Diffraction Effects in Reflected Ultrasound Spectral Estimates

This paper explores the effect of diffraction on estimates of the spectra of signals reflected from interfaces located at a given range from the transducer. When an ultrasound pulse is transmitted into a medium, the power spectra of reflected signals vary as a function of range from the transducer. These spectral changes are caused by diffraction effects, due to the finite-sized aperture, and by the properties of the medium. When the properties of the medium are to be determined from the backscattered signals, the diffraction effects tend to mask these properties, especially in the near field. Estimates of the diffraction effects are determined by accurately estimating the reflected signal spectra at a set of ranges from the transducer. The observed signals are reflections from media that are composed of randomly distributed scatterers and have a linear-with-frequency attenuation. The two random media considered are the in vivo liver and a tissue-equivalent phantom. The results indicate that the diffraction effects produce a high-frequency enhancement in the reflected spectra which diminishes with range. Similar results are observed for the reflections from plane and randomly roughened surfaces located at the same ranges. Simulated results using a transducer model verify the plane surface results. The effect of diffraction is examined in the clinical problem of estimating the slope of the acoustic attenuation coefficient of the in vivo liver. Diffraction introduces a bias into the observed slope estimate. The bias is a function of the range interval over which the reflected data are acquired.