Quantitative assessment of liver fat with magnetic resonance imaging and spectroscopy

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 non-alcoholic fatty liver disease (NAFLD). Other conditions associated with steatosis include alcoholic liver disease, viral hepatitis, 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. Non-invasive 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 magnetic resonance (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.

Liver MRI: From basic protocol to advanced techniques

“…European Journal of Radiology 93 (2017) [30][31][32][33][34][35][36][37][38][39] that appear hypointense compared to the normal parenchyma [2,5,10,16]. There are two Gd-based hepato-specific agents, gadobenate dimeglumine (Gd-BOPTA, Multihance ® , Bracco Imaging, Milan, Italy) and gadoxetic acid (Gd-EOB-DTPA, Primovist ® , Bayer Schering Pharma AG, Berlin, Germany), with different profiles [14].…”

“…Knowing a priori the resonance frequencies of fat and water protons, their concentrations can be quantified directly from their spectral signal [36]. Fat (triglyceride) signal has multiple frequency components, the dominant one being located at a frequency shift of 420 Hz (1.46 ppm) relative to water peak, on 3T magnetic filed.…”

“…Fat proton density results from the sum of these diverse multiple fat peaks. MRS is an accurate and reproducible method for quantification of proton density fat fraction (PDFF) [36]. However, it is time consuming and it is only available in specialized hospital or research centres.…”

“…Multi-echo chemical shift encoded (MECSE) GRE sequences, performed with more than 3 echoes (usually between 6 and 12), have recently emerged as an accurate tool for PDFF quantification [36,37] (Fig. 9).…”

“…These sequences take advantage of the chemical shift of water and fat protons. For precise fat quantification, these sequences must be corrected for the main confounding factors (T1 bias, effect of T2* relaxation, fat spectral complexity) [36,37]. Fat protons have a shorter T1 relaxation time than water protons, which may introduce a significant bias in fat fraction estimation if the acquisition is T1-w.…”