Measurement of fat/water ratios in rat liver using 3D three‐point dixon MRI

Abstract

Hepatic steatosis, or fatty liver, is commonly observed during the animal phase of drug safety studies. A noninvasive threedimensional (3D) three-point Dixon method was used to quantitatively evaluate the fatty livers of rats induced by an experimental microsomal transfer protein (MTP) inhibitor, in an effort to develop a safety biomarker that could be translated to human studies. The method was implemented at 2.0 T for in vivo studies, and at 7.1 T for higher-resolution magnetic resonance (MR) histologic studies. In three separate protocols to study dose response and longitudinal evolution, intrahepatic fatty accumulation was detected by this method and confirmed by chemical and histologic assessments. Consistent with the pathologic changes, the fat/water ratios estimated by the MR technique increased significantly at doses of 1 mg/kg and 100 mg/kg of MTP inhibitor after 14 days of continuous administration. Among the more important findings were: 1) with the 3D threepoint Dixon method, in vivo longitudinal studies of liver fat distribution can be conducted at significantly higher resolution than has previously been reported; 2) MR histology allows delineation of distribution at the microscopic scale of 0.0024 mm 3 resolution; and 3) the 3D three-point Dixon technique provides relative estimates of liver fat content and distribution at a high confidence level. This technique will be applicable in future studies in which fatty liver is a potential safety issue. A common finding in drug safety studies is fatty liver, which is the result of diet, stress, or compounds that modify lipid metabolism in the liver. The development of noninvasive methods to detect the onset and progression of, and recovery from fatty liver would be a valuable aid for the early determination of compound toxicity.The use of decomposing fat and water signals to discriminate between fat and water protons based on their resonant frequency difference was first introduced by Dixon (1). This method uses two acquisitions with a delay between the radiofrequency (RF) and gradient echoes, such that the phase shift between water and fat is either 0 or radians (in-phase and out-of-phase, respectively). Separate water and fat images can be obtained by adding and subtracting the in-phase and out-of-phase images. This method was subsequently enhanced to accommodate magnetic field (B 0 ) and RF inhomogeneity through the use of a third acquisition, leading to the three-point Dixon method (2,3).The microsomal transfer protein (MTP) inhibitor family of drugs has subnanomolar potency in the inhibition of lipid transfer and apolipoprotein B secretion of hepatocytes. Thus, MTP inhibitors have a potential application as therapeutic agents to lower atherogenic lipoprotein in humans. MTP inhibitor studies (both experimental and clinical) (4 -6) have provided visual and biochemical evidence of hepatocellular triglyceride (TG) accumulation. Quantification of the TG-filled droplets within the hepatocytes is of interest in the evaluation of MTP inhibitors under metabo...

Pancreatic Lipomatosis Is a Structural Marker in Nondiabetic Children With Mutations in Carboxyl-Ester Lipase

“…The three-point Dixon method has been shown to be highly reproducible and accurate for the estimation of true fat volume ratios (19,24). The method is useful to quantify fat in lean tissues such as liver (17), skeletal muscle, and pancreas (18). Further independent support of lipomatosis comes from the T1-weighted, fat-saturated MRI images (VIBE), which demonstrated altered signal intensities, with a hypointense pancreatic gland relative to liver in mutation carriers.…”

Altered Upper Airway and Soft Tissue Structures in the New Zealand Obese Mouse

“…Previous studies in rodents have used physical methods, such as whole-body and regional dissection and analysis of fat deposits (42-44), or histological (microscopic) analysis of tissues (45). Whereas chemical analysis and dissection methods have been used for determining fat content in rodents, recent studies indicate that quantitative MRI provides more reliable measurements of fat content in rats and mice (43,46).…”

“…Particularly in smaller animals, where dissection cannot easily separate organs and regional boundaries, noninvasive MRI techniques for measuring fat volume and distribution are preferable (46). They provide detailed anatomical information in addition to fat distribution maps (45). They also allow in-vivo measurements, thereby permitting longitudinal studies in the same animals, and provide for physiologic conditions during the image acquisition (47).…”

“…They provided a validation of the quantitative Dixon-method MRI by comparing the Dixon liver fat measurements with histological and chemical analysis of fat infiltration in rat livers (45).…”

Overload hepatitides: quanti-qualitative analysis

“…The different relaxation times between water and fat results in a significant bias in the estimation of fat fraction. Fat has a shorter T1 than water (343 ms vs. 586 ms at 1.5 T) and its signal is relatively amplified due to its higher signal [13].…”