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 multibanded radiofrequency (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 multibanded slice acceleration has been limited by available peak power.

Theory and methods: Multibanded "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 3 T.

Results: For typical sinc pulses, time-shifted multibanded 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 multibanded 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.

Figure 1

From left to right: single-banded (SB) sinc RF pulse (R = 5.2) for band #1 (no frequency offset), SB pulse for band #4 (~3 kHz frequency offset), conventional four-banded (MB4) pulse with matched duration t, conventional MB4 pulse with stretched duration 1.75·t, time-shifted MB4 pulse generated with the proposed technique with duration 1.75·t (25% temporal shift between bands). For all plots, solid lines represent the magnitude, dashed lines the real component, and dotted lines the imaginary component.

Figure 2

Bipolar diffusion-weighted EPI sequence diagram using time-shifted multi-banded refocusing RF pulses. No time shift is used for the multi-banded excitation RF pulse.

Figure 3

Monopolar diffusion-weighted EPI sequences using time-shifted multi-banded refocusing and excitation RF pulses. The top diagram (a) shows the “aligned-echo” variant, where all spin echoes form at the same time, but the effective T_E of each band differs by 2Δ_S. The bottom diagram (b) shows the “aligned-T_E” variant, where the T_E of each band is the same, but the formation of the spin echoes for each band is separated in time by Δ_S.

Figure 4

Plot of required peak B₁ vs. total pulse duration for four-banded pulses with 1.25 kHz inter-band frequency offsets: stretched conventional pulse (dark blue), time-shifted pulse (green), time-shifted with static inter-band phase offsets (light blue), and time-shifted with optimized phase offsets for each shift (red). B₁ and duration are shown relative to the base single-banded sinc pulse.

Figure 5

Slice profiles measured in a cylindrical oil phantom. For reference, slice profiles acquired separately with single-band pulses at the minimum T_E of 28 ms are shown in gray. The six slices simultaneously acquired with a time-shifted six-banded RF pulse are shown in red (T_E = 38 ms); this is compared to single-band acquisition at the same T_E is shown in blue. The same six slices acquired with a stretched conventional six-banded RF pulse are shown in green (T_E = 52 ms).

Figure 6

Comparison of human brain images acquired with multi-band slice accelerations of 3–6 (MB3-MB6) using conventional multi-banded RF pulses (left column) and time-shifted RF pulses (right column) with equivalent effective bandwidth. The display window and level are the same for all images.