Adiabatic localized correlation spectroscopy (AL‐COSY): Application in muscle and brain

Abstract

Purpose: To describe an enhanced version of a localized correlation spectroscopy (L-COSY) by introducing adiabatic radiofrequency (RF) pulses for localization in two dimensions. Adiabatic pulses will improve slice selection profile and reduce chemical shift artifacts. Optimized Mao and adiabatic hyperbolic secant pulses are tested in vivo. Materials and Methods:Region of interest is localized by a 90 nonselective adiabatic RF pulse followed by two pairs of adiabatic RF pulses and a terminal 90 RF sinc pulse. Slice profiles for both refocusing pulses and chemical shift artifacts are measured in a water-oil phantom for L-COSY and AL-COSY. In vivo results of both COSY sequences are shown from muscle and brain on a 3 Tesla (T) scanner.Results: Chemical shift artifacts were reduced with AL-COSY compared with L-COSY. Slice profiles of adiabatic pulses were found to be sharper and more symmetrical than those of traditional Mao pulses. One-dimensional (1D) phantom studies showed longer T2 values using AL-COSY sequence. Comparison of 2D spectra obtained revealed spectroscopic peak volume improvements in AL-COSY and less residual water. In vivo 1D comparison showed more inphase and sharper peaks in AL-COSY spectrum. Conclusion:The AL-COSY sequence is an improved sequence due to sharper slice selection profiles, reduction of chemical shift artifacts, peak volume improvements in 2D techniques, and less J-modulation.

Magnetic Resonance Spectroscopy for Detection of 2-Hydroxyglutarate as a Biomarker for IDH Mutation in Gliomas

“…The loss of the H3/H3′ peak as seen in TE 97 ms PRESS was circumvented by the use of adiabatic refocusing pulses in the semi-LASER sequence. Application of such refocusing pulses result in significantly reduced compartmental artefacts that would normally arise from extended J-evolution times under enhanced chemical shift displacements seen when narrow bandwidth RF pulses are applied [51,66]. The overall effect is total refocusing of the resonance at 1.9 ppm (H3 spins), which, when combined with diminished Glu and Gln peaks, allows for reliable detection of 2-HG (Figure 6).…”

Two-dimensional semi-LASER correlation spectroscopy with well-maintained cross peaks

“…Compared with conventional L‐COSY , sLASER‐first‐COSY yielded most of peaks with higher SNR and sLASER‐last‐COSY yielded some peaks with higher SNR in the spectra of GE MRS Braino phantom and pig brain tissue (L‐COSY spectra not shown), though the durations from the excitation to the starting of data acquisition in sLASER‐first‐COSY and sLASER‐last‐COSY were 10 and 13 ms longer than that in L‐COSY, respectively. This is because a pair of AFP pulses can provide better pulse profiles than a nonadiabatic regular pulse .…”

“…Both sequences contain two pairs of AFP pulses for slice selections in two orthogonal planes. In sLASER‐last‐COSY sequence , the mixing pulse (i.e., the second 90° pulse) is used for selection of the third orthogonal plane. In contrast, in sLASER‐first‐COSY sequence the excitation pulse (i.e., the first 90° pulse) is used for selection of the third orthogonal plane while the second 90° pulse is only used as the mixing pulse for COSY but not for slice selection.…”

“…However, the peak intensities also depend on pulse profiles, the apparent transverse relaxation time T 2 *, and the duration from excitation to starting of data acquisition in the sequence. It was reported that due to better pulse profiles and possibly longer T 2 *, improved SNR was observed using adiabatic sequences compared with a nonadiabatic regular sequence .…”

“…A basic COSY sequence is composed of two 90° RF pulses. Recently, adiabatic localization were incorporated into COSY sequences: one is LASER, which stands for localization by adiabatic selective refocusing and uses a non–slice‐selective excitation pulse followed by three pairs of adiabatic full‐passage (AFP) pulses for signal refocusing as well as volume selection, so we call the sequence “LASER‐COSY” , and the other is semi‐LASER, which uses two pairs of AFP pulses and the second 90° RF pulse for volume localization , so we call the sequence “sLASER‐last‐COSY”. As LASER‐COSY has disadvantages including significantly longer echo time (TE) and higher specific absorption rate (SAR), semi‐LASER seems to be a more appropriate scheme for adiabatically localized COSY in in vivo application.…”