Doppler Optical Coherence Tomography for Otology Applications: From Phantom Simulation to In Vivo Experiment

: In otology, visualization and vibratory analysis have been crucial to enhance the success of diagnosis and surgical operation. Optical coherence tomography (OCT) has been employed in otology to obtain morphological structure of tissues non-invasively, owing to the ability of measuring the entire region of tympanic membrane, which compensates the limitations of conventional methods. As a functional extension of OCT, Doppler OCT, which enables the measurement of the motion information with structural data of tissue, has been applied in otology. Over the years, Doppler OCT systems have been evolved in various forms to enhance the measuring sensitivity of phase difference. In this review, we provide representative algorithms of Doppler OCT and various applications in otology from preclinical analysis to clinical experiments and discuss future developments.


Introduction
Visualization and functional analysis for vibratory properties of the ear structure (e.g., middle and inner ear) are required to diagnose various ear-related diseases (e.g., acute otitis media, hearing loss, middle ear effusion, bullous myringitis, and traumatic perforation of the tympanic membrane (TM)) in otology [1][2][3][4][5]. As an aspect of the conventional techniques to visualize otologic pathology for otolaryngologists, otoscopes and surgical microscopes have been utilized as diagnostic and operative tools [6,7]. Although otoscopy has been utilized as one of the preferred methods to visualize the surface of TM, field-ofview and visualization of middle ear are limited, which leads to low diagnostic accuracy for ear-related diseases [8]. Though the surgical microscope has made progressed in otological visualization, it is hard to provide the anatomical structure of the middle ear sub-surface, which is essential to succeed a subjective surgical procedure [9]. To provide the structural information in otology to assist the surgery, computed tomography (CT) has been employed to detect the abnormalities of bone [10], and magnetic resonance imaging (MRI) has been utilized to measure the differences of soft tissue [11]. However, applications of these methods are limited because of the low differentiation ability of soft tissue, required radiation exposure, low resolution to visualize the microstructure of TM (MRI), and poor patient tolerance.
Optical coherence tomography (OCT) is a non-invasive and high-resolution imaging technique, which provides three-dimensional morphological information of biological tissues in real-time [12][13][14]. OCT has been widely utilized in various fields, such as ophthalmology [15][16][17], dentistry [18][19][20], otolaryngology [21][22][23][24], industrial fields [25][26][27][28], and even in agriculture [29,30]. Among various otolaryngology applications, OCT has been readily applied in otology, owing to its several advantages over the current standard imaging techniques for TM and the middle ear structure [31][32][33]. Since the resolution of OCT (around 10 µm in the lateral direction and 1 µm to over 10 µm in the axial direction) is comparably higher than other otological methods, such as ultrasound imaging (150 µm with 10 MHz probe), MRI (300 µm), and CT (400 µm), OCT enables characterization of the TM and middle ear structure without the radiation exposure of CT and the impedance matching medium of ultrasound imaging [34,35]. In addition, measuring the anatomical structure of the middle ear is limited using ultrasound imaging due to the highly attenuated ultrasound signal in the air [35].
Doppler OCT has been widely studied as one of the functional OCT modalities to provide movement information of a sample by detecting the phase shift of the obtained interfered signal in addition to the morphological structure [36,37]. The initially proposed Doppler OCT, which was developed in a time-domain, was involved in the imaging of the dynamic blood flow in vivo [36] and bi-directional color Doppler imaging for the visualization of blood flow in tissues [37]. Following the initial introduction of the Doppler technique to OCT, various approaches to calculate the velocity with phase-resolved measurement were investigated [38][39][40]. After the introduction of Fourier-domain OCT (FD-OCT) [41,42], the scanning speed and sensitivity of Doppler OCT were significantly improved [43,44]. Since the initiation of Doppler OCT in the otology field, it has been widely studied and applied to the simulation of membrane-resembled samples [45,46], ex vivo studies including small animal and human cadaveric experiments [47,48], and in vivo experiments for both animals and humans [49,50], while providing a morphological structure with motion information of tissue.
As an aspect of the methodology of Doppler OCT, numerous approaches to improve the sensitivity of the measurement have been demonstrated, including phase-resolved color Doppler [51][52][53], phase-resolved Doppler variance [54][55][56], intensity-based Doppler variance [57][58][59], and optical coherence vibrometry (OCV) [60][61][62]. Phase-resolved color Doppler is one of the widely utilized methods, and extracts the Doppler frequency shift by comparing the phase differences of adjacent A-lines [36]. To utilize both intensity and phase information, there are other phase-resolved techniques, such as phase-resolved Doppler variance [54,55] and filtering-based operations to the original signal [63,64]. The phase-resolved Doppler variance is an autocorrelation algorithm-based method, which uses the complex Doppler signal of each position. The intensity-based Doppler variance method involves both intensity and phase data, while increasing the time interval of each A-line [57,58]. Moreover, OCV enables the measurement of the motion of the cellular structure on a sub-nanometer scale, which uses the phase changes of localized tissue motion by applying a Fourier transform analysis [65].
In this review, we present the representative applications of Doppler OCT for otology from preliminary studies using a phantom membrane to in vivo experiments, including a brief description of each representative principle of the method. Furthermore, we focus on timely updated clinical applications of Doppler OCT in otology and discuss the potential of preclinical and clinical imaging.

