Lightdosimetric Quantitative Analysis of the HumanPetrous Bone: Experimental Study for Laser Irradiationof the CochleaStefan Tauber, MD,1* Reinhold Baumgartner, PhD,2 Karin Schorn, MD,1 and Wolfgang Beyer, PhD2

1Department of Otolaryngology, Head and Neck Surgery, University of Munich, D-81377 Munich, Germany2Laser-Research-Laboratory, Department of Urology, University of Munich, D-81377 Munich, Germnay

Background and Objective: Application of laser irra-diation targeting the inner ear has to be investigated fortherapeutic effectiveness in cochlear injury and dysfunc-tion. In vitro data demonstrate low-level laser-inducedphotochemical and photobiologic cell response, dependingon cell type and irradiation parameters such as light dose.The aim of the presented study was to determine the lightdose received by the cochlear hair cells by using differentirradiation modalities for the human petrous bone.Study Design/Materials and Methods: Lightdosimet-ric assessment was performed in human cadaver temporalbones (n 4 13) after removing the cochlear membranouslabyrinth. The external auditory meatus, the tympanicmembrane (quadrants), and the mastoid bone were illu-minated by a helium-neon laser (l 4 593 nm) and diodelasers of different wavelengths (l 4 635, 690, 780, and830 nm). The spatial distribution of transmitted light inthe cochlear windings was measured by means of a retro-cochlearly positioned endoscopic CCD camera for imageprocessing and was assigned to acoustic frequencies ac-cording to the tonotopic organization of the cochlea. For anestimation of the corresponding space irradiance in an in-tact cochlea, correction factors have been calculated by aMonte Carlo procedure on the basis of experimentally de-termined optical properties of skull bone.Results: The transmission of light across the tympaniccavity and the promontory depends strongly on wave-length of the laser and the position of the radiator. Trans-tympanal irradiation results in spatial intensity varia-tions of a factor 4 to 10 within the cochlear windings. Thespace irradiance in an intact cochlea is 10 to 20 times themeasured irradiance. For an irradiation of the mastoid,the light transmission within the cochlea is 103 to 105

times smaller compared with an irradiation of the tym-panic membrane and is extremely variable for differentspecimens.Conclusion: The strong dependence of the cochlear lightdistribution on various irradiation parameters demon-strates the impact of preclinical lightdosimetric investiga-tions for effective individual laser irradiation of the hu-man cochlea. Because of the observed spatial intensityvariations, the optimal external light dose has to be cho-sen with regard to the tonotopy of the human cochlea. Theobtained results are enabling us to apply defined laser

light doses to different cochlear winding areas. Mastoidalirradiation leads to therapeutically insufficient light doseswithin reasonable treatment times, whereas transmeatalirradiation is recommendable. Further studies are man-datory for development of clinical devices for transmeatalirradiation of the cochlea. Lasers Surg. Med. 28:18–26,2001. © 2001 Wiley-Liss, Inc.

Key words: cochlear light distribution; light dosimetry;low-level laser therapy; human cochlea; tinnitus aurium;sensorineural hearing loss; Monte Carlo calculation

INTRODUCTION

Low-level laser therapy (LLLT) has been reported for avariety of medical applications such as promotion ofwound-healing, musculoskeletal diseases, and therapeuticpain control [1–4]. The application of photo-irradiation atlow energy levels has been documented to generate bio-logic effects, which are manifested in biochemical andphysiologic phenomena in various enzymes, cells, tissues,organs, and organisms. There are laser-induced effects oncell proliferation [5–7], synthesis of ATP [8] and collagen[6], decrease of hypoxic injury and reductive stress [9],release of growth factors [10], and enhancement of woundhealing [11].

Neurophysiologic studies demonstrated that laser irra-diation can alter nerve conduction and nerve repair byaccelerating the healing of injured peripheral nerves [12–16]. However, other authors did not always confirm thoseeffects [17,18]. In vivo and in vitro experiments demon-strated that biologic effects of nonthermal laser irradia-tion are dependent on irradiation parameters such aswavelength (l 4 630–904 nm), waveform (cw, pulsed, Q-switched), power (10–100 mW), light dose (1–10 J/cm2),duration, time lapse between injury and irradiation, andtype of irradiated cell [7,12,17,19,20]. The basic photo-chemical cellular mechanisms of laser biostimulation are

*Correspondence to: Stefan Tauber, MD, Department ofOtolaryngology, Head and Neck Surgery, University of Munich,Marchioninistr. 15, 81377 Munich, Germany.E-mail: stauber@hno.med.uni-muenchen.de

Accepted 3 April 2000

Lasers in Surgery and Medicine 28:18–26 (2001)

© 2001 Wiley-Liss, Inc.

merely defined, and the clinical application and effective-ness remains still controversial.

