Independent estimation ofT*₂for water and fat for improved accuracy of fat quantification

Abstract

Noninvasive biomarkers of intracellular accumulation of fat within the liver (hepatic steatosis) are urgently needed for detection and quantitative grading of nonalcoholic fatty liver disease, the most common cause of chronic liver disease in the United States. Accurate quantification of fat with MRI is challenging due the presence of several confounding factors, including T* 2 decay. The specific purpose of this work is to quantify the impact of T* 2 decay and develop a multiexponential T* 2 correction method for improved accuracy of fat quantification, relaxing assumptions made by previous T* 2 correction methods. A modified Gauss-Newton algorithm is used to estimate the T* 2 for water and fat independently. Improved quantification of fat is demonstrated, with independent estimation of T* 2 for water and fat using phantom experiments. The tradeoffs in algorithm stability and accuracy between multiexponential and single exponential techniques are discussed. Magn Reson Med 63: [849][850][851][852][853][854][855][856][857] 2010. V C 2010 Wiley-Liss, Inc. Key words: fat quantification; chemical shift imaging; T 2 * decay; hepatic steatosis; IDEAL Nonalcoholic fatty liver disease (NAFLD) is now recognized as the most common cause of chronic liver disease, afflicting up to 30% of all Americans (1). It is an emerging condition closely related to obesity and insulin resistance. Importantly, NAFLD's prevalence among children is reported to be up to 10% overall, and as high as 53% in obese children (2-4). NAFLD is expected by many experts to become a leading cause of end-stage liver disease as the prevalence of obesity increases in the general population, both in the US and worldwide.The hallmark feature of NAFLD is intracellular accumulation of triglycerides within hepatocytes (steatosis). In many patients, steatosis leads to inflammation and fibrosis, and ultimately to cirrhosis, with subsequent liver failure or development of hepatocellular carcinoma. In such patients, liver transplant is the only definitive option for cure. Nontargeted liver biopsy, which is the current gold standard for diagnosis of NAFLD, is limited by its high cost, morbidity, and importantly, its high sampling variability due to the heterogeneous nature of intracellular lipid accumulation. Quantitative assessment of liver fat using MRI is attractive because it can assess fat over the entire liver, thus avoiding the sampling variability, as well as the risks and high cost of biopsy.Chemical shift-based MRI techniques are currently under development by many groups for the quantification of liver fat (5-10). These methods exploit the differences in chemical shift between water and fat (3.29 ppm between water and the main resonance peak of fat, 210 Hz at 1.5 T). Chemical shift-based methods are often used to estimate the concentration of triglycerides through the use of the fat fraction, which is independent of amplitude of radiofrequency field coil sensitivities and therefore is a useful metric of fat concentration (11).Two-point methods acquire tw...

Optimized in-phase and opposed-phase MR imaging for accurate detection of small fat or water fractions: theoretical considerations and experimental application in emulsions

“…The applied biexponential method described by Bydder et al [27] takes the apparent T 2 * of water and fat into account. However, this procedure tends to be more instable in cases with limited SNR [28,29] For measurements of low water fractions inside fatty tissue, the proposed correction for the DBs becomes essential, because the signal contribution of DBs might be similar or even higher than the water signal contribution. The amount of DBs of lipids can slightly differ for different locations and between healthy and impaired tissue (for example in liver or bone marrow as reported in [41,47,48]).…”

“…Correction methods for fat signal contributions from vinyl groups with a Larmor frequency close to the water resonance have already been examined [24][25][26]. Furthermore, accuracy of the derived fat content has been investigated for different echo timings and strategies to consider multiple relaxation effects and spectral components [27][28][29][30][31][32].…”

Quantification of low fat contents: a comparison of MR imaging and spectroscopy methods at 1.5 and 3 T

“…The latter assumption is justified since macroscopic inhomogeneities within the voxel are supposed to affect fat and water protons equally. The results from Chebrolu et al [24] indicate that the single decay model fails in situations where T 2 * is short, but the single T 2 * -decay model has the advantage of being more stable, in particular when the FF is low [25]. With our model, the stability of the single decay model is maintained, while it is still being able to account for different relaxation rates of fat and water protons.…”

“…For T 2 * correction, both single [8] and dual decay [24] models have been proposed and compared [25]. We have chosen a model with different a priori T 2 relaxations for water and fat, while the estimated R′ 2 relaxation rate is assumed equal for water and fat.…”

“…Different models have been proposed to estimate the T 2 -and T 2 * -decay rates on a voxel-by-voxel basis: a common decay rate for water and fat [8], independent decay rates for water and fat [24], or different decay rates but with a known, fixed difference [12]. In a recent comparative study, Hernando et al found that estimation of individual decay rates can improve the accuracy of fat quantification if the signal-tonoise ratio (SNR) is high, but that the single-decay model is superior at low SNR and when the fat fraction (FF) approaches 0 or 100% [25].…”