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

Balanced steady-state driven trajectory imaging (#105)

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 & Electronic Engineering, University of Melbourne, Melbourne, Australia

Continuous wave (CW) RF excitation has been shown to drive the spin-system into information-rich steady-state trajectories1,2. In recent work, we have demonstrated the ability to estimate underlying spin-system properties such as chemical shift1 and relaxation rates2 from a series of measured steady-state trajectories. Here we propose a pseudo-continuous method to efficiently image steady-state trajectories, providing novel tissue contrast and the potential to estimate spin-system parameter maps. Our method, balanced steady-state driven trajectory (bSSDT) imaging, is analogous to balanced steady-state free precession (bSSFP) methods in the pulsed-excitation regime.

In bSSDT imaging, a pseudo-continuous acquisition protocol is achieved by switching rapidly between excitation and measurement, as previously demonstrated by time-interleaved implementations of Sweep Imaging with Fourier Transformation (SWIFT)3. Balanced frequency encoding gradients are applied during the measurement gap of the pseudo-continuous protocol to measure a series of 3D radial k-space projections, across a range of Rabi-modulated excitation parameters. From these sets of projections, a time series of image volumes is reconstructed, providing voxelwise steady-state magnetisation trajectories.

The duration of a bSSDT experiment is longer than bSSFP due to the measurement of a periodic trajectory as opposed to a steady-state point. If we consider bSSFP to be hardware limited by a minimum repetition time, TR, and the trajectory measured by bSSDT to have a period, T0, then a bSSDT acquisition will take approximately N=T0/TR times longer than bSSFP. Low power excitation envelopes are selected to address SAR limitations.

We have circumvented the drawbacks of a fully CW acquisition, including 1) hardware design difficulties due to magnitude mismatch between RF excitation and NMR signals, 2) the constraints imposed by modern spectrometer hardware designed for pulsed FT methods and 3) the difficulty to localise information efficiently under CW excitation4, by introducing a pseudo-continuous imaging technique that captures the novel contrast in the CW steady-state trajectories. Future work will evaluate the ability to estimate relaxation maps from steady-state trajectories in response to Rabi modulated excitation using the bSSDT imaging method.

  1. Korte, J. C., Layton, K. J., Tahayori, B., Farrell, P. M., Moore, S. M., and Johnston, L. A. (2017). NMR spectroscopy using Rabi modulated continuous wave excitation. Biomedical Signal Processing and Control, 33:422-428.
  2. Korte, J. C., Tahayori, B., Farrell, P. M., Moore, S. M., and Johnston, L. A. (2017). Relaxometry via steady-state ring-locked trajectories. In Proceedings of the 25th Annual Meeting of ISMRM, Honolulu, USA.
  3. Idiyatullin, D., Corum, C., Park, J.Y., and Garwood, M. (2006). Fast and quiet MRI using a swept radiofrequency. Journal of Magnetic Resonance, 181:342-349.
  4. Korte, J. C., Tahayori, B., Farrell, P. M., Moore, S. M., and Johnston, L. A. (2016). Rabi modulated continuous wave imaging. In Proceedings of the 24th Annual Meeting of ISMRM, Singapore.