The first two authors contributed equally to this paper.
The author is now working at Department of Biomedical Engineering, Ulsan National Institute of Science and Technology (UNIST), Ulsan, Korea
Doppler optical coherence tomography (DOCT) is useful for both, the spatially resolved measurement of the tympanic membrane (TM) oscillation and high-resolution imaging. We demonstrated a new technique capable of providing real-time two-dimensional Doppler OCT image of rapidly oscillatory latex mini-drum and
Using DOCT system, the oscillation of sample was measured at frequency range of 1–4 kHz at an output of 15 W. After the sensitivity of the DOCT system was verified using a latex mini-drum consisting of a 100 μm-thick latex membrane, changes in displacement of the umbo and contacted area between TM and malleus in normal and pathologic conditions.
The oscillation cycles of the mini-drum for stimulus frequencies were 1.006 kHz for 1 kHz, 2.012 kHz for 2kHz, and 3.912 kHz for 4 kHz, which means that the oscillation cycle of the mini-drum become short in proportional to the frequency of stimuli. The oscillation cycles of umbo area and the junction area in normal TM for frequencies of the stimuli showed similar integer ratio with the data of latex mini-drum for stimuli less than 4 kHz. In the case of middle ear effusion condition, the Doppler signal showed a tendency of attenuation in all frequencies, which was prominent at 1 kHz and 2 kHz.
The TM vibration under sound stimulation with frequencies from 1 kHz to 4 kHz in normal and pathologic conditions was demonstrated using signal demodulation method in
Laser Doppler vibrometry (LDV) is the gold standard method for obtaining noncontact vibration measurements in hearing research [
Optical interferometry is ideally suited for precise measurements of sub-nanometer-level vibration [
LDV applied in many studies has shown to be a promising technique and as an exclusive tool for the diagnosis and differentiation of intact and diseased middle ears. But, it is usually a single point measurement of sound induced vibration at a point near the umbo of the tympanic membrane (TM). This method lacks information about the structural features of the TM, that is in-depth visualization is not available. Furthermore, LDV cannot provide a high-resolution depth resolved cross-sectional map of structural and vibrational information because of the interference caused by the long coherence length of laser.
Sound is collected by the external ear and sent to the cochlea via vibration of the TM and ossicles. Because the structure of the middle ear, which includes the TM and ossicles, makes it more accessible for an instrument that measures vibration than the inner ear, experiments on sound transmission have been performed on animals using LDV [
The recently developed graphics processing unit (GPU) allows very fast processing of OCT signals, in addition to executing parallel, general-purpose numerical solutions, thus surpassing the functionality of central processing units [
The schematic diagram of the DOCT system is shown in
The Doppler frequency shift generated by vibration of the sample was detected by a phase signal and the effect was calculated for each corresponding pixel of the M-mode image. We computed the phase difference between adjacent A-lines, which is proportional to the modulation of the difference in optical path induced by the acoustic stimulation. The phase information was extracted from real and imaginary values. The Doppler phase shift
where
Where λ0 is the center wavelength,
The Doppler signal was measured using the Kasai autocorrelation algorithm [
To test the sensitivity of the DOCT system, a latex drum was constructed by stretching a 100-μm-thick latex membrane over and attaching it to a 50-mm-diameter glass tube. Sound stimuli comprising sinusoidal signals of 1, 2, and 4 kHz from a functional generator were broadcast through the loudspeaker, causing the latex drum to vibrate. To minimize the effect of reflected sound, all procedures were performed in a 250×200×120-mm sound-proof chamber. A calibrated probe microphone was placed near the latex drum to record the sound pressure level at the surface of the latex drum. The OCT beam scanned the entire drum and detected the Doppler frequency shift caused by vibration.
Sprague Dawley rats (5 weeks old, 60 g) were anesthetized by intramuscular injection of a mixture of tiletamine-zolazepam (1.8 mg/100 g) and xylazine hydrochlorate (0.7 mg/100 g). After local injection of 2% lidocaine HCl and epinephrine (1:100,000), a postauricular skin incision was made, the auricle and cartilaginous external auditory canal were removed, and the bony external auditory canal was widened by drilling to expose the entire TM.
During
Our study was permitted by Institutional Animal Care and Use Committee of Kyungpook National University Hospital (No. 2014-12). All tests were performed within the guidelines of the Animal Care Ethics Committee of Kyungpook National University and the National Institutes of Health guidelines.
The oscillation pattern of the contact area in the simulated effusion condition is shown in
The errors from the mean of the results for the five samples were as follows: 0.006% (latex drum), –0.107% (umbo), and –0.001% (contact area) at 1 kHz, 0.006% (latex drum), –0.036% (umbo), and –0.264% (contact area) at 2 kHz, 0.022% (latex drum), 0.044% (umbo), and 0.044% (contact area) at 4 kHz, each.
To gain a rigorous evaluation of the proposed method, we obtained 3D DOCT images of the area in the white boxes with arrows in
Our system performance was verified through the acquired 3D reconstruction phase shift images of the contact area between TM and malleus in normal TM and simulated effusion condition
The oscillating frequencies were compared among latex drum, umbo, and contact area in normal TM at 1, 2, and 4 kHz pure tone stimuli, each (
These results suggest that the oscillation pattern provided more detailed information on the Doppler shift that the 3D phase pattern. The 3D phase pattern provides a more comprehensive view of the condition of the TM for identifying pathological features and making a diagnosis.
