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Optimized in vivo brain glutamate measurement using long‐echo‐time semi‐LASER at 7 T
Author(s) -
Wong Dickson,
Schranz Amy L.,
Bartha Robert
Publication year - 2018
Publication title -
nmr in biomedicine
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.278
H-Index - 114
eISSN - 1099-1492
pISSN - 0952-3480
DOI - 10.1002/nbm.4002
Subject(s) - glutamate receptor , nuclear magnetic resonance , in vivo , chemistry , physics , materials science , biology , biochemistry , receptor , microbiology and biotechnology
A short echo time ( T E ) is commonly used for brain glutamate measurement by 1 H MRS to minimize drawbacks of long T E such as signal modulation due to J evolution and T 2 relaxation. However, J coupling causes the spectral patterns of glutamate to change with T E , and the shortest achievable T E may not produce the optimal glutamate measurement. The purpose of this study was to determine the optimal T E for glutamate measurement at 7 T using semi‐LASER (localization by adiabatic selective refocusing). Time‐domain simulations were performed to model the T E dependence of glutamate signal energy, a measure of glutamate signal strength, and were verified against measurements made in the human sensorimotor cortex (five subjects, 2 × 2 × 2 cm 3 voxel, 16 averages) on a 7 T MRI scanner. Simulations showed a local maximum of glutamate signal energy at T E = 107 ms. In vivo, T E = 105 ms produced a low Cramér‐Rao lower bound of 6.5 ± 2.0% across subjects, indicating high‐quality fits of the prior knowledge model to in vivo data. T E = 105 ms also produced the greatest glutamate signal energy with the smallest inter‐subject glutamate‐to‐creatine ratio (Glu/Cr) coefficient of variation (CV), 4.6%. Using these CVs, we performed sample size calculations to estimate the number of participants per group required to detect a 10% change in Glu/Cr between two groups with 95% confidence. 13 were required at T E = 45 ms, the shortest achievable echo time on our 7 T MRI scanner, while only 5 were required at T E = 105 ms, indicating greater statistical power. These results indicate that T E = 105 ms is optimum for in vivo glutamate measurement at 7 T with semi‐LASER. Using long T E decreases power deposition by allowing lower maximum RF pulse amplitudes in conjunction with longer RF pulses. Importantly, long T E minimizes macromolecule contributions, eliminating the requirement for acquisition of separate macromolecule spectra or macromolecule fitting techniques, which add additional scan time or bias the estimated glutamate fit.