Low-level laser therapy in otologic diseases has previ-ously been discussed and investigated by clinical laser-gingko studies for treatment of chronic cochlear tinnitusaurium and sensorineural hearing loss [21–27]. In most ofthe studies, the patients were irradiated by a combinedhelium-neon (l 4 632 nm) and gallium-arsenide laser (l4 904 nm) at low powers (9–20 mW), accompanied byintravenous gingko biloba extracts [21–23]. The laserbeam was directed from the corresponding mastoid to thelateral rim of the opposite orbita, according to the beamdirection of Schuller’s X-ray technique. Other authorsused gingko biloba extracts with a helium-neon (l 4 632nm) and infrared laser (l 4 830 nm), irradiating the mas-toidal bone and external auditory meatus at the same time[27].

The results of other studies were controversial, becausesome reported on reductions of chronic tinnitus in 15 to67% of treated cases [21,23,27], but there was also missingstatistical therapeutic effectiveness by using a combinedhelium-neon and infrared diode laser in a placebo-controlled double-blind study [26]. In a preliminary inves-tigation, 38 patients suffering from chronic tinnitus wereirradiated with a diode laser (l 4 830 nm) by use of amodified transmeatal laser administration at the externalauditory meatus, expecting higher effective light dosewithin the injured cochlea. By this method of application,effective attenuation of tinnitus symptoms such as dura-tion in 26%, loudness in 58%, and degree of annoyance in55% was demonstrated [24]. The authors concluded that aprospective randomly assigned controlled study is manda-tory, because LLLT could represent a therapeutic modal-ity in intractable tinnitus. Possible reasons leading tovarious results of the former otologic LLLT studies arisefrom differences in study designs, treatment parameters,and evaluation of therapeutic outcome. In addition, thelaser dose applied to the mastoidal area, to the externalauditory meatus, or both, and to the human cochlea cer-tainly varied among the former studies, because laser out-put power and emitter positions differed.

The concept of laser-induced photochemical effects fortreatment in cochlear dysfunction and injury seems rea-sonable, provided that both cellular photoceptors beingsensitive to photoirradiation and reversibly damaged co-chlear structures and cells (inner/outer hair cells, afferentneurons etc.) are present. Clinical studies are necessary toevaluate the effectiveness of laser-induced photobiostimu-lation for therapy of the injured inner ear. Regarding pre-vious investigations, however, there is information miss-ing about lightdosimetric data of the target organ cochlea.Only Wedel et al. performed light transmission measure-ments with illumination of the mastoidal bone [26]. Theapplied laser beam was nearly completely absorbed by thebony structures of the mastoid, indicating that there ismerely laser irradiation of the human cochlea. The trans-mission was dependent on the pneumatization of the mas-toid.

The human temporal bone represents complex anatomicstructures. A well-defined lightdosimetric analysis ismandatory for preclinical determination of the irradiationparameters such as wavelength, light dose, and site to betreated. This kind of preclinical assessment is even moreimportant, because in vitro studies defined fluences of1–10 J/cm2 to be effective in LLLT and lower or exceedingirradiation within the cochlea should be avoided to mini-mize bioinhibitory results or laser-induced damages[12,17,19,20,28]. The purpose of this study was to inves-tigate the laser light transmission to the human cochleaand to quantitatively analyse the light distribution withinthe cochlea, depending on the anatomic structures of thepetrous bone, the site of irradiation, and the laser wave-length.

MATERIALS AND METHODS

Specimen Preparation

For the experimental study, human anatomic freshlyformalin-conserved specimen of the petrous bone (n 4 13)of female and male adults were used. After macroscopicexamination of the specimen and microscopic inspection ofthe external auditory meatus confirming intact and trans-parent tympanic membrane, a high resolution computedtomographic (CT) scan (1 mm tomography) of the petrousbones was performed to exclude malformations and to con-firm proper anatomic structures (external auditory me-atus, tympanic cavity, auditory ossicles, labyrinth) withregular pneumatization of the mastoid of all specimen.During the CT scan, the specimen were immobilised byfixing on the platform of the tomographic device. Subse-quently, the internal auditory meatus was prepared in adistal direction with careful removal of the vestibuloco-chlear nerve, approaching the cochlear bony labyrinth un-der microscopic control. The basal winding of the cochleawas partially opened, and small bony parts were removedto inspect the cochlear labyrinth. The cochlear duct andthe modiolus were removed. The bony cochlear labyrinth,the vestibular labyrinth, the tympanic membrane, thetympanum, and the round and oval windows were notaffected and remained intact during the preparation pro-cedure.

