The tympanic membrane is one of the major structures of the ear that aids in the hearing process, giving humans one of the five major senses. It is hypothesized that sound induced displacements of the membrane, which allow humans to hear, are directly related to the membrane's medial layer which is comprised of a network of collagen fibers. Limitations in available medical imaging techniques have thus far inhibited the further study of these fibers. In this paper we detail an imaging system that we developed with the capability to quantitatively and noninvasively image the internal structures of biological tissues in vitro through spatial domain optical coherence tomography (OCT). By utilizing spatial OCT, we can correlate the characteristics of internal collagen fibers to sound induced displacements in the tympanic membrane. This will eventually lead to improved modeling of the middle-ear and a better understanding of hearing mechanics.

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In this paper, we propose a multi-pulsed double exposure (MPDE) acquisition method to quantify in full-field-of-view the transient (i.e., >10 kHz) acoustically induced nanometer scale displacements of the human tympanic membrane (TM or eardrum). The method takes advantage of the geometrical linearity and repeatability of the TM displacements to enable high-speed measurements with a conventional camera (i.e., <20 fps). The MPDE is implemented on a previously developed digital holographic system (DHS) to enhance its measurement capabilities, at a minimum cost, while avoiding constraints imposed by the spatial resolutions and dimensions of high-speed (i.e., >50 kfps) cameras. To our knowledge, there is currently no existing system to provide such capabilities for the study of the human TM. The combination of high temporal (i.e., >50 kHz) and spatial (i.e., >500k data points) resolutions enables measurements of the temporal and frequency response of all points across the surface of the TM simultaneously. The repeatability and accuracy of the MPDE method are verified against a Laser Doppler Vibrometer (LDV) on both artificial membranes and ex-vivo human TMs that are acoustically excited with a sharp (i.e., <100 μs duration) click. The measuring capabilities of the DHS, enhanced by the MPDE acquisition method, allow for quantification of spatially dependent motion parameters of the TM, such as modal frequencies, time constants, as well as inferring local material properties.