Work

Global brain metabolic quantification with whole‐head proton MRS at 3 T

Year: 2017

Type: article

Abstract: Total N‐acetyl‐aspartate + N‐acetyl‐aspartate–glutamate (NAA), total creatine (Cr) and total choline (Cho) proton MRS (1H–MRS) signals are often used as surrogate markers in diffuse neurological pathologies, but spatial coverage of this methodology is limited to 1%–65% of the brain. Here we wish to demonstrate that non‐localized, whole‐head (WH) 1H–MRS captures just the brain's contribution to the Cho and Cr signals, ignoring all other compartments. Towards this end, 27 young healthy adults (18 men, 9 women), 29.9 ± 8.5 years old, were recruited and underwent T1‐weighted MRI for tissue segmentation, non‐localizing, approximately 3 min WH 1H–MRS (TE/TR/TI = 5/10/940 ms) and 30 min 1H–MR spectroscopic imaging (MRSI) (TE/TR = 35/2100 ms) in a 360 cm3 volume of interest (VOI) at the brain's center. The VOI absolute NAA, Cr and Cho concentrations, 7.7 ± 0.5, 5.5 ± 0.4 and 1.3 ± 0.2 mM, were all within 10% of the WH: 8.6 ± 1.1, 6.0 ± 1.0 and 1.3 ± 0.2 mM. The mean NAA/Cr and NAA/Cho ratios in the WH were only slightly higher than the “brain‐only” VOI: 1.5 versus 1.4 (7%) and 6.6 versus 5.9 (11%); Cho/Cr were not different. The brain/WH volume ratio was 0.31 ± 0.03 (brain ≈ 30% of WH volume). Air‐tissue susceptibility‐driven local magnetic field changes going from the brain outwards showed sharp gradients of more than 100 Hz/cm (1 ppm/cm), explaining the skull's Cr and Cho signal losses through resonance shifts, line broadening and destructive interference. The similarity of non‐localized WH and localized VOI NAA, Cr and Cho concentrations and their ratios suggests that their signals originate predominantly from the brain. Therefore, the fast, comprehensive WH‐1H‐MRS method may facilitate quantification of these metabolites, which are common surrogate markers in neurological disorders.

Source: NMR in Biomedicine

DOI: 10.1002/nbm.3754