Flow Fields and Acoustics in a Unilateral Scarred ... - Semantic Scholar

Annals ofOtohiiy. Rhimhgy & Laryngology 118( I ):44-5n, © 2009 Annals Publinhing Company. All rights reserved.

Flow Fields and Acoustics in a Unilateral Scarred Vocal Fold Model Shanmugam Murugappan, PhD; Sid Khosla. MD; Keith Casper, MD; Liran Oren; Ephraim Gutmark, PhD Objectives: From prior work in an excised canine larynx model, it has been shown thai intraglottal vortices form between the vocal folds during the latter part of closing. It has also been shown that the vortices generate a negative pressure between the folds, producing a suction force that causes sudden, rapid closing of the folds. This rapid closing will produce increased loudness and increased higher harmonics. We used a unilateral scarred excised canine larynx nutdel to determine whether the intraglottal vortices and resulting acoustics were changed, compared to those of norma! laryngés. Methods: Acoustic, flow field, and high-speed imaging measurements from 5 normal and 5 unilaterally scarred canine laryngés are presented in this report. Scarring was produced hy complete resection of the vocal fold mucosa and superficial layer of the lamina propria on the right vocal fold only. Two months later, these dogs were painlessly sacrificed, and testing was done on the excised laryngés during phonation. High-speed video imaging was then used to measure vocal fold displacement during different phases. Particle image velocimetry and acoustic measurements were used to describe possible acoustic effects of the vortices. Results: A higher phonation threshold was required to excite the motion of the vocal fold in scarred laryngés. As the suhglottal pressure increased, the strength of the vortices and the higher harmonics both consistently increased. However. it was seen that increasing the maximum displacement of the scarred fold did not consistently increase the higher harmonics. The improvements that result from increasing subglottal pressure may be due to a combination of increasing the strength of the intraglottal vortices and increasing the maximum displacement of the vocal fold; however, the data in this study suggest ihat the voiiices play a much more important role. Conclusions: The current study indicates that higher subglottal pressures may excite higher harmonics and improve loudness for patients with unilateral vocal fold scarring. This finding implies that therapies that raise the subglottal pressure may be helpful in improving voice quality. Key Words: phonation, vocal fold scarring, voice production, vortex.

INTRODUCTION

in the higher harmonics. Khosla et al^ showed that there is a strong correlation between the acoustic energy in the higher-frequency harmonics and the negative pressures generated by the intraglottal vortices duritig closing. The authors also found that these intraglottal vortices were decreased in asymmetric motion because of asymmetric stiffness: the cause of this stiffness was unknown, but the mucosa of the fold did not appear to be scarred.-

Iti previous work. Khosla et al ' showed that dtiritig the latter part of vocal fold clositig, vortices fortned betweeti the folds. The.se vottices are areas of rotational motion in the glottal airflow, and are of significance because they produce significant negative pressure that generates an additional closing force on the vocal folds near the end of the phonation cycle. This rapid closing contributes to flow shutoff, or the reduction of airflow exiting the glottis; this is important clinically, since it is known that increasing the rate of flow shutoff will increase voice intensity and will increase the relative acoustic energy

The purpose of this study was to investigate the effects of unilateral vocal fold scarring on the intraglottal vortices. The mechanism for the fomiation of intraglottal vortices is known as flow separation.

From the Department of Otolaryngology-Head and Neck Surgery. University of Cincinnati Medical Center (Murugappan. Khosla, Cusper, Gutmark), and ihc Department of Aerospace Engineering and Engineering Mechanics. University of Cincinnati (Oren. Guimark). Cincinnati. Ohio. Supported by grant 5K()8DCO()Í42 ! from the National Instiiutcs of Health/National Institute on Dealncss and Other Communication Disorders. This study was perlbrmed in accordance with ihc PHS Policy on Humane Care and Use iif laibonitory Animais, the NIH Giiitk-for the Care and Use c)f Laboratory Atiimal.s. and ihe Animal Welfare Act (7 U.S.C. et seq.); the animal use protocol was approved by the Institutional Animal Care and Use Committee (IACUC) of the University of Cincinnati. Presented at the meeting of the American Broncho-Esophagological Association. Orlando. Florida. May 1 -2. 2008. Correspondence: Shanmugam Murugappan. PhD. Dept of Otolaryngology-Head and Neck Surgery. University of Cincinnati Medieal Center. 2.11 Albert B. Sabin Way. ML ().S28. Cincinnati. OH 43267.