Laser Light Transmission Studies

The experiments were performed with a helium-neonlaser with a wavelength of 593 nm and four different diodelasers of 635, 690, 780, and 830 nm. For the irradiation amicrolens fiber (AOC Medical Systems, South Plainfield,NJ) was used delivering a flat homogeneously illuminatedcircular irradiation field with a diameter of 57 mm at adistance of 100 mm. The lasers were operated at an outputpower between 2 mW and 8 mW. The experimental setupwas localized in a darkened room.

Transmeatal irradiation. In this experiment, thespatial distribution of light transmitted and emitted fromthe cochlea windings was measured while performing la-ser irradiation by means of the external auditory meatus.

LIGHTDOSIMETRIC QUANTITATIVE ANALYSIS OF THE HUMAN PETROUS BONE 19

For this purpose, the tympanic membrane was illumi-nated by the microlens fiber positioned in the center of theexternal auditory meatus and directed to the umbo of thetympanic membrane. Both, the petrous bone specimenand the emitter were immobilised by a vice combined withmicrodistance adjusting screws. Distances between fibertip and the umbo of the tympanic membrane of 1, 1.5, and2 cm were adjusted. A 0-degree-endoscope (Karl StorzGmbH, Tuttlingen, Germany) was connected to a slowscan CCD camera and positioned retrocochlearly as shownin Figure 1. This unit was linked to a 16-bit frame grabberfor computerized image analysis. The obtained pixel val-ues are proportional to the irradiance emitted locally fromthe cochlea surface and do not depend on the distance. Incase of a cosine characteristic of radiation, which is ex-pected in case of scattering matter like bone, it does notdepend on the orientation of the surface relative to thedirection of camera view, too. For calibration of the endo-scopic camera signals, a flat reflection standard with back-scattering coefficient of 95%, and defined area was illumi-nated homogeneously with a defined power of eachwavelength used. Signal errors caused by nonlinear re-sponse and shading of the camera came out to be neglect-able. Signal offset was eliminated by dark image subtrac-tion. Because of the large dynamic range of signals,exposure times between 50 msec and 5 seconds were nec-essary. By this method, the spatial distributions of theemitted irradiance were obtained absolutely. For identifi-cation of corresponding anatomic structures, a white lightimage by using the endoscopic illumination was taken foreach bone specimen. Image processing software(OPTIMAS) was used for the calculation of the irradiancealong manually defined spiral lines inside the cochlearwindings. The obtained values of irradiance (2,000 singlemeasurement points) were correlated with the cochlearangle (0 to 900 degrees starting with the oval window at 0degrees) and the tonotopic organization (acoustic fre-quency from 100 to 10,000 Hz) of the cochlea.

In the subsequent experiment, the influence of the lightdistribution across the tympanic membrane was investi-gated for all specimen (n 4 13). For this purpose, themicrolens fiber was positioned eccentrically to illuminateeach of the four single quadrants of the tympanic mem-

brane separately (I, ventrocranial; II, ventrocaudal; III,dorsocaudal; IV, dorsocranial). The diode laser with awavelength of 635 nm was applied, and the fiber tip waspositioned by use of microdistance adjusting screws to il-luminate the chosen tympanic membrane quadrant. Thetransmitted light was measured with a bare fiber posi-tioned in the center of the cochlea and connected with aphotodiode. Performance checks and calibration proce-dures were performed in a similar manner to those men-tioned before for the camera device.

Mastoidal irradiation. In a following experiment, thelight transmission for irradiation of the mastoidal bonewas investigated. For this purpose, the emitting microlensfiber was positioned retroauricularly with distances of 1–2cm between the fiber tip and the skin of the mastoidalregion. The light was directed from the correspondingmastoid to the lateral rim of the opposite orbita, accordingto the Schuller X-ray technique as performed in formerclinical LLLT otologic studies mentioned above. For mea-surement of transmitted light, a bare fiber connected witha photomultiplier tube was positioned in the center of thecochlea. A black box and additional shieldings were nec-essary to prevent light from reaching the detector fiber bypassing the bony specimen or passing the tympanic mem-brane.

