###### Abstract  
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. optical coherence tomography; optical Doppler tomography; optical coherence vibrometry; otology; Doppler effect +
Footnote †: journal:  
OCT (around 10 \(\upmu\)m in the lateral direction and 1 \(\upmu\)m to over 10 \(\upmu\)m in the axial direction) is comparably higher than other ontological methods, such as ultrasound imaging (150 \(\upmu\)m with 10 MHz probe), MRI (300 \(\upmu\)m), and CT (400 \(\upmu\)m), 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].