Enhanced refocusing of fat signals using optimized multipulse echo sequences

Abstract

Endogenous magnetic resonance contrast based on the localized composition of fat in vivo can provide functional information. We found that the unequal pulse timings of the Uhrig’s Dynamical Decoupling (UDD) multipulse echo sequences significantly alter the signal intensity compared to conventional, equal-spaced Carr-Purcell-Meiboom-Gill (CPMG) sequences. The signal increases and decreases depending on the tissue and sequence parameters, as well as on the interpulse spacings; particularly strong differences were observed in fatty tissues, which have a highly structured morphology and a wide range of chemical shifts and J-couplings. We found that the predominant mechanism for fat refocusing under multipulse echo sequences is the chemical structure, with stimulated echoes playing a pivotal role. As a result, specialized pulse sequences can be designed to optimize refocusing of the fat chemical shifts and J-couplings, where the degree of refocusing can be tailored to specific types of fats. To determine the optimal time delays, we simulated various UDD and CPMG pulse sequence timings, and these results are compared to experimental results obtained on excised and in vivo fatty tissue. Applications to intermolecular multiple-quantum coherence (iMQC) imaging, where the improved echo refocusing translates directly into signal enhancements, are presented as well.

Effect of J coupling on 1.3-ppm lipid methylene signal acquired with localised proton MRS at 3 T

“…Evidence of J-coupling effects on the fat signal has been observed in MRI, where the signal from fat is higher from fast spin echo or Carl-Purcell-Meiboom-Gill (CPMG) sequences than from a conventional spin echo sequence as a result of a reduction in J-coupling modulation by the refocusing pulses (20)(21)(22)(23), particularly as the interpulse timings are shortened (21,22), in agreement with theoretical predictions made by Allerhand (24). More recently, further dependence on interpulse timings in multipulse echo MRI sequences has been demonstrated (19). In addition, in singlevoxel spectroscopy experiments, PRESS and STEAM yield differences in measures of the 1.3-ppm methylene protons and their T 2 values (16).…”

“…The objective of this work was to investigate, at 3 T, the impact of J ‐coupling interactions on the quantification and T 2 determination of 1.3‐ppm lipid methylene protons. Effects caused by the scalar coupling interactions have been observed previously ; however, the monoexponential nature of the signal decay with increasing TE tends to alleviate concerns about quantification errors . In this work, we investigated the response of the 1.3‐ppm protons to PRESS and STEAM in fatty acids of different chain lengths to gain an insight into the decay response.…”

“…The 1.3‐ppm protons are also strongly coupled to neighbouring 1.6‐ppm protons with J / δ ≈ 0.18 . In addition, in the presence of double bonds, 1.3‐ppm protons are weakly coupled to neighbouring 2.1‐ppm allylic protons with J / δ ≈ 0.07 , where δ is about 102 Hz.…”

“…Lipid methylene protons in vivo and in phantoms representative of in‐vivo lipid compositions have been shown to exhibit monoexponential decay with increasing TE , which provides the impression that the effects of J coupling on the quantification of the 1.3‐ppm lipid signal and its estimated T 2 are minimal . However, some of the 1.3‐ppm lipid methylene protons do exhibit J ‐coupling interactions, namely with the ≈ 0.90‐ppm methyl, the ≈1.6‐ppm methylene and the ≈ 2.1‐ppm allylic proton groups . Evidence of J ‐coupling effects on the fat signal has been observed in MRI, where the signal from fat is higher from fast spin echo or Carl–Purcell–Meiboom–Gill (CPMG) sequences than from a conventional spin echo sequence as a result of a reduction in J ‐coupling modulation by the refocusing pulses , particularly as the interpulse timings are shortened , in agreement with theoretical predictions made by Allerhand .…”

Multi-spin echo spatial encoding provides three-fold improvement of temperature precision during intermolecular zero quantum thermometry

“…A separate simulation was performed for d = 0, 0.25 0.5, 0.75, 1.0. H 1 ( d = 0) corresponds to the Hamiltonian specified in Table of reference . H 1 ( d = 1) corresponds to the ChemBioDraw Ultra (Perkin Elmer, Waltham, Massachusetts, USA) simulation of oleic acid.…”