Determination of Optical Parameters in Bone andLight Dosimetric Correction Factors

For a comparison of the measured irradiance with cellexperimental data, the space irradiance, assuming an in-tact cochlea, has to be considered. For its determination,three dosimetric correction factors have to be taken intoaccount: (1) Due to the removal of cochlear tissue mate-rial, the index of refraction changes along the way of lightacross the bony material to the detector. This results intotal reflection of light. Therefore, the light dose inside ishigher than the light dose measured outside. (2) Tissuematerial being removed by the preparation procedurewould usually contribute by additional backscattering ef-fects to the total light dose. Therefore, the measured lightdose is reduced, compared with the nonexperimental clini-

Fig. 1. Experimental setup of lightdosi-metric assessment. Laser irradiation wasperformed with a microlens fiber posi-tioned nearby the mastoid (mastoidal irra-diation) or within the external auditorymeatus, illuminating the tympanic mem-brane (tm; transmeatal irradiation). Thespatial distribution of transmitted light in-side the cochlear windings was measuredby means of a 0-degrees endoscope on theretrocochlear site connected with a chargecoupled device (CCD) camera for imageprocessing. By suitable calibration, the ir-radiance emitted by the cochlea windingswas obtained. PC, personal computer.

20 TAUBER ET AL.

cal situation. (3) The quantity we measured and that isusually discussed in dosimetry is the irradiance. It is de-fined as the power per area hitting the surface of a flatdetector. Light entering different from perpendicular in-cidence contributes with reduced impact. Light from belowdoes not contribute at all. On the other hand, the cellularlight absorption does not depend on the angle of light in-cidence. The corresponding adequate lightdosimetricquantity is the space irradiance, defined as the lightpower hitting a sphere divided by its cross-section. For anisotropic light field, the space irradiance is four times theirradiance. For a plane light wave, as often used in cellexperiments, both the space irradiance and the irradianceare equal in size.

For our experiments, the overall correction factor foreach wavelength was calculated by Monte Carlo calcula-tion [29] assuming a simplified geometry. The transmittedirradiance E of a plane light wave, hitting perpendicularan infinite layer of bony matter with a defined thickness,was calculated and compared with the space irradiance atthe same point inside a specimen of infinite thickness.

The optical properties of skull bone necessary for thecalculation were obtained by a measurement of the remis-sion R (diffuse reflection) and the transmission T of lighthitting perpendicular skull specimen of known thickness.The signals of a light detector in a certain distance werecompared with the backscattering signal from the reflec-tion standard mentioned above. The calculation of the ab-sorption coefficient ma and the reduced scattering coeffi-cient ms8 4 ms (1−g) from the measured R and T wasperformed by a simplified procedure valid for a thickness× of the bone with meff × > 2. In this case, T follows theequation T 4 Teff exp(-meff x). Teff as well as R and C/Edepend in good approximation only from the ratio ma/ms8.All three functions were obtained by Monte Carlo calcu-lation for a set of certain values of ma/ms8. The desiredcorrection factors C/E and Teff were calculated by a simpleinterpolation procedure by using these three functions andthe known R. The knowledge of Teff allows the determina-tion of meff from the equation for T. This way, meff × >2 couldbe checked. Finally, ma and ms8 follow from meff

2 4 3ma (ma +ms8) and the known ratio ma/ms8. All Monte Carlo calcula-tions were performed assuming an index of refraction of n4 1.35 and an anisotropy factor g 4 0.8. Considering onlyms8 instead of ms, the precise value of g is of minor impor-tance.

RESULTS

Laser Light Transmission Studies

Transmeatal irradiation. The typical light distribu-tion within the cochlear windings as taken by the endo-scopic camera is demonstrated in Figure 2A–C (OPTIMASsystem). Figure 3 shows the measured irradiance trans-mitted and emitted from the cochlea as function of thelaser wavelength and the measurement position in thecochlear windings described by the cochlear angle. Zerodegrees correspond with the oval window and 900 degreeswith the helicotrema after 2.5 windings. The microlens

