Ultrasound characterization of coronary artery wall in vitro using temperature-dependent wave speed

Temperature dependence of the speed of sound, partial partial differential c/partial partial differential T, is examined as a parameter to characterize tissue-equivalent phantoms and coronary artery tissue in vitro. The experimental system comprises an ultrasound biomicroscope, operating at center frequency of 50 MHz, and a temperature controlled micropositioning sample cell. Radio frequency (RF) backscattered signals were recorded, with a digital oscilloscope, from 64 independent positions and at 5 temperatures starting at 31 degrees C (phantom) and 36 degrees C (tissue) in steps of one degree. Time shift per degree Celsius (delta t/delta T) was obtained with a correlation technique applied between gated sections of two RF-signals collected with one degree temperature difference from the same location in the sample. The average (delta t/delta T), calculated for every position of the gated sections along the propagation axis of the ultrasound beam, has the slope proportional to the difference between the linear coefficient of thermal expansion and the thermal sensitivity of the speed of sound. Calibration measurements of partial partial differential c/partial partial differential T, made with single- and three-layer tissue equivalent phantoms, correlated well (r > or = 0.91) with those measured by the time-of-flight substitution method. The partial partial differential c/partial partial differential T was estimated for the three layers on the wall of eight samples of human coronary arteries, obtained at autopsy from four individuals. The partial partial differential c/partial partial differential T for the intima layers decreases as the disease progresses from mild intimal thickening to a more advanced atherosclerosis.