Transverse relaxation time constants of the five major metabolites in human brain measured in vivo using LASER and PRESS at 3 T

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Excerpt

Proton magnetic resonance spectroscopy (1H MRS) is a noninvasive technique that allows the measurement of multiple metabolites in the brain in vivo at the same time. These MR visible metabolites are located primarily in the intracellular compartments. The most commonly measured brain metabolites at relatively short echo time (TE) are N‐acetyl aspartate (NAA), total creatine (tCr, creatine plus phosphocreatine), choline‐containing compounds (tCho, phosphorylcholine plus glycerophosphorylcholine), glutamate (Glu), and myo‐inositol (mIns). These metabolites are preferentially concentrated in certain cell types. For instance, NAA and Glu are predominantly located in neurons, tCr and tCho are found in both neuronal and glial cells, and mIns is thought to be localized exclusively in astrocytes 1. By measuring the transverse relaxation time constants (T2), which are sensitive to changes in the molecular motion primarily through interaction of metabolites with structural or cystolic macromolecules 2, the cellular microenvironment of these metabolites can be probed.
Two spin‐echo pulse sequences commonly used in MRS studies are localization by adiabatic selective refocusing (LASER) 3 and point‐resolved spectroscopy (PRESS) 4. Both sequences provide full‐intensity signal. However, on clinical 3 T and above systems, PRESS suffers from larger chemical shift displacement (CSD) error ( > 10%/ppm) because of the limited bandwidth of its refocusing pulses than the LASER sequence, which uses broad‐bandwidth adiabatic full‐passage (AFP) refocusing pulses.
Many studies have reported the apparent T2 relaxation time constants of singlet resonances (e.g., NAA, tCr, and tCho in the human brain from 1.5 to 7 T using PRESS and LASER sequences) 2. However, so far fewer human studies have measured the T2 of J‐coupled metabolites such as Glu and mIns using those techniques (5–8,11). Interestingly, it was previously demonstrated that the apparent T2 of NAA, tCr, and water were lengthened with the CP‐LASER sequence, a variant of LASER with an additional Carr‐Purcell pulse train 12, compared with PRESS in the human brain at 4 and 7 T 13.
There have not been any studies comparing the T2 values for intracellular metabolites measured with LASER and PRESS. Therefore, the aim of the current study was to measure and compare the apparent T2 relaxation times of five major metabolites using LASER and PRESS sequences in the human brain on a clinical 3 T scanner. High signal‐to‐noise ratio (SNR) spectra were acquired over a large TE range with both sequences such that there was no bias in estimating the T2 relaxation of metabolites.

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