High-Speed Shape and Transient Response Measurements of Tympanic Membrane

Abstract

We are developing a High-speed Digital Holographic (HDH) system to measure acoustically induced transient displacements of live mammalian Tympanic Membranes (TM) for research and clinical applications. To date, the HDH can measure one-dimensional displacements along a single 1-D sensitivity vector. However, because of the TM’s tent-like shape and angled orientation inside the ear canal, 1-D measurements need to be combined with measurements of the shape and orientation of the TM to determine the true surface normal (out-of-plane) displacements. Furthermore, TM shape also provides invaluable information for better diagnostic and modelling of the TM. To introduce shape measurements capabilities into our HDH, a tunable laser (line width <300 kHz, 770.2–790 nm, <8 nm/s) is incorporated in tandem with the single frequency (line width <100 MHz, 532.3 nm) laser used for displacement measurements in a manner that provides a common sensitivity vector for the two measurements. Interferograms gathered with continuous high-speed optical phase sampling and wavelength tuning allow the reconstruction of the TM shape with <50 μm measuring resolution and <120 μm repeatability. With our modified HDH, shape measurements immediately follow transient displacement measurements (>67 kHz temporal and <15 nm displacement resolutions) in response to broadband acoustic click excitations (50 μs duration). Both shape and displacement can be measured in less than 150 ms, which avoids slow disturbances introduced by breathing and heartbeat, with the promise of future measurements in vivo. Representative shape and displacement measurements capabilities are demonstrated on cadaveric human temporal bones.