Scatterer-induced frequency variations in reflected acoustic pulses: implications for tissue characterization.

A Hilbert transform approach to the detection of the instantaneous center frequency of reflected ultrasonic pulses was performed for simulated reflected ultrasonic pulses, and digitized ultrasonic waveforms from tissue phantoms. Considerable center frequency modulation was observed for both simulated and recorded ultrasonic waveforms. This phenomenon of frequency modulation can be explained with the aid of a phasor model, in which variations in instantaneous frequency results from the interaction of a multitude of overlapping ultrasonic pulses arising from different scattering centers in tissue. The observed frequency modulation of ultrasonic signals has important implications for tissue characterization techniques, including the measurement of attenuation by the determination of the shift in ultrasonic center frequency.