T1 Contrast Mechanisms at Ultra-High Magnetic Field

ABSTRACT Martin John MacKinnon: T1 Contrast Mechanisms at Ultra-High Magnetic Field (Under thedirection of Dr. Yen-Yu Ian Shih) This work investigates two different spin-lattice relaxation (T1) contrast mechanisms at ultrahighmagnetic field strength. Oxidative stress, a sustained imbalance between the production of reactive oxygen species andthe antioxidant capacity of the cell, is commonly proposed as a pathogenic factor in a plethora oflife-changing morbidities including neurodegenerative disease cancer, and neurodevelopmentaldisorders. Currently, no non-invasive tool is capable of spatially mapping regions of oxidativestress. Some MR-studies speculatively attribute measured signal changes to a a subset of ROS, freeradicals, due to their hypothesized T1-shortening effect. However, as proof-of-concept studies arelacking, interpretation of data is confounded. We began to address this ambiguity by measuring thespin-lattice-relaxivity of a free radical, superoxide, for the first time. We show that the spin-latticerelaxivity of the superoxide free radical is 0.3 mM−1s−1, which is of a similar order of magnitudeto molecular oxygen. This work acts as the first step in a line of future validation studies.Next, we developed a Zero-Echo-Time (ZTE) - fMRI technique, which is sensitive to T1-shortening hemodynamic effects. The gold-standard fMRI technique, gradient-echo echo planarimaging (GRE-EPI), suffers from five major drawbacks: (1) high acoustic noise (2) ghosting andmotion artifact; (3) sensitivity to magnetic field inhomogeneities; (4) low sensitivity comparedto other, non fMRI, neuroimaging modalities and (5) poor spatial specificity to the actual site ofneuronal activation. Although alternative fMRI techniques address some of these limitations, theyall sacrifice sensitivity compared to GRE-EPI. We demonstrate how a ZTE sequence, designed forfMRI applications, addresses the weaknesses of GRE-EPI without sacrificing functional sensitivity.