Central and Peripheral Neuroimaging in Magnetic Resonance-guided Focused Ultrasound

Magnetic resonance-guided focused ultrasound (MRgFUS) is a non-invasive treatment modality which uses acoustic waves to generate precise thermal lesions. There is growing interest in its application to the treatment of neurological conditions. Conventional MR contrasts can provide anatomical references for targeting and lesion identification but do not impart information about the cellular environment or neural microstructure. Diffusion weighted imaging (DWI) is a technique sensitive to the motion of water within tissue that can be used to visualize fiber bundles and quantitatively assess treatment changes to nerve architecture in vivo. The main aims of this thesis are to: 1) develop an imaging and acoustic sonication procedure using DWI and tractography; and 2) incorporate DWI in the thermal lesioning of intracranial white matter and peripheral nerves to enhance targeting and understand the treatment effect on tissue microstructure. Specific aims of this thesis are to: 1) investigate the DWI characteristics of ex vivo animal brain tissue as a model for neurosurgical treatment development; 2) generate thermal lesions on brain white matter in an in vivo animal model and determine the treatment effect via diffusion tensor imaging (DTI) metrics, tractography, and histological validation; 3) reconstruct the sciatic nerves via tractography to enhance target determination; and 4) produce focal ablations on the sciatic nerve and measure the microstructural changes via DTI metrics. Imaging results from Study I reveal that non-fixed ex vivo brain tissue does not differ from in vivo tissue in terms of diffusion anisotropy, demonstrating its potential use in neurosurgical technique characterization. In Study II, lesion size and location were automatically determined in vivo using Mean DWI and confirmed by histology. DTI metrics in the lesion revealed a pattern of treatment-related anisotropy and diffusivity alterations. Affected fiber pathways were visualized via tractography. In Study III, higher order tractography methods permitted consistent reconstruction of sciatic nerves and precise MRgFUS target placement. Significant microstructural changes in the nerve were observed due to lesioning. Taken together, this thesis demonstrates significant advancements in the visualization of MRgFUS treatment targets and the assessment of microstructural changes that occur following thermal ablation within the central and peripheral nervous systems.Ph.D