The extracellular matrix is an important source of ultrasound backscatter from myocardium

Ultrasound tissue characterization with measurement of backscatter has been employed in numerous experimental and clinical studies of cardiac patholog y, yet the cellular components responsible for scattering from cardiac tiss ues have not been unequivocally identified. This laboratory has preposed a mathematical model for myocardial backscatter that postulates the fibrous e xtracellular matrix (ECM) as a significant determinant of backscatter. To d emonstrate the importance of ECM, this group sought to determine whether me asurements of backscatter from the isolated ECM could reproduce the known d irectional dependence, or anisotropy of backscatter, from intact cardiac ti ssues in vitro. Segments of left ventricular free wall from ten formalin fi xed porcine hearts were insonified at 50 MHz, traversing the heart wall fro m endo- to epicardium to measure the anisotropy of myocardial backscatter, defined as the difference between peak (perpendicular to fibers) and trough (parallel to fibers) backscatter amplitude. The tissue segments were then treated with 10% NaOH to dissolve all of the cellular components, leaving o nly the intact ECM. Scanning electron micrographs (SEM) were obtained of ti ssue sections to reveal complete digestion of the cellular elements. The di mensions of the residual voids resulting from cell digestion were approxima tely the diameter of the intact myocytes (10-30 mu m). These samples were r einsonified after seven days of treatment to compare the anisotropy of inte grated backscatter. The magnitude of anisotropy of backscatter changed from 15.4+/-0.8 to 12.6+/-1.1 dB for intact as compared with digested specimens . Because digestion of the myocardium leaves only extracellular sources of ultrasonic scattering, and because the isolated ECM exhibits similar ultras onic anisotropy as does the intact myocardium, it is concluded that there i s a direct association between the ECM and the anisotropy of backscatter wi thin intact tissue. Thus, it is suggested that ultrasonic tissue characteri zation represents a potentially clinically applicable method for delineatin g the structure and function of the ECM. (C) 2000 Acoustical Society of Ame rica. [S0001-4966(00)05501-6].