Original contributionSemi-adiabatic Shinnar–Le Roux pulses and their application to diffusion tensor imaging of humans at 7T☆
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.
References (30)
- et al.
Diffusion-weighted imaging of the brain at 7 T with echo-planar and turbo spin echo sequences: preliminary results
Magn Reson Med
(2011) - et al.
Readout-segmented EPI for rapid high resolution diffusion imaging at 3 T
Eur J Radiol
(2008) Double-spin-echo diffusion weighting with a modified eddy current adjustment
Magn Reson Med
(2010)- et al.
Highly selective π/2 and π pulse generation
J Magn Reson
(1984) - et al.
Optimization of modulation functions to improve insensitivity of adiabatic pulses to variations in B1 magnitude
J Magn Reson
(1988) - et al.
Improved performance of frequency-swept pulses using offset-independent adiabaticity
J Magn Reson
(1996) - et al.
The human connectome project and beyond: initial applications of 300 mT/m gradients
Neuroimage
(2013) - et al.
Pushing the limits of in vivo diffusion MRI for the Human Connectome Project
Neuroimage
(2013) - et al.
Designing adiabatic RF pulses using the Shinnar–Le Roux algorithm
Magn Reson Med
(2010) - et al.
Signal to noise ratio and uncertainty in diffusion tensor imaging at 1.5, 3.0, and 7.0 Tesla
J Magn Reson Imaging
(2011)
Advances in ultra-high field MRI for the clinical management of patients with brain tumors
Curr Opin Neurol
Simultaneous quantification of T2 and T'2 using a combined gradient echo-spin echo sequence at ultrahigh field
Magn Reson Med
Diffusion imaging in humans at 7 T using readout-segmented EPI and GRAPPA
Magn Reson Med
Improvement in Diffusion MRI at 3 T and Beyond with the Twice-Refocused Adiabatic Spin Echo (TRASE) Sequence
Adiabatic refocusing pulses in 3 T and 7 T diffusion imaging
Cited by (7)
UltraHigh Field MR Imaging in Epilepsy
2021, Magnetic Resonance Imaging Clinics of North AmericaCitation Excerpt :The resultant inhomogeneity can be particularly problematic in the visualization of deep brain structures and temporal lobe, complicating ultrahigh field imaging–based assessment of epilepsy. Multiple techniques have emerged for mitigating the effects of this inhomogeneity including B1 insensitive adiabatic pulses72–74 and parallel transmit (PTx) coils, which use multiple transmission elements working independently and predetermined B1 field maps to compensate for inhomogeneity with hardware.75 The use of high permittivity dielectric pads, such as a deuterium-based suspension of barium titanate,76,77 have demonstrated improved coverage in regions with poor B1-homogeneity at ultrahigh field strengths.
Diffusion MRI of the human brain at ultra-high field (UHF): A review
2018, NeuroImageCitation Excerpt :Although adiabatic RF pulses can address the problem of B1-inhomogeneity - they are typically associated with an increase in RF power deposition, meaning that SAR restrictions at ultra-high field become an even greater issue. It has recently been demonstrated that it is possible to design semi-adiabatic refocusing pulses which specifically address the problem of B1-inhomogeneity at ultra-high field (Balchandani and Qiu, 2014) - allowing to trade-off some of the adiabaticity in exchange for lower peak B1 and SAR. Even with these improvements, the SAR threshold can remain restrictive at ultra-high field when two semi-adiabatic pulses are required per readout.
B<sup>+</sup><inf>1</inf> inhomogeneity mitigation for diffusion weighted MRI at 7T using TR-FOCI pulses
2024, Magnetic Resonance in MedicineUltrahigh field MR Neuroimaging
2019, Topics in Magnetic Resonance ImagingUltrahigh field single-refocused diffusion weighted imaging using a matched-phase adiabatic spin echo (MASE)
2016, Magnetic Resonance in MedicineA SEmi-Adiabatic matched-phase spin echo (SEAMS) PINS pulse-pair for B<inf>1</inf>-insensitive simultaneous multislice imaging
2016, Magnetic Resonance in Medicine
- ☆
This work was supported by Lucas Foundation, NIH R01 MH080913, NIH-NINDS K99/R00 NS070821, P41 EB015891, and GE Healthcare.