New laryngoscope for quantitative high-speed imaging of human vocal folds vibration in the horizontal and vertical direction

External Validity of Calibrated Measurements from a Laser-Projection Transnasal Fiberoptic High-Speed Videoendoscopy System

OBJECTIVE

Evidence-based practice and precision medicine can significantly benefit from the ability to perform calibrated spatial measurements (eg, mm) from endoscopic images. However, calibrated measurements are not readily available from laryngeal images. Laser-projection endoscopes can provide the required information for performing calibrated spatial measurements, but their applications require a process known as calibration. During calibration, a set of benchtop recordings are used to determine the effect of confounding factors of spatial measurements, and also to learn their proper compensation strategies. Calibration benchtop recordings are acquired from flat surfaces and at a perpendicular imaging angle which is significantly different from in-vivo situations, where a three-dimensional (3D) surface gets recorded at a semi-unknown imaging angle. The aim of this study was to quantify changes in calibrated vertical and horizontal measurement accuracies as we move from the controlled condition of calibration to more realistic and uncontrolled settings.

METHOD

A flat surface was positioned in front of a calibrated laser-projection transnasal fiberoptic endoscope at different working distances and imaging angles. Calibrated vertical and horizontal measurement errors were computed from each condition. Multiple linear regression analyses were used to quantify the dependence of vertical and horizontal measurement errors on the imaging angle and working distance. Next, a 3D-printed surface was positioned in front of the laser-projection endoscope at different working distances. Calibrated vertical and horizontal measurement errors were computed from each condition and then they were compared to measurement errors from a flat surface positioned at comparable working distances.

RESULTS

The outcome of analyses supported a significant effect of imaging angle on calibrated vertical measurement accuracy, while no significant effect of imaging angle on calibrated horizontal measurement accuracy was established. Additionally, the result of multiple linear regression analyses showed that the coefficient of imaging angle was two times larger than the working distance, which further highlights the significant effect of imaging angle on vertical measurement accuracy. Comparing the magnitude of calibrated vertical and horizontal measurement errors between the 3D surface and a flat surface suggested a significant effect of surface topology on calibrated measurement accuracies.

CONCLUSIONS

The mean percent magnitude of error of vertical and horizontal measurement errors from the 3D surface were respectively around 6% and 11%, at most working distances, which are acceptable for many applications. However, the significant effect of imaging angle and surface topology on measurement errors highlights the need for further research on these confounding factors. It also suggests that significant improvements in measurement accuracies may be achieved if these factors are properly accounted for during the calibration process. Last but not least, this study highlights the need for the evaluation of laser-projection endoscopes in uncontrolled and more realistic settings. Specifically, evaluations of laser-projection endoscopes in very controlled settings could significantly overestimate their accuracies and hence it will not represent their actual performances during in-vivo data acquisitions.

Analyzing Vocal Fold Frequency Dynamics Using High-Speed 3D Laser Video Endoscopy

OBJECTIVE

To examine changes in lateral and vertical vibratory motion along the anterior, middle, and posterior sections of the vocal folds, as a function of vocal frequency variations.

METHODS

Absolute measurements of vocal fold surface dynamics from high-speed videoendoscopy with custom laser endoscope were made on 23 vocally healthy adults during sustained /i:/ production at 10%, 20%, and 80% of pitch range. The 3D parameters of amplitude (mm), maximum velocity opening/closing (mm/s), and mean velocity opening/closing (mm/s) were computed for the lateral and vertical vibratory motion along the anterior, middle, and posterior sections of the vocal folds. Linear mixed model analysis was conducted to evaluate the differences in (a) vocal frequency levels (high vs. normal vs. low pitch), (b) axis level (vertical vs. lateral), (c) position level (anterior vs. middle vs. posterior), and (d) gender differences (male vs. female).

RESULTS

Overall, the superior surface vertical motion of the vocal fold is greater compared with the lateral motion, especially in males. Along the superior surface, the mean and maximum closing velocities are greater posteriorly for low pitch. The location (anterior, middle, and posterior) along the superior surface is relevant only for vocal fold closing rather than opening, as the dynamics are different along the various locations.

CONCLUSIONS

The study highlights the significance of assessing the vertical motion of the superior surface of the vocal fold to understand the complex dynamics of voice production.

LEVEL OF EVIDENCE

NA Laryngoscope, 134:3267-3276, 2024.

