Intravascular ultrasound cannot be used to detect and quantify specific histologic contents. However, a zone of reduced echogenicity may also be attributable to a necrotic zone within the plaque, an intramural hemorrhage, or a thrombus. Most soft plaques contain minimal collagen and elastin.
Fibrous plaques represent the majority of atherosclerotic lesions.
In general, the greater the fibrous tissue content, the greater the echogenicity of the tissue. Very dense fibrous plaques may produce sufficient attenuation or acoustic shadowing to be misclassified as calcified. See section on calcium measurements. Thrombi may appear relatively echolucent or have a more variable gray scale with speckling or scintillation. Stagnant blood flow can simulate a thrombus with a grayish-white accumulation of specular echoes within the vascular lumen.