Large gains in SNR through the application of Shinnar-Le Roux RF pulse design to short-TR spin-echo fMRI acquisitions at 7 T

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 (T₁/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 B₁⁺ inhomogeneity, should however be kept in mind, especially at ultra-high field strengths.