Vocal fold wave velocity in the cover and body layers measured in vivo using dynamic sonography

Traditionally, application of ultrasound imaging on vocal fold vibration has been limited by the frame rate of dynamic sonography. When the frame rate fs is lower than half of the vibration frequency, ambiguity in the vibration frequency evident from dynamic sonography occurs. This ambiguity is known as aliasing. We exploited the aliasing effect in dynamic 2D-mode sonography to capture the vibratory pattern of the vocal fold during modal phonation. The phonation frequency f was tuned to be close to an integer multiple of fs, so that f = Nfs + p, with N = 2, 3, 4, 5, and 1 Hz < p < 4 Hz. The vibration frequency evident from dynamic sonography was p, and slowly-traveling waves were observed on the vocal fold surface and in the vocal ligament. To measure their propagation velocities, we derived the equation relating the wavelength evident from dynamic sonography, the line-scanning velocity of 2D-mode ultrasound, the phonation frequency, and the wave velocity. The wavelengths evident from dynamic sonography were measured for 90 Hz < f < 203 Hz in a male subject. Our data of the vocal-fold surface wave velocity agreed with previous experiments, whereas measurements of the wave velocity in the vocal ligament agreed with the shear wave velocity estimated from its Young’s modulus and density.