Elsevier

Magnetic Resonance Imaging

Volume 32, Issue 7, September 2014, Pages 804-812
Magnetic Resonance Imaging

Original contribution
Semi-adiabatic Shinnar–Le Roux pulses and their application to diffusion tensor imaging of humans at 7T

https://doi.org/10.1016/j.mri.2014.04.003Get rights and content

Abstract

The adiabatic Shinnar–Le Roux (SLR) algorithm for radiofrequency (RF) pulse design enables systematic control of pulse parameters such as bandwidth, RF energy distribution and duration. Some applications, such as diffusion-weighted imaging (DWI) at high magnetic fields, would benefit from RF pulses that can provide greater B1 insensitivity while adhering to echo time and specific absorption rate (SAR) limits. In this study, the adiabatic SLR algorithm was employed to generate 6-ms and 4-ms 180° semi-adiabatic RF pulses which were used to replace the refocusing pulses in a twice-refocused spin echo (TRSE) diffusion-weighted echo planar imaging (DW-EPI) sequence to create two versions of a twice-refocused adiabatic spin echo (TRASE) sequence. The two versions were designed for different trade-offs between adiabaticity and echo time. Since a pair of identical refocusing pulses is applied, the quadratic phase imposed by the first is unwound by the second, preserving the linear phase created by the excitation pulse. In vivo images of the human brain obtained at 7 Testa (7 T) demonstrate that both versions of the TRASE sequence developed in this study achieve more homogeneous signal in the diffusion-weighted images than the conventional TRSE sequence. Semi-adiabatic SLR pulses offer a more B1-insensitive solution for diffusion preparation at 7 T, while operating within SAR constraints. This method may be coupled with any EPI readout trajectory and parallel imaging scheme to provide more uniform coverage for diffusion tensor imaging at 7 T and 3 T.

Introduction

Adiabatic radiofrequency (RF) pulses can be powerful replacements for conventional windowed sinc or Shinnar–Le Roux (SLR) pulses in MR pulse sequences to provide greater immunity to B1 inhomogeneity when operating at high magnetic fields or when using surface coils with non-uniform B1 profiles. One of the advantages of the adiabatic SLR algorithm described in 2010 [1], is that the designer has greater control over pulse and spectral profile characteristics. Therefore trade-offs can more easily be made between aspects of pulse performance such as spatial selectivity, bandwidth, duration, maximum amplitude and adiabaticity. Adiabatic pulses often have high time–bandwidth (TBW) products resulting in good selectivity, but also high RF power deposition as measured by the specific absorption rate (SAR). Sometimes, a certain amount of B1 insensitivity is desired but bandwidth (BW) and duration of the RF pulse need to be lower in order to adhere to echo time (TE) and SAR limits. Using the adiabatic SLR algorithm, these parameters can be traded off for degree of adiabaticity. Furthermore, the adiabatic SLR method allows the pulse designer to apply additional quadratic phase across the pulse profile in order to achieve a more uniform distribution of RF energy. One application in which a short, low-BW, B1-insensitive pulse would be useful is diffusion tensor imaging (DTI) in the presence of the non-uniform B1 field that is observed at 7 Tesla (7 T).

The higher signal-to-noise ratio (SNR) offered at 7 T has been shown to increase the SNR and directional certainty of DTI-based parameters such as fractional anisotropy (FA) [2], [3]. However, DTI at 7 T faces major challenges including B0 and B1 inhomogeneity and shortening of white matter T2 values, from 71 ms at 3 T to 47 ms at 7 T [4], offsetting the SNR benefits offered by the higher magnetic field. Nonetheless, development of an optimized DTI sequence is essential for developing a complete neuroimaging protocol at 7 T. Although some techniques have been proposed that provide solutions for better excitation or readout of magnetization at 7 T [5], [6], [7], [8], [9], the optimal sequence structure for 7 T DTI has yet to be established.

Readout-segmented echo planar imaging (EPI) combined with parallel imaging may be used to contend with the severe B0 artifact [5]. Fast spin echo (FSE or TSE) sequences have also been used at 7 T instead of EPI to reduce susceptibility artifacts [6]. However, in order to reduce signal loss due to B1 inhomogeneity, alternative approaches for excitation of the magnetization must be explored. A simple Stejskal–Tanner pulse sequence utilizes a single refocusing pulse and allows for shorter TEs but is not easily translated to an adiabatic sequence and must be coupled with post-processing eddy current compensation methods that result in some data blurring. Looking forward, as stronger gradient sets are used to shorten echo times and achieve higher b values, diffusion gradients will be driven to a higher maximum value, increasing the severity of eddy current distortions which scale with maximum gradient amplitude.

