Quantifying the mechanical properties in thin layered structures poses a challenge due to the limited spatial resolution of the available mechanical analysis tools. Here, we propose a broadband (0.1 – 10 kHz) Rayleigh-wave optical coherence elastography technique and a bilayer-based inverse model to determine the elastic moduli of all three skin layers in vivo with high reliability. Our results show that 4 – 10 kHz, a previously unexplored frequency range, is critical to resolve the thin epidermis. Our proposed method is valuable for depth-resolved mechanical characterization of various layered materials and tissues.
Optical coherence tomography-based elastography (OCE) can perform localized, quantitative measurements of biomechanical properties. One of the most promising applications of OCE is to measure corneal stiffness, which has been linked to keratoconus, corneal crosslinking, and laser vision correction, and can help improve diagnosis, screening and treatment monitoring. Various techniques have been demonstrated to determine the speed of elastic waves traveling in the cornea and thereby to measure the shear modulus of corneal tissues. Here we present a new approach based on a contact probe with a piezo-electrically vibrating tip. This wave generation approach is robust, provides extensive control over the temporal and spectral profiles of the mechanical stimulus, and allows us to measure traveling wave velocities a frequency range of 1 to 15 kHz. The shorter wavelengths obtained at high frequencies can improve the resolution of traveling wave elastography and enable measurements of stiffer tissues such as the sclera. Direct contact with the corneal surface are routinely performed for intraocular pressure measurements, which suggest that this approach has a path to clinical translation. Interestingly, we found that mechanical stimulation tends to excite a combination of guided and non-guided elastic waves, which must be considered for accurate calculation of the shear modulus and may affect other OCT elastography techniques.
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.
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