Whole-brain quantitative mapping of metabolites using short echo three-dimensional proton MRSI

Abstract

Purpose To improve the extent over which whole brain quantitative 3D-MRSI maps can be obtained and be used to explore brain metabolism in a population of healthy volunteers. Materials and Methods Two short TE (20 ms) acquisitions of 3D Echo Planar Spectroscopic Imaging at two orientations, one in the anterior commissure – posterior commissure (AC-PC) plane and the second tilted in the AC-PC +15° plane were obtained at 3T in a group of ten healthy volunteers. B1+, B1−, and B0 correction procedures and normalization of metabolite signals with quantitative water proton density measurements were performed. A combination of the two spatially normalized 3D-MRSI, using a weighted mean based on the pixel wise standard deviation metabolic maps of each orientation obtained from the whole group, provided metabolite maps for each subject allowing regional metabolic profiles of all parcels of the automated anatomical labeling (AAL) atlas to be obtained. Results The combined metabolite maps derived from the two acquisitions reduced the regional inter-subject variance. The numbers of AAL regions showing NAA SD/Mean ratios lower than 30% increased from 17 in the AC-PC orientation and 41 in the AC-PC+15° orientation, to a value of 76 regions out of 116 for the combined NAA maps. Quantitatively, regional differences in absolute metabolite concentrations (mM) over the whole brain were depicted such as in the GM of frontal lobes (cNAA=10.03+1.71, cCho=1.78±0.55, cCr=7.29±1.69; cmIns=5.30±2.67) and in cerebellum (cNAA=5.28±1.77, cCho=1.60±0.41, cCr=6.95±2.15; cmIns=3.60±0.74). Conclusion A double-angulation acquisition enables improved metabolic characterization over a wide volume of the brain.

Metabolic voxel‐based analysis of the complete human brain using fast 3D‐MRSI: Proof of concept in multiple sclerosis

“…Two axial fast 3D-EPSI (4) were included with two different orientations, first in the AC-PC plane and secondly in the AC-PC +15° plane (Figure A) (8). RF power settings and head position were identical to the MRI protocol, with the addition of CHESS water suppression (40 Hz), lipid inversion nulling with TI = 198 ms, TE/TR = 20ms/1710ms, flip angle = 73°, 50*50*18 k-space points, voxel size = 1×1×1cm 3 , FOV = 280*280*180mm 3 , acquisition time = 17:48min.…”

“…As explained in details by Lecocq and coworkers (8), the two angles of MRSI orientations were chosen in order to get sufficient differences in acquisition planning between the two orientations to obtain good quality spectra on as large a brain area as possible (optimally whole brain) on at least one acquisition, but also keeping a reasonable angulation to allow accurate automatic normalization procedure for the two orientations (SPM8, Welcome Trust center for Neuroimaging at University College London). The post-processing pipeline described in Figure B , used the Metabolite Imaging and Data Analyses System (MIDAS) software (19) including B 0 map correction, lipid suppression, tissue volume fraction through T 1 segmentation, spectral fitting, exclusion of outlier voxels based on Cramer Rao bounds (CRB) and signal normalisation with the interleaved water signal acquired (5).…”

“…In order to give an estimate of the absolute concentrations of metabolites, we used as reference the GM concentration of NAA of 10mM obtained in (8) using the same 3D-EPSI sequence. The correction factor from the arbitrary unit (AU) to mM was then derived from mean GM NAA value of controls and applied to all metabolite maps first expressed in AU, a unit already normalized by the water signal and the number of protons.…”

“…Nevertheless, partial brain sampling has been reported to be sufficient to conduct statistical mapping analyses comparing metabolic maps between different group of healthy subjects (7). Concurrently, a combination of two fast short TE MRSI acquired in two different orientations (AC-PC and AC-PC +15°) has been used to obtain high quality cerebral metabolite maps covering the whole brain of healthy controls in about 45min (8). These complete brain metabolite maps may be of value to determine the brain topography of metabolic abnormalities associated with various brain diseases such as MS.…”

Density‐weighted concentric circle trajectories for high resolution brain magnetic resonance spectroscopic imaging at 7T

“…For FID‐MRSI the combination of high spatial resolution and spatial low‐pass filtering has led to substantial improvements of the point spread function (PSF), and hence a reduction of lipid artifacts, which enabled high‐resolution MRSI with full‐slice coverage, but at the expense of prolonged scan times 1. This has triggered the need for MRSI acceleration methods suitable for $B_0\ge 7$ T. At $B_0\le 3$ T, MRSI acceleration is dominated by spatial‐spectral encoding (SSE) via echo‐planar spectroscopic imaging (EPSI) and spiral spectroscopic imaging, which offer acceleration factors of up to two orders of magnitude 8, 9. Yet, only a single article on SSE has been published so far at 7T using EPSI with a non‐standard head‐only gradient insert 10.…”