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Measuring the motions in the human middle ear by Laser Doppler Vibrometry
Optics and Lasers in Engineering 25 (1996) 289-301 Copyright 0 1996 Elsevier Science Limited Printed in Northern Ireland. All rights reserved 0143-8166/96/SlS~XI 014%8166(95)ooo76-3
ELSEVIER
Measuring the Motions in the Human Middle Ear by Laser Doppler Vibrometry Hans-Jochen Department
Foth, Christian Huthoff, Matthias Brenner, Silke Ftirber
of Physics, University of Kaiserslautern, Erwin-Schrbdinger-StraBe, Kaiserslautern, Germany
67663
Norbert Stasche Department
of ENT, Klinikum of Kaiserslautern, Friedrich-Engels-StraRe, Kaiserslautern, Germany
Antonio Baker-Schreyer Department
of ENT, University
& Karl Hiirmann
Hospital of Mannheim, Mannheim, Germany
(Received 26 January
67655
Theodor-Kutzer-Ufer,
68167
1995; accepted 19 May 1995)
ABSTRACT For the detection of tiny motions which are caused by the tympanic membrane under normal hearing conditions, the touch-free method of Laser Doppler Vibrometry was used. Spectra containing information about the motions of the middle ear bones were recorded within 1 min when the umbo was chosen as the detection point and acoustic stimulation was performed via white noise excitation. It was observed that these spectra correlate to middle ear diseases, which had been artificiaily induced by manipulations in the chain of the middle ear bones in human temporal bones. The dosimetry of the applied laser radiation was found to be harmless to the tympanic membrane, which opened the way for successful in vivo measurements. Copyright 0 1996 Elsevier Science Ltd.
1 INTRODUCTION The human ear comprises the inner, the middle and the external ear, the latter consisting of the auricle and the auditory canal (Fig. 1). The auditory canal has the shape of a slightly bent tube with a typical length 289
H.-J. Foth et al.
290
Auricle pinna
Semicircular
Fig. 1.
canals
Vertical section through the human ear.
of 30 mm and a diameter of 6-8 mm and is closed to the middle ear by the ear drum (tympanic membrane). This membrane is the shape of a flat funnel; the deepest point of the funnel, which is also nearly the centre, is called the umbo. The sophisticated construction of the human middle ear is needed to match the index of refraction of acoustic waves propagated in air versus that in the liquid of the inner ear. Without the mechanics of the middle ear bones the reflectivity would be at 97%. The acoustic waves guided by the ear canal stimulate the ear drum to move, which is then transmitted by the three middle ear bones (ossicular chain) to the fluid medium in the inner ear (cochlea). These three bones, the hammer (malleus), the anvil (incus) and the stirrup (stapes) typically have masses of 23, 27 and 2.5 mg, respectively. Under normal hearing conditions (40-70 dB SPL; Sound Pressure Level) the amplitude of the motion is in the range of several tens of nanometers. The best point to monitor the motion of the tympanic membrane is the umbo, since at this point the manubrium of the malleus is fixed to the tympanic membrane. Therefore measurements at this point give information not only about the dynamics of the tympanic membrane but also about the dynamics of the middle ear bones. A touch-free method is needed to detect the motions of the middle
Measuring motions in the human middle ear
291
ear without perturbing them. While the earliest measurements have been performed with a capacitive probe;’ the laser technique was used in 1986 by Tonndorf & Khanna. * These authors used at first Laser Doppler Vibrometry, in the following years mainly holographic techniques3 to analyse the motion of the tympanic membrane came back after several years to Laser Doppler Vibrometry. In their recent projects4 they showed that the motion of the manubrium of the malleus of cats has to be described as a superposition of a rotational and a translational motion. Aside from diseases of the inner ear, there are also multiple cases where the hearing of a person is decreased by imperfect sound transmission in the middle ear. The intention of our work is to test whether Laser Doppler Vibrometry is a suitable technique to be applied for the diagnosis of middle ear diseases.
2 EXPERIMENTAL
SET-UP
The experimental set-up is shown in Fig. 2. The temporal bones were clamped in an adjustable vice to ensure a stable mounting and to adjust the bone in a proper way to the laser beam. Previously frozen human temporal bones were used; after warming up, they were cleaned and permanently moistened to avoid dehydration. In some cases the outer
Microscope LaserDoppler Vibrometer
-
Fourier Analyzer
Fig. 2.
Experimental
set-up.
292
H.-J. Foth et al.
