6.0 mSv per enrollee per year; however, a wide distribution was noted, with a median effective dose of 0.1 mSv per enrollee per year (interquartile range, 0.0 to 1.7). Overall, moderate effective doses of radiation were incurred in 193.8 enrollees per 1000 per year, whereas high and very high doses were incurred in 18.6 and 1.9 enrollees per 1000 per year, respectively. In general, cumulative effective doses of radiation from imaging procedures increased with advancing age and were higher in women than in men. Computed tomographic and nuclear imaging accounted for 75.4% of the cumulative effective dose, with 81.8% of the total administered in outpatient settings. Imaging procedures are an important source of exposure to ionizing radiation in the United States and can result in high cumulative effective doses of radiation. 2009 Massachusetts Medical Society