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

Spectroscopic imaging of the human head at short echo times (<or=15 ms) typically requires suppression of signals from extracerebral tissues. However, at 7 T, decreasing efficiency in B1+ 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 B1+ field distribution that excites a ring about the periphery of the head and leaves central brain regions largely unaffected. We have used this novel B1+ distribution to provide in-plane outer volume suppression (>98% suppression of extracerebral lipids) without the use of gradients. This novel B1+ 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.