Feasibility of Noninvasive Magnetic Resonance Temperature Imaging of Fat and Water Based on Methylene Proton Spin-lattice Relaxation Time and Water Proton Resonance Frequency

Abstract

:A noninvasive magnetic resonance temperature imaging technique for fat-water mixed tissues was proposed. This technique uses the temperature dependence of the spin-lattice relaxation time( T (cid:50876) ) of protons originated from methylene chain(CH (cid:50877) )of fat as well as the resonance frequency shift of water proton(H (cid:50877) O). A multiple point Dixon method in conjunction with a multiple flip angle method enables simultaneous calculation of T (cid:50876) of CH (cid:50877) and the resonance frequency change of H (cid:50877) O. A phantom with two mayonnaise tubes,one heated by microwave while the other kept at room temperature was imaged at 3 Tesla during the cooling process by a spoiled gradient recalled acquisition in steady state (SPGR)of the following conditions;field of view,32 × 32 cm (cid:50892) ;matrix,64 × 64;parallel imaging factor, 2;repetition time,36 ms;echo time spacing,1.15 ms;and flip angles,20,50 and 70 degrees. Signals obtained with each flip angle were processed by IDEAL(Iterative Decomposition of water and fat with Echo Asymmetry and Least squares estimation)algorithm to obtain H (cid:50877) O,CH (cid:50877) and CH (cid:50878) images. The smaller components of fat were ignored for simplicity. Temperature distribution of fat in the phantom was imaged by T (cid:50876) of CH (cid:50877) obtained from the three CH (cid:50877) images with different flip angles,while that of water with the change in the phase difference between H (cid:50877) O and CH (cid:50877) or the relative phase change in H (cid:50877) O. Those temperature images were then fused as a weighted sum of H (cid:50877) O and CH (cid:50877) fractions in each voxel. The resultant images highly correlated with the probe-measured temperature elevation demonstrating that simultaneous fat-water temperature imaging is feasible and is expected to be sufficient for clinical practice. . The other study evaluated the stability of the water-fat chemical shifts using line scan echo planar spectroscopic imaging(LSEPSI) (cid:50896)(cid:50903) . The deviation of the results demonstrated that the water-fat PRF difference is not a useful indicator for breast temperature. Moreover,both water and fat contents have to be high enough in this type of spectroscopic technique. These studies were conducted at relatively low magnetic field(0.5-1.5T)aiming at observation of total fat signal without separating the fatty acid components.