Method for Vertical Calibration of Laser-Projection Transnasal Fiberoptic High-Speed Videoendoscopy

The ability to provide absolute calibrated measurement of the laryngeal structures during phonation is of paramount importance to voice science and clinical practice. Calibrated three-dimensional measurement could provide essential information for modeling purposes, for studying the developmental aspects of vocal fold vibration, for refining functional voice assessment and treatment outcomes evaluation, and for more accurate staging and grading of laryngeal disease. Recently, a laser-calibrated transnasal fiberoptic endoscope compatible with high-speed videoendoscopy (HSV) and capable of providing three-dimensional measurements was developed. The optical principle employed is to project a grid of 7 × 7 green laser points across the field of view (FOV) at an angle relative to the imaging axis, such that (after calibration) the position of each laser point within the FOV encodes the vertical distance from the tip of the endoscope to the laryngeal tissues. The purpose of this study was to develop a precise method for vertical calibration of the endoscope. Investigating the position of the laser points showed that, besides the vertical distance, they also depend on the parameters of the lens coupler, including the FOV position within the image frame and the rotation angle of the endoscope. The presented automatic calibration method was developed to compensate for the effect of these parameters. Statistical image processing and pattern recognition were used to detect the FOV, the center of FOV, and the fiducial marker. This step normalizes the HSV frames to a standard coordinate system and removes the dependence of the laser-point positions on the parameters of the lens coupler. Then, using a statistical learning technique, a calibration protocol was developed to model the trajectories of all laser points as the working distance was varied. Finally, a set of experiments was conducted to measure the accuracy and reliability of every step of the procedure. The system was able to measure absolute vertical distance with mean percent error in the range of 1.7% to 4.7%, depending on the working distance.

High-speed videolaryngoscopy in early glottic carcinoma patients following transoral CO2 LASER cordectomy

PURPOSE

To compare high-speed videolaryngoscopy (HSV) findings, like open quotient (OQ), vocal fold vibratory onset delay (VFVOD), amplitude symmetry index (ASI) and phase symmetry index (PSI), after 6 months of cordectomy with that after 1 year, and to compare later with the control group.

METHODS

Retrospective analysis of HSV recordings of 33 patients of early glottic carcinoma after cordectomy was performed after 6 months and 1 year of cordectomy with the help of videokymogram and digital kymogram. The control group of ten individuals was selected from patients who came to hospital with complaints other than larynx. The comparison was done for different types of cordectomy separately.

RESULTS

The mean of OQ, VFVOD, ASI and PSI was found to be significantly higher after 6-month follow-up than after 1-year follow-up, the later was in turn found to be significantly higher than that of the control group, for type II and III cordectomy. The mean of OQ, VFVOD, ASI and PSI for other types of cordectomy also showed similar results, but the significance was not calculated due to less sample size.

CONCLUSION

There was significant improvement in OQ, VFVOD, PSI and ASI following CO₂ LASER cordectomy after 1 year of follow-up, making it a good management option for early glottic carcinoma. The ability of the HSV to measure variations in the vocal cord vibration, following cordectomy, was well established by this study.

Synchronized, concurrent optical coherence tomography and videostroboscopy for monitoring vocal fold morphology and kinematics

Voice disorders affect a large number of adults in the United States, and their clinical evaluation heavily relies on laryngeal videostroboscopy, which captures the medial-lateral and anterior-posterior motion of the vocal folds using stroboscopic sampling. However, videostroboscopy does not provide direct visualization of the superior-inferior movement of the vocal folds, which yields important clinical insight. In this paper, we present a novel technology that complements videostroboscopic findings by adding the ability to image the coronal plane and visualize the superior-inferior movement of the vocal folds. The technology is based on optical coherence tomography, which is combined with videostroboscopy within the same endoscopic probe to provide spatially and temporally co-registered images of the mucosal wave motion, as well as vocal folds subsurface morphology. We demonstrate the capability of the rigid endoscopic probe, in a benchtop setting, to characterize the complex movement and subsurface structure of the aerodynamically driven excised larynx models within the 50 to 200 Hz phonation range. Our preliminary results encourage future development of this technology with the goal of its use for in vivo laryngeal imaging.

