Sodium MRI of the Human Brain: Application to Ischemic Stroke and the Development of Multiple Quantum Filtering.

摘要

MRI of sodium in the brain is much more challenging compared to hydrogen. However, imaging of brain tissue sodium has been suggested to provide temporal information in acute ischemic stroke that may benefit patients with unknown onset time such as those awake with symptoms. Furthermore, selective imaging of intracellular sodium may provide compartment specific changes early after onset prior to the increase of tissue sodium as demonstrated previously in animal models. Both inversion recovery (IR) and triple-quantum-filtering (TQF) methods have been proposed to probe sodium signal weighted toward intracellular. Unfortunately, signal is greatly reduced (more so for TQF) using compartment specific sodium imaging sequences. Consequently, intracellular-weighted sodium images, especially for TQF, are poor in spatial resolution with low signal-to-noise ratio (SNR).;TQF signal is only ~10% of tissue sodium, or single quantum (SQ), signal in brain. In this thesis, SNR optimization for TQF sodium brain imaging is presented. The strategy of using longer RF pulses with smaller first flip angle and shorter repetition time is shown to improve SNR relative to the 'standard' implementation. In addition, inhomogeneous B0 field causes TQF signal loss. Results are shown to demonstrate that TQF signal loss due to off-resonance in most of the cerebrum is well within 10% and thus implementation of correction methods that incur significant scan time increase is not necessary.;One aspect of potential sodium signal loss, which has mostly been ignored, is caused by sodium interactions in anisotropic ordered environments. Anisotropic sodium signal can be detected using the double-quantum magic angle (DQ-MA) sequence, which is similar to TQF with different flip angle and RF pulse phase cycling. Preliminary results show DQ-MA signals to appear throughout the brain and future studies are required to investigate the distribution of this signal.;Sodium MRI performed in acute stroke patients in this thesis demonstrated that SQ sodium signal was unchanged in the 'at-risk' tissue but increased in the lesion core. However, the increase was not correlated with perfusion deficits. Additionally, IR sodium signal showed greater increase than SQ signal within the first fourteen hours after onset suggesting IR signal to reflect intracellular compartment changes.