3-LAYER APPEARANCE OF THE ARTERIAL-WALL IN INTRAVASCULAR ULTRASOUND IMAGING

A three-layer appearance of the arterial wall in intravascular ultrasound imaging has been described, but its correspondence with a characteristic histologic structure is still controversial. Some studies emphasize the imprecision of vessel-wall measurements carried out on the intravascular ultrasound image, while other studies cast doubt upon the reality of the three-layer appearance and its correspondence with a characteristic histologic structure. The consequences of this disputed point are important with respect to the interpretation of such images. We performed an in vitro study using phantoms and heighty postnecroptic arteries to determine the relations existing between histology, acoustic properties of arterial wall tissue, and intravascular ultrasound image formation. Measurements carried out on phantoms showed a significant overestimation of wall thickness, equal to axial resolution and due to the radial impulse response of the imaging system. Measurements performed on normal arterial walls of muscular arteries with a three-layer appearance or normal arterial walls of elastic arteries with a single-layer appearance showed very clearly that there was no correlation with histologic measurements of the thickness of intima and media taken individually. However, measurements (n = 53), which include the intima plus the media, exhibit excellent correlation with histologic measurements: r = 0. 95 +/- 0.03, y = 1.02 + 11.2. The shiny internal ring of the muscular artery wall on the image corresponded always to the internal elastic lamina (IEL). Thus, the three-layer appearance is not an artifact. However, specific alterations, which this appearance may undergo during image formation such as beam incidence angle, lateral and radial impulse responses, and the effects of proximity of the transducer to the arterial wall, modified the shape and sizes characterizing the IEL on intravascular ultrasound images. The description and understanding of such image alterations are necessary for both interpreting them and determining which are the only reliable measurements that can be performed.