Characteristics of blood vessel wall and atherosclerosis using high frequency ultrasound signals
In today's medical field, many techniques like electrocardiogram, x-rays, ultrasound, electroencephalogram, computed tomography and magnetic resonance imaging use signal and image processing techniques to determine the physical and biological properties of the tissues and organs. The underlying principle in signal processing in medicine is that the response of a tissue to a signal applied to it can be used to derive inherent characteristics of the tissue. The characterization is done effectively by applying certain signal processing algorithms that vary from one application to another. This ever-growing field has opened up exploration into many research areas. Ultrasound is commonly used in non-destructive evaluation of materials and in medical imaging to evaluate tissues and organs. Many characteristics of the ultrasound signal, such as velocity, attenuation, and backscatter coefficient are useful to characterize the material or tissues. Atherosclerosis is a condition that results in the thickening and hardening of arteries. It involves deposits of fatty substances, cholesterol, cellular products, calcium and other substances in the inner lining of an artery. This build-up, called plaque, reduces and eventually blocks the blood flow to the heart muscle, which leads to heart attack and failure. Different techniques such as electrocardiogram, coronary angiography, computer tomography scan and ultrasound exist to diagnose and evaluate the risk and presence of atherosclerosis. Various ultrasound techniques, including intravascular ultrasound imaging, elastography and acoustic microscopy have been applied for atherosclerosis diagnosis. The focus of this thesis was to develop data-acquisition and processing techniques for high frequency (50 Mhz) focused ultrasound probe to characterize small tissue specimens such as vessel wall of a few mm thickness. We attempted characterization of normal and atherosclerotic tissue specimens using 50 MHz ultrasound. Techniques for measurement of ultrasound velocity, attenuation and backscatter parameters of signals were developed. Rabbit and monkey tissues with varying intensities of atherosclerosis were used as specimens for the research work and a relationship between the severity of atherosclerosis and the parameters was observed.