A twice-refocused spin echo (TRSE) diffusion preparation which utilizes two 180° RF pulses, is regularly used in clinical scans and has shown value in substantially reducing eddy current distortions [10], [11]. A TRSE approach cancels eddy current effects through the use of appropriately chosen bipolar diffusion gradients [10], [12]. It is particularly well suited to the use of adiabatic refocusing pulses because the second identical adiabatic 180° pulse perfectly refocuses quadratic phase induced by the first one, leaving no net phase in the final echo. This is important because quadratic phase across the final slice would result in signal loss. Echo time for TRSE sequences may be shortened by the use of stronger diffusion gradients, while inherently compensating for the resultant increase in eddy current effects. Furthermore, the twice-refocused approach may be used to perform reduced field of view imaging at high resolutions [13]. In this case a second 180° pulse is necessary to reset inverted spins outside the desired slice or to provide a third dimension of selectivity. Therefore it is of value to investigate twice-refocused approaches to diffusion-weighted imaging at 7 T, in addition to single-refocused approaches.

Our objective was to increase the B1 insensitivity of the TRSE sequence while not significantly increasing the effective TE and operating within SAR limits for similar acquisition times. This was achieved by replacing the 180° RF pulses in the TRSE sequence with semi-adiabatic SLR pulses which were specifically designed to minimize pulse duration and peak B1 at adiabatic threshold, while maintaining a high degree of adiabaticity. The semi-adiabatic SLR pulses achieve higher-quality slice profiles with lower SAR and peak amplitude than conventional hyperbolic secant (HS) [14], [15], [16] pulses designed to have similar BW and pulse duration. Using a twice-refocused adiabatic spin echo (TRASE) sequence has been previously proposed and shown to improve image homogeneity in phantoms at 3 T and 7 T and DWI of animals at 14.1 T [7], [8], [17].

This work shows that semi-adiabatic SLR pulses can substantially improve the performance of the standard TRSE sequence at 7 T and provide more uniform SNR in the presence of B1 inhomogeneity. Two different variants of the semi-adiabatic SLR pulses with different pulse durations, BW, and B1 insensitivity were tested. Although this method could be applicable at both 3 T and 7 T, results are obtained in the human brain at 7 T in order to demonstrate improved performance in the presence of severe B1 inhomogeneity.

Section snippets

RF pulse and pulse sequence design

Our first step was to design an adiabatic SLR 180° pulse to replace the 180° pulses in the TRASE DWI sequence. The adiabatic SLR algorithm [1] was used to generate an adiabatic full-passage 180° pulse.

The pulse was designed to have physical spectral bandwidth of 2.32 kHz, pulse duration of 6 ms and fractional transition width (FTW) of 0.5. The sampling rate, fs, in Hz for the filter was set to 119 kHz. Values used as inputs for the finite impulse response filter design function “firls” in MATLAB

Results

Fig. 4 shows diffusion-weighted images in one direction for a slice of the brain of a representative volunteer obtained using (A) product TRSE and (B) Version 1 of our TRASE sequence for DWI. The B1 profile of the slice is shown in Fig. 4D. The main area of signal loss due to a peak in the B1 profile at the center of the brain is indicated by the white arrow in Fig. 4A. This signal is largely recovered by the TRASE sequence as shown by Fig. 4B. Central cross sections through these two images

Discussion

In vivo data obtained at 7 T demonstrate that semi-adiabatic SLR pulses provide significant improvement in the SNR and uniformity of diffusion-weighted images acquired using a twice-refocused sequence while maintaining similar TEs and acquisition times. This method for diffusion preparation may be coupled with improved readout trajectories that compensate for B0 inhomogeneity, resulting in robust DTI sequences for use at 7 T. Using the adiabatic SLR method, it was possible to systematically

Acknowledgments

We would like to thank Mehdi Khalighi for his help with B1 transmit and receive sensitivity maps and Brian Rutt, Jennifer McNab, Gary Glover and Daniel Spielman for helpful discussions.

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    This work was supported by Lucas Foundation, NIH R01 MH080913, NIH-NINDS K99/R00 NS070821, P41 EB015891, and GE Healthcare.

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