Optical characterization of vocal folds using optical coherence tomographyOptische Charakterisierung von Stimmlippen mit Optischer Kohärenztomographie (OCT)Caracterización óptica de las cuerdas vocales mediante el uso de Tomografía de Coherencia Òptica
Introduction
Optical coherence tomography (OCT) is well suitable for application on vocal folds. The depth relevant for its examination correlates well with the penetration depth of the applied weak infrared laser beam, which is approximately 1–2 mm. The three layers of the lamina propria mucosae of the vocal fold differ from each other by their varying content of elastic and collagenous fibers. The so-called Reinke space, the superficial layer of the lamina propria mucosae, has a fibrous contingent which is loosely organized. The epithelium of a human vocal fold has a diameter of about 0.1 mm. The lamina propria mucosae has a diameter of about 1 mm (Fig. 1).
According to the well-accepted “body-cover-model” introduced by Hirano and coworkers [1], [2], [3], these layers perform certain movements as a function of vocal fold tension. Vocal fold activity also relies on a functional separation of muscle (body) and the lamina propria mucosae (cover). This separation has to be respected whenever performing surgery [4], [5], [6], [7], [8].
To indicate phonosurgery it is of key importance to differentiate the quality of structural changes on vocal folds beforehand, such as the expansion of lesions into the depth or tissue loss. The latter results in a concave vocal fold, which concomitantly leads to a loss of aerodynamics. The voice becomes instable and will be limited in performance of frequency and dynamic range. Moreover, due to scaring of the tissue, the already present voice deficit becomes worse by a stiff glottis [6]. So it becomes evident that an exact preoperative diagnosis is a prerequisite for optimizing protection of the epithelium.
OCT is already well established in dermatology and ophthalmology [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22]. First clinical studies in the respiratory and digestive tracts were also already performed at the Nizhny Novgorod Medical Academy in Russia [11], [23]. The most detailed and comprehensive study of laryngeal tissue was done by the medical academy and the Semashko Regional Clinical Hospital in Nizhny Novgorod [24].
Section snippets
Materials and methods
OCT is a method for imaging different layers of transparent or scattering tissue by scanning the area of interest with a weak (<1 mW) laser beam. The measuring principle is similar to ultrasound imaging. The measurement signal consists of light which is directly reflected from the surface of the tissue as well as light which is back-scattered from different layers from inside the tissue. As a typical light source, superluminescent diodes with a broad emission band (several tens of nanometers)
Results
Fig. 5 shows a comparison of an OCT image, which was taken from the porcine vocal fold and a histological section from the same area. The image clearly shows the distinction of the epithelium mucosae from the loose collagenous structure of the lamina propria mucosae. In these pictures, the epithelium mucosae has a thickness of approximately 50 μm. Inside the loose collagenous layer, structures can be detected which can be interpreted as vessels (arrows). Since the resolution of the OCT image is
Conclusion
During laryngoscopy, the question whether or not a lesion is malignant is a common challenge. One of the main criteria for the decision between phonosurgery versus tumorsurgery depends on the depth extension of the lesion. This cannot be decided by sole examination of the surface structure. It is important to get a view under the surface of the vocal fold. Alterations which penetrate the lamina propria mucosae into the deep layer are defined to be malignant. The diameter of the lamina propria
Acknowledgements
We thank H. Gasse, R. Koch from Hanover Vetenary School for preparing the histological sections, and we thank ISIS Optronics as well as Carl Zeiss Surgical Instruments for enabling us the OCT measurements and we thank H. Nawe from Hanover Medical School for preparing the human histological section.
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