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Murugcippan et al. Flow Fields & Acoustics in Unilateral Scarred Voca! Fold Model

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will produce inereased loudness,an inereased HNR. and increased energy in the higher harmonics. These flndings may have i nplications for characterizing and treating the dysphonia seen in vocal fold scarring.

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Fig I. Schematic of diverging duct shows flow separation.

and thus we will refer to the intraglottal vortices as flow-separation vortices. Flow separation describes a phenomenon, well known in fluid mechanics, in which now separates from a confining wall. Because of the vertical wave, the glottis forms a divergent duct during closing; during this phase of the phonation cycle, flow separation from the medial surface can occur inside the glottis, as is shown in Fig 1. This is due to the fact that when the diverging angle of the duct exceeds a certain value, the ilow cannot follow the duct walls and will separate from them.^ The result of this intraglottal flow separation is the production of rotational motion, or vortices, in the glottis. Flow-separation vortices will not occur between the vocal folds during opening, because the vocal folds are not divergent. Our earlier work on laryngés with asymmetric mucosal wave motion showed weaker or absent intraglottal vortices-; in these laryngés, the asymmetry was primarily one of magnitude and nol of phase. Since scarring is expected to produce significant asymmetries in both magnitude and phase, we hypothesized that unilateral vocal fold scarring would also reduce the intraglottal vortices, thereby reducing the energy in the higher-frequency harmonics and the harmonics-to-noise ratio (HNR). In this study, we aiso aimed to investigate the effects of different subglottal pressures. In fluid mechanics- it is well known that increasing the pressure will increase the strength of the flow-separation vortices; Alipour and Scherer* showed similar findings in a computational model of the vocal folds. If the intraglottal vorticity increases with subglottai pressure, more rapid flow shutoff is expected, which

To measure the intragiottal vortices, we obtained the velocity fields during phonation by the particle imaging veiocimetry (PIV) method described previously.'•'^ In our PIV method, we used micronsized (5 to 15 \im) aerosolized olive oil panicles that are injected into the flow. Illumination of these particles is produced by a laser beam that is spread into an essentially 2-iimensionai planar light sheet. The laser is pulsed sach that 2 sheets are produced microseconds apart. Both images are recorded on a specialized catnera. ( 'omputer analysis of the resultant images correlates the particles in the 2 images; using this information, we can calculate a displacement and velocity flold. The advantage of the PIV technique is that it is noninvasive and can give the magnitude and direction of velocity vectors over the entire measurement ¡ilane at a given instant of time. METHODS The method of pre taring the excised laryngés and obtaining velocity fields will be briefly described. Further details have been published.'-'' As discussed in those reports, other investigators have used single sensor hot-wire probes to measure velocity flelds in the canine larynx Major disadvantages of these measurements are that velocity fields could not be measured within the irst centimeter above the folds, and that vortical structures could not be detected. For our work, the PIV method was chosen because it can detect rotational flow structures immediately above the glottis. Five male dogs, ranging from 20 to 22 kg, were used in the scarring experiments. General anesthesia was obtained with xylazine hydrochloride. a shoiiacting intramusculai anesthetic. Once an adequate plane of anesthesia was obtained and verified, tbe larynx was exposed via a pédiatrie Lindholm laryngoscope. After adequate visualization was obtained, the laryngoscope was suspended and an operative microscope was advanced into position. Using microlaryngeal instruments, we carefully excised the mucosa of the righi true vocal fold approximately to the depth of ti e intermediate lamina propria. This method for creating scarring is similar to techniques used by We>ler et al^ and Rousseau et al.' Two tnonths after th s procedure, the dogs were euthanized and the larjnges were removed. In addition to the 5 scarred laryngés, we also report measurements Irom 5 normal laryngés: 3 from

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Murugappan e! al. Flow Fields & Acoustics in Unilateral Scarred Vocal Fold Model