ear canal was removed, while the laser beam was usually brought through the intact ear canal to represent a situation most comparable to clinical conditions. The point where the laser beam irradiated the tympanic membrane was checked by a binocular (Zeiss OPM 9). The experiments were carried out with a commercial Laser Doppler Vibrometer (Polytec, Model OFV 3000). The He-Ne laser beam, with a total power of less than 1 mW, was transmitted by a flexible optical fibre and focused onto the tympanic membrane by the commercial optics of Polytec or by a microprobe, which was developed at the Department of Physics in cooperation with the company Polytec. This microprobe with a length of -50 mm and an outer diameter of 1.5 mm is small enough to be inserted into the ear canal; at the end of the microprobe a gradient index lens (Selfoc@ lens) was used to generate a beam waist of a diameter of 9 pm at a focus length of 3 mm.’ The electric output of the Laser Doppler Vibrometer is proportional to the time depending component of velocity onto the direction of the laser beam of the irradiated spot u,(t). A Fast-Fourier-Transformer (FFI, Advantest: Model 9211 B) was used to transform uz(t) into an excitation spectrum u,(o), which shows at which frequencies w the velocity or the amplitude of the motion has a maximum. Usually spectra in the frequency range O-10 kHz were recorded, and up to 100 transformed spectra were averaged to obtain a good SNR. An IEEE bus was used to transfer the data to a PC for data handling and to guide the Laser Doppler Vibrometer, as well as the FFT-analyser and the functions generator, which was used to generate the sinus tone for acoustic stimulation of the tympanic membrane. At the beginning the acoustic stimulation was performed with sinus tone excitation; but the extremely long measuring time of up to 7 h led to an excitation by white noise; this cut down the measuring time to less than 60 s for a spectrum of comparable resolution and SNR.6 The generation of white noise was done by a card in the FFI analyser (digital white noise), with the effect that the spectrum contained only these frequencies, which are used for the digital Fourier transformation. The electric output of the functions generator or of the white noise generator was amplified to drive a broad band loudspeaker (Dynaudio, model: Gemini). The sound pressure level of the acoustic field was checked by a microphone (Bruel & Kjaer, model: 4165, 2639, and 5935). Great care was taken not to perturb the acoustic field by the mounting of the microphone and furthermore to be aware of standing waves. The whole set-up was placed inside of an acoustic insulated cabin (IAC, Industrial Acoustic Company).
Measuring motions in the human middle ear
3 EXPERIMENTAL
293
RESULTS
3.1 Results with temporal bones Figure 3 shows a typical result obtained by an acoustic excitation with a sinus tone of 3 kHz and a sound pressure level of 93 dB. Figure 3(a)
100.0 flV
Xa
Gaa
Lwl
0.0 Vr
-...adL-
0.0
.
I
i
FlaQmlcY
82
2oJk
Fig. 3. Top: velocity of the tympanic membrane observed in the area of the light reflex by stimulating with a sinus tone of 3 kHz at 93 dB SPL. Scale of the vertical axis: 1 V = 5 mm s-l. Bottom: Fourier transformation of signal as shown above (stimulation with a sinus tone of 3 kHz, 75 dB SPL).
H.-J. Foth et al.
294
shows the directly observed signal v,(t) while Fig. 3(b) shows the Fourier transformed spectrum u,(o); obviously in this case the motion can be described in good approximation by an harmonic oscillation. From the measured signal u,(t) or u,(o) the amplitude of the motion z(t) resp. z(w) is easily calculated by time integration. These technical developments have been the basis used to study the dependency of the resonance spectrum at the umbo on disorders in the middle ear. In experiments with temporal bones, various changes had been artificially induced, in the sense to simulate mechanical immobility in the chain of the middle ear bones, which are well known from the clinical routine:
0) Fixation of the malleus head, which happens in reality by an
(2)
(3)
(4)
inflammation and swelling in the middle ear. Here it was simulated by using a piece of foam rubber. After removing the piece of rubber, the same spectrum was recorded as before starting this manipulation. Fixation of the stapes footplate, described as otosclerosis, when a ring of bone had grown around the footplane and hinders any motion of the stapes. Here it was simulated by using bone cement. Dislocation of a joint, which may be caused by an acoustic shock front, like a bang. Here the joint between incus and stapes was intersected mechanically. Finally, the incus was removed to create a greater loss of mass in the ossicular chain.
The recorded spectra in correlation to the described manipulations are shown in Fig. 4. Obviously each of these changes introduced a specific change in the resonance spectrum. More details were described in a recent publication.7 3.2 Dosimetry
of the applied laser radiation
Before the technique of Laser Doppler Vibrometry was applied to living persons, it had to be ensured that the irradiated He-Ne laser beam does not do any harm to the tympanic membrane. Even if daily experience indicates that a continuous laser beam of 1 mW power focused onto the skin introduces no obvious thermal effects, the problem has to be investigated carefully. Using the conventional optic of the Laser Doppler Vibrometer, the laser beam is focused down to a spot size of 40 pm diameter reaching a laser intensity of 80 W cm-‘. The thresholds for thermal effects are at
Measuring motions in the human middle ear
r 012
-
‘. :..’ :
010 -
295
-........ without manipulation -fixation of malleus -------fixation of stapes ------dislocation of joint
_ 0083 m 006.* B
004
-
002
-
>”
\___x’ 1
I.
0
2000
I
L
4000
6000
I
6000
I
(
10000
Frequency [Hz] Fig. 4.
l l l
Velocity of the umbo in correlation to artificially induced manipulations middle ear bones to simulate middle ear diseases.
100 W cmm2 for coagulation, 2000 W cm-’ for vaporization, lo6 W cm-* for photoablation.
at the
and
Obviously vaporization or photoablation is not possible, but the risk of coagulation had to be checked seriously. The experiment was performed with a cw ring dye laser (Coherent 699-21) pumped by an 8 Watt Ar’ laser. The wavelength was tuned to 633 nm and controlled by a travelling Michelson interferometer. The laser power, which was sent onto the specimens was selected by turning a Nicol’s prism in the optical light path. The laser beam was focused by a plan convex lens with a focus length of 100 mm to a spot size of 80 pm diameter. This spot size was determined by a microscope lens and a CCD camera; the measured intensity profile was fitted by a Gaussian profile. Therefore, the described width is the full width at l/e’. In a first series 17 specimens of human tympanic membrane were irradiated with power densities between 1000 and 12 000 W cm-‘. During the irradiation time of up to 6Os, the transmitted laser light and the diffraction pattern was on-line monitored by the CCD camera. The threshold at which this pattern became unstable, i.e. moving and changing, was taken at the threshold for thermal effects. Generation of holes was easily recognized by the transmitted light and ensured by inspection with a microscope. The results of these measurements are
H.-J. Foth et al.