Principles of Doppler OCT for Otology
The measured interfered spectrum (I(k)) is simply expressed by subtracting the DC term: where k = 2π/λ, z is the in-depth coordinate direction, ϕ is the random phase noise, and I R and I S are the obtained back-scattered signals from the reference arm and sample arm, respectively. To obtain an A-scan of OCT, which is a depth-resolved signal, a Fourier transform is applied to I(k) over k into the time-domain interfered signal (i(z)): where z is the depth and can be expressed as z = 2ct (c is the speed of light and the refractive index of the sample is assumed as 1). In addition, A(z) and Φ(z) are the amplitude and the phase of signals located at depth z. The optical phase angle is expressed as: where δz and f 0 are the amplitude and vibration frequency, respectively, and λ is the center wavelength of the source. To extract the value of Doppler phase shift, Doppler OCT uses the following equation: where θ i,z and θ i+1,z are phases of the obtained signal at the z pixel in-depth direction of i and (i + 1) and T is the time interval between adjacent A-lines. Moreover, the equation of the Doppler frequency shift is expressed after differentiation of Equation (3) as: where ξ(t) is the intrinsic phase noise, which is comparably larger than ∆Φ(z, t), and ∆t is reciprocal of the sampling frequency. According to the autocorrelation algorithm, Equation (4) becomes: where Re(i i ) and Im(i i ) are the real and imaginary parts of the obtained complex signal, respectively. As shown in Equation (6), the phase-resolved Doppler OCT method utilized the calculated phase information to extract a Doppler frequency shift.
To enhance the signal-to-noise ratio (SNR) of the Doppler system, the averaging mask, which consists of multiple sequential A-lines in the transverse direction with pixels in the axial direction, was applied to Equation (6). Averaging in the lateral (temporal) direction for N lines and in the depth direction for M lines can be expressed as: By averaging for both directions (lateral and depth), Equation (7) reduces the background subtle noise, which leads to the image quality enhancement. Though a larger size of the phase mask (N and M) increases SNR, it requires a large processing time and reduces resolution. Therefore, the appropriate size of the phase mask is needed to choose according to applications.
As another approach to enhance the sensitivity for measuring phase changes, the Fourier frequency analysis approach is applied to Equations (5) and (7). Fourier transformed Equation (5) becomes: where f is the frequency variable and |p( f )| and θ( f ) are the magnitude and the phase of the vibration at the specific frequency, respectively. Therefore, the magnitude of the vibration source at a specific frequency ( f 0 ) is expressed as: By applying a Fourier transform to Equation (7), the obtained phase noise of the system was able to be assumed as the Gaussian noise and phase data were chosen by the applied specific frequency ( f ) of the input sound, which enhances SNR for the signal of interest. Therefore, the signal of the region-of-interest was amplified without effect on Gaussian noise, which enables to measure the phase changes at a subnanometer scale [60][61][62].
In this section, we briefly introduced principles of phase-resolved Doppler OCT methods to measure the phase difference and magnitude of vibration in otology. There are various methods of Doppler-related OCT techniques, such as phase-resolved color Doppler, phase-resolved Doppler variance, intensity-based Doppler variance, and optical coherence vibrometry. The phase-resolved color Doppler technique was the first introduced method of Doppler OCT. However, phase-resolved color Doppler is based on time-domain OCT, which has a comparable low scanning speed that is comparably slow. Therefore, other methods of Doppler OCT have been widely studied to compensate for these limitations. Phase-resolved Doppler variance, which applied an autocorrelation algorithm for complex data of each position, has been introduced to measure the phase variance of the sample. In addition, intensity-based Doppler variance was presented, which increases the time interval between adjacent A-lines, to enhance the sensitivity. However, variance-based Doppler methods have been primarily applied to measure the flow. Therefore, we focused on the vibration-related phase and magnitude differences, which are appropriate for otology applications.

