MRI Fat Quantification a Phase Sweep B-SSFP Approach
Author: Sarah Larmour
Publisher:
Published: 2015
Total Pages: 93
ISBN-13:
DOWNLOAD EBOOKFats are a fundamental building block of the human body, but accumulation of unwanted fat in and around tissues is a common pathology related to many disease mechanisms. Magnetic Resonance Imaging (MRI) offers a host of methods to differentiate water and fat signals in images or spectra, for both high fat content visceral and adipose fat, and the lower concentration intra-cellular fat pools. Accurate quantitative measurements of small fat concentrations and small changes in fat concentration within the heart and liver would enable the early detection of disease, evaluation of disease progression, and assessment of the effectiveness of prescribed treatments. Current methods, such as Dixon fat-water methods, have poor performance at low fat-fraction (FF), while 1H Nuclear Magnetic Resonance (NMR) spectroscopy methods are difficult to apply in the heart, and are not widely available. The goals of this thesis were primarily to develop and validate a new method called Phase Sweep b-SSFP for the simultaneous quantification of FF, water T1 and T2, and off-resonance frequency using multiple b-SSFP images with incremented radio frequency (RF) pulse phase (Phase Sweep b-SSFP), and secondly to characterize the effects of fat on commonly used T1 mapping sequences and evaluate a new method for quantitative FF imaging, based on the modulation of T1 values by the fat pool. For the purpose of validation of these methods, the proposed work on fat quantification addressed accuracy and precision for the case of small concentrations of fat in the 0 - 10% range. Methods included numerical simulations, phantom experiments and application in skeletal muscle for validation. This range reflects the intended future clinical application of the techniques in the heart, kidney and liver to provide early diagnosis of disease and assessment of prescribed treatment effectiveness. Skeletal muscle provides a good surrogate for the diffuse and heterogeneous fat deposits found in the heart, kidney and liver while allowing us to develop methods without the added complication of excessive movement and need for free breathing pulse sequences. The results of this work show that 1) by using the proposed Phase Sweep b-SSFP method it is feasible to acquire quantitative results for FF, water T1 and T2 and off-resonance frequency, for which the variability in each parameter is largely independent of all other parameters and 2) low FF in tissues result in relatively large negative or positive shifts in native tissue T1 measured with MOLLI and SASHA T1 mapping methods as a function of off-resonance frequency, and that these resulting T1 shifts can be used to accurately quantify FF.
