Intravascular Ultrasonography: Image Interpretation and Limitations

Intravascular ultrasonography is a new imaging modality in which a high-frequency ultrasound transducer is placed on the tip of a catheter and inserted directly into an artery.’-3 This results in high-resolution images of arterial structure that have not been possible in the past. By depicting normal and abnormal arterial structure, intravascular ultrasonography has potential for: (1) quantitating severity of atherosclerotic disease; (2) assessing type and composition of atherosclerotic disease; and (3) assessing response to catheter-based interventions.* In initial studies of intravascular ultrasonography, it was observed that three distinct layers were present on the ultrasonogram of normal arteries (Fig. 1). A thin, bright inner layer was thought to represent the intima. A thicker, dark middle layer was thought to represent the media and a bright outer layer, the adventitia. As experience was gained from a larger number of studies, it became apparent that the three-layered appearance was not always present in normal a r t e r i e ~ . ~ . ~ Additionally, atherosclerotic plaque frequently “blended” in with, or obscured, the underlying ~ t r u c t u r e s . ~ ~ These findings have important implications for the use of this imaging modality in guiding the catheter-based therapies that remove atherosclerotic structure, such as atherectomy or laser therapy. As with any new imaging modality, it is essential to correlate the ultrasonograms with tissue structures as well as to determine the limitations of imaging.