Preclinical Simulations Using a Phantom Membrane
To evaluate the Doppler OCT system performance and characterize the effects of the external sound pressure on the membrane, phantom-based simulations are required. As an approach of phantom simulation in otology, the latex membrane, which mimics TM, was utilized as a representative sample with external stimulating speakers. As an initial approach to measure the vibrating pattern of the phantom membrane, Chang et al. [45] captured the rapid periodic oscillating motions of latex membrane using OCV, with a sub-nanometer scale for the motion sensitivity and a micro-second scale for the temporal resolution. By using the experimental configuration shown in Figure 1a, a homogeneous vibrating pattern was measured at opposite vibration phases (0 and π) and a cutaway view (Figure 1b,c), respectively. This study demonstrated the feasibility of Doppler OCT to visualize the vibrating mode according to the external sound stimulation.
To evaluate the performance of the developed Doppler OCT, Jeon et al. [66] preliminarily measured the vibrating patterns of the latex membrane, while the obtained results were compared with in vivo assessments. Before the in vivo experiment, a preliminary study using a latex drum was conducted to evaluate the performance of the Doppler OCT system. Additionally, this result was employed for analysis data to improve the objectivity of the obtained experimental results.  Ref. [45]. © The Optical Society.
To evaluate the performance of the developed Doppler OCT, Jeon et al. [66] preliminarily measured the vibrating patterns of the latex membrane, while the obtained results were compared with in vivo assessments. Before the in vivo experiment, a preliminary study using a latex drum was conducted to evaluate the performance of the Doppler OCT system. Additionally, this result was employed for analysis data to improve the objectivity of the obtained experimental results.
As a different approach to compensate the equivalent anatomical structural characteristics of the latex membrane, a set of two speakers were utilized to make a localized sound distribution to evoke distinct characteristics of the phantom membrane by external sound stimulation by Seong et al. [46]. Measured phase differences of each cross-sectional image and combined enface phase map by Doppler OCT are shown in  As a different approach to compensate the equivalent anatomical structural characteristics of the latex membrane, a set of two speakers were utilized to make a localized sound distribution to evoke distinct characteristics of the phantom membrane by external sound stimulation by Seong et al. [46]. Measured phase differences of each cross-sectional image and combined enface phase map by Doppler OCT are shown in Figures 2a-d, respectively. According to the increasing applied frequency of sound (2, 2.2, 2.8, 3.1, and 3.2 kHz), an escalation of the order of circular mode was measured as shown in Figure 2e-h. This study showed the feasibility of Doppler OCT for measuring a complex micro-vibration of the membrane.
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Ex Vivo Studies
Following the results of phantom simulations, ex vivo experiments, including small animals and human cadaveric samples, have proceeded, as a clinical application of

Ex Vivo Studies
Following the results of phantom simulations, ex vivo experiments, including small animals and human cadaveric samples, have proceeded, as a clinical application of Doppler OCT.

