Mechanism of the view-dependence of ultrasonic backscatter fromnormal myocardium

摘要

Anisotropy of ultrasonic scattering and attenuation in heartntissue depends on the specific orientation of myofibers with respect tonangle of insonification. The present study was designed to delineate theneffect of the angle of insonification with respect to the alignment ofnmyofibers on measurements of integrated backscatter. A transmural cubenof myocardium was cut from the anterior wall of the left ventricles fromn5 normal canine hearts and their transmural scattering behavior wasnstudied with the use of reflection acoustic microscopy at 50 MHz. Antheoretical model for scattering based on the Born approximation (weaknscattering) was employed to predict the relationship between backscatternand angle of insonification. Insonification of the basal face (basalnview) demonstrated a wide transmural variation in integrated backscattern(~15 dB), while insonification of the lateral face (lateral view) hadnmuch reduced variation (~4 dB), despite an equivalent overall shift inntransmural fiber angle of ~85° across the ventricular wall.nIntegrated backscatter was greatest in the midmyocardium when the basalnface was viewed and least in the midmyocardium when the lateral face wasnviewed. The backscatter in the subepicardial and subendocardial regionsnwas similar for both views. The maximum difference in backscatter fromnbasal and lateral views at the midmyocardial level was approximately 18ndB, which represents a 64-fold change in the intensity of ultrasonicnbackscatter. The mathematical model developed for scattering based onnthe Born approximation (weak scattering) predicted the observednrelationship between backscatter and angle of insonification. The rapidnangular variation of integrated backscatter perpendicular to the fiberndirection and the slow variation at parallel incidence observednexperimentally were predicted by the model. This angular variation isndue to the specific shape and elastic properties assumed for thenpredominant myocardial scatterer. There was a strong relationshipnbetween backscatter and fiber orientation, indicating that the viewnchosen for insonification of myocardium in clinical imaging mayninfluence the estimation of scattering behavior. The mathematical modelnutilized here predicts the anisotropic behavior of scattering andnsuggests that the principal scattering structure in normal myocardiumnmay be a stiff collagen shell surrounding a more compliant myocyte. Thisnmodel might provide a valid approach for the study of materialnproperties of the heart with the use of ultrasound