7T MR spectroscopic imaging for human epilepsy

Yale University, New Haven CT

Investigators

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Abstract

DESCRIPTION (provided by applicant): While challenges of SNR, hardware, and pulse sequence have limited the penetration of MRSI into clinical use, it remains among the most sensitive avenues towards assessing cerebral function and an important motivation for ongoing 7T development. However at any field strength, MRSI has challenges for spectral quality, acceptable acquisition time and spatial coverage. Specifically, while 3T MRSI has reported excellent SNR for NAA in supraventricular locations, there remain acknowledged problems for spectral quality in critical brain regions including the temporal and frontal lobes. 7T MRS has shown the expected doubling in SNR, which with the >2-fold greater spectral resolution effectively gives a total 16x reduction for scan time in comparison to 3T. However, problems at 7T focus on rf coil technology and B0 inhomogeneity. At 300MHz, the dielectric constant of tissue results in marked axial and longitudinal B1 inhomogeneities, simultaneous to a linear increase in required power for equivalent B1 generation. With a goal of developing and implementing MR spectroscopic imaging at 7T, our group has developed a transceiver detector which as used with RF shimming, has shown excellent performance at 7T. In collaboration with Resonance Research Inc., we have also shown that with higher order shim mapping and corrections, outstanding field homogeneity can be achieved over extended brain regions. Thus far this success has been primarily achieved over single slice regions. In this project, we will continue to develop this work for wide brain and multi-slice MRSI at 7T. This will be achieved through Aim 1 that extends the longitudinal coverage of the transceiver and further improves large volume Bo homogeneity, and Aim 2 which develops the pulse sequences (B1 based localization, Hadamard and SENSE encoding with the J-refocused acquisition), our goal being high SNR multi-slice spectroscopic imaging with low SAR (~2W/kg). Because methodologic development ideally occurs with real-world targets, we will test these developments with the challenging problem of neocortical epilepsy (NE). Since many NE patients are clinically complex, their evaluation commonly requires intracranial EEG (icEEG), a neurosurgical procedure where intracranial electrodes are used to localize seizures. For this process, it is clear that as much advanced knowledge on where to place electrodes is needed, so as to not "miss" the seizure onset zone. Yet even with this complex process, the post-surgical outcome is that ~40-50% of patients continue with significant seizures. With the variable etiologies in NE, there are major challenges for MRSI coverage (seizures can arise from any cortical location), volume resolution (typical size of ictal onset zone), and optimal metabolite pattern (is glutamate better than NAA). These unknowns likely explain why MRSI is not routinely used at 3T, but even in anatomically well defined medial temporal lobe epilepsy, there are spectral quality problems at 3T. In Aim 3, we will test the hypothesis that in regions of seizure onset and propagation (as defined by icEEG) the NAA/Cr and Glu/Cr will be abnormal, thus determining the typical voxel size needed for such identification, and whether NAA or glutamate may be more accurate. To bring this work into greater implementation, Aim 4 will take the parameters identified at 7T into a collaboration with O Gonen PhD, New York Univ., a leader in the development and application of 3T wide brain coverage MRSI. We will compare extended volume coverage MRSI at 3T and 7T in healthy controls and in a limited group of patients, allowing us to define the optimum methods at 3T to achieve identification of ictogenic regions. This project proposes a coordinated development in hardware and pulse sequences for 7T MRSI. We believe that this project's impact is broad, not just for improved neurosurgical management of NE, but also for improved imaging and MRSI at 3 and 7T. As stated, 3T MRSI, while successful for supra- ventricular regions, is inconsistent in the temporal lobes. This will improve with our proposed work in higher order shims and algorithms that optimally correct for and redistribute B0 homogeneity. At 7T, the transceiver work is critical as presently there is no clear solution to the problem of homogeneous and extended rf (~20uT) coverage. Thus while the impact of this project is clearly for 7T MRSI, the proposed work in B1 methods and B0 shimming will be highly relevant for many aspects of high field MR, both 7 and 3T.