The Effects of Frequency and Angle on Ultrasonic Backscattering from the Upper Airway

In vitro experiments on dog tracheas were performed at 5, 8, 11.7, 18, and 26 MHz to evaluate the backscattering as functions of frequency and transducer orientation angles with respect to the tissue to air surface. Additionally, more idealized flat and convex curved specular reflecting surfaces were also studied. The importance of transducer aperture size, frequency, and target distance in determining the maximum angle over which backscattering can be received from a flat specular surface is highlighted. Convex curvature causes a further increase in this maximum angle. The findings from the tissue studies were: 1) backscattering from the trachea became more diffuse in nature as frequency Increased, 2) backscattering from the anterior section of the trachea was detectable over larger angles than that from the posterior, and 3) at frequencies of 11.7 MHz and above, diffuse scattering was a major consideration in scattering from the anterior surface of the tracheas tested.