| Cardiovascular disease is the leading cause of death in the industrialized world. It results in about 1 million deaths in the US each year, with acute coronary heart disease accounting for the majority. The underlying mechanism of most acute coronary events is the disruption of atherosclerotic coronary plaques. These rupture-prone plaques, or "vulnerable plaques", are mainly characterized by a lipid-rich core and a thin fibrous cap.; An ultrasonic tissue differentiation technique, thermal strain imaging (TSI), has been proposed for vulnerable plaque detection. It employs cross-correlation algorithms to create thermal strain images based on the temperature dependence of sound speed. Due to the strong contrast between water-bearing and lipid-bearing tissue, it has the potential to distinguish a lipid-laden lesion from the arterial vascular wall. Excised rat and pig tissues were tested using 10MHz and 85MHz single element transducers and a surface microwave antenna as the heating source. Results demonstrated strong thermal strain contrast between water-based liver and fat, suggesting the feasibility of TSI in tissue separation. A phantom experiment using an intravascular ultrasound (IVUS) imaging array was performed to further validate the concept, where the phantom consisted of rubber and gelatin layers to mimic plaque and the arterial wall, respectively. Results agree reasonably well with theoretical predictions, and estimated layer boundaries match phantom physical dimensions. Simulations performed on an atherosclerotic artery model including a lipid-filled plaque also verified experimental results.; The major challenge for in vivo application of TSI is cardiac motion, including bulk motion and tissue deformation. Simulations based on the same artery model demonstrate effective bulk motion compensation can be achieved within a certain motion range using spatial interpolation. We also propose a practical imaging scheme to minimize mechanical strains caused by cardiac motion based on a linear least squares fitting strategy. This scheme was tested on clinical IVUS data by artificially superimposing thermal strains, corresponding to different temperature rises, on strain images generated from the clinical data. Results suggest a 1--2°C temperature rise is required to detect lipids in an atherosclerotic plaque in vivo. |
