Characterization of collagen by high-frequency ultrasound: evidence for different acoustic properties based on collagen fiber morphologic characteristics.

Fibrous tissue on conventional ultrasound images appears as an echo-bright area. We have observed that on high-frequency ultrasonography images of thin sections of myocardium, fibrous tissue may appear as either a dark or light area. This study was designed to test the hypothesis that echo characteristics of fibrous tissue on high-frequency ultrasonography are determined by collagen fiber morphologic characteristics. We examined 16 tissue specimens from human beings and rats containing different forms of fibrosis. The specimens were sectioned at 5 microns, placed on a glass slide, and imaged with a 600 MHz transducer. On ultrasound images, collagen appeared either as a dark amorphous area or a light area that had a fibrillar pattern. The same specimens were then stained with picrosirius red and examined with polarized light. When viewed with polarized light microscopy, thick collagen fibers appear red or orange and thin fibers appear green or yellow. Polarized light microscopy revealed that dark areas on ultrasound images corresponded to thick collagen fibers that were predominantly longitudinally sectioned. In contrast, light areas corresponded to regions of thin, loosely packed fibers, or to thick collagen fibers that were obliquely sectioned. Collagen has different appearances on high-frequency ultrasound images depending on collagen fiber morphologic characteristics. If such variation in echo intensity also occurs with lower frequency transducers used in clinical echocardiography, the differentiation between normal myocardium and immature scar may be difficult.