MRI-Based Quantification of Intra-Myocellular Fat Content

Abstract: "Introduction Cardiac steatosis, or overstorage of fat in the cardiac muscle cells, is a possible side effect from drugs treating diabetes mellitus. Therefore, a non-invasive means of fat quantification is needed during drug development. The aim of this master thesis is to develop a method to quantify the fraction intra-myocellular fat in the cardiac muscle of rats using magnetic resonance imaging (MRI). Simulations, in vitro experiments on phantoms and in vivo experiments on rats were conducted during development and the quantification accuracy of the method was compared to that of magnetic resonance spectroscopy (MRS). Theory Multi-echo imaging is based on the Dixon technique which is a method for fat/water separation. The method uses the chemical shift between water and fat to separate the species from a number of acquired gradient echoes with different TEs. The reconstruction includes built-in corrections for T2'-relaxation, off-resonance effects and the multi-resonance signal of fat. To obtain a quantitative measure of the volume fat fraction, corrections for bias from differences in relaxation times and proton density are included. Material and methods Initially, simulations were performed in Matlab to learn more about the method and its properties. Intralipid phantoms were created with volume fat fractions ranging from 0 % to 21.7 % and measured with multi-echo imaging and spectroscopy at 1.5 T and 3 T Siemens scanners. The method was also tested in vivo by animal experiments on rats fed with a steatosis-inducing drug using a 4.7 T Bruker system. Results Simulations showed that the use of a lower magnetic field strength and a fewer number of echoes is less sensitive to inaccuracies in the modelled fat spectrum. In vitro, the method successfully quantified fat fractions as low as 0.2 % with quantification accuracy similar to that of spectroscopy. In vivo, however, no correlation could be detected between spectroscopy and imaging results. Conclusion Although the method is still to be successfully tested in vivo, it does show potential to be applicable also in living tissue. This potential may be greater in organs were the fat content is higher and the movements less problematic than in the heart."