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RF shimming for spectroscopic localization in the human brain at 7 T

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Hetherington, H., Avdievich, N., Kuznetsov, A., & Pan, J. (2010). RF shimming for spectroscopic localization in the human brain at 7 T. Magnetic Resonance in Medicine, 63(1), 9-19. doi:10.1002/mrm.22182.


Cite as: http://hdl.handle.net/21.11116/0000-0002-7771-9
Abstract
Spectroscopic imaging of the human head at short echo times (≤15 ms) typically requires suppression of signals from extracerebral tissues. However, at 7 T, decreasing efficiency in Burn:x-wiley:07403194:media:MRM22182:tex2gif-stack-1 generation (hertz/watt) and increasing spectral bandwidth result in dramatic increases in power deposition and increased chemical shift registration artifacts for conventional gradient‐based in‐plane localization. In this work, we describe a novel method using radiofrequency shimming and an eight‐element transceiver array to generate a Burn:x-wiley:07403194:media:MRM22182:tex2gif-stack-2 field distribution that excites a ring about the periphery of the head and leaves central brain regions largely unaffected. We have used this novel Burn:x-wiley:07403194:media:MRM22182:tex2gif-stack-3 distribution to provide in‐plane outer volume suppression (>98% suppression of extracerebral lipids) without the use of gradients. This novel Burn:x-wiley:07403194:media:MRM22182:tex2gif-stack-4 distribution is used in conjunction with a double inversion recovery method to provide suppression of extracerebral resonances with T1s greater than 400 ms, while having negligible effect on metabolite ratios of cerebral resonances with T1s > 1000 ms. Despite the use of two adiabatic pulses, the high efficiency of the ring distribution allows radiofrequency power deposition to be limited to 3‐4 W for a pulse repetition time of 1.5 sec. The short echo time enabled the acquisition of images of the human brain, displaying glutamate, glutamine, macromolecules, and other major cerebral metabolites.