Small Animal Studies
Hong et al. [67] initially measured the cochlea of small animal (Mongolian gerbil) ex vivo using Doppler optical coherence microscopy (OCM). In addition to the obtained anatomical structure of the middle ear using OCT, the functional vibrating motion of the middle ear is captured by applying the Doppler technique. This experiment presented the ability of Doppler OCM to discriminate the motion of the cochlea structure (basilar membrane, tectorial membrane, reticular lamina, and outer hair cells).
In addition to the measured two-dimensional (2D) cross-sectional motion data of cochlea in [67], Chang et al. [68] measured the three-dimensional (3D) vibrating pattern of the TM and ossicles of the chinchilla ear using OCV with sub-nanometer scale sensitivity. By measuring the volumetric vibrating information of the manubrium, the distinctive characteristic of each region was obtained. This study revealed the different simulated pathologies derived from the simultaneously measured ossicular motion information with the movement of the TM.
As an aspect of the sensitivity of the Doppler OCT system, Park et al. [69] demonstrated phase-sensitive OCT with a Vernier-tuned distributed Bragg reflector swept laser and measured the vibrations of the mouse middle ear to validate the sensitivity enhancement. Since sensitivity is one of the crucial factors of Doppler OCT performance, multiple attempts have been made to study and improve the sensitivity. Unlike conventional algorithm-based research to enhance the performance, Park et al. [69] utilized a swept laser, which provides a highly linear sweeping rate with high stability and repeatability. Based on these advantages of the Vernier-tuned distributed Bragg reflector swept laser, a simplified systematic design was used and demonstrated the near shot-noise-limited phase sensitivity.
To overcome the frequency limit of applied sound and random phase noise drift, Ramier et al. [47] developed an OCT system for the precise measurement of the phase delay. With this system, both the sound-induced vibrating motion of the TM and ossicles of the chinchilla ear can be simultaneously measured and visualized. As shown in Figure 3, different characteristics of the prominent structures (i.e., manubrium of the malleus, incus, and stapes) were measured according to the applied frequencies (0.5, 6.4, and 15.0 kHz). This study revealed an undescribed vibration mode of chinchilla ossicles measured at high frequencies.

Human Cadaveric Experiments
Subhash et al. [62] initially employed phase-sensitive OCT to detect the micro-vibrations in the TM and ossicular chain of the cadaveric temporal bone. This study demonstrated the possibility of Doppler OCT to diagnose the middle ear abnormalities by comparing the corresponding vibrating pattern of normal and ossicle-fixed cadavers.
In addition to the obtained 2D vibration data in [62], the 3D vibration patterns of a human cadaver using Doppler OCT with repeated sound stimuli was presented in Burkhardt et al. [48]. As shown in Figure 4, different characteristic of each typical point (e.g., umbo and manubrium of malleus) was measured according to the applied external sound pressure, which is one of the factors to distinguish the normal and abnormal condition. Based on these results, the feasibility of Doppler OCT as a promising technique to investigate and diagnose in otology was verified.
of the chinchilla ear can be simultaneously measured and visualized. As shown in Figure  3, different characteristics of the prominent structures (i.e., manubrium of the malleus, incus, and stapes) were measured according to the applied frequencies (0.5, 6.4, and 15.0 kHz). This study revealed an undescribed vibration mode of chinchilla ossicles measured at high frequencies. Reprinted with permission from Ref. [47]. © The Optical Society.

