Detection of wall vibrations by means of cepstrum analysis.

Preliminary studies on the usefulness of the cepstrum analysis of Doppler signals for detecting vibrations of periodically vibrating structures, such as aortic valves in children with innocent heart murmurs, were carried out in two stages. First, by cepstrum analysis of computer simulated frequency modulated signals, and second, by the analysis of real Doppler signals generated by a vibrating reflector. Simulation and experimental studies were performed for modulating frequencies from 10 Hz to 60 Hz while changing the amplitude of vibrations from 0.2 mm to 1 mm. The striking similarity between computed cepstra for simulated and for real Doppler signals encouraged further studies in patients. Cepstra computed from two Doppler recordings in children with weak innocent vibratory heart murmurs showed characteristic cepstral lines for modulating frequencies at 21 Hz and 30 Hz in the first and second child, respectively, which were interpreted as the vibration frequencies of the aortic valve leaflets.