Detection of moving flow separation in pulsatile flow and the degree of stenosis by power of Doppler shift signals

Evaluation of distal turbulence intensity for the detection of both plaque ulceration and stenosis grade in the carotid bifurcation using clinical Doppler ultrasound

OBJECTIVES

To determine the interrelationship of stenosis grade and ulceration with distal turbulence intensity (TI) in the carotid bifurcation measured using conventional clinical Doppler ultrasound (DUS) in vitro, in order to establish the feasibility of TI as a diagnostic parameter for plaque ulceration.

METHODS

DUS TI was evaluated in a matched set of ulcerated and smooth-walled carotid bifurcation phantoms with various stenosis severities (30, 50, 60 and 70 %), where the ulcerated models incorporated a type 3 ulceration.

RESULTS

Post-stenotic TI was significantly elevated owing to ulceration in the mild and moderate stenoses (P < 0.001). TI increased with stenosis severity in both the ulcerated and non-ulcerated series, with a statistically significant effect of increasing stenosis severity (P < 0.001). Whereas TI in the mild and non-ulcerated moderate stenoses was less than 20.4 ± 1.3 cm s(-1), TI in the ulcerated moderate and severe models was higher than 25.6 ± 1.3 cm s(-1), indicating a potential diagnostic threshold.

CONCLUSION

We report a two-curve relationship of stenosis grade and ulceration to distal TI measured using clinical DUS in vitro. Clinical DUS measurement of distal TI may be a diagnostic approach to detecting ulceration in the mild and moderately stenosed carotid artery.

KEY POINTS

• Patients with carotid artery plaque ulcerations are at higher risk of stroke. • Clinical Doppler ultrasound is routinely used to detect carotid artery stenosis. • Doppler ultrasound turbulence intensity can detect ulceration in realistic flow models. • Turbulence intensity also increases with stenosis severity independent of ulceration. • Doppler ultrasound should help in assessing both stenosis severity and ulceration.

Flow separation in a computational oscillating vocal fold model

A finite-volume computational model that solves the time-dependent glottal airflow within a forced-oscillation model of the glottis was employed to study glottal flow separation. Tracheal input velocity was independently controlled with a sinusoidally varying parabolic velocity profile. Control parameters included flow rate (Reynolds number), oscillation frequency and amplitude of the vocal folds, and the phase difference between the superior and inferior glottal margins. Results for static divergent glottal shapes suggest that velocity increase caused glottal separation to move downstream, but reduction in velocity increase and velocity decrease moved the separation upstream. At the fixed frequency, an increase of amplitude of the glottal walls moved the separation further downstream during glottal closing. Increase of Reynolds number caused the flow separation to move upstream in the glottis. The flow separation cross-sectional ratio ranged from approximately 1.1 to 1.9 (average of 1.47) for the divergent shapes. Results suggest that there may be a strong interaction of rate of change of airflow, inertia, and wall movement. Flow separation appeared to be "delayed" during the vibratory cycle, leading to movement of the separation point upstream of the glottal end only after a significant divergent angle was reached, and to persist upstream into the convergent phase of the cycle.

Characterization of arterial stenosis and elasticity by analysis of high-frequency pressure wave components

We describe a novel method for assessing stenosis severity based on pressure wave measurements. Pressure waves for several degrees of stenosis at different distances proximal to the stenosis were recorded from in vitro and in vivo models. Signal analysis was performed using power spectral density, and radial compliance was also measured. Pressure wave components at the acoustic frequency band (400-2500Hz) changed gradually and were dependent upon the distance from the stenosis and its severity. The shift of the pressure components could also demonstrate the elastic properties of tubes and arteries and explain the effect of a bifurcation in the system.