Oral Presentation Australian & New Zealand Society of Magnetic Resonance Conference 2017

B1 insensitive RF excitation using off-resonant spin-lock correction (#17)

Edward M Green 1 , James C Korte 1 , Bahman Tahayori 2 , Peter M Farrell 3 , Leigh A Johnston 1
  1. Dept. Biomedical Engineering, University of Melbourne, Melbourne, Australia
  2. Dept. Electrical and Computer Systems Engineering, Monash University, Melbourne, Australia
  3. Dept. Electrical and Electronic Engineering, University of Melbourne, Melbourne, Australia

Inhomogeneity in B1 excitation fields can be caused by either hardware properties, in the case of surface transmit coils, or by tissue properties, particularly prevalent in high field MRI systems1. B1 inhomogeneity results in flip angle variation when using conventional RF pulses, which produces signal strength non-uniformity1. Current attempts to address B1 inhomogeneity include 1) the use of parallel transmit systems2, which are expensive, require time consuming subject-specific computation and are potentially dangerous if the subject moves, and 2) B1 insensitive pulses, including composite pulses3 and adiabatic pulses4, which can be slow to apply and can be limited by specific absorption rate constraints. In addition to these acquisition adaptations, B1 inhomogeneity can be measured and compensated for during post-processing5.  Significant signal drop-out, however, cannot be counteracted by post-processing, and therefore improved excitation pulse designs are sought.

Variations in B1 field strength cause bulk magnetisation to acquire a distribution of path-lengths on the Bloch sphere, leading to non-uniformity in image intensity.  We demonstrate a new class of B1 insensitive RF pulses that use off-resonant spin-lock correction (ORSLC).  ORSLC pulses combine on-resonance and off-resonance excitation, making it possible to maintain control over the spin trajectories in order to achieve a bulk magnetisation response that is accurately localised on the Bloch sphere.  We show that composite pulses made up of an interleaved series of on-resonant flips and off-resonant spin-locking corrections can achieve similar B1 insensitivity performance to adiabatic pulses.  As with adiabatic pulses, however, slice selection is challenging, and therefore we initially focus on the use of off-resonant spin-lock correction in 3D imaging sequences.

ORSLC pulses provide the opportunity to deliver B1 insensitivity performance comparable to adiabatic pulses, while addressing concerns of temporal efficiency and specific absorption rate limitations.

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  2. Lattanzi, R., et al., Electrodynamic Constraints on Homogeneity and Radiofrequency Power Deposition in Multiple Coil Excitations. Magnetic Resonance in Medicine, 2009. 61(2): p. 315-334.
  3. Levitt, M.H., COMPOSITE PULSES. Progress in Nuclear Magnetic Resonance Spectroscopy, 1986. 18: p. 61-122.
  4. Garwood, M. and L. DelaBarre, The return of the frequency sweep: Designing adiabatic pulses for contemporary NMR. Journal of Magnetic Resonance, 2001. 153(2): p. 155-177.
  5. van Schie, J.J.N., et al., Feasibility of a fast method for B-1-inhomogeneity correction for FSPGR sequences. Magnetic Resonance Imaging, 2015. 33(3): p. 312-318.