Collagen fiber morphology determines echogenicity of myocardial scar: implications for image interpretation

Fibrous tissue appears as echo dense areas on conventional ultrasound images; however, the determinants of such echo brightness have not been assessed. We demonstrated previously, using 600-MHz ultrasound images, that collagen fiber morphology determines echo brightness. In the current study, we tested the hypothesis that collagen fiber morphology also determines echogenicity of myocardial scar at the lower transducer frequencies used in conventional ultrasound. We examined both the infarcted and noninfarcted regions of rat hearts 1 and 3 weeks after permanent coronary artery occlusion. Ultrasound images obtained from excised hearts were digitized to quantify echocardiographic brightness. The hearts were then sectioned, stained with picrosirius red, and examined with polarized light microscopy. We found that myocardial scar tissue appeared either hyperechoic or normoechoic depending on collagen fiber morphology. Specifically, the hyperechoic areas corresponded to the 3-week-old infarcted regions containing thick collagen fibers that appeared predominantly orange when viewed with polarized light. In contrast, normoechoic areas corresponded to the 1-week-old infarcted regions containing thin collagen fibers that appeared predominantly green and also to noninfarcted myocardium. We conclude that the echogenicity of tissue early after infarction is similar to that found in normal, noninfarcted myocardium, thus compromising the ability of echocardiography to distinguish between these two states based on echotexture.