296 I
.
-A-
-
’
I
I
’
1
-o-
ThermalEffects
’
1
”
Hole or Coagulation
100
.
2
4
6 Inten&ty
Fig. 5.
Percentage
8
10
12
[kW/cm’]
of damages in the irradiated specimens.
summarized in Fig. 5 showing that the threshold for holes was found at 2000 W cm-2.8 In a second series, 38 specimens of tympanic membranes of fresh slaughtered pigs were irradiated with laser intensities up to 20 000 W cm*. Damage was observed in none of the specimens at laser intensities below of 2500 W cm-2.9 In current experiments, the power density at which 100% of the irradiated specimens get damaged is investigated, since this threshold was not reached in the experiments so far. However, the laser intensity of 80 W cm-‘, applied by the Laser Doppler Vibrometer, is a factor of 30 below the lowest value at which damage occurred. Furthermore, the sticker of retro foil protects the tympanic membrane against the laser radiation. This was the basis to perform in uiuo measurements. 3.3 In uiuo measurements Figure 6 shows the velocity of the umbo of a 25year-old healthy male with a normal ear.” The spectrum had been obtained at a sound pressure level of around 80 dB and stimulation by white noise. The dominant resonance around 3-5 kHz is mainly caused by the resonance of a standing wave in the ear canal. This result shows that the technique of Laser Doppler Vibrometry is suitable for clinical application.
Measuring motions in the human middle ear
297
1.5 i
.
5 1.0 .P 8 z a 0.5 -
0.0 I 0
h
I
,
2000
I,
4000
I
Frequency [ Hz
Fig. 6.
,
6000
,
8000
.
I 10000
]
Velocity of the umbo in a healthy ear of a 25year-old
male.
4 DISCUSSION The results presented here show that Laser Doppler Vibrometry can be performed successfully while bringing the laser beam through the intact outer ear canal. Furthermore, experiments with temporal bones have shown that the spectra U,(W) observed at the umbo depend on the dynamics of the middle ear bones. Manipulations in the chain of the middle ear bones, by which middle ear diseases were simulated, generated characteristic changes in the spectra. These changes were easily recognized by comparing the spectra of one temporal bone before and after manipulation. Comparing the spectra of different temporal bones showed that the individual variation is sometimes larger than the chances due to the manipulations. The results of the dosimetry opened the way to in viuo measurements as shown in Fig. 6. Here also, spectra of different persons show a broad variety. Common in all spectra is the dominant resonance at 3 kHz. By a total length of 30 mm of the outer ear canal, the first standing wave in this tube would have a wavelength of 120mm and a frequency of around 28 kHz (the speed of sound is at 340.3 m s-l). Therefore, this resonance is seen to be mainly due to the outer ear canal. Resonances, which are more sensitive on the condition of the middle ear bones show up at higher frequencies, for example 4-8 kHz. Since the amplitudes of these resonances are much smaller, as can be seen in Fig. 6, care was taken to improve the SNR. This ratio is mainly limited by the amount of light reflected or scattered at the tympanic membrane and then
H.-J. Foth et al.
298
coupled back into the interferometer. formed in two ways: (1) (2)
An improvement
can be per-
increase the solid angle of the detection optics, and increase the reflectivity of the irradiated spot on the tympanic membrane.
4.1 Increase
of the solid angle
The solid angle of the outer ear canal would be 4-l X 1O-4 sr. However, this value cannot be reached experimentally by an optic outside of the ear. While the first problem of hairs in the outer part of the ear canal, which scatter the laser light and destroy the optical quality of the beam, can be solved by using an otoscope, the other problem is much more strict. The ear canal is bent and the plane of the tympanic membrane is not perpendicular to the axis of the ear canal. Therefore, the realistic angle of acceptance of the optic outside of the ear is significantly smaller than mentioned above. A much better version is to use a microprobe, which was constructed with such a small diameter, that it could be inserted into the ear canal. The optic of the microprobe can be placed at a very short distance from the tympanic membrane allowing a much larger angle for irradiation and collecting of the backscattered light. The end of the probe is built from a stainless steel tube of l-5 mm diameter and 50 mm length. The grid lens (Selfoc@ lens) of 1 mm diameter at the end of the probe focuses the laser beam onto a spot of 9 Frn diameter. The beam waist is at a distance of 3 mm to the end of the microprobe, which corresponds to a solid angle of 6.9 X low3 sr. So far, this microprobe was used only in experiments with temporal bones. Although the short focus length of 3mm is helpful for the detection of light it introduces some critical points for in vim measurements. Even with an endoscope close to the microprobe, the outer metal tube of the probe blocks the view onto the laser spot. Furthermore, the short distance to the tympanic membrane needs a special mechanical mount to avoid any relative motion between probe and head of the person. Finally, the spot size 9 pm diameter gives a power density of l-5 kW cm-‘, which means the dosimetry has to be investigated for this spot size. 4.2 Increase the reflectivity The optical properties of the tympanic membrane are quite comparable to that of skin. In our own experiments, the reflectivity of the tympanic
Measuring motions in the human middle ear
299
