Geometrical Spectrum Broadening in Ultrasonic Doppler Systems

The bandwidth of the output spectrum of blood flow meters may prove of value in estimating flow parameters such as the degree of turbulence. It is therefore important to determine the various factors which affect this bandwidth. It is shown that scatterers moving in the near field of the sound beam generated by a single transducer ultrasound Doppler system produce a spectrum whose breadth is primarily determined by the range of angles over which backscattered ultrasound is received by the transducer. An empirical method of calculating the bandwidth of the output spectrum is described, and the results are compared with direct Doppler measurements. Since the spectrum broadening depends on the angle between the target path and the ultrasound beam, it is suggested that the phenomenon may be usable to provide an independent measure of this angle.