Inflammatory diseases of the middle ear are frequently accompanied by a pathological condition that causes morphological and functional changes to the structure of the middle ear, including the TM. In general, conventional diagnostic tools such as tympanometry, audiometry, otoscope examination, and LDV are used to evaluate the status of the TM. However, the TM and the tympanic cavity behind it cannot be visualized in depth and the oscillation of the TM cannot be spatially measured with these tools. In this study, we showed that DOCT could resolve the structure and motion of the TM and parts of the ossicular chain simultaneously under normal and MEE-simulated conditions. The results of this study support the possibility of using DOCT as an optimal diagnostic tool by simultaneously measuring TM and ossicular motion, as previously reported [
Recently, studies on the oscillation patterns of TM and ossicular motion have been performed using LDV [
We analyzed the vibrational characteristics of the TM and malleus with respect to frequency and identified a reduced vibration pattern in the middle ear in a simulated effusion condition compared with the pattern of the middle ear under normal condition. Effusion and middle ear static pressure reduce the oscillation of the TM by decreasing middle ear compliance. The functional information about the TM acquired by DOCT can improve diagnostic accuracy. Vibration of the TM is affected by mass at high frequencies and by stiffness of the TM and the ossicular chain at low frequencies [
Vibration characteristics were analyzed using a signal demodulation method, in contrast with previous studies, and phase differences of the TM and malleus were visualized via 3D mapping. In addition, we performed
There are some problems with the application of DOCT to the human TM that need to be resolved. The thickness of the human TM and the complicated structure of the tympanic cavity reduce the sensitivity of DOCT in measuring ossicular motion. In addition, the time for acquiring DOCT images and the underestimation of vibration not perpendicular to the optical beam are issues that need attention.
Recently, Chang et al. [
For the diagnosis of human TM, it is significant to develop the diagnostic sample arm to be of probe type which will also allow a free path for sound waves to travel in the ear that does not obstructing the optical path, without surgery is also essential. OCT has been advanced enough to be applicable for clinical use. We have performed a study to diagnose the vibration of the eardrum using DOCT to observe the surface of the eardrum with an otoscope and which is unlike the conventional method of relying on the information given by the patient in response to the sound they hear using headphones.
This preliminary study used a signal demodulation method to demonstrate TM vibration under sound stimulation at frequencies of 1, 2, and 4 kHz in a normal ear and an ear under simulated pathological condition
This study was performed to test the possibility of DOCT signal extraction. Especially, we focused on the 2D/3D OCT imaging and signal restoration in accordance to the difference of normal and abnormal cases. We will conduct detailed studies on abnormal models and further animal and human experiments [
▪ Doppler optical coherence tomography is useful for the measurement of the tympanic membrane oscillation
▪ The oscillation cycles in normal tympanic membrane for frequencies of the stimuli drum become short in proportional to the frequency of stimuli less than 4 kHz.
▪ Doppler signal showed a tendency of attenuation in all frequencies, which was prominent at 1 kHz and 2 kHz in fluid in middle ear condition.
No potential conflict of interest relevant to this article was reported.
This research was supported by the Bio & Medical Technology Development Program of the National Research Foundation of Korea (NRF) funded by the Korean Government, MSIP (2017 M3A9E2065282), and “Development of Micro-surgical Apparatus Based on 3D Tomographic Operating Microscope” funded by the Ministry of Trade, Industry and Energy (MI, Korea; grant no. 10047943). Also, this study was supported by BK21 Plus project funded by the Ministry of Education, Republic of Korea (21A20131600011).
Schematic diagram of the Doppler optical coherence tomography system. BS, broadband source; C, collimator; DG, diffraction grating; FL, focusing lens; LSC, line scan camera; FC, fiber coupler; M, mirror; GS, galvanometer scanner; S, speaker; SAC, sound-absorbing chamber.
Correspond DOCT two-dimensional images and extracted phase signals of stimulated 1-kHz, 2-kHz, and 4-kHz pure tone stimuli in latex drum (A-C) and umbo of tympanic membrane (TM) (D-F). Solid rectangular box: extracted phase; white box with arrow: region of interest for signal demodulation. OCT, optical coherence tomography; DOCT, Doppler optical coherence tomography; TM, tympanic membrane; M, malleus.
Two-dimensional images and phase signals of pure tone 1-kHz, 2-kHz, and 4-kHz sine wave stimuli at 15 W: contact area between tympanic membrane and ossicles in normal tympanic membrane (TM) (A-C) and simulated effusion condition (D-F). Solid rectangular box: extracted phase; white box with arrow: region of interest for signal demodulation. TM, tympanic membrane; M, malleus; EF, effusion.
Three-dimensional reconstruction images of the detected phase shift (area in the white boxes in
Three-dimensional reconstruction images of the detected phase shift (area in the white boxes in
Comparison in latex mini drum, umbo and ossicles by 1, 2, and 4 kHz frequency for each rats. TM, tympanic membrane.