| The last several decades have witnessed the breakthrough of early cancer detection techniques. Earlier detection facilitates earlier treatment. Promising methods for early treatment are minimally invasive thermal therapies, where the treatment is directly applied to the tumor without significant damage to healthy tissues. Magnetic resonance imaging (MRI) is a promising modality for guiding minimally invasive therapies. In this dissertation, several new methods are proposed for improved MRI monitoring of and evaluation after minimally invasive therapies.;The first part of the dissertation focuses on a new MR imaging method using diffusion-weighted imaging to visualize the acoustic radiation force (MR-ARFI) in therapeutic ultrasound applications where there is no significant temperature change. Current MR-ARFI techniques suffers from background phase distortion and low signal-to-noise ratio when measuring a local displacement less than 1 mum. This dissertation describes the optimization of the encoding gradients in order to address both of these problems. Improved accuracy and precision are achieved and demonstrated in simulation and phantom experiments.;The second part of the dissertation investigates the use of diffusion-weighted MRI (DWI) to evaluate the acute and chronic treatment results after high intensity ultrasound and cryo ablation. The current gold standard for lesion assessment is contrast-enhanced MRI, which detects the hemodynamic disruption in necrotic tissue. However, this method is not repeatable and requires administration of contrast material. In this dissertation, the sensitivity of DWI in assessing cellular damage in the canine prostate immediately after either cryoablation or therapeutic ultrasound treatment is evaluated. A 36% drop in the apparent diffusion coefficient (ADC) was measured in 19 in vivo experiments, providing high contrast between the acute lesion and healthy tissue. In chronic studies, the ADC value gradually increased as the tissue recovered and regenerated. Furthermore, the competing effects between the ADC decrease due to tissue damage and the ADC increase due to elevated temperature are discussed for DWI based thermometry. |
