Article

Self-refocused adiabatic pulse for spin echo imaging at 7 T

Department of Radiology, Stanford University, Stanford, CA, USA.
Magnetic Resonance in Medicine (Impact Factor: 3.4). 04/2012; 67(4):1077-85. DOI: 10.1002/mrm.23089
Source: PubMed

ABSTRACT Spin echo pulse sequences are used to produce clinically important T(2) contrast. However, conventional 180° radiofrequency pulses required to generate a spin echo are highly susceptible to the B(1) inhomogeneity at high magnetic fields such as 7 Tesla (7 T), resulting in varying signal and contrast over the region of interest. Adiabatic 180° pulses may be used to replace conventional 180° pulses in spin echo sequences to provide greater immunity to the inhomogeneous B(1) field at 7 T. However, because the spectral profile of an adiabatic 180° pulse has nonlinear phase, pairs of these pulses are needed for proper refocusing, resulting in increased radiofrequency power deposition and long minimum echo times. We used the adiabatic Shinnar Le-Roux method to generate a matched-phase adiabatic 90°-180° pulse pair to obviate the need for a second adiabatic 180° pulse for phase refocusing. The pulse pair was then reformulated into a single self-refocused pulse to minimize the echo time, and phantom and in vivo experiments were performed to validate pulse performance. The self-refocused adiabatic pulse produced transmit profiles that were substantially more uniform than those achieved using a conventional spin echo sequence.

Full-text

Available from: Daniel M Spielman, Jun 04, 2015
1 Follower
 · 
156 Views
  • [Show abstract] [Hide abstract]
    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 are 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 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 DTI at 7 T as well as 3 T.
    Magnetic Resonance Imaging 09/2014; 32(7). DOI:10.1016/j.mri.2014.04.003 · 2.02 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Applications of broadband pulses for EPR have been reported for FID, echo detection and inversion pulses recently. Here we present a broadband Hahn, stimulated and refocused echo sequence derived from adiabatic pulses. The formation of echoes is accomplished by using variable chirp rates and pulse lengths. In all three broadband echo experiments the complete spectral shape of a nitroxide (about 70 Gauss at X-band frequency) could be recovered by Fourier transformation of the quadrature detected echo signals. Such broadband echoes provide an exciting opportunity to optimize pulse sequences where a full excitation of the spectrum is mandatory for an optimum performance. We applied our pulses to the SIFTER (single frequency technique for refocusing dipolar couplings) experiment, a solid echo based pulse sequence to measure the dipolar coupling between two unpaired electron spins. By employing our broadband Hahn echo sequence on a nitroxide biradical we could achieve an artifact free dipolar evolution time trace in the SIFTER experiment with 95 % modulation depth at X-band frequency and of 10 % modulation depth at Q-band frequency.
    Journal of Magnetic Resonance 11/2014; 250. DOI:10.1016/j.jmr.2014.10.017 · 2.32 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: To propose a new phase-based B1 -mapping method that exploits phase information created by hyperbolic secant (HS) pulses in conventional 2D spin-echo imaging. In this B1 -mapping method, HS pulses are used to accomplish π/2 excitation and π refocusing in standard multislice spin-echo imaging. When setting the ratio of pulse lengths of the π/2 and π HS pulses to 2:1, the spin-echo phase is independent of offset frequency and varies as a function of B1 strength. To eliminate undesired phase accumulations induced by unknown factors other than the B1 strength, two spin-echo images are acquired using HS pulses applied with opposite frequency-sweep directions, and the resulting phase images are subtracted from each other. To demonstrate the performance of the proposed method, phantom and in vivo experiments were performed using a surface coil and a volume coil. The B1 maps obtained by using the proposed method were in accordance with the B1 maps obtained using previous methods in both phantom and in vivo experiments. The proposed method is easy to implement without any sequence modification, is insensitive to B0 inhomogeneity and chemical shift, and is robust in a reasonably wide range of B1 field strength. Magn Reson Med, 2014. © 2014 Wiley Periodicals, Inc.
    Magnetic Resonance in Medicine 01/2015; 73(1). DOI:10.1002/mrm.25110 · 3.40 Impact Factor