Human Cadaveric Experiments
Subhash et al. [62] initially employed phase-sensitive OCT to detect the microvibrations in the TM and ossicular chain of the cadaveric temporal bone. This study demonstrated the possibility of Doppler OCT to diagnose the middle ear abnormalities by comparing the corresponding vibrating pattern of normal and ossicle-fixed cadavers.
In addition to the obtained 2D vibration data in [62], the 3D vibration patterns of a human cadaver using Doppler OCT with repeated sound stimuli was presented in Burkhardt et al. [48]. As shown in Figure 4, different characteristic of each typical point (e.g., umbo and manubrium of malleus) was measured according to the applied external sound pressure, which is one of the factors to distinguish the normal and abnormal condition. Based on these results, the feasibility of Doppler OCT as a promising technique to investigate and diagnose in otology was verified. Moreover, in terms of detecting the effusions in the tympanic cavity, which is required to choose the appropriate therapeutic strategies, Kirsten et al. [70] demonstrated the possibility of Doppler OCT as a tool to indirectly measure the scattering and transparent fluids. Since non-invasive detection of the fluid in the tympanic cavity is difficult when using otoscopy and tympanometry, the selection of proper therapeutic strategy depends on the expertise of the physician. In contrast, by applying Doppler OCT, oscillation of the TM is able to be non-invasively measured, which enables to detect the scattered and transparent fluids of the tympanic cavity. This study showed the possibility of Doppler OCT as a tool for classifying the effusion in the tympanic cavity.
To provide the functional movement information using otoscopy, the new probe, combining otoscopy with phase-sensitive OCT, was presented in Park et al. [71]. Otoscopy has been widely utilized as a conventional diagnostic tool in otology providing superficial information of the middle ear with limited range. The results of this study obtained using a cadaveric human temporal bone model demonstrated the capability of visualizing the middle ear anatomy on a micro-scale and measuring the vibration on a sub-nanometer scale.  Reprinted with permission from Ref. [48].
Moreover, in terms of detecting the effusions in the tympanic cavity, which is required to choose the appropriate therapeutic strategies, Kirsten et al. [70] demonstrated the possibility of Doppler OCT as a tool to indirectly measure the scattering and transparent fluids. Since non-invasive detection of the fluid in the tympanic cavity is difficult when using otoscopy and tympanometry, the selection of proper therapeutic strategy depends on the expertise of the physician. In contrast, by applying Doppler OCT, oscillation of the TM is able to be non-invasively measured, which enables to detect the scattered and transparent fluids of the tympanic cavity. This study showed the possibility of Doppler OCT as a tool for classifying the effusion in the tympanic cavity.
To provide the functional movement information using otoscopy, the new probe, combining otoscopy with phase-sensitive OCT, was presented in Park et al. [71]. Otoscopy To verify the effect of the moisture degree on the TM, and to understand how it relates to chronic otorrhea and abnormal TM, Jeon et al. [72] analyzed the dependency of the moisture degree with the vibrating phase. As shown in Figure 5, the homogenously moisturized cadaveric TM was exposed and the Doppler signal to each frequency was measured every 30 min. This study revealed a depth-dependent vibration tendency of the applied frequencies according to the declined moisture level on the TM.
To verify the effect of the moisture degree on the TM, and to understand how it relates to chronic otorrhea and abnormal TM, Jeon et al. [72] analyzed the dependency of the moisture degree with the vibrating phase. As shown in Figure 5, the homogenously moisturized cadaveric TM was exposed and the Doppler signal to each frequency was measured every 30 min. This study revealed a depth-dependent vibration tendency of the applied frequencies according to the declined moisture level on the TM.

In Vivo Experiments
As clinical applications of Doppler OCT to otology, various in vivo experiments from mouse to human have been presented.

Small Animal Studies
Gao et al. [73] presented the first measurement of vibrating motion from the apex of an intact mouse cochlea in vivo. To minimize the motion artifact caused by animal motion, a pre-experiment of collecting the data of six different sound stimulus levels and nine frequencies for averaging was conducted. This study demonstrated the advantage of phase-sensitive OCT to detect vibrations of intracochlear tissues in vivo without the risk of opening compared to the laser Doppler vibrometry.
In an attempt to apply proper systems with a different lateral resolution for each application, Page et al. [74] complementary utilized Doppler OCT and Doppler OCM to measure the micromechanical mechanisms of the mammalian middle ear. The cochlear apexes of the gerbil, mouse, and guinea pig were well-measured by cross-section, motion, and phase map, which verified the applicability of Doppler OCT and Doppler OCM to in vivo vibration measurements.
To measure the wide-range dynamics and vibration distribution of tissues, the multifrequency swept-source OCM using wide-field heterodyne detection was developed in Choi et al. [75]. To capture a full-field volumetric TM data and mapping the spatial information of vibration (i.e., amplitude and phase), an ultrahigh-speed CMOS camera and working-distance enhanced microscope were utilized. The results of this study showed the applicability of multi-frequency swept-source OCT for vibration measurement in vivo experiments and surgeries.
In addition to the obtained wide-range in vivo vibrating motion data of the middle ear, Jeon et al. [76] measured the relative vibrating distribution of distinctive sections of the TM (e.g., malleus, thick region, and thin region) by applying external sound from 1 to 8 kHz. This study demonstrated the feasibility of Doppler OCT for measuring the vibration tendency and analyze the characteristics of each section in the middle ear according to the frequency of externally induced sound.
To verify the feasibility of Doppler OCT as a tool for functional and structural assessment, Jeon et al. [66] measured the vibrating motion of both normal and pathologic TM, as well as the ossicles of rats in vivo. As shown in Figure 6, 3D reconstructed phase shift maps were obtained at the contact area (between the TM and malleus) in the normal condition and in the simulated effusion condition. The results of this study verify the possibility of Doppler OCT extracting the signal-by-signal demodulation method, which enables to distinguish the normal and abnormal bio-tissues.