Laser-Calibrated System for Transnasal Fiberoptic Laryngeal High-Speed Videoendoscopy

The design specifications and experimental characteristics of a newly developed laser-projection transnasal flexible endoscope coupled with a high-speed videoendoscopy system are provided. The hardware and software design of the proposed system benefits from the combination of structured green light projection and laser triangulation techniques, which provide the capability of calibrated absolute measurements of the laryngeal structures along the horizontal and vertical planes during phonation. Visual inspection of in vivo acquired images demonstrated sharp contrast between laser points and background, confirming successful design of the system. Objective analyses were carried out for assessing the irradiance of the system and the penetration of the green laser light into the red and blue channels in the recorded images. The analysis showed that the system has irradiance of 372 W/m² at a working distance of 20 mm, which is well within the safety limits, indicating minimal risk of usage of the device on human subjects. Additionally, the color penetration analysis showed that, with probability of 90%, the ratio of contamination of the red channel from the green laser light is less than 0.002. This indicates minimal effect of the laser projection on the measurements performed on the red data channel, making the system applicable for calibrated 3D spatial-temporal segmentation and data-driven subject-specific modeling, which is important for further advancing voice science and clinical voice assessment.

Dynamic vocal fold parameters with changing adduction in ex-vivo hemilarynx experiments

Ex-vivo hemilarynx experiments allow the visualization and quantification of three-dimensional dynamics of the medial vocal fold surface. For three excised human male larynges, the vibrational output, the glottal flow resistance, and the sound pressure during sustained phonation were analyzed as a function of vocal fold adduction for varying subglottal pressure. Empirical eigenfunctions, displacements, and velocities were investigated along the vocal fold surface. For two larynges, an increase of adduction level resulted in an increase of the glottal flow resistance at equal subglottal pressures. This caused an increase of lateral and vertical oscillation amplitudes and velocity indicating an improved energy transfer from the airflow to the vocal folds. In contrast, the third larynx exhibited an amplitude decrease for rising adduction accompanying reduction of the flow resistance. By evaluating the empirical eigenfunctions, this reduced flow resistance was assigned to an unbalanced oscillation pattern with predominantly lateral amplitudes. The results suggest that adduction facilitates the phonatory process by increasing the glottal flow resistance and enhancing the vibrational amplitudes. However, this interrelation only holds for a maintained balanced ratio between vertical and lateral displacements. Indeed, a balanced vertical-lateral oscillation pattern may be more beneficial to phonation than strong periodicity with predominantly lateral vibrations.

Volume estimation of tonsil phantoms using an oral camera with 3D imaging

Three-dimensional (3D) visualization of oral cavity and oropharyngeal anatomy may play an important role in the evaluation for obstructive sleep apnea (OSA). Although computed tomography (CT) and magnetic resonance (MRI) imaging are capable of providing 3D anatomical descriptions, this type of technology is not readily available in a clinic setting. Current imaging of the oropharynx is performed using a light source and tongue depressors. For better assessment of the inferior pole of the tonsils and tongue base flexible laryngoscopes are required which only provide a two dimensional (2D) rendering. As a result, clinical diagnosis is generally subjective in tonsillar hypertrophy where current physical examination has limitations. In this report, we designed a hand held portable oral camera with 3D imaging capability to reconstruct the anatomy of the oropharynx in tonsillar hypertrophy where the tonsils get enlarged and can lead to increased airway resistance. We were able to precisely reconstruct the 3D shape of the tonsils and from that estimate airway obstruction percentage and volume of the tonsils in 3D printed realistic models. Our results correlate well with Brodsky's classification of tonsillar hypertrophy as well as intraoperative volume estimations.

State of the art laryngeal imaging: research and clinical implications

PURPOSE OF REVIEW

This study provides a review of the latest advances in videostroboscopy, videokymography and high-speed videoendoscopy, and outlines the development of new laryngeal imaging modalities based on optical coherence tomography, laser-depth kymography, and magnetic resonance imaging (MRI), published in the past 2 years.

RECENT FINDINGS

Videostroboscopy and videokymography: Image quality has improved and several image processing and measurement techniques have been published. High-speed videoendoscopy: Significant progress has been made through increased sensitivity and frame rates of the cameras, and the development of facilitative playbacks, phonovibrography and several image segmentation and measurement methods. Clinical evidence was presented through applications in phonosurgery, comparisons with videostroboscopy, normative data, and better understanding of voice production. Optical coherence tomography: Latest developments allow the capture of dynamic high-resolution cross-sectional images of the vibrating vocal fold mucosa during phonation. Depth kymography: New laser technique allowing recording of the vertical movements of the vocal folds during phonation in calibrated spatial values. Laryngeal magnetic resonance: New methods allow high-resolution imaging of laryngeal tissue microstructure, or measuring of dynamic laryngeal structures during phonation.

