Development of New Technique of Tissue Characterization of Atherosclerotic Plaque by Use of Intravascular Ultrasound

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Background - The thickness of the fibrous cap is a major determinant in the vulnerability of atherosclerotic plaque to rupture. It has been demonstrated that intravascular ultrasound (IVUS) backscatter from fibrous tissue is strongly dependent on the ultrasound beam angle of incidence. This study investigated the feasibility of using a new IVUS color mapping technique representing the angle-dependent echo-intensity variation to determine the thickness of the fibrous cap in atherosclerotic plaque. Methods and Results - Nineteen formalin-fixed noncalcified human atherosclerotic plaques from necropsy were imaged in vitro with a 30 MHz IVUS catheter. The IVUS catheter was moved coaxially relative to the plaque. The images showing maximum and minimum echo-intensity of the plaque surface were selected to calculate the angle-dependent echo-intensity variation. A colorized representation of the echo-intensity variation in the plaque was obtained from the two IVUS images. A clearly bordered area with large variation in echo-intensity was revealed for each plaque surface in the colorized IVUS image. The thickness (x: mm) of this area correlated significantly with that of fibrous cap (y: mm) measured from histologically prepared sections, as: y = 1.05x - 0.01 (γ=0.81, p<0.0001). Bland-Altman analysis also supported the reliability of this method (mean difference, 0.00±0.10 mm). Conclusions -This novel technique for color mapping the echo-intensity variation in IVUS provided an accurate representation of the thickness of the fibrous cap in atherosclerotic plaque. This method may be useful in assessing plaque vulnerability to rupture in atherosclerosis.