Sodium MRI optimization for the human head with application to acute stroke

  • Author / Creator
    Stobbe, Robert
  • 25 years after the first sodium images of the human brain were created, sodium MRI remains on the periphery of MR research, despite intimate sodium involvement in cellular metabolism and implicated abnormal concentrations in numerous disorders. The difficulties of sodium MRI include not only tissue concentration, ~1750x less than proton, but also rapid biexponential signal decay. The purpose of this work was to optimize human brain sodium MRI and facilitate a study of sodium increase following onset of acute human stroke, with potential ‘timing’ application for those patients who present with unknown time-of-onset, as effective treatment is currently bound by a 4.5 hour time-window. Optimization begins with radial ‘center-out’ k-space acquisition, which minimizes echo time (TE) and signal loss, and in particular concerns the twisted projection imaging (TPI) technique, which has not found wide use. This thesis first considers a new application of TPI, i.e. k-space filtering by sampling density design to minimize detrimental ringing artifact associated with cerebral spinal fluid. Image noise correlation is addressed next, and a method for measuring volumes of statistical noise independence presented, as this correlation together with signal-to-noise ratio (SNR) defines the confidence of signal-averaging measurements. Radial acquisition is then considered with respect to a new imaging metric, i.e. the minimum object volume that can be precisely (with respect to noise) and accurately (with respect to image intensity modulation with object volume) quantified. It is suggested that TPI is a highly beneficial radial acquisition technique when implemented with ‘long’ readout duration (countering common thought), reduced SNR (i.e. small voxel volumes), and in particular small TPI parameter p. Sequence optimization for bulk-tissue sodium analysis demonstrates a large SNR/voxel-volume advantage for TPI implementation in a steady-state approach, even though excitation pulse length and TE must be increased to mitigate power deposition. Finally, an inversion-recovery based fluid-nulling method that facilitates sodium environment separation based on rapid relaxation during soft inversion pulses is presented, with possible application for intracellular weighted imaging. On ‘high quality’ sodium images a clear trend of lesion intensity increase with time-after-onset is demonstrated for the first time in acute stroke patients, as expected from animal models.

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  • Degree
    Doctor of Philosophy
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    This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.