In Vivo Human Experiments
As a first approach to measure the Doppler OCT signal of the live human middle ear, highly phase-stable Vernier-tuned distributed Bragg-reflector laser with a graphics processing unit were utilized by MacDougall et al. [49]. As shown in Figure 7, representative cross-sectional image and 3D reconstructed volume data were clearly obtained with vibration displacement on the sub-nanometer scale in vivo. The results of this study suggested the clinical applicability of Doppler OCT to detect the vibrating motion of tissues.

In Vivo Human Experiments
As a first approach to measure the Doppler OCT signal of the live human middle ear, highly phase-stable Vernier-tuned distributed Bragg-reflector laser with a graphics processing unit were utilized by MacDougall et al. [49]. As shown in Figure 7, representative cross-sectional image and 3D reconstructed volume data were clearly obtained with vibration displacement on the sub-nanometer scale in vivo. The results of this study suggested the clinical applicability of Doppler OCT to detect the vibrating motion of tissues.
In addition, the clinical results for distinguishing normal, otosclerosis TM using Doppler OCT was presented in MacDougall et al. [77]. Following the condition of the TM (i.e., normal and otosclerosis), the ratio of umbo to incus vibration level was welldistinguished by comparing the obtained data of each group. This study demonstrated the feasibility of Doppler OCT as a clinical tool of diagnosis and investigation of the status of the middle ear non-invasively. In addition, the clinical results for distinguishing normal, otosclerosis TM using Doppler OCT was presented in MacDougall et al. [77]. Following the condition of the TM (i.e., normal and otosclerosis), the ratio of umbo to incus vibration level was welldistinguished by comparing the obtained data of each group. This study demonstrated the feasibility of Doppler OCT as a clinical tool of diagnosis and investigation of the status of the middle ear non-invasively.
Moreover, in terms of the scanning range of endoscopic Doppler OCT, an endoscopic phase-resolved Doppler OCT with a wide field-of-view, which allows 3D visualization of the nearly entire TM and measurement of the vibrating motion, was presented in Kirsten et al. [50]. As shown in Figure 8, distinctive oscillating patterns were measured according to the externally induced sound frequencies. This study demonstrated the imaging capability of Doppler OCT as a promising tool of diagnosis in otology.
As an aspect of the clinical application of Doppler OCT, MacDougall et al. [78] demonstrated the comparison results between normal control and ears with an otosclerotic stapes fixation. By combining the cross-sectional structural imaging with vibration measurement, the reduction of vibration motion with otosclerosis was measured at umbo. The results of this study verified the clinical feasibility of Doppler OCT compared to laser Doppler vibrometry, which has been a widely utilized method in otology. Moreover, in terms of the scanning range of endoscopic Doppler OCT, an endoscopic phase-resolved Doppler OCT with a wide field-of-view, which allows 3D visualization of the nearly entire TM and measurement of the vibrating motion, was presented in Kirsten et al. [50]. As shown in Figure 8, distinctive oscillating patterns were measured according to the externally induced sound frequencies. This study demonstrated the imaging capability of Doppler OCT as a promising tool of diagnosis in otology. In addition to the clinical applications with Doppler OCT as a diagnostic tool, Kim et al. [79] presented the highly phase-stable OCV, which was attached to a surgical microscope as an accessory to apply for clinical diagnosis and surgeries in the operation room. The phase sensitivity of this system is under five picometers, which sufficiently exploits the tuning curve of the human TM according to the threshold of hearing. This study demonstrated the possibility of Doppler OCT for providing vibrational information with simultaneous monitoring.
OR PEER REVIEW 13 of 18 As an aspect of the clinical application of Doppler OCT, MacDougall et al. [78] demonstrated the comparison results between normal control and ears with an otosclerotic stapes fixation. By combining the cross-sectional structural imaging with vibration measurement, the reduction of vibration motion with otosclerosis was measured at umbo. The results of this study verified the clinical feasibility of Doppler OCT compared to laser Doppler vibrometry, which has been a widely utilized method in otology.
In addition to the clinical applications with Doppler OCT as a diagnostic tool, Kim et al. [79] presented the highly phase-stable OCV, which was attached to a surgical microscope as an accessory to apply for clinical diagnosis and surgeries in the operation room. The phase sensitivity of this system is under five picometers, which sufficiently