fiber was positioned centrally in the external auditory me-atus. The distance between fiber tip and the umbo of thetympanic membrane was 1.5 cm, causing a homoge-neously illuminated area with a diameter of 8.5 mm at thetympanic membrane. The transmitted irradiance is givenfor a laser power of 1 mW, corresponding with an irradi-ance of 1.76 mW/cm2 at the tympanic membrane. Becauseof the large variation of transmission for different speci-mens, the geometrical mean values (n 4 13) have beencalculated. In this case, the resulting standard deviationof the mean displayed on a logarithmic scale is equivalentto a certain factor, which amounts 2.5 for 593 nm and 1.5for 830 nm. To obtain the space irradiance in an intactcochlea, the transmitted irradiance must be multiplied bythe dose correction factors (Table 2). The transmitted ir-radiance increases with the wavelength until 780 nm by afactor 5 to 15. The wave-shaped spatial distributions ofthe transmitted irradiance are similar for all wavelengths.In each winding, a maximum occurs at the position closeto the oval window, i.e., at approximately 400, 1,500, and8,000 Hz. The observed spatial variations reduce from afactor 20 at 593 nm to a factor 5 at 830 nm. The standarderror of the mean (SEM) decreases with increasing wave-length (Fig. 3).

In Figure 4, the transmitted irradiances for differentdistances d between fiber tip and the umbo are shown. Theresults for 593 nm and 830 nm are presented. Increasingthe distances d results in a decline of transmitted irradi-ance within the cochlea. However, between d 4 1.5 cmand d 4 2.0 cm, the attenuation of irradiance is strongerthan between d 4 1.0 cm and d 4 1.5 cm. The influence ofthe distance is much smaller compared with a 1/d2 depen-dence.

Irradiation of the different tympanic membrane quad-rants demonstrates that the light transmission to the co-chlear center depends on the position of the microlens fi-ber (Fig. 5). Illuminating the caudal quadrants leads tosignificantly (P < 0.05) higher transmission (quadrant II:0.062 ± 0.023 mW/cm2, quadrant III: 0.079 ± 0.033 mW/cm2) than irradiation of the cranial tympanic membranequadrants (quadrant I: 0.019 ± 0.007 mW/cm2, quadrantIV: 0.022 ± 0.005 mW/cm2). There is no significant differ-ence of light transmission between both the lower quad-rants II and III and both the upper quadrants I and IV.For statistical comparison between the different quad-rants the Mann-Whitney U test was used because of non-parametric distribution. Values of P < 0.05 were consid-ered significant and are marked by asterisks (Fig. 5).

Mastoidal irradiation. The transmission for an irra-diation of the mastoid at different specimen varied over arange of 4 and 3 orders of magnitude for 593 and 830 nm,respectively. The geometric mean values are presented inFigure 6. The transmitted irradiance at 593 nm is morethan 105 times and at 830 nm more than 103 times smallercompared with an irradiation of the tympanic membrane(shown in Fig. 5).

LIGHTDOSIMETRIC QUANTITATIVE ANALYSIS OF THE HUMAN PETROUS BONE 21

Determination of Optical Parameters in Bone andLight Dosimetric Correction Factors

Table 1 shows the remission R and the transmission Tfor the wavelengths of interest measured at three skullbone specimens of different thickness. Teff, R, and C/Ehave been calculated by Monte Carlo calculation as func-tion of the ratio ma/ms8. An anisotropy factor g 4 0.8 andan index of refraction of n 4 1.35 are assumed. The re-sulting curves, shown in Figure 7 were used for the deter-mination of the optical parameters of bone listed in Table2 for the different wavelengths together with the resultingdosimetric correction factors C/E.

The scattering and absorption in Table 2 reveal a de-cline with increasing wavelength until 780 nm. The opti-cal parameters for 780 and 830 nm are identical withinSEM, which is in good agreement with the behavior of thecorresponding curves in Figure 3. The obtained dosimetriccorrection factor C/E ranges from approximately 10 to 20.An analysis of the calculated light distribution inside thebony layer shows that the contribution of light being back-

scattered from the cochlear material removed during thepreparation procedure ranges from a factor approximately2 to 4. A decrease of the ratio ma/ms8 increases backscat-tering leading to the observed increase of C/E with in-creasing wavelength. The residual factor 5 is the ratio ofthe space irradiance inside the bone of the opened cochleato the transmitted irradiance and follows as a conse-quence of reflection due to the change of index of refractionand angular factors from the different definition of irradi-ance and space irradiance (Fig. 8).

