Abstract
Ultrasound may be used to determine the acoustic structure of tissue on a scale corresponding to the wavelengths employed. The underlying concept is the relation between the spacing of tissue elements, ultrasonic wavelength, and scattered signal. There is selective reinforcement or cancellation of certain frequencies depending on the relation of wavelength and reflector spacing. A Fourier analysis of the received signal as a function of frequency reveals the variation in acoustic impedance which corresponds to structure. By employing a swept-frequency ultrasonic signal, data similar to that obtained by angle scanning at a fixed frequency in x-ray diffraction is obtainable. Frequency scanning appears more readily applicable for diagnostic purposes in patients. The theory is illustrated by two specific reflector configurations. One is for regular periodic structures in which point reflectors are represented as rapid changes in impedance and yield well-known Bragg diffraction effects. The other is for a random variation in acoustic impedance in which the structure information is obtained as an impedance correlation function. The frequency scanning results are identical with those for angle scanning when the medium is statistically regular or isotropic.
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