membrane was determined to be slightly above the value of parchment paper and a factor of 16 lower than that of aluminium.’ Even when it was possible to obtain a sufficient amount of backscattered light for successful measurements in the case of working with temporal bones, the daily work was quite often disappointing since alignment was very delicate and not stable over a time period of some minutes. Performing in uiuo measurements needs an optical set-up, which absolutely ensures a proper technical working of the system for obtaining data. In principle, several materials can be used to increase the reflectivity of a surface. To work with these materials on top of the tympanic membrane, they have to be biocompartible and stick tightly to the surface without changing the dynamical properties. In their earlier measurements Tonndorf & Khanna2*3 used bronze powder to increase the reflection of the tympanic membrane. Our test with aluminium flakes was not satisfying. When the flakes were in close contact to the tympanic membrane, they were parallel to the surface and, since the tympanic membrane tilted towards the axis of the outer ear canal, the laser beam was reflected away and finally absorbed by the skin of the ear canal. On the other hand, flakes which stood at an orientation to reflect the laser beam back into the interferometer, had no tight contact to the tympanic membrane. In principle, metallic powders have to be investigated concerning their biocompartibility; therefore, other materials were tested. At the beginning of our experiments, the laser was focused onto a spot in the area of the light reflex; only with this alignment, was a sufficient amount of light collected by the optic and coupled back into the interferometer. As mentioned above, a much better point to monitor the motion of the middle ear is to focus the laser onto the umbo. This was practicable after fixing a small, 1 by 1 mm, sticker of retro foil on the tympanic membrane. The total mass of this small sticker of retro foil is in the range of several pg and thereby too small to influence the resonance frequencies of the tympanic membrane and the ossicular chain. But the area of 1 mm’ beneath the sticker had a totally different tension coefficient. It has to be checked whether the solid sticker also changes the tension in a much larger area than just in the geometric size of the sticker. This means, even if all measurements have been successfully performed using a sticker of retro foil, it is worthwhile looking for alternative solutions, for example, increasing the reflectivity by using scattering materials. This could be small crystallites, with a typical size smaller than the spot size of the laser beam or a white powder. In the case of temporal bones, we obtained good results with flour. The two spectra
H.-J. Foth et
300
al.
Retro-Foil Flour
-80 1 0
I 2000
4000
8000
8000
10000
Frequency [Hz] Fig. 7.
Motion of the tympanic membrane observed at a sound pressure level near the hearing level.
shown in Fig. 7 have been obtained at an acoustic field close to the hearing threshold, one by using the sticker of retro foil and the other with a thin layer of flour on the tympanic membrane; both traces have a comparable SNR.
5 CONCLUSION During the last years Laser Doppler Vibrometry became an established measuring technique in hearing research. Beside recording the motions of the tympanic membrane,“,” it is also used for measuring the motion of hair cells in the inner ear.13 The goal of our work was to show that this technique is not limited to in vitro measurement at temporal bones or experiments with animals. We could demonstrate, that a sufficient SNR can be obtained by bringing the laser beam through the intact ear canal. This and the results concerning the dosimetry of the laser radiation had opened the way to perform successfully in vivo measurements. Furthermore, the observation of the motion of the umbo does not only give information about the motion of the tympanic membrane, but also about the dynamics of the middle ear bones, without opening the middle ear. This is seen as a promising route for the diagnosis of middle ear diseases.
Measuring motions in the human middle ear
301
ACKNOWLEDGEMENT The financial support of the ‘Ministerium fiir Wissenschaft bildung von Rheinland-Pfalz’ is gratefully acknowledged.
und Weiter-
REFERENCES 1. BCkCsy, G. V., iiber die Messungen der Schwingungsamplitude der Gehorknbchelchen mittels einer kapazitiven Sonde. Akw Z., 6 (1941) 1-16. 2. Tonndorf, J. & Khanna, S. M., Submicroscopic displacement amplitude of the tympanic membrane (cat) measured by laser interferometer. J. Acoust. Sot. Am., 44 (1968) 1546-54. 3. Tonndorf, J. & Khanna, S. M., Tympanic membrane vibrations in human cadaver ears studied by time-averaged holography. J. Acoust. Sot. Am., 52 (1972) 1221-3. 4. Decraemer, W. F. & Khanna, S. M., Modelling the malleus vibration as a rigid body motion with one rotational and one translational degree of freedom. Hear. Res., 72 (1994) 1-18. 5. Huthoff, Ch., Laser-Audiometrie: Bertihrungsfreie Messung der Schwingungen der Mittelohrmechanik am Trommelfell. Diplom-Thesis, Department of Physics, University of Kaiserslautern, 1993. 6. Foth, H.-J., Huthoff, Ch., Stasche, N. & Hormann, K., Measuring the motion of the human tympanic membrane by laser-Doppler-vibrometry: Basic principles and technical aspects, SPIE, 2083 (1994) 250-62. 7. Stasche, N., Foth, H.-J., Hormann, K., Baker, A. & Huthoff, Ch., Middle ear transmission disorders-tympanic membrane vibration analysis by laser Doppler vibrometry. Actu Otolaryngol., 114 (1994) 59-63. 8. Foth, H.-J., Huthoff, Ch., Gauer, A., Baker, A., Stasche, N. & Hormann, K., Dosimetry of He-Ne radiation on specimens of human tympanic membrane. SPIE, 2084 (1994) 178-86. 9. Foth, H.-J., Farber, S., Gauer, A., Becker, K. & Wagner, R., Dosimetry of tympanic membrane of pig versus laser radiation at 633 nm. SPIE, 2323 (1994) 110-6. 10. Stasche, N., Foth, H.-J. & Hormann, K., Laser-Doppler-Vibrometrie des Trommelfells-Erste In-uiuo-Messungen. Otorhinokzryngol Nova, 167 (1993) 223-4. 11. Vlaming, M. S. M. & Feenstra, L., Studies on the mechanics of the normal human middle ear. Cfin. Otoluryngol, 11 (1986) 353-63. 12. Suter, J. S., Poret, J. C. & Mattox, E. D., Laser interferometry for ossicullar motion detection. SPZE, 2131 (1994) 525-30. 13. Gummer, W. A., Hemmert, W. & Zenner, H.-P., Laser interferometric measurement of the micromechanics of the inner ear. SPIE, 2329 (1994) 65-73.
