Multiband accelerated spin‐echo echo planar imaging with reduced peak RF power using time‐shifted RF pulses

Abstract

Purpose To evaluate an alternative method for generating multi-banded RF pulses for use in multiband slice-accelerated imaging with slice-GRAPPA unaliasing, substantially reducing the required peak power without bandwidth compromises. This allows much higher accelerations for spin-echo methods such as SE-fMRI and diffusion-weighted MRI where multi-banded slice acceleration has been limited by available peak power. Theory and Methods Multi-banded “time-shifted” RF pulses were generated by inserting temporal shifts between the applications of RF energy for individual bands, avoiding worst-case constructive interferences. Slice profiles and images in phantoms and human subjects were acquired at 3T. Results For typical sinc pulses, time-shifted multi-banded RF pulses were generated with little increase in required peak power compared to single-banded pulses. Slice profile quality was improved by allowing for higher pulse bandwidths, and image quality was improved by allowing for optimum flip angles to be achieved. Conclusion A simple approach has been demonstrated that significantly alleviates the restrictions imposed on achievable slice acceleration factors in multiband spin-echo imaging due to the power requirements of multi-banded RF pulses. This solution will allow for increased accelerations in diffusion-weighted MRI applications where data acquisition times are normally very long and the ability to accelerate is extremely valuable.

Whole brain, high resolution multiband spin-echo EPI fMRI at 7T: A comparison with gradient-echo EPI using a color-word Stroop task

“…Another limitation related to the hardware rather than the safety limits is the peak voltage which also increases linearly with the flip angle and the number of slices. In this study, in line with previously reported RF peak power reduction methods (Auerbach et al, 2013;Goelman, 1997;Hennig, 1992) we optimized the phases of the multiplexed slices along the lines of (Wong, 2012) and achieved a 26% decrease in the RF peak amplitude. The low signal intensity observed at the center of SE EPI images is caused by B1 inhomogeneity which is a typical feature of high field.…”

Magnetic Resonance Imaging at Ultrahigh Fields

“…Efforts based on using different RF pulses [142], [143] or parallel transmit pulse design [64], [118] has been important advances to overcome these limitations. The parallel transmit pulse design approach provides the unique capability to improve the flip angle distribution over the targeted volume while constraining power parameters [64], [118]; this has been demonstrated first for total RF power [64], [118] and recently extended to global and local power deposition [144]–[146].…”

Three‐dimensional echo planar imaging with controlled aliasing: A sequence for high temporal resolution functional MRI

“…Such approaches have shown their potential to provide sub-second temporal resolution in fMRI. The main penalty of the multiband excitation approach, especially at high fields, is the peak power of multiband pulses (18), although various methods have been developed to address this limitation (19)(20)(21)(22)(23).…”