DISCUSSION

Low-level laser therapy as a possible novel treatmentmodality in chronic inner ear dysfunction and diseases,e.g., tinnitus aurium or sensorineural hearing loss, hasbeen previously discussed in different clinical studiesdemonstrating controversial treatment outcome [21–27].Investigating the LLLT, there is increasing evidence thatlow-level laser-induced effects are dose-dependent with atypical efficient range of 1–10 J/cm2 [12,17,20]. However,

Fig. 2. A: Endoscopic image of a petrous bone specimenwith an opened cochlea by a retrocochlear view. Note theoval window (1) and the basal (2), the middle (3), and theupper (4) cochlear winding. B: Same endoscopic view of thesame specimen with a 635-nm laser illuminating the tym-panic membrane (fiber tip distance to umbo of tympanicmembrane d 4 1.5 cm). Note highest laser transmission(red light) at the oval window and inside the middle andupper cochlear windings between the oval window and thehelicotrema. Anatomic structures were identified by addi-tional white light emitted from the endoscope. C: Endo-scopic image of a petrous bone specimen demonstratinglight intensity variation by computer-controlled OPTIMASsystem. Visualization of different light transmission inten-sities by colour ranges. Note highest intensities (yellow-and red-coloured areas) at the oval window and betweenthe oval window and the helicotrema, whereas basal andmiddle cochlear windings demonstrate lower emission oflight (blue-coloured area).

22 TAUBER ET AL.

dosimetric assessment of laser light transmission withinthe human cochlea has been missing in previous studies[21–27], even though the target of irradiation representscomplexity of anatomic structures.

This study first describes a dosimetry for quantificationof light transmission to the human cochlea when irradi-ating human cadaveric petrous bones by external laserlight at different irradiation modalities. The use of a CCDcamera enabled us to quantify light transmission and dis-tribution within the cochlea, depending on the position ofthe radiator and the wavelength of the laser. By comput-erized image analysis, the cochlear light distribution wascorrelated with the tonotopic organization of the cochlea.The results of this study demonstrate that the light dosestransmitted into the cochlea depend strongly on the exactlocalization of the laser fiber and the point of measure-ment within the cochlear winding.

One part of our experiments was the analysis oftransmeatal irradiation by positioning the microlens laserfiber at different distances centrally and eccentrically tothe eardrum. When irradiating the tympanic membrane,the transmitted light crosses anatomic structures of themiddle-ear such as the eardrum, the auditory ossicles, theoval window, and the promontory bone. The central illu-

mination of the tympanic membrane results in a charac-teristic light distribution within the cochlea (Figs. 3, 4).Maxima occur nearby the oval and round window andclose to the helicotrema. Our data confirm the suppositionof Shiomi et al. expecting light penetration through theround window, when irradiating the external auditory me-atus [24]. The increase of the transmitted irradiance withincreasing wavelength follows from the known decrease ofthe absorption and scattering leading to a decrease of meff.The observed spatial variations reduce with increasingwavelength. This finding follows from an approximatelyexponential dependence of the transmission of light fromthe thickness of the material in combination with a gen-eral mathematic property of an exponential function. Incase of a decrease of meff a certain variation of × in exp(-meff

x) leads to a decreased variation of this exponential term.The same effect reduces also the obtained SEM with in-creasing wavelength.

Increasing the distance d between the microlens fibertip and the umbo of the tympanic membrane from 1 to 2cm leads to attenuation of transmitted irradiance,whereas the characteristic light distribution within thecochlear windings remains unchanged. The influence ofthe distance d is much smaller compared with a 1/d2 de-pendence. The change of the irradiance between d 4 1.0cm and d 4 1.5 cm is even smaller than between d 4 1.5

Fig. 3. Transmitted irradiance for 1 mW laser power as afunction of the position in the cochlea characterized by co-chlear angle (0–900 degrees) and the corresponding acousticfrequency (100–10,000 Hz) for different wavelengths. Noteirradiance maxima at acoustic frequencies of 8,000, 1,500,and 400 Hz. The large variations demonstrate the necessity ofa light dosimetry according to the tonotopy of the ear. For thispurpose, the dosimetric correction factors of Table 2 have tobe taken into account. The data given are means and ex-amples of the SEM.

Fig. 4. Transmitted irradiance for 1 mW laser power (l 4635 nm and 830 nm) for different distances d between themicrolens fiber tip and the umbo of the tympanic membrane.Increasing distances result in attenuation of transmitted ir-radiance, whereas the characteristic light distribution withinthe cochlear windings remains unchanged. The observed in-fluence of the distance is smaller compared with a 1/d2 de-pendence. The data given are means.