ELSEVIER
Measuring the Motions in the Human Middle Ear by Laser Doppler Vibrometry Hans-Jochen Department
Foth, Christian Huthoff, Matthias Brenner, Silke Ftirber
of Physics, University of Kaiserslautern, Erwin-Schrbdinger-StraBe, Kaiserslautern, Germany
67663
Norbert Stasche Department
of ENT, Klinikum of Kaiserslautern, Friedrich-Engels-StraRe, Kaiserslautern, Germany
Antonio Baker-Schreyer Department
of ENT, University
& Karl Hiirmann
Hospital of Mannheim, Mannheim, Germany
(Received 26 January
67655
Theodor-Kutzer-Ufer,
68167
1995; accepted 19 May 1995)
ABSTRACT For the detection of tiny motions which are caused by the tympanic membrane under normal hearing conditions, the touch-free method of Laser Doppler Vibrometry was used. Spectra containing information about the motions of the middle ear bones were recorded within 1 min when the umbo was chosen as the detection point and acoustic stimulation was performed via white noise excitation. It was observed that these spectra correlate to middle ear diseases, which had been artificiaily induced by manipulations in the chain of the middle ear bones in human temporal bones. The dosimetry of the applied laser radiation was found to be harmless to the tympanic membrane, which opened the way for successful in vivo measurements. Copyright 0 1996 Elsevier Science Ltd.
1 INTRODUCTION The human ear comprises the inner, the middle and the external ear, the latter consisting of the auricle and the auditory canal (Fig. 1). The auditory canal has the shape of a slightly bent tube with a typical length 289
H.-J. Foth et al.
290
Auricle pinna
Semicircular
Fig. 1.
canals
Vertical section through the human ear.
of 30 mm and a diameter of 6-8 mm and is closed to the middle ear by the ear drum (tympanic membrane). This membrane is the shape of a flat funnel; the deepest point of the funnel, which is also nearly the centre, is called the umbo. The sophisticated construction of the human middle ear is needed to match the index of refraction of acoustic waves propagated in air versus that in the liquid of the inner ear. Without the mechanics of the middle ear bones the reflectivity would be at 97%. The acoustic waves guided by the ear canal stimulate the ear drum to move, which is then transmitted by the three middle ear bones (ossicular chain) to the fluid medium in the inner ear (cochlea). These three bones, the hammer (malleus), the anvil (incus) and the stirrup (stapes) typically have masses of 23, 27 and 2.5 mg, respectively. Under normal hearing conditions (40-70 dB SPL; Sound Pressure Level) the amplitude of the motion is in the range of several tens of nanometers. The best point to monitor the motion of the tympanic membrane is the umbo, since at this point the manubrium of the malleus is fixed to the tympanic membrane. Therefore measurements at this point give information not only about the dynamics of the tympanic membrane but also about the dynamics of the middle ear bones. A touch-free method is needed to detect the motions of the middle
Measuring motions in the human middle ear
291
ear without perturbing them. While the earliest measurements have been performed with a capacitive probe;’ the laser technique was used in 1986 by Tonndorf & Khanna. * These authors used at first Laser Doppler Vibrometry, in the following years mainly holographic techniques3 to analyse the motion of the tympanic membrane came back after several years to Laser Doppler Vibrometry. In their recent projects4 they showed that the motion of the manubrium of the malleus of cats has to be described as a superposition of a rotational and a translational motion. Aside from diseases of the inner ear, there are also multiple cases where the hearing of a person is decreased by imperfect sound transmission in the middle ear. The intention of our work is to test whether Laser Doppler Vibrometry is a suitable technique to be applied for the diagnosis of middle ear diseases.
2 EXPERIMENTAL
SET-UP
The experimental set-up is shown in Fig. 2. The temporal bones were clamped in an adjustable vice to ensure a stable mounting and to adjust the bone in a proper way to the laser beam. Previously frozen human temporal bones were used; after warming up, they were cleaned and permanently moistened to avoid dehydration. In some cases the outer
Microscope LaserDoppler Vibrometer
-
Fourier Analyzer
Fig. 2.
Experimental
set-up.
