Verifying the Multilayered Structures of Vocal Folds in Ultrasound Imaging

Although clinical ultrasound imaging has been widely used in various fields of practice, its application on the laryngeal examination has been limited due to the spatial resolution and the dynamic response to tiny high frequency movements. Among the basic functions of human larynx, phonation is one of the most complex and the least understood activity. The 3D vibratory movements of the vocal folds（VF）, the stiffness of the mucosa and the aerodynamic forces acting on the VF play important roles in the formation of voice. Therefore, sophisticated noninvasive technologies that combine photoglottography (PGG), electroglottography (EGG) or video laryngostroboscopy with aerodynamic methods need to be developed to investigate the laryngeal function and the neurophysiology of phonation.

To visualize the laryngeal activity in vivo under normal articular movements, fiberoptics were inserted into the lower pharynx via the nasal cavity. The ultrasonography of the larynx would provide further details of the VF cartilaginous and endolaryngeal structures. The true vocal folds and the vocal muscles appeared as hypoechoic structures that could not be adequately visualized in US scanning (Raghavendra et al. 1987). Morphologically, the use of B-mode US is not superior to other imaging techniques, such as CT or MRI in visualizing laryngeal structures, especially the true VF. Therefore, the studies of phonation have been limited.

The vibration of VF periodically interrupts the glottal airflow and forms the acoustic signal that is perceived as the voice. Due to its unique multilayered structures, the VF can be divided into the “cover” and the “body”. The cover includes mucosa and the superficial layer of lamina propria, called the Reinke’s space. The body consists of vocal ligament and vocal muscle. Functionally, the vibration of VF is confined mainly to the cover and the mucosal wave is propagated vertically from the lower to the upper margin of the VF. The vertical mucosal traveling wave is the summation of sequential horizontal tissue displacement waves from the infraglottic to supraglottic extent of the VF structure.

The status of the VF cover, especially the stiffness of Reinke’s space, plays the key important role in the pathogenesis of dysphonia. In this study, we use the new generation ultrasonography scanner with higher resolution (HDI-5000, ATL, Bothell, WA) and higher frame rate (33 Hz) in B-mode to study the multilayered structures of vocal folds from fresh excised human larynx after laryngectgomy. We studied the echogenic features of each layer. The multilayered structure of the vocal fold was stripped off layer by layer and was confirmed by histopathology. And the multilayered structures of vocal folds in ultrasound imaging were verified. The cover of the vocal folds was an almost nonechogenic band.

The medical ultrasound imaging is easy to perform and analyze, safe, non-invasive, well tolerated, generally available, real-time and with a minimum disturbance to the voice production. It seems to be one of the best choices to meet the requirements for routine laryngeal examination. This is the first time the multilayered structures of vocal folds, especially the cover were verified by medical ultrasound imaging. By doing so, we hoped that medical ultrasound imaging can someday become a promising tool in diagnosing the causes of dysphonia in routine laryngeal examination.