Peter Adany1, Phil Lee1,2, and In-Young Choi1,2,3
1Hoglund Brain Imaging Center, University of Kansas Medical Center, Kansas City, KS, United States, 2Department of Molecular & Integrative Physiology, University of Kansas Medical Center, Kansas City, KS, United States, 3Department of Neurology, University of Kansas Medical Center, Kansas City, KS, United States
Synopsis
Reliable
localization of 1H MRS in gray and white matter of the human brain
was achieved using the BASE-SLIM technique. Distinctive
spectral patterns of gray and white matter were measured from
all subjects using BASE-SLIM, which were consistent with those measured in both
tissue types using single-voxel 1H MRS. BASE-SLIM spectroscopy promises
1) accurate and robust acquisition of MR spectra from gray and white matter
with minimum cross contamination from other compartments, 2) shorter scan time
and 3) flexibility in compartmental shapes that match anatomical structures. Target Audience
Scientists and technologists
who are interested in advanced in vivo
1H MRS methods to
measure neurochemicals in a region-specific and/or tissue type-specific manner.
Introduction
Metabolic processes differ between
gray (GM) and white matter (WM) in the human brain, and significantly different
spectral patterns and metabolite concentrations were measured in
1H
MRS. Differential GM and WM pathology
and tissue-type specific neurochemical alterations are often observed in neurological
disorders. Thus, reliable
1H MRS measurement of GM or WM is
important in clinical applications. However, current
1H MRS
acquisition strategies present significant challenges in ascertaining exclusive
metabolite quantification of GM and WM. One of commonly used methods, CSI regression
analysis with GM and WM tissue fractions, which extrapolates metabolite concentrations
from each compartment
1, suffers from extrapolation errors due to a
narrow range of tissue fraction values. BASE-SLIM
2, an advanced form
of Spectral Localization by Imaging (SLIM) promises reliable measurements of 1H
MRS in GM and WM
3 as BASE-SLIM incorporates information of anatomical
contours of GM and WM boundaries. In this study, we demonstrate significantly
improved localization accuracy in compartments with arbitrary shapes, e.g., GM
and WM, and consistent and reliable measurement of metabolites in the human
brain using the BASE-SLIM technique.
Methods
Eleven healthy subjects (29 ± 4
yrs, mean±SD) were studied at 3 T (Skyra, Siemens) with a 16 channel head receive
coil. The
1H CSI was acquired using a semi-LASER sequence
4
(TE/TR=35/1600ms, matrix=16x16, FOV=20cm) with the slab positioned across
the prefrontal to parietal lobes. B0 and coil sensitivity (B1)
maps were acquired using gradient echo sequences (TE=4.92/7.38ms for B0,
TE=2.07ms for B1). BASE-SLIM reconstruction was performed using the
CSI k-space data, B0 and B1 maps, and high resolution GM
and WM segmentation masks. Coil combination of each
1H MR spectrum
from 16 channels was performed to maximize SNR with a maximal ratio combining
scheme using the signal and noise spectral power estimates. Metabolite
concentrations were quantified using LCModel
3 and non-suppressed
water scan as a concentration reference. CSI voxels with greater than 75%
tissue fraction were selected for the regression analysis. The GM and WM regions
covered in BASE-SLIM were identical to the CSI ROI in the regression analysis.
BASE-SLIM reconstruction was implemented as previously described
2,
involving an expanded geometry matrix for multiple coil sensitivity profiles
and a time-dependent solution for B0 corrections.
Results and Discussion
BASE-SLIM
reconstruction was successfully applied to obtain
1H MR spectra from
GM and WM in the human brain. Distinctive spectral patterns of GM and WM were
observed in all spectra acquired from the fronto-parietal regions of eleven
subjects (Fig. 1), which showed a lower ratio of Cho (3.17 ppm) to Cr (3.03
ppm), and higher Glu+Gln (around 2.25 ppm) in GM, compared with those in WM. Consistent
and reliable measurement of
1H MR spectra in GM and WM resulted in
excellent averaged spectra over eleven subjects (Fig. 2).
Quantification
of metabolites in GM and WM showed that NAA and tCho in GM were 8% (p=0.03) and
22% (p=0.00001) lower than those in WM, and Cr, Glu+Gln, and myo-inositol in GM were 16% (p=0.001),
148% (p=0.00005), and 13% (p=0.004) higher in GM compared with those in WM.
Metabolite concentrations from BASE-SLIM reconstruction
were in agreement with the reported
values using the extrapolated CSI regression for GM and WM
5. For
the BASE-SLIM method, MR scan time can be significantly shorter than
conventional CSI because BASE-SLIM requires much lower number of k-space
encodings compared with the CSI regression analysis, which requires high
spatial resolution for reliable determination of metabolite concentrations in
GM and WM. Another important advantage of BASE-SLIM is the ability to choose
any arbitrary shapes of compartments that match anatomical structures such as
lesions and specific brain regions.
In conclusion, BASE-SLIM based
1H MRS provides reliable
quantication of metabolites in GM and WM with minimum cross contamination.
Acknowledgements
This work is
partly supported by the National Institutes of Health (S10RR29577, UL1TR000001)
and the Hoglund Family Foundation.References
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al. MRM (2008) 1-6.
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