Abstract
Objective: In clinical practice, obvious differences exist between the Doppler theory which does not allow a frequency shift at right angles of insonation, and the fact that a frequency is generated. Possible technical and physical causes were evaluated in our study. Methods: A laminar, non-pulsatile flow model was insonated between 35° and 155° with pulsed wave (pw) Doppler sonography incorporated in a duplex scanner. Results: For all angles tested the maximum frequencies were higher than predicted by the Doppler formula. The frequency shift observed at perpendicular insonation cannot be explained by the Doppler theory. Conclusions: The results obtained can be explained by intrinsic spectral broadening (ISB) caused by the transit time effect or equivalently by the geometry of the ultrasonic beam. These effects have to be considered when the maximum frequency is employed to quantify flow volume or when spectral broadening is used as an indicator of disturbed flow.
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