Mechanisms causing narrow-band sounds associated with aneurysms and arterial lesions

Abstract

Many investigators have reported sounds of nearly pure tone, with frequencies of the order of 500 Hz and Q's of the order of 20, associated with flow in human carotid and cerebral arteries. These sounds are apparently caused by vascular lesions, but there is not yet a satisfactory explanation. Olinger and Wasserman [Surg. Neurol. 8, 298–312 (1977)] advanced the hypothesis that aneurysms act as Helmholtz resonators. Simple order of magnitude calculations support this hypothesis, but the resonator's acoustic compliance must be associated with wall elasticity, rather than fluid compressibility. Aaslid and Nornes [J. Neurosurg. 60, 32́36 (1984)], using angiography and Doppler flow velocimetry, found no tangible link between narrow peaks in the spectrum of fluctuations in the flow velocity and the presence of aneurysms. They postulated instead that the peaks are due to periodic vortex shedding; the vortex-shedding hypothesis is supported by a strong dependence of frequency of spectrum peaks on blood flow velocity. The present paper addresses the question of whether such narrow-band sound can result solely from mechanisms of a fluid mechanical nature that do not involve the elasticity of the arteries. An alternate mechanism is also discussed involving weakened elastic spots or patches on the arterial walls that have intrinsic resonance frequencies. [Work supported by the William E. Leonhard endowment to Pennslyvania State University. The author acknowledges the advice of A. D. Pierce.]