Discussion and Conclusions
In otology, OCT has been developed as a promising tool for optical biopsy to diagnose and investigate the ear condition based on the obtained morphological volume data. In addition, as a functional approach to otology applications, Doppler OCT was introduced to identify the vibrating motion of biofilm with anatomical information. Unlike other conventional techniques, including laser Doppler vibrometry (LDV) and stroboscopic holography, Doppler OCT non-invasively provides phase variance data of the large area of the ear with precise targeting in a comparably short time to process. Vibrations stimulated by external sound are not only different for each part of the ear, but also vary depending on various factors, including the condition of the ear, thickness, degree of moisture, and the presence of disease. Therefore, as an aspect of importance for measuring the vibrationrelated information in otology, based on these advantages, Doppler OCT has demonstrated the potential of a tool, which is based on the phase variance differences of tissue, for clinical applications including rapid diagnosis and assessment of conductive hearing loss (i.e., otosclerosis and tympanosclerosis), cholesteatoma, dimeric TM, hyperkeratosis, cochlear mechanics, and other ossicular diseases. Moreover, Doppler OCT has the capability of emphasizing vibration-based ear condition evaluating factor for hearing-related tests based on the various experimental results, including phantom simulations, ex vivo, and in vivo studies. Due to the fast scanning rate of Doppler OCT, the ear's responses to the stimulus sound are identified, and these findings enable the feasibility of the method as a new diagnostic approach in otology.
Although continuous development has been studied and various pathologies have been conducted, many other challenges remain to be improved to be able to apply Doppler OCT to the clinical approach of otology. First, the SNR of the system should be improved to obtain a high phase stability, which is degraded by the limited penetration in soft and hard tissue of OCT. Second, selecting an appropriate scanning interval and noise floor level for each system is required to enhance the accuracy of the measuring phase data. Third, the scanning and processing time to obtain vibrating information need to be shortened to provide data in real-time, which is required in otologic surgery. Fourth, reducing the effect of motion artifact of the patient, caused by heartbeat, respiration, and body movement, needs to be addressed. In spite of these challenges, Doppler OCT in otology can be pushed forward through further studies by many research groups on the common clinical use.
In this review, we introduced recent advances of Doppler OCT in otology for clinical and preclinical approaches from a phantom simulation to in vivo experiments. Doppler OCT has been developed over the years to enhance the image contrast, sensitivity, and imaging speed and reduce the processing time. Various experimental development using Doppler OCT has been reported to diagnose and operate in otology, whereas many other challenges remain. Nevertheless, Doppler OCT provides vibration-related information with anatomical morphology for preclinical and clinical diagnosis in otology, which is one of the factors to validate the ear condition. According to the preclinical and clinical recent results of Doppler OCT in the otology field, the potential of Doppler OCT for investigating ear-related diseases and discriminating ear conditions was verified. Therefore, with further technological development focused on the clinical application, Doppler OCT will provide meaningful data for physicians to conduct diagnosis and operations in otology.

Conflicts of Interest:
The authors declare no conflict of interest.