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

Zero-Gradient-Excitation Ramped Hybrid Encoding (zGRF-RHE) Sodium MRI (#95)

Yasmin Blunck 1 , Bradford A. Moffat 2 , Scott C. Kolbe 2 , Roger J. Ordidge 2 , Jon O. Cleary 2 , Leigh A. Johnston 1
  1. Department of Biomedical Engineering, University of Melbourne, Parkville, Victoria, Australia
  2. Department of Anatomy and Neuroscience, University of Melbourne, Parkville, Victoria, Australia

Sodium MRI offers promise as a clinical imaging modality, but currently provides comparatively low image quality resulting from the low MR-sensitivity and fast biexponential signal decay of sodium1. The latter necessitates dedicated fast sampling schemes like ultra-short (UTE) and zero (zTE) echo time sequences. UTE yields only limited encoding time (tenc) efficiency due to finite gradient slew rates, and zTE sampling efficiency improvements are not readily achievable in sodium MRI due to B1 limitations2-4.   A further restriction on the acquisition of fast signal decays is the finite transmit/receive switching time that introduces a sampling onset delay (~40-100μs on clinical systems). This constrains the achievable tenc and results in a sampling gap in central k-space for unsynchronized gradient and ADC onsets. 

In this work, we propose a zero-gradient-excitation ramped hybrid encoding (zGRF-RHE) sequence that provides 1) gradient-free excitation for high flip angle, artifact-free excitation profiles and 2) gradient ramping during deadtime for the optimisation of tenc.

Simulations and experiments in both phantoms and in vivo brain were performed with standard 3D-radial UTE and 3D-radial zGRF-RHE. Simulation and 7T-MR-measurement parameters were: FA=90°, TE=0.3ms, TR=160ms, resolution=(3.1mm)3, FOV=20cm, readout=2-8ms, 8000 projections. Central k-space in zGRF-RHE was acquired through single point measurements at the minimum achievable TE=0.3ms. T2* blurring and image SNR were assessed.

zGRF-RHE enabled 90° flip angle excitation, while gradient pre-ramping provided greater tenc efficiency (equivalent to system deadtime) at all readout bandwidths. Experiments confirmed simulation results, revealing sharper edge characteristics particularly at short readout durations. Significant SNR improvements of up to 4.8% were observed for longer readouts. 

In conclusion, we have shown zGRF-RHE allows for artifact-free high flip angle excitation with time-efficient encoding improving on image characteristics. Furthermore, the hybrid encoding concept with gradient pre-ramping is trajectory independent and can be introduced in any center-out UTE trajectory design.

 

  1. Thulborn KR. Quantitative sodium MR imaging: A review of its evolving role in medicine. Neuroimage 2016.
  2. Balcom BJ, Macgregor RP, Beyea SD, Green DP, Armstrong RL, Bremner TW. Single-Point Ramped Imaging with T1 Enhancement (SPRITE). J Magn Reson A 1996;123(1):131-134.
  3. Romanzetti S, Halse M, Kaffanke J, Zilles K, Balcom BJ, Shah NJ. A comparison of three SPRITE techniques for the quantitative 3D imaging of the 23Na spin density on a 4T whole-body machine. J Magn Reson 2006;179(1):64-72.
  4. Heid O, Deimling, M. Rapid Single Point (RASP) Imaging. 1995; Nice, France. p 684.