| Magnetic susceptibility, the intrinsic property of an object to affect an applied magnetic field, is determined by the object's chemical composition. By measuring the local changes in the magnetic field with MRI, we can characterize the tissue architecture of the brain and spinal cord.;This dissertation primarily focuses on the development of methods to measure the resonance frequency across the central nervous system of normal control volunteers in order to create quantitative susceptibility maps. Traditionally, mapping the magnetic field is accomplished using phase images from gradient-recalled echo (GRE) imaging, a relatively standard sequence on almost all clinical MRI systems. However, these phase images are prone to errors from subject motion and the main field drifting over time. The images themselves have phase wraps, which complicate the signal. These problems also cause difficulties for QSM in the spinal cord.;Instead, the local magnetic field can be measured by creating a resonance frequency map with direct saturation imaging by using the WAter Saturation Shift Referencing (WASSR) method. WASSR uses direct saturation of water protons as a function of frequency irradiation offset to generate frequency maps without phase wraps, which can be combined with any image or spectroscopy acquisition. By utilizing a series of fast short-echo-time GRE images with multiple radiofrequency offsets, a frequency correction for field drift can be applied based on the individual GRE image phases. Because these images can be acquired quickly, motion artifacts are also minimized. This method was used to create, to the best of our knowledge, the first quantitative magnetic susceptibility maps in the human spinal cord, a small structure that is especially prone to motion.;We also developed an automated segmentation method for atlas-based analysis of quantitative susceptibility maps, and were able to efficiently determine the average susceptibility for over 60 structures in the brain.;Analyzing the average quantitative susceptibility in regions of interest within the brain was also correlated with age-normalized iron concentration, showing that QSM may provide useful biomarkers in determining the pathology of Alzheimer's Disease, Huntington's Disease, Multiple Sclerosis, and several other neurodegenerative disorders that correspond with an increase in brain iron. |
