Intravascular ultrasound: a histological study of vessels during life. The new 'gold standard' for vascular imaging.

uring the last several years, dramatic developD ments have taken place in the area of cardiac imaging techniques. From an era of imaging by silhouettes (chest roentgenography, fluoroscopy, angiocardiography, angiography), we have emerged into an era of imaging by tomographic scanning (echocardiography, radionuclide and computed tomography, magnetic resonance).l-3 Two-dimensional intravascular echocardiography is one of these exciting new techniques capable of providing cross-sectional tomographic images of coronary arteries. Although visualization of the major epicardial coronary arteries by transthoracic or transesophageal echocardiography has been possible for nearly 15 years,4-17 intravascular ultrasound has, for the first time, allowed visualization of the coronary lumen and various normal and abnormal layers of the coronary arteries. In essence, intravascular echocardiographic imaging represents a histological study of the coronary artery during life. The usual three-layer structure (intima, media, adventitia) of muscular arteries such as the coronary artery system is acoustically distinctive, which permits ultrasound imaging. The relative echo lucency of the media compared with the intimal and adventitial layers permits visualization of this multilayer architecture first described in vitro by Meyer et a118 in 1988 and in vivo by Yock et a119 in 1989. The bright inner (luminal) lining of the ultrasound image in the normal coronary artery is caused principally by the internal elastic membrane separating the intima from the media.20-22 The intimal layer in the normal coronary artery is probably not thick enough to generate a distinct ultrasonic layer.3 Further refinement in the interpretation of images from the internal and external elastic membranes as well as the luminal surface of vessels is under way. Several in vitro and in vivo studies of arterial vessels (with and without histology) have confirmed the ultrasonic appearance of these arterial structures and have validated measurements of layer thickness and luminal cross-sectional area.23-35 Attention is now focusing on the clinical applications of intravascular echocardiography. At least three major areas of diagnostic and therapeutic use of intravascular