The tympanic membrane (TM) and ossicular chain play a central role in hearing by providing acoustic impedance matching between the air-filled ear canal and the fluid-filled inner ear. Vibrometric measurement of the ossicles and TM has been critical for advancing our understanding of the hearing mechanics and improving treatments such as middle-ear prosthetics. It also holds promise for diagnosis of ossicular disorders and planning surgical interventions. Phase-sensitive optical coherence tomography (OCT) is a promising tool in hearing research and otology because it can simultaneously image the anatomical structure of the middle ear and measure sound transduction along the TM and ossicular chain with nanometer level sensitivity. Up to now, the demonstrations of OCT-based middle ear vibrometry have been largely focused on vibration magnitude, and vibration phase has been generally overlooked. Here we show OCT vibrography, in which the data acquisition is synchronized with sound excitation and beam scanning, is well suited for volumetric, vibrational imaging of the ossicles and TM. The acquired vibrography data provide intuitive motion pictures of the ossicular chain and how they vary with sound frequency. We investigated the chinchilla ear over 100 Hz to 15 kHz. The vibrography images reveal a previously undescribed mode of motion of the chinchilla ossicles at high frequencies, involving the rotation of the ossicular chain around a secondary axis parallel to the manubrium. We also found evidence of bending and torsion of the manubrium.
The eardrum or tympanic membrane (TM) transforms acoustic energy at the ear canal into mechanical motions of the ossicles. The acousto-mechanical transformer behavior of the TM is determined by its shape, three-dimensional (3-D) motion, and mechanical properties. We have developed an optoelectronic holographic system to measure the shape and 3-D sound-induced displacements of the TM. The shape of the TM is measured with dual-wavelength holographic contouring using a tunable near IR laser source with a central wavelength of 780 nm. 3-D components of sound-induced displacements of the TM are measured with the method of multiple sensitivity vectors using stroboscopic holographic interferometry. To accurately obtain sensitivity vectors, a new technique is developed and used in which the sensitivity vectors are obtained from the images of a specular sphere that is being illuminated from different directions. Shape and 3-D acoustically induced displacement components of cadaveric human TMs at several excitation frequencies are measured at more than one million points on its surface. A numerical rotation matrix is used to rotate the original Euclidean coordinate of the measuring system in order to obtain in-plane and out-of-plane motion components. Results show that in-plane components of motion are much smaller (<20%) than the out-of-plane motions’ components.
Current methodologies for characterizing tympanic membrane (TM) motion are usually limited to either average acoustic estimates (admittance or reflectance) or single-point mobility measurements, neither of which suffices to characterize the detailed mechanical response of the TM to sound. Furthermore, while acoustic and single-point measurements may aid in diagnosing some middle-ear disorders, they are not always useful. Measurements of the motion of the entire TM surface can provide more information than these other techniques and may be superior for diagnosing pathology. We present advances in our development of a new compact optoelectronic holographic otoscope (OEHO) system for full field-of-view characterization of nanometer-scale sound-induced displacements of the TM surface at video rates. The OEHO system consists of a fiber optic subsystem, a compact otoscope head, and a high-speed image processing computer with advanced software for recording and processing holographic images coupled to a computer-controlled sound-stimulation and recording system. A prototype OEHO system is in use in a medical research environment to address basic science questions regarding TM function. The prototype provides real-time observation of sound-induced TM displacement patterns over a broad frequency range. Representative time-averaged and stroboscopic holographic interferometry results in animals and human cadaver samples are shown, and their potential utility is discussed.