SPECTRAL-SPATIAL PULSES FOR REDUCED DISPLACEMENT ARTIFACTS AT 7 TESLA

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This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. For localized MR spectroscopy (MRS) at high field (>= 3 T), chemical shift misregistration of the excited/refocused volume is a major concern. This is because the peak achievable RF amplitude is typically similar to lower field systems (with the same sized transmit coil), but the range of chemical shifts to be imaged is proportionally larger. This is reduced using high bandwidth saturation pulses to trim the misregistered edges of the volume of interest, at the cost of RF heating. We have been exploring the approach of using spectral-spatial pulses, which have no shift artifact. Methods and Discussion: Conventional slice-selective excitation and refocusing pulses are transmitted along with a constant slice-select gradient. Invoking the excitation k-space formalism [1], this gradient results in a single, angled line through kz vs. time. The Fourier transform of this pattern is tilted by the same angle. This tilt is the source of chemical-shift mis-registration. With the oscillating gradient used with spectral-spatial pulses [2,3], which gives a zig-zag trajectory through kz vs. time, this tilt can be entirely removed over the bandwidth defined by the oscillation frequency.