TE-Averaged two-dimensional proton spectroscopic imaging of glutamate at 3 T
Introduction
Glutamate and glutamine are important neurotransmitters in the central nervous system. The neurotoxic properties of excess glutamate have been associated with several neurodegenerative diseases. Glutamate and glutamine are strongly coupled systems and resonate at multiple frequencies. Due to similar chemical components, these metabolites overlap significantly in the 1H resonance spectrum. Higher magnetic field strength offers a possibility of resolving glutamate and glutamine resonances due to larger frequency dispersion and simplification of J coupling patterns (Mason et al., 1994). J-refocused coherence transfer methodology has also been used for the spectroscopic imaging of glutamate at 4.1 T using 1H (Pan et al., 1996) and 13C spectroscopy (Pan et al., 1997).
At 3 T, it is challenging to measure glutamate and glutamine separately using conventional one-dimensional spectroscopic methodologies. Techniques such as chemical shift selective filter (Schulte et al., 2005), 2D constant time PRESS (CT-PRESS) (Mayer and Spielman, 2005), and 2D J-resolved (Thomas et al., 1996) spectroscopy have been developed to increase the spectral resolution of strongly coupled metabolites by the measurement of an additional spectral dimension—also known as the J-resolved dimension. Recently, we have shown (Hurd et al., 2004) that the zeroth component of a 2D J-resolved spectrum resulted in an unobstructed detection of glutamate at 2.35 ppm that was distinct from glutamine and NAA. This technique called TE-Averaged PRESS has been implemented as a single voxel technique and was used to show differences in glutamate levels between normal subjects and Multiple Sclerosis patients (Srinivasan et al., 2005). While TE-Averaged PRESS results in a unobstructed glutamate detection, it does not differentiate between intra- and extra-cellular glutamate.
Due to the additional time required for the acquisition of a second spectral dimension to isolate the glutamate resonance, spatially resolved, two-dimensional spectral acquisitions involve prohibitive scan times when conventional phase encode methods are used. To incorporate J-resolved techniques for spectroscopic imaging, rapid acquisitions using time varying gradients during the readout window have been used (Adalsteinsson and Spielman, 1999), which encode some of the spatial information simultaneously with the spectral readout.
In this study, we developed a spectroscopic imaging technique (TE-Averaged MRSI) by combining TE-Averaged PRESS scheme for the measurement of an unobstructed glutamate resonance with fast spectroscopic imaging to obtain glutamate estimates over a 2D region of interest within a clinically reasonable time. To speed up the data acquisition for TE-Averaged MRSI, we used the flyback echo planar gradient trajectory (Mulkern and Panych, 2001, Feinberg et al., 1990) that was recently optimized for 3 T MRSI studies by our group (Cunningham et al., 2005). Use of this fast trajectory to simultaneously encode 16 spatial encode steps and the spectral dimension makes the scan time for a two-dimensional (10 × 16) TE-Averaged MRSI acquisition equivalent to a one-dimensional acquisition resulting in a dramatic reduction in scan time making J-resolved acquisitions feasible within a clinically acceptable time.
Section snippets
Materials and methods
All data were acquired on a 3 T GE SIGNA scanner on an EXCITE platform using an 8-channel phased array coil with body coil transmit.
Results
Fig. 1 shows spectra from a subset of MRSI voxels from TE-Averaged MRSI data obtained from phantoms that contained one metabolite and their relationship to an in-vivo spectrum. As observed, the TE-Averaged MRSI acquisition scheme preserves unobstructed detection of glutamate signal at 2.35 ppm as our previous single voxel implementation. In vivo TE-Averaged MRSI data were obtained with good quality and SNR (Fig. 2) over the entire spectral array with a well detected glutamate peak. The SNR for
Discussion
The main goal of this study was to provide a clinically feasible technique at 3 T for the detection of glutamate over a two-dimensional region using a spectroscopic editing method called TE-Averaged PRESS which has been shown by our group to provide an unobstructed detection of glutamate in single voxel studies. Compared to single voxel 1H MRS, multi-voxel proton magnetic resonance spectroscopy imaging (1H MRSI) increases spatial coverage and offers clear advantages for application to
Conclusion
This study has demonstrated the successful implementation of the TE-Averaged MRSI scheme for detection of unobstructed glutamate within a 2D region of interest with good SNR and spatial resolution. Significantly higher glutamate levels were obtained in GM relative to WM, validating our acquisition and quantification methodologies. Use of a fast spectroscopic imaging technique resulted in a significant reduction in scan time (∼21 min) making TE-Averaged MRSI a viable technique for brain
Acknowledgments
The authors would like to thank Dr. Napapon Sailasuta for helpful discussions. Work was supported as part of the University of California Industry-University Cooperative Research Program (LSIT01-10107) and National Multiple Sclerosis Society (RG3517A2, PI. Daniel Pelletier).
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