LIGHTDOSIMETRIC QUANTITATIVE ANALYSIS OF THE HUMAN PETROUS BONE 23

cm and d 4 2.0 cm. The reason may be the increased sizeof the illuminated area with increasing distance. Thisway, certain parts of the lower tympanic membrane quad-rants with higher transmission to the cochlea could obtainmore light compensating partially for the larger distance

d. However, for exact calculation of light transmission inclinical application the small differences of the obtainedvalues should not be neglected as far as it is possible todefine a certain distance of the laser fiber to the tympanicmembrane in vivo. The distance between the microlensfiber and the tympanic membrane in a range of 1–2 cm isof minor influence in contrast to the choice of the irradi-ated quadrant of the tympanic membrane. Illumination ofthe lower tympanic membrane quadrants induces highercochlear light doses than irradiation of the upper tym-panic membrane quadrants. This result is in agreementwith the anatomic localization of promontorium and co-chlea relative to the tympanic membrane. The distancebetween the lower tympanic membrane quadrants and thepromontory is smaller than between promontory and up-per tympanic membrane quadrants. Light illuminatingthe upper tympanic membrane quadrants mainly irradi-ates the epitympanic and the upper mesotympanic area,whereas the backscattered light hits the promontory.These results indicate that a large portion of transme-atally applied light seems to be directly transmitted acrossthe promontory, whereas light being backscattered withinthe tympanum is of minor impact.

Laser irradiation of the mastoidal bone with a lightbeam direction according to Schuller’s X-ray technique re-sulted in extremely small light transmission comparedwith transmeatal laser application. This result is in con-cordance with a previous study describing nearly completeabsorption of laser light by the mastoid [26] and the sup-position by Shiomi et al. expecting higher light transmis-sion by transmeatal irradiation [24].

Calculating our light transmission data for mastoidalirradiation with power and time of laser irradiation offormer clinical studies [21–23,26,27], the dose applied tothe cochlea of the treated patients was less than 3 mJ/cm2.Indeed, this dose is quite lower than light doses between 1and 10 J/cm2 supposed to be efficient in low-level lasertherapy [17]. The value of 3 mJ/cm2 was determined forspecimen with regular mastoid pneumatization confirmedby high resolution CT scans. The possibly impaired orvarying pneumatization of the mastoidal bone in the clini-

TABLE 1. Remission R (upper number) andTransmission T (lower number) Measured at ThreeBony Specimens of the Skull WithDifferent Thickness

l [nm]

Thickness (mm)

4.2 4.5 6.5

593 0.703 0.633 0.5280.021 0.005 0.004

635 0.734 0.708 0.6390.013 0.013 0.006

690 0.789 0.803 0.7310.036 0.038 0.024

780 0.802 0.816 0.7570.071 0.067 0.049

830 0.796 0.822 0.7570.077 0.069 0.051

Fig. 6. Transmitted irradiance for a 1 mW laser illuminatingthe mastoidal bone as function of the wavelength. The trans-mission is 3 to 5 orders of magnitude smaller compared withan irradiation of the tympanic membrane. The data given aremeans ± SEM.

Fig. 5. Transmitted irradiance for 1 mW laser power at 633nm for the irradiation of different tympanic membrane quad-rants I-IV. The laser fiber is positioned eccentrically insidethe external auditory meatus and illuminates one the fourtympanic membrane quadrants. The observed variations fol-low from the anatomic localization of the promontory and thecochlea and demonstrate the necessity of a precise emitterposition for a well-defined light dosimetry. The data given aremeans ± SEM; asterisks indicate P < 0.05 vs. tympanic mem-brane quadrants II and III.

24 TAUBER ET AL.

cal situation would even additionally attenuate or modu-late the light transmission to the cochlea. In conclusion,transmeatal laser irradiation should be preferred for suf-ficient and quantified light dose within reasonable treat-ment time.

To predict the light distribution within an intact co-chlea, a theoretical analysis was performed on the basis ofexperimental determination of optical skull bone proper-ties. The study showed that the expected space irradianceis 10 to 20 times the measured irradiance (Figs. 3–6) emit-ted from the opened cochlea, a factor which is far frombeing neglectable. For determining the absolute light dosein the cochlea in a clinical situation and for comparisonwith cell experimental data the radiance values from Fig-ures 3 to 6 have to be multiplied with the suitable dosi-metric correction factor (Table 2).

Our results indicate that LLLT has to be performed bymeans of the external auditory meatus according to theobtained lightdosimetry data. In previous studies on

LLLT the external auditory meatus was partially irradi-ated without definition of the exact laser positioning at theauricle, at or within the external auditory meatus or eventhe tympanic membrane [24,27]. Hence, the authors werenot able to perform lightdosimetrically controlled irradia-tion according to defined irradiation parameters of LLLT,e.g., the light dose at the site of the target organ. This factcould represent a contributing factor to the widespreadand different therapeutic outcome ranges of previousstudies.

