## 4 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  
Figure 8: (**a–e**) Exemplary oscillation patterns in vivo at characteristic frequencies, which were calculated by postprocessing of one functional OCT measurement, show the deflection of the tympan membrane. The displacement scale and color map are adjusted to the maximum amplitude of each oscillation pattern. (**f**) Depth projection of the corresponding OCT volume scan for comparison. The x and y scales are applicable for the working distance of 8 mm. Reprinted with permission from Ref. [50]. 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 vibration-related 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.