292
H.-J. Foth et al.
ear canal was removed, while the laser beam was usually brought through the intact ear canal to represent a situation most comparable to clinical conditions. The point where the laser beam irradiated the tympanic membrane was checked by a binocular (Zeiss OPM 9). The experiments were carried out with a commercial Laser Doppler Vibrometer (Polytec, Model OFV 3000). The He-Ne laser beam, with a total power of less than 1 mW, was transmitted by a flexible optical fibre and focused onto the tympanic membrane by the commercial optics of Polytec or by a microprobe, which was developed at the Department of Physics in cooperation with the company Polytec. This microprobe with a length of -50 mm and an outer diameter of 1.5 mm is small enough to be inserted into the ear canal; at the end of the microprobe a gradient index lens (Selfoc@ lens) was used to generate a beam waist of a diameter of 9 pm at a focus length of 3 mm.’ The electric output of the Laser Doppler Vibrometer is proportional to the time depending component of velocity onto the direction of the laser beam of the irradiated spot u,(t). A Fast-Fourier-Transformer (FFI, Advantest: Model 9211 B) was used to transform uz(t) into an excitation spectrum u,(o), which shows at which frequencies w the velocity or the amplitude of the motion has a maximum. Usually spectra in the frequency range O-10 kHz were recorded, and up to 100 transformed spectra were averaged to obtain a good SNR. An IEEE bus was used to transfer the data to a PC for data handling and to guide the Laser Doppler Vibrometer, as well as the FFT-analyser and the functions generator, which was used to generate the sinus tone for acoustic stimulation of the tympanic membrane. At the beginning the acoustic stimulation was performed with sinus tone excitation; but the extremely long measuring time of up to 7 h led to an excitation by white noise; this cut down the measuring time to less than 60 s for a spectrum of comparable resolution and SNR.6 The generation of white noise was done by a card in the FFI analyser (digital white noise), with the effect that the spectrum contained only these frequencies, which are used for the digital Fourier transformation. The electric output of the functions generator or of the white noise generator was amplified to drive a broad band loudspeaker (Dynaudio, model: Gemini). The sound pressure level of the acoustic field was checked by a microphone (Bruel & Kjaer, model: 4165, 2639, and 5935). Great care was taken not to perturb the acoustic field by the mounting of the microphone and furthermore to be aware of standing waves. The whole set-up was placed inside of an acoustic insulated cabin (IAC, Industrial Acoustic Company).
Measuring motions in the human middle ear
3 EXPERIMENTAL
293
RESULTS
3.1 Results with temporal bones Figure 3 shows a typical result obtained by an acoustic excitation with a sinus tone of 3 kHz and a sound pressure level of 93 dB. Figure 3(a)
100.0 flV
Xa
Gaa
Lwl
0.0 Vr
-...adL-
0.0
.
I
i
FlaQmlcY
82
2oJk
Fig. 3. Top: velocity of the tympanic membrane observed in the area of the light reflex by stimulating with a sinus tone of 3 kHz at 93 dB SPL. Scale of the vertical axis: 1 V = 5 mm s-l. Bottom: Fourier transformation of signal as shown above (stimulation with a sinus tone of 3 kHz, 75 dB SPL).
H.-J. Foth et al.
294
shows the directly observed signal v,(t) while Fig. 3(b) shows the Fourier transformed spectrum u,(o); obviously in this case the motion can be described in good approximation by an harmonic oscillation. From the measured signal u,(t) or u,(o) the amplitude of the motion z(t) resp. z(w) is easily calculated by time integration. These technical developments have been the basis used to study the dependency of the resonance spectrum at the umbo on disorders in the middle ear. In experiments with temporal bones, various changes had been artificially induced, in the sense to simulate mechanical immobility in the chain of the middle ear bones, which are well known from the clinical routine:
0) Fixation of the malleus head, which happens in reality by an
(2)
(3)
(4)
inflammation and swelling in the middle ear. Here it was simulated by using a piece of foam rubber. After removing the piece of rubber, the same spectrum was recorded as before starting this manipulation. Fixation of the stapes footplate, described as otosclerosis, when a ring of bone had grown around the footplane and hinders any motion of the stapes. Here it was simulated by using bone cement. Dislocation of a joint, which may be caused by an acoustic shock front, like a bang. Here the joint between incus and stapes was intersected mechanically. Finally, the incus was removed to create a greater loss of mass in the ossicular chain.
The recorded spectra in correlation to the described manipulations are shown in Fig. 4. Obviously each of these changes introduced a specific change in the resonance spectrum. More details were described in a recent publication.7 3.2 Dosimetry
of the applied laser radiation
Before the technique of Laser Doppler Vibrometry was applied to living persons, it had to be ensured that the irradiated He-Ne laser beam does not do any harm to the tympanic membrane. Even if daily experience indicates that a continuous laser beam of 1 mW power focused onto the skin introduces no obvious thermal effects, the problem has to be investigated carefully. Using the conventional optic of the Laser Doppler Vibrometer, the laser beam is focused down to a spot size of 40 pm diameter reaching a laser intensity of 80 W cm-‘. The thresholds for thermal effects are at
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Velocity of the umbo in correlation to artificially induced manipulations middle ear bones to simulate middle ear diseases.
