Abstract
PURPOSE: The optimal excitation flip angle (FA) for short-TR spin-echo acquisitions can be well above 90°, far beyond the small FAs suited for commonly-used sinc RF pulses. The goal of this study was to characterize the benefits of Shinnar-Le Roux (SLR) over sinc pulses for these acquisitions, which may lead to improvements in the temporal and spatial specificity of fMRI. METHODS: Slice profiles were obtained either through Bloch simulation or from scans of an oil phantom at 7 T (T(1)/TR = 1500/300 ms). Spatial integrals of the slice profiles were used as measures of the resulting (relative) SNR. We also measured the spatial profile of spin-echo "linescan" acquisitions, which are of increasing interest for in vivo studies of cortical layers. RESULTS: For 2D acquisitions with the parameter values used here, the high-quality slice profiles provided by the SLR pulses resulted in an SNR gain of ∼100% relative to sinc pulses. For 1D linescan acquisitions, the SNR gains were even higher: ∼150%. CONCLUSIONS: The large gains in SNR described here should enhance any studies using short-TR spin-echo acquisitions; in particular, we anticipate application of these SLR pulses to fMRI studies that target the microvasculature with both high spatial and high temporal resolution. Potential limitations, due to high SAR or B(1)(+) inhomogeneity, should however be kept in mind, especially at ultra-high field strengths.