Multiband RF pulse design for realistic gradient performance

Abstract

PurposeSimultaneous multi‐slice techniques are reliant on multiband RF pulses, for which conventional design strategies result in long pulse durations, lengthening echo‐times so lowering SNR for spin‐echo imaging, and lengthening repetition times for gradient echo sequences. Pulse durations can be reduced with advanced RF pulse design methods that use time‐variable selection gradients. However, the ability of gradient systems to reproduce fast switching pulses is often limited and can lead to image artifacts when ignored. We propose a time‐efficient pulse design method that inherently produces gradient waveforms with lower temporal bandwidth.MethodsEfficient multiband RF pulses with time‐variable gradients were designed using time‐optimal VERSE. Using VERSE directly on multiband pulses leads to gradient waveforms with high temporal bandwidth, whereas VERSE applied first to singleband RF pulses and then modulated to make them multiband, significantly reduces this. The relative performance of these approaches was compared using simulation and experimental measurements.ResultsApplying VERSE before multiband modulation was successful at removing out‐of‐band slice distortion. This effectively removes the need for high frequency modulation in the gradient waveform while preserving the benefit of time‐efficiency inherited from VERSE.ConclusionWe propose a time‐efficient RF pulse design that produces gradient pulses with lower temporal bandwidth, reducing image artifacts associated with finite temporal bandwidth of gradient systems.

Time optimal control‐based RF pulse design under gradient imperfections

“…The GIRF alterations can be iteratively corrected for a given RF and G s shape 33 or reduced limiting the G s frequency content. 17 These methods, however, do not guarantee compliance with respect to the hardware constraints. Alternatively, the presented direct inclusion of the measured GIRF in the optimization compensated for gradient imperfections and resulted in a matched RF pulse that is linked to the filtered slice selective gradient shape G s .…”

“…Since neither system had both capabilities, SMS imaging was performed by scanner 2 using pulses corrected for the GIRF measured from scanner 1. Though not ideal, prior experience suggests that the gradient performance of both systems is similar 17 ; errors that may arise from assuming this equivalence will be discussed later. Figure 1 depicts the idealized (H i ) and the measured transfer functions along the y (H y ) and z (H z ) direction with frequency resolution δ f = 76.3 Hz.…”

“…The good experimental performance implies that the gradient systems do have similar frequency responses, which we have observed before. 17 However, more work is needed to understand the limits of using generic GIRFs to improve the applicability of the proposed method.…”

“…This reduction of aliased slices is in agreement with previously published work. 17 The proposed method provides a new approach for GIRFcorrected RF pulse design. The incorporation of the GIRF into the optimization framework generates a pre-emphasized G s shape and a matched RF pulse that can be applied to further push the limits of SMS sequences such as clinically important turbo spin echo (TSE) based sequences, 36 functional imaging 37 or short SE diffusion applications.…”

“…5 The increased T E easily becomes a limiting issue for spin echo (SE) based applications such as turbo spin echo 7 or high resolution diffusion imaging. 8 Therefore, different RF pulse design methods have been introduced to reduce peak RF amplitudes [9][10][11] or RF power by applying variable-rate slice selective excitation (VERSE) [12][13][14][15][16][17] or power independent number of slices (PINS). 18,19 Alternatively, SMS RF pulses can be computed via optimal control.…”