previous studies and 2 from laryngés tested at the same time as the scarred laryngés. For all of our experiments, excised canine laryngés were harvested from shared research dogs. Immediately after euthanasia, the larynx was placed in normal saline solution (0.9% sodium chloride) and refrigerated. After 12 to 24 hours, all cartilage and soft tissue above the vocal folds were removed to produce an unobstructed view of the vocal folds. For all animals, the inferior 4 cm of the traehea was placed over a rigid tube. A suture was used to adduct the vocal processes; this suture was placed through both vocal processes at the same level. The stitch was tied with the minimal tension needed to create a prephonatory width of 0 mm between the vocal processes. Special care was taken to position tbe suture symmetrically in both the anterior-posterior and inferior-superior directions. The posterior glottis was left open. The larynx was fixed in space by a square mounting apparatus that had double-prong pins on each side. Each pin was inserted into the cricoid cartilage. A microphone was placed 5 cm to the side of the glottal exit in such a way that it did not interfere with iaryngeal airflow or with the laser illumination. Electronic pressure gauges, a pressure regulator, a thermocouple, a mass tlowmeter, and an electronic control valve were used to regulate and monitor the air upstream. The air was moistened by a humidifier with thermostat control. The air was then mixed with seeding aerosolized olive oil particles in a settling chamber. A high-speed video camera was placed approximately 13 cm above the glottal exit in order to visualize vocal fold vibration. The vocal folds were illuminated by a 150-W fiber optic illuminator (model MI 150, Dolan Jenner. Boxborough, Massachusetts). The high-speed video images were taken at a frame rate of approximately 26.000 frames per second with an exposure time of 2 ms. A 2-dimensionaI PIV system was used. Because the motion we studied was periodic, the microphone signal was used as a trigger source, which allowed us to collect images at a desired phase. A totai of 30 phases were obtained, and 10 images were acquired at every phase to reconstruct the phase-averaged images over the equivalent of I period of the acoustic signal. A data acquisition card was used to record the acoustic, microphone (trigger source), and electroglottograph (EGG) signals. The laser was focused and spread to produce a laser light sheet in the coronal plane at a position halfway between the vocal process and the anterior commissure. The thickness of the laser sheet was 1 mm. Rhodamine G powder was placed on the paraglottic tissue and thyroid cartilage to reduce contam-

ination and saturation of the raw seeded image tbat arose from laser light reflections. The powder never came in contact with the vocal folds. The high-speed imaging and PIV measurements for all phases were obtained during 1 phonation trial lasting over 3 minutes. The vocal fold vibrations were periodic during the measurement, as evidenced by the jitter rates measured over a 20-second period (4,000 cycles for a 20()-Hz fundamental frequency): less than 3.4%. Subglottal pressures were used that gave periodic, stable vibration and reliable microphone and EGG signals. For 4 of the 5 scarred laryngés and 2 of the normal laryngés, we obtained measurements for at least I low and 1 high subglottal pressure. The position of the vocal folds for each phase was evaluated from the high-speed images of vocal fold vibration for the duration of data collection. The phonation threshold pressures (PTPs) were determined by use of an automated process that slowly increased the subglottal pressure until periodic vocal fold vibrations were obtained. The acoustic signals recorded during phonation were digitized and sampled at 22 kHz with a 16-bit National Instruments data acquisition card. A longterm average spectrum (LTAS) was performed t)n the acoustic time series to obtain the spectral characteristics.^ The fundamental frequency and its harmonics were then identified by use of an autocorrelation function. Both the high-speed video and the EGG were used to confirm the fundamental frequency obtained from the spectrum. From the LTAS, the average amplitudes ofthe first 8 harmonic peaks (in decibels) were identified; the first 8 were chosen because we found they were identifiable for all laryngés. Using linear regression, we determined the best-fit line connecting those peaks; the spectral slope was the slope of that best-fit line in decibels per octave. The HNR was computed on the basis of the algorithm of Qi and Hillman.'^The negative pressures produced by the flow-separation vortices were calculated by use of a steady Bernoulli's law for the vortex streamlines, as described by Khosla et a!.' RESULTS Figure 2 shows the average and standard deviation (SD) of the PTPs for tbe normal and scarred laryngés. The average for the scaired folds was 14.9 cm H2O (SD, L94 cm H2O; range, 8.8 to 16.9 cm H2O). For the normal laryngés, the average was 9.2 cm H2O (SD, 1.3 cm H2O; range, 7.65 to 10 cm H2O). The vocal fold displacement was determined from instantaneous high-speed images and refers to the lateral displacement of the vocal fold. The dis-

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Murugappan et al. Flow Fields & Acoustics in Unilateral Scarred Vocal Fold Model

ment ofthe scarred fold versus the normal fold for a scarred larynx. Each vibration cycle is defined as 360°, with opening starting at O*". Because ofa phase difference, the timing ofthe maximal displacement could differ between the normal and scarred folds: this is seen for the larynx in Fig .Mi, in which maximal displacement oci urred for the scarred fold before the normal fold. In Fig 3B, it is seen that the difference in displacement between the two vocal folds occured predominantly during vocal fold closing. Normal Scarred Fig 2. Mean phonation threshold pressure for normal and scarred laryngcs.

