ARE SOFT ECHOES REALLY SOFT - INTRAVASCULAR ULTRASOUND ASSESSMENT OF MECHANICAL-PROPERTIES IN HUMAN ATHEROSCLEROTIC TISSUE

To examine the accuracy of intravascular ultrasound (IVUS) in assessin g the biophysical properties of atherosclerotic plaque, 33 human iliac arteries were imaged with a 25 MHz IVUS transducer and classified int o four groups on the basis of IVUS appearance: minimally diseased arte rial wall, bright echogenic plaque with acoustic shadowing, bright ech ogenic plaque without shadowing, and hypoechogenic plaque (so-called ' 'soft echoes''). The hardness of each plaque was assessed with an ultr asensitive compression ergonometer. The radial static stress-strain re lations fit well (r > 0.98) to exponential curves, providing a compres sion stiffness constant(K) defined as the coefficient of the exponenti al power. K for bright echogenic plaque with shadowing was significant ly greater than that of the other tissues. However, K among minimally diseased entire arterial wall, hypoechogenic plaque, and bright echoge nic plaque without shadowing was not significantly different, but thes e tissues are not physically soft compared with adipose tissue. Theref ore, tissue characterization by IVUS distinguishes calcified from nonc alcified plaque and accurately predicts its biomechanical hardness. Ho wever, soft echoes, although less firm than calcium, do not necessaril y correspond to soft tissue.