| Ischemic stroke is a highly prevalent disease. The administration of the only approved treatment, tissue plasminogen activator, is associated with a substantial risk of hemorrhage. In addition, it is not always obvious who will benefit from treatment. Advances in neuroimaging may assist in selecting appropriate patients for treatment. This thesis explores one MR technique---diffusion imaging---as a method to assess ischemic stroke progression.;The model-under-test proposes FA increases in ischemic stroke at a time after the well-described and accepted decrease in ADC. It is proposed that the FA increases as a result of the onset of blood brain barrier permeability.;To test the proposed model, a canine model of embolic stroke was developed; as it was necessary to serially image each subject to examine the evolution of diffusion maps. Measurement characteristics of ADC and FA were examined, and substantial observer variability was found when performing observer-based region-of-interest analysis. An exploratory study provided preliminary evidence to support the proposed model; however, it was determined that more sophisticated and better controlled experiments would be required. A refined method was implemented and the results of this more intensive study did not show a delay in FA changes. However, these experiments provide the foundation of a revised, improved understanding of ADC and FA evolution in acute ischemic stroke.;The revised model proposes that ADC decrease and FA increase occur at the same time. The decrease in FA occurs independent of any ADC change and is likely associated with an increase in blood brain barrier permeability.;MR diffusion imaging can be used to assess net diffusion with apparent diffusion coefficient (ADC) maps. Diffusion directionality is imparted by cellular structures and can be depicted by, for example, fractional anisotropy (FA) maps. ADC is used extensively in the detection of acute ischemic stroke, but FA has had limited study. Here, the progression of FA in acute ischemic stroke is examined. A proposed biological model is tested and, at the conclusion of this work, a revised model is described. |