placement we measured was obtained by the following method. The reference line connects Ihe anterior commissure to the posterior commissure, and the reference point lies hallway along this line. The displacement is defined as the distance from the reference point to the medial edge ofthe vocal fold at the mid-membranous location. Two voice scientists and I laryngotogist visually evaluated the location of the medial edge from the high-speed images. For each of the 5 scaned and 5 normal laryngés. 150 instantaneous images (spanning 5 cycles) were presented in random order to all of Ihe raters. We repeated 20% of the images to evaluate the intra-rater reliability. The reliability of identifying the medial edge within 2% of the total glottal length was computed on the basis of the percent agreement between the raters (inter-rater reliability) and the repeatability of tbe same rater (intra-rater reliability). We observed that both the intertater (97%) and intra-ratcr (98%) reliabilities were at least 97%. Figure 3B shows a typical graph for the displace-

The phase-averaged mid-coronal velocity field for scarred dog 2 is ihown in Fig 4A and Fig 4B. These correspond to lîhase angles of 267° and 311" at subglottal pressures of 18.5 and 24.5 cm H20. respectively; therefore, these velocity fields were taken near the end of the closing phase. In the velocity fields, the length of the vector is proportional to the velocity magnitude. Vortical stnictures are represented by quasi-cin ular isocontours that represent rotational motion. Vortices that can be identified in phase-averaged imagos have to be consistent in both phase and spatial lot^ation in all of the individual flow-field images taten at each phase. It is important to note that this is not a confmed flow, because all tissue above the vocal folds was removed. On the X-axis. 0 refers to a ¡joint halfway between the superior medial edges of the folds at the mid-coronal plane. The pair of flow-separation vortices is seen in Fig 4A, whereas I'ig 4B only shows one. Since images are taken evei y 30°, an image with both vortices may not ahvay? be captured. It can be seen in Fig 4 that the size of the tlow-separation vortex increases as the subgloLtal pressure increases. For scarred laryngés 1 and 2, we used 3 subglottal pressures. Two subglottal pressures were used for scarred laryngés î and 5 and normal laryngés 4 and 5, and 1 subgiottal pressure was used for scarred

Antcrior commissure erence line DiSlacement (Ô) MedftI edge at the midmeinbi ;iiious location Rt iVrence point

Posterior commissure

150 200 250 Phas5 (degrees)

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Fig 3. A) Method of determining displacement (see text). B) Average maximum displacement of normal and scarred folds over I phonal ion cycle. ,

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Murugappan et al. Flow Fields & Acoustics in Unilateral Scarred Vocal Fold Model

1.5

E

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0.5

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x(mm)

x(mm)

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Fig 4. Flow fields for scarred dog 2 during closing A) when phase angle is 267° and subglottal pressure is 18.54 cm H2O and B) when phase angle is 311° and subglottal pressure is 24.48 cm H2O.

larynx 4 and normal laryngés 1,2, and 3. Figure 5 shows the calculated negative pressure produced by the flow-separation vortices as a function of the subglottal pressure for normal and scaired laryngés. For scarred laryngés 1, 2, and 3, increasing subglottal pressure increased the strength ofthe intraglottal vortices. The 5 normal laryngés had a higher magnitude of intraglottal negative pressures at lower subglottal pressures, compared to the scarred laryngés. For example, subglottal pressures of approximately 19 cm H2O produced negative pressures 3.75 times higher in normal larynx 4 than in scarred larynx 2.

this value of spectral slope is significantly less negative than the spectral slopes for the scarred laryngés at any subglottal pressure. The value of spectral slope seen in the normal laryngés is consistent with normal human phonation.'" Figure 6 also shows that for the scarred laryngés, increasing subglottal pressure produced a less negative spectral slope. Figure 7 shows the HNR as a function of the subglottal pressure. It is seen that higher subglotlal pressure resulted in a higher HNR for both the normal and scarred laryngés.

Figure 6 shows the measured spectral slope as a function of subglottal pressure. A less negative spectral slope corresponds to greater energy in the higher harmonics. In our 5 normal animals, an average spectral slope of-6.2 dB per octave was observed for an average subglottal pressure of 13.6 cm H2O;

Figure 8 shows the spectral slope as a function of maximum displacement. The method for measuring maximum displacement is described above, in relation to Fig 3A. For scarred laryngés 2 and 4, the magnitude ofthe spectral slope decreased as the maximal displacement increased. For scarred larynx 1, the magnitude ofthe spectral slope decreased sig-

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Fig 5. Negative pressure generated by intraglottal vortex as function of subglottal pressure for normal and scarred laryngés.

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-25 10 15 20 Subglottal pressure (cm HîO)

Fig 6. Spectral slope as function of subglottal pressure for normal and scarred laryngés.

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Murugappan et at. Flow Fields