Comparison of brain gray and white matter macromolecule resonances at 3 and 7 Tesla

Abstract

Purpose: In proton MR spectra of the human brain, relatively broad macromolecule (MM) resonances underlie the narrower signals from metabolites. The purpose of this study was to quantify the MM profile in healthy human brain at 3T and 7T, both in gray matter (anterior cingulate cortex [ACC]) and white matter (centrum semiovale [CSO]).

Methods: A water-suppressed, inversion-recovery pulse sequence was used to null metabolite signals and acquire MM spectra in 20 healthy volunteers using very similar methodology at both field strengths (n = 5 per region and field). The MM spectra were fitted with multiple Gaussian functions and quantified relative to the unsuppressed water signal from the same volume.

Results: MM proton concentration values were in the range of 5-20 mmol/kg. No significant differences were found between the MM proton concentration measurements by region (P ≈ 0.8) nor by field strength (P ≈ 0.5). Linewidths of the well-resolved M1 peak were slightly more than double at 7T (43.0 ± 4.7 Hz in ACC, 45.6 ± 4.1 Hz in CSO) compared with 3T (19.8 ± 3.5 Hz in ACC, 20.0 ± 4.3 Hz in CSO).

Conclusion: The absence of differences in MM concentrations between white and gray matter implies that a single MM "baseline" may be adequate for spectral fitting of multiple brain regions when determining metabolite concentrations. Visibility of MM signals is similar at 3T and 7T.

Keywords: 7 Tesla; brain; in vivo 1H MRS; macromolecular baseline; macromolecules.

Figure 1

Conventional water-suppressed in vivo spectra at (A) 3T and (B) 7T acquired with the semi-LASER pulse sequence from a 25×25×25 mm³ voxel located in the anterior cingulate cortex (ACC - inset). The major metabolite resonances include: tCr, total creatine (3.90 ppm and 3.01 ppm); tCho, total choline (3.19 ppm); tNAA, total N-acetylaspartate (2.01 ppm). The MM peak labeled M1 (0.9 ppm) can be clearly distinguished, whereas the other MM peaks are overlapped by the metabolite resonances. MM-spectra acquired with an inversion-recovery, water-suppressed semi-LASER pulse sequence from the same region as shown in (C) 3T (TR/TI 1559/600 ms) and (D) 7T (TR/TI 3000/900 ms). All spectra were line broadened by 2 Hz. Conventional water-suppressed in vivo spectra at (E) 3T and (F) 7T acquired with the semi-LASER pulse sequence from a 30×25×25 mm³ voxel located in the centrum semiovale (CSO - inset). MM spectra acquired with an inversion-recovery, water-suppressed semi-LASER pulse sequence from the same region as shown in (G) 3T (TR/TI 1559/600 ms) and (H) 7T (TR/TI 3000/900 ms). All spectra were line broadened by 2 Hz.

Figure 2

Example results of the Gaussian curve-fitting of the MM spectra in the ACC and in the CSO at 3T (respectively A and B) and at 7T (respectively C and D). The MM peak clusters (M1–M7) were fitted using 19 Gaussian lineshapes and integrated. The full red lines represent the fits of the metabolite-nulled spectra.

Figure 3

MM resonance proton concentrations for ACC and CSO at 3T and 7T (mmol/kg wet weight, mean ± st.dev). No corrections for relaxation time effects were applied. No significant differences are found between the MM proton concentrations by region (p ≈ 0.8), nor by field strength (p ≈ 0.5).

Figure 4

Example inversion recovery spectra from the anterior cingulate cortex recorded at 7 Tesla in one subject. (A) shows the conventional spectrum without inversion, while (B-E) show spectra as a function of inversion time (TI) ranging from (B) 900 to (E) 700 ms. Major metabolite peaks and MM peaks M1–M7 are indicated.