The presented data are obtained by use of cadaver ma-terial, which is not completely comparable to the in vivoclinical situation. Errors of the reported values could oc-cur, because tissue material (membranous cochlear laby-rinth, parts of bony cochlear labyrinth) was removed in allspecimens. Tissue vascularization, inner ear fluids, andmiddle-ear mucosa were not present. Differences betweenthe measured data and the in vivo situation are expected

TABLE 2. Optical Properties and Dosimetric Correction Factor C/E of Skull Bone for Different Wavelengths*

l [nm] ma/ms8 Teff ma [1/mm] ms8 [1/mm] meff [1/mm] c/E

593 0.0135 ± 0.0048 0.585 ± 0.054 0.0561 ± 0.0108 4.71 ± 0.83 0.870 ± 0.091 10.0 ± 1.5635 0.0072 ± 0.0018 0.501 ± 0.038 0.0371 ± 0.0022 5.57 ± 0.91 0.780 ± 0.044 12.9 ± 1.4690 0.0035 ± 0.0008 0.398 ± 0.026 0.0169 ± 0.0011 5.11 ± 0.73 0.504 ± 0.032 17.7 ± 1.7780 0.0028 ± 0.0005 0.367 ± 0.026 0.0107 ± 0.0009 4.00 ± 0.46 0.356 ± 0.018 19.5 ± 1.6830 0.0028 ± 0.0005 0.367 ± 0.027 0.0104 ± 0.0009 3.88 ± 0.44 0.345 ± 0.016 19.4 ± 1.6

*To obtain the space irradiance C in an intact cochlea, the curves from Fig. 3–6 have to be multiplied with C/E. Values arereported as mean ± SEM.

Fig. 7. The remission R and Teff as a function of the ratioma/ms8 calculated by Monte Carlo calculations for perpendicu-lar light incidence at a layer with infinite thickness. Thesedata were used for the determination of the optical propertiesof bone from measurements of R and the transmission T. n 41.35 and g 4 0.8 were assumed.

Fig. 8. Space irradiance C in an intact cochlea divided by themeasured transmitted irradiance E is shown in Figures 3–6as function of the optical properties. Dosis correction factorC/E was defined by Monte Carlo calculations. For determina-tion of the light doses to be applied to the patients’ cochlea,the data from Figures 3–6 have to be multiplied with C/E.

LIGHTDOSIMETRIC QUANTITATIVE ANALYSIS OF THE HUMAN PETROUS BONE 25

because of the additional absorption of blood especially forthe shorter wavelength. The calculated dose correctionfactors have been determined approximately, assuming asimplified plane geometry instead of a simulation of thecomplex anatomy of the cochlea. The observed large varia-tions of light transmission among the different specimenslead to errors concerning patient specific dosimetry, too.The obtained dosimetric data should not be expected to beabsolutely precise in view of the complex anatomy and thedifficult estimation of light propagation in tissue and boneby multiple scattering (in contrast to X-rays for instance).For clinical light dosimetry, the presented data shouldonly be considered as coarse guidelines. The variationrange of obtained transmission values also should be dis-cussed as a possible factor for the different and controver-sial clinical outcomes of the previous studies [21–27].

On the basis of the presented data for transmeatal tym-panic membrane irradiation, the amount of light that isnecessary to irradiate a chosen target area inside the co-chlea with a defined light dose can be determined approxi-mately. This determination enables us to irradiate, with aspecific light dose, a defined cochlear area, functionallymediating the perception of defined acoustic frequencies.If further experimental in vivo and in vitro studies con-firm the dependence of LLLT photostimulatory effects onlight dose and irradiation parameters, the secure and ex-act application of laser light to the human cochlea shouldbe oriented to the data obtained in this study. For clinicalinvestigation of therapeutic effectiveness of low-level la-sers in cochlear dysfunction, further development of asuitable laser device for transmeatal light application willbe mandatory. Placebo-controlled double-blind studies arethen necessary to analyse the effectiveness of lightdosi-metric-controlled LLLT as therapeutic regimen for the hu-man cochlea. Moreover, the future development of photo-reactive substances mediating cellular photochemicallaser-induced effects could possibly influence or even en-hance the photochemical response of irradiated cells.

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