The Laryngoscope
Lippincott Williams & Wilkins, Inc., Philadelphia
©
2002 The American Laryngological,
Rhinological and Otological Society, Inc.
High-Precision Measurement of the Vocal
Fold Length and Vibratory Amplitudes
Stefan Schuberth, Dipl Phys; Ulrich Hoppe, PD Dr Dr; Michael D o¨ llinger, Dipl Math;
J o¨ rg Lohscheller, Dipl Ing; Ulrich Eysholdt, Prof Dr Dr
Objective/Hypothesis: Standard laryngoscopy suf-
fers from a lack of information about the actual size of
the observed objects (i.e., vocal fold length and oscillat-
ing amplitudes). However, there is much interest in ab-
solute measures for both clinical and research pur-
poses. Therefore, a laser projection device has been
developed that enables the precise determination of ab-
solute units in endoscopic investigation during respira-
tion and phonation. Study Design: An experimental
study in which 9 adults underwent high-speed endos-
copy combined with a laser projection device. Methods:
The projection system consists of two parallel laser
beams with a distance of 3.8 mm. It is mounted on the tip
of a rigid endoscope, which is attached to a digital high-
speed camera. During development and design, exami-
nation situations were taken into account. Two laser
spots are projected onto the vocal folds and enable the
definition of a metric scale within the endoscopic im-
age. Knowledge-based image processing algorithms
were used for evaluation. Results: First measurements
of the vocal fold length during phonation were per-
formed in a group of nine healthy male students. The
determination of glottal length during phonation
agrees with former results. Quantifying vocal fold veloc-
ities in absolute units makes it possible to estimate the
initial collision forces. Conclusions: The presented laser
projection system allows the determination of absolute
measures in the larynx. Because of the simple func-
tional principle, the system is open for use without dig-
ital high-speed recording as well. Absolute units may
also be helpful in voice diagnosis and in monitoring
during voice therapy. Key Words: laryngoscopy, phona-
tion, vocal fold length, high-speed camera, laser projec-
tion system.
of voice disorders. Standard laryngoscopy as the most
commonly used imaging method for vocal fold examina-
tion allows rough estimation of the vibratory patterns.
Using digital high-speed glottography, examinations were
1
extended to irregular vocal fold oscillations. Both meth-
ods fail to procure an absolute scale and allow only a
determination of the relative size of anatomical struc-
tures. Therefore, laryngoscopic recordings performed in
separate sessions are not comparable. Consequently, long-
term effects (e.g., during voice therapy) usually cannot be
observed.
Introducing an absolute scale may avoid these disad-
vantages. If both the optical properties of the endoscopic
optic and the distance between endoscope and the glottal
plane were known, an absolute scale could be derived.
However, these parameters are not known in the exami-
nation situation. Therefore, many attempts were made to
yield an absolute scale by using sophisticated techniques.
For example, stereo endoscopy, where simultaneously two
endoscopes are brought into the throat, provides absolute
2
scaling information. Another technique to receive metric
information is the projection of objects with known size
into the larynx.
The purpose of the current report is to present a new
laser projection system (LPS) for determining metric in-
formation. It consists of two parallel laser beams in a fixed
distance, which are projected on the glottal plane. The
system allows derivation of anatomical structures, vocal
fold elongations, and vocal fold velocities. The design of
the LPS is described, and the accuracy of this method is
estimated. First measurements in a group of nine male
subjects are presented.
Laryngoscope, 112:1043–1049, 2002
INTRODUCTION
The recording of vocal fold oscillations during phona-
tion by endoscopic methods is important for the diagnosis
MATERIALS AND METHODS
Laser Projection System
The LPS was designed by our group to enable determination
of absolute measures. It consists of a portable laser unit and a
projection device. The laser unit is a battery-driven semiconduc-
tor laser diode with a wavelength of 633 nm (GaAs). A light pipe
channels the laser beam to the projection device (Figs. 1 and 2).
This device consists of a metal casing with a height of 10
mm, length of 38 mm, and width of 6 mm. The bottom is a quartz
glass plate, which is passed by the laser light. A mirror of 50%
reflectivity splits the laser beam from the light pipe. One part of
From the Department of Phoniatrics and Pediatric Audiology, Uni-
versity of Erlangen-N u¨ rnberg, Erlangen, Germany.
Supported by the Deutsche Forschungs Gemeinschaft (project DFG
EY 15/8-3).
Editor’s Note: This Manuscript was accepted for publication Febru-
ary 1, 2002.
Send Correspondence to Stefan Schuberth, Dipl-Phys, Department
of Phoniatrics and Pediatric Audiology, Bohlenplatz 21, D-91054 Erlangen,
Germany. E-mail: Stefan.Schuberth@gmx.de
Laryngoscope 112: June 2002
Schuberth et al.: Vocal Fold Measurements
1043