SUMMARY

The endoscopic laryngeal imaging techniques have made significant advances increasing their clinical value, whereas techniques providing new types of potentially clinically relevant information have emerged.

In Vivo measurement of pediatric vocal fold motion using structured light laser projection

OBJECTIVE

The aim of the study was to present the development of a miniature structured light laser projection endoscope and to quantify vocal fold length and vibratory features related to impact stress of the pediatric glottis using high-speed imaging.

STUDY DESIGN

The custom-developed laser projection system consists of a green laser with a 4-mm diameter optics module at the tip of the endoscope, projecting 20 vertical laser lines on the glottis. Measurements of absolute phonatory vocal fold length, membranous vocal fold length, peak amplitude, amplitude-to-length ratio, average closing velocity, and impact velocity were obtained in five children (6-9 years), two adult male and three adult female participants without voice disorders, and one child (10 years) with bilateral vocal fold nodules during modal phonation.

RESULTS

Independent measurements made on the glottal length of a vocal fold phantom demonstrated a 0.13mm bias error with a standard deviation of 0.23mm, indicating adequate precision and accuracy for measuring vocal fold structures and displacement. First, in vivo measurements of amplitude-to-length ratio, peak closing velocity, and impact velocity during phonation in pediatric population and a child with vocal fold nodules are reported.

CONCLUSION

The proposed laser projection system can be used to obtain in vivo measurements of absolute length and vibratory features in children and adults. Children have large amplitude-to-length ratio compared with typically developing adults, whereas nodules result in larger peak amplitude, amplitude-to-length ratio, average closing velocity, and impact velocity compared with typically developing children.

Kymographic imaging of laryngeal vibrations

PURPOSE OF REVIEW

Kymographic imaging is a modern method for displaying and evaluating vibratory behaviour of the vocal folds which is crucial for voice production. This review summarizes the state of the art of this method, and focuses on the progress in this area within the last 5 years.

RECENT FINDINGS

Videokymography, using a special videocamera, offers high-speed (video)kymographic images in real time, which is advantageous in daily clinical practice. Two other methods use software to create kymograms retrospectively: digital kymography processes high-speed videolaryngoscopic recordings and offers numerous research possibilities, whereas strobovideokymography processes videostroboscopic recordings, and its use is limited to regular vibration patterns. Current studies reveal that high-speed kymographic images allow more reliable visual evaluation of vibrations than by watching video recordings. Image analysis procedures have been advanced to quantify the vibration properties of the vocal folds. New information has been obtained on asymmetry, mucosal waves, irregularities, phonation onset, and nonlinear dynamic phenomena in voice disorders, as well as in singing.

SUMMARY

High-speed kymography visualizes vibratory features which are not simply observable via traditional methods. It shows large potential in better understanding the functional origin of hoarseness and unsteady phonatory states. Further research in this area is envisioned.

Laser projection imaging for measurement of pediatric voice

OBJECTIVES/HYPOTHESIS

The aim of the study was to present the development of a miniature laser projection endoscope and to quantify vocal fold length and vibratory amplitude of the pediatric glottis using high-speed digital imaging coupled with the laser endoscope.

STUDY DESIGN

For this prospective study, absolute measurement of entire vocal fold length, membranous length of the vocal fold, and vibratory amplitude during phonation were obtained in one child (9 years old), one adult male (36 years old), and one adult female (20 years old) with the use of high-speed digital imaging, coupled with a custom-developed laser projection endoscope.

METHODS

The laser projection system consists of a module slip-fit sleeve with two 3-mW 650-nm laser diodes in horizontal orientation separated by a distance of 5 mm. Calibration involved projecting the laser onto grid patterns at depths ranging from 6 to 10 cm and tilt angles of 15 to -5 degrees to obtain pixel-to-millimeter conversion templates. Measurements of vocal fold length and vibratory amplitude were extracted based on methods of image processing.

RESULTS

The system demonstrated a method for estimating vocal fold length and vibratory amplitude with a single laser point with high measurement precision. First measurements of vocal fold length (6.8 mm) and vibratory amplitude (0.25 mm) during phonation in a pediatric participant are reported.

CONCLUSIONS

The proposed laser projection system can be used to obtain absolute length and vibratory measurements of the pediatric glottis. The projection system can be used with stroboscopy or high-speed digital imaging systems with a 70-degree rigid endoscope.