Non-invasive in vivo measurement of the shear modulus of human vocal fold tissue

Abstract

Voice is the essential part of singing and speech communication. Voice disorders significantly affect the quality of life. The viscoelastic mechanical properties of the vocal fold mucosa determine the characteristics of the vocal folds oscillations, and thereby voice quality. In the present study, a non-invasive method was developed to determine the shear modulus of human vocal fold tissue in vivo via measurements of the mucosal wave propagation speed during phonation. Images of four human subjects' vocal folds were captured using high speed digital imaging (HSDI) and magnetic resonance imaging (MRI) for different phonation pitches, specifically fundamental frequencies between 110 and 440 Hz. The MRI images were used to obtain the morphometric dimensions of each subject's vocal folds in order to determine the pixel size in the high-speed images. The mucosal wave propagation speed was determined for each subject and at each pitch value using an automated image processing algorithm. The transverse shear modulus of the vocal fold mucosa was then calculated from a surface (Rayleigh) wave propagation dispersion equation using the measured wave speeds. It was found that the mucosal wave propagation speed and therefore the shear modulus of the vocal fold tissue were generally greater at higher pitches. The results were in good agreement with those from other studies obtained via in vitro measurements, thereby supporting the validity of the proposed measurement method. This method offers the potential for in vivo clinical assessments of vocal folds viscoelasticity from HSDI.

Keywords: High-speed imaging; Mucosal wave propagation; Non-invasive measurement; Shear modulus.

Figure 1

Comparison between the anterior-posterior lengths of the vocal fold while performing a similar task. (a) L_M, the length in MRI image (b) L_H, the length in high-speed image.

Figure 2

Image processing algorithm, Subject 1 phonating at 130 Hz: (a) pre-processed high-speed image, region of interest and non-vertical glottal midline; (b) magnified region of interest with vertical glottal midline; (c) binary image; (d) perimeter of the binary image, where boundaries represent the vocal fold edge; (e) the search window for mucosal wave detection, obtained vocal fold edge is overlapped on the gray scale image; (f) obtained mucosal wave overlapped on the gray scale image.

Figure 3

The intensity gradient on a medial-lateral line passing through the vocal folds’ midpoint. (a) Subject 1 phonating at 130 Hz (the arrow line in Figure 2 (e)), and (b) Subject 1 phonating at 110 Hz. The location of the mucosal wave peak and the vocal folds edges are detected by the diagram extrema.

Figure 3

The intensity gradient on a medial-lateral line passing through the vocal folds’ midpoint. (a) Subject 1 phonating at 130 Hz (the arrow line in Figure 2 (e)), and (b) Subject 1 phonating at 110 Hz. The location of the mucosal wave peak and the vocal folds edges are detected by the diagram extrema.

Figure 4

Simplified schematic of a coronal section structure of human vocal fold tissue, and the motion of different locations of the vocal fold surface due to the mucosal wave propagation. ● and ○ denote the cross section of elastin and collagen fibers, respectively.

Figure 5

The maximum of the horizontal displacement of vocal fold upper edge during the opening phase at different phonation pitches from 110 to 440 Hz. : Subject 1; : Subject 2; : Subject 3; : Subject 4; : regression for Subject 1; : regression for Subject 2; : regression for Subject 3; : regression for Subject 4. (Regressions are to guide the eye)

Figure 6

The mucosal wave propagation speed at different phonation pitches from 110 to 440 Hz. : Subject 1; : Subject 2; : Subject 3; : Subject 4; : regression for Subject 1; : regression for Subject 2; : regression for Subject 3; : regression for Subject 4. (Regressions are to guide the eye)

Figure 7

The shear modulus of human vocal fold mucosa at different phonation pitches from 110 to 440 Hz. : Subject 1; : Subject 2; : Subject 3; : Subject 4; : regression for Subject 1; : regression for Subject 2; : regression for Subject 3; : regression for Subject 4. (Regressions are to guide the eye)

Figure 8

The means and standard deviations (upper error bars) of the shear modulus of human vocal fold mucosa versus frequency in the range from 100 to 260 Hz. ● : the mean value of three male subjects from in vivo measurements (present study); □ : the mean value of seven specimens obtained from in vitro measurements (Chan and Rodriguez, 2008).