Short echo spectroscopic imaging of the human brain at 7T using transceiver arrays

Abstract

Recent advances in magnet technology have enabled the construction of ultra high-field magnets (7T and higher) that can accommodate the human head and body. Despite the intrinsic advantages of performing spectroscopic imaging at 7T, increased SNR and spectral resolution, few studies have been reported to date. This limitation is largely due to increased power deposition and B1 inhomogeneity. To overcome these limitations, we have utilized an 8 channel transceiver array with a short TE (15 ms) spectroscopic imaging sequence. Utilizing phase and amplitude mapping and optimization schemes, the 8 element transceiver array provided both improved efficiency, (17% less power for equivalent peak B1) and homogeneity (SD(B1) =±10% versus ±22%) in comparison to a transverse electromagnetic (TEM) volume coil. To minimize the echo time to measure J-modulating compounds such as glutamate, we developed a short TE sequence utilizing a single slice selective excitation pulse followed by a broad band semi-selective refocusing pulse. Extracerebral lipid resonances were suppressed with an inversion recovery pulse and delay. The short TE sequence enabled visualization of a variety of resonances, including glutamate in both a control subject and a patient with a grade II oligodendroglioma.

Improvement of resolution for brain coupled metabolites by optimized 1H MRS at 7 T

“…As the coupling strength, which governs the overall width of a multiplet, is independent of the external magnetic field, the spectral resolution between coupled resonances is improved with increasing field strength (19)(20)(21)(36)(37)(38). Moreover, the multiplet pattern of the C4 proton resonances of Glu, Gln and GSH, which are strongly coupled to the C3 protons at low fields, is Figure 7.…”

“…With these benefits, previous short-TE, single-voxel MRS (19,36) and spectroscopic imaging (20) studies have reported improved measurement of Gln and GSH by spectral fitting. It appears that the precise detection of Gln and GSH in the frontal brain using short TE at 7 T is challenging because of difficulties with B 0 shimming, the large width of the multiplets and the increased vulnerability to motion artifacts.…”

“…hippocampus) appear to be even more challenging at 7 T, most probably because of large B 0 and B 1 inhomogeneities. It may be improved with B 1 shimming of multichannel RF transmission (20,21). These technical challenges could be alleviated by employing an optimized, long-TE acquisition, with benefits from improved resolution and reduced MM signals.…”

Proton Magnetic Resonance Spectroscopy of the Central Nervous System

“…Magnetic field inhomogeneities result primarily from susceptibility differences between different tissues and between tissue and air cavities, which are scaled non-linearly in ultra-high magnetic fields [47]. Thus, voxels that are placed in inhomogeneous regions of the brain, such as the temporal poles, are difficult to shim due to their close proximity to the sinuses.…”

Adaptively optimized combination (AOC) of magnetic resonance spectroscopy data from phased array coils

“…The unsuppressed water signal was selected as the reference for computing the coil sensitivities and phase shifts of individual coil elements in this study (12)(13)(14)17,18,21), that is,…”