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

A three-layer appearance of the arterial wall in intravascular ultraso und imaging has been described, but its correspondence with a characte ristic histologic structure is still controversial. Some studies empha size the imprecision of vessel-wall measurements carried out on the in travascular ultrasound image, while other studies cast doubt upon the reality of the three-layer appearance and its correspondence with a ch aracteristic histologic structure. The consequences of this disputed p oint are important with respect to the interpretation of such images. We performed an in vitro study using phantoms and heighty postnecropti c arteries to determine the relations existing between histology, acou stic properties of arterial wall tissue, and intravascular ultrasound image formation. Measurements carried out on phantoms showed a signifi cant overestimation of wall thickness, equal to axial resolution and d ue to the radial impulse response of the imaging system. Measurements performed on normal arterial walls of muscular arteries with a three-l ayer appearance or normal arterial walls of elastic arteries with a si ngle-layer appearance showed very clearly that there was no correlatio n with histologic measurements of the thickness of intima and media ta ken individually. However, measurements (n = 53), which include the in tima plus the media, exhibit excellent correlation with histologic mea surements: r = 0. 95 +/- 0.03, y = 1.02 + 11.2. The shiny internal rin g 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 tran sducer to the arterial wall, modified the shape and sizes characterizi ng the IEL on intravascular ultrasound images. The description and und erstanding of such image alterations are necessary for both interpreti ng them and determining which are the only reliable measurements that can be performed.