100 W cmm2 for coagulation, 2000 W cm-’ for vaporization, lo6 W cm-* for photoablation.
at the
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Obviously vaporization or photoablation is not possible, but the risk of coagulation had to be checked seriously. The experiment was performed with a cw ring dye laser (Coherent 699-21) pumped by an 8 Watt Ar’ laser. The wavelength was tuned to 633 nm and controlled by a travelling Michelson interferometer. The laser power, which was sent onto the specimens was selected by turning a Nicol’s prism in the optical light path. The laser beam was focused by a plan convex lens with a focus length of 100 mm to a spot size of 80 pm diameter. This spot size was determined by a microscope lens and a CCD camera; the measured intensity profile was fitted by a Gaussian profile. Therefore, the described width is the full width at l/e’. In a first series 17 specimens of human tympanic membrane were irradiated with power densities between 1000 and 12 000 W cm-‘. During the irradiation time of up to 6Os, the transmitted laser light and the diffraction pattern was on-line monitored by the CCD camera. The threshold at which this pattern became unstable, i.e. moving and changing, was taken at the threshold for thermal effects. Generation of holes was easily recognized by the transmitted light and ensured by inspection with a microscope. The results of these measurements are
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summarized in Fig. 5 showing that the threshold for holes was found at 2000 W cm-2.8 In a second series, 38 specimens of tympanic membranes of fresh slaughtered pigs were irradiated with laser intensities up to 20 000 W cm*. Damage was observed in none of the specimens at laser intensities below of 2500 W cm-2.9 In current experiments, the power density at which 100% of the irradiated specimens get damaged is investigated, since this threshold was not reached in the experiments so far. However, the laser intensity of 80 W cm-‘, applied by the Laser Doppler Vibrometer, is a factor of 30 below the lowest value at which damage occurred. Furthermore, the sticker of retro foil protects the tympanic membrane against the laser radiation. This was the basis to perform in uiuo measurements. 3.3 In uiuo measurements Figure 6 shows the velocity of the umbo of a 25year-old healthy male with a normal ear.” The spectrum had been obtained at a sound pressure level of around 80 dB and stimulation by white noise. The dominant resonance around 3-5 kHz is mainly caused by the resonance of a standing wave in the ear canal. This result shows that the technique of Laser Doppler Vibrometry is suitable for clinical application.
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4 DISCUSSION The results presented here show that Laser Doppler Vibrometry can be performed successfully while bringing the laser beam through the intact outer ear canal. Furthermore, experiments with temporal bones have shown that the spectra U,(W) observed at the umbo depend on the dynamics of the middle ear bones. Manipulations in the chain of the middle ear bones, by which middle ear diseases were simulated, generated characteristic changes in the spectra. These changes were easily recognized by comparing the spectra of one temporal bone before and after manipulation. Comparing the spectra of different temporal bones showed that the individual variation is sometimes larger than the chances due to the manipulations. The results of the dosimetry opened the way to in viuo measurements as shown in Fig. 6. Here also, spectra of different persons show a broad variety. Common in all spectra is the dominant resonance at 3 kHz. By a total length of 30 mm of the outer ear canal, the first standing wave in this tube would have a wavelength of 120mm and a frequency of around 28 kHz (the speed of sound is at 340.3 m s-l). Therefore, this resonance is seen to be mainly due to the outer ear canal. Resonances, which are more sensitive on the condition of the middle ear bones show up at higher frequencies, for example 4-8 kHz. Since the amplitudes of these resonances are much smaller, as can be seen in Fig. 6, care was taken to improve the SNR. This ratio is mainly limited by the amount of light reflected or scattered at the tympanic membrane and then
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coupled back into the interferometer. formed in two ways: (1) (2)
An improvement
can be per-
increase the solid angle of the detection optics, and increase the reflectivity of the irradiated spot on the tympanic membrane.
4.1 Increase
of the solid angle
The solid angle of the outer ear canal would be 4-l X 1O-4 sr. However, this value cannot be reached experimentally by an optic outside of the ear. While the first problem of hairs in the outer part of the ear canal, which scatter the laser light and destroy the optical quality of the beam, can be solved by using an otoscope, the other problem is much more strict. The ear canal is bent and the plane of the tympanic membrane is not perpendicular to the axis of the ear canal. Therefore, the realistic angle of acceptance of the optic outside of the ear is significantly smaller than mentioned above. A much better version is to use a microprobe, which was constructed with such a small diameter, that it could be inserted into the ear canal. The optic of the microprobe can be placed at a very short distance from the tympanic membrane allowing a much larger angle for irradiation and collecting of the backscattered light. The end of the probe is built from a stainless steel tube of l-5 mm diameter and 50 mm length. The grid lens (Selfoc@ lens) of 1 mm diameter at the end of the probe focuses the laser beam onto a spot of 9 Frn diameter. The beam waist is at a distance of 3 mm to the end of the microprobe, which corresponds to a solid angle of 6.9 X low3 sr. So far, this microprobe was used only in experiments with temporal bones. Although the short focus length of 3mm is helpful for the detection of light it introduces some critical points for in vim measurements. Even with an endoscope close to the microprobe, the outer metal tube of the probe blocks the view onto the laser spot. Furthermore, the short distance to the tympanic membrane needs a special mechanical mount to avoid any relative motion between probe and head of the person. Finally, the spot size 9 pm diameter gives a power density of l-5 kW cm-‘, which means the dosimetry has to be investigated for this spot size. 4.2 Increase the reflectivity The optical properties of the tympanic membrane are quite comparable to that of skin. In our own experiments, the reflectivity of the tympanic
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membrane was determined to be slightly above the value of parchment paper and a factor of 16 lower than that of aluminium.’ Even when it was possible to obtain a sufficient amount of backscattered light for successful measurements in the case of working with temporal bones, the daily work was quite often disappointing since alignment was very delicate and not stable over a time period of some minutes. Performing in uiuo measurements needs an optical set-up, which absolutely ensures a proper technical working of the system for obtaining data. In principle, several materials can be used to increase the reflectivity of a surface. To work with these materials on top of the tympanic membrane, they have to be biocompartible and stick tightly to the surface without changing the dynamical properties. In their earlier measurements Tonndorf & Khanna2*3 used bronze powder to increase the reflection of the tympanic membrane. Our test with aluminium flakes was not satisfying. When the flakes were in close contact to the tympanic membrane, they were parallel to the surface and, since the tympanic membrane tilted towards the axis of the outer ear canal, the laser beam was reflected away and finally absorbed by the skin of the ear canal. On the other hand, flakes which stood at an orientation to reflect the laser beam back into the interferometer, had no tight contact to the tympanic membrane. In principle, metallic powders have to be investigated concerning their biocompartibility; therefore, other materials were tested. At the beginning of our experiments, the laser was focused onto a spot in the area of the light reflex; only with this alignment, was a sufficient amount of light collected by the optic and coupled back into the interferometer. As mentioned above, a much better point to monitor the motion of the middle ear is to focus the laser onto the umbo. This was practicable after fixing a small, 1 by 1 mm, sticker of retro foil on the tympanic membrane. The total mass of this small sticker of retro foil is in the range of several pg and thereby too small to influence the resonance frequencies of the tympanic membrane and the ossicular chain. But the area of 1 mm’ beneath the sticker had a totally different tension coefficient. It has to be checked whether the solid sticker also changes the tension in a much larger area than just in the geometric size of the sticker. This means, even if all measurements have been successfully performed using a sticker of retro foil, it is worthwhile looking for alternative solutions, for example, increasing the reflectivity by using scattering materials. This could be small crystallites, with a typical size smaller than the spot size of the laser beam or a white powder. In the case of temporal bones, we obtained good results with flour. The two spectra
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Motion of the tympanic membrane observed at a sound pressure level near the hearing level.
shown in Fig. 7 have been obtained at an acoustic field close to the hearing threshold, one by using the sticker of retro foil and the other with a thin layer of flour on the tympanic membrane; both traces have a comparable SNR.
5 CONCLUSION During the last years Laser Doppler Vibrometry became an established measuring technique in hearing research. Beside recording the motions of the tympanic membrane,“,” it is also used for measuring the motion of hair cells in the inner ear.13 The goal of our work was to show that this technique is not limited to in vitro measurement at temporal bones or experiments with animals. We could demonstrate, that a sufficient SNR can be obtained by bringing the laser beam through the intact ear canal. This and the results concerning the dosimetry of the laser radiation had opened the way to perform successfully in vivo measurements. Furthermore, the observation of the motion of the umbo does not only give information about the motion of the tympanic membrane, but also about the dynamics of the middle ear bones, without opening the middle ear. This is seen as a promising route for the diagnosis of middle ear diseases.
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ACKNOWLEDGEMENT The financial support of the ‘Ministerium fiir Wissenschaft bildung von Rheinland-Pfalz’ is gratefully acknowledged.
und Weiter-
REFERENCES 1. BCkCsy, G. V., iiber die Messungen der Schwingungsamplitude der Gehorknbchelchen mittels einer kapazitiven Sonde. Akw Z., 6 (1941) 1-16. 2. Tonndorf, J. & Khanna, S. M., Submicroscopic displacement amplitude of the tympanic membrane (cat) measured by laser interferometer. J. Acoust. Sot. Am., 44 (1968) 1546-54. 3. Tonndorf, J. & Khanna, S. M., Tympanic membrane vibrations in human cadaver ears studied by time-averaged holography. J. Acoust. Sot. Am., 52 (1972) 1221-3. 4. Decraemer, W. F. & Khanna, S. M., Modelling the malleus vibration as a rigid body motion with one rotational and one translational degree of freedom. Hear. Res., 72 (1994) 1-18. 5. Huthoff, Ch., Laser-Audiometrie: Bertihrungsfreie Messung der Schwingungen der Mittelohrmechanik am Trommelfell. Diplom-Thesis, Department of Physics, University of Kaiserslautern, 1993. 6. Foth, H.-J., Huthoff, Ch., Stasche, N. & Hormann, K., Measuring the motion of the human tympanic membrane by laser-Doppler-vibrometry: Basic principles and technical aspects, SPIE, 2083 (1994) 250-62. 7. Stasche, N., Foth, H.-J., Hormann, K., Baker, A. & Huthoff, Ch., Middle ear transmission disorders-tympanic membrane vibration analysis by laser Doppler vibrometry. Actu Otolaryngol., 114 (1994) 59-63. 8. Foth, H.-J., Huthoff, Ch., Gauer, A., Baker, A., Stasche, N. & Hormann, K., Dosimetry of He-Ne radiation on specimens of human tympanic membrane. SPIE, 2084 (1994) 178-86. 9. Foth, H.-J., Farber, S., Gauer, A., Becker, K. & Wagner, R., Dosimetry of tympanic membrane of pig versus laser radiation at 633 nm. SPIE, 2323 (1994) 110-6. 10. Stasche, N., Foth, H.-J. & Hormann, K., Laser-Doppler-Vibrometrie des Trommelfells-Erste In-uiuo-Messungen. Otorhinokzryngol Nova, 167 (1993) 223-4. 11. Vlaming, M. S. M. & Feenstra, L., Studies on the mechanics of the normal human middle ear. Cfin. Otoluryngol, 11 (1986) 353-63. 12. Suter, J. S., Poret, J. C. & Mattox, E. D., Laser interferometry for ossicullar motion detection. SPZE, 2131 (1994) 525-30. 13. Gummer, W. A., Hemmert, W. & Zenner, H.-P., Laser interferometric measurement of the micromechanics of the inner ear. SPIE, 2329 (1994) 65-73.