Combined high-speed holographic shape and full-field displacement measurements of tympanic membrane

Abstract

The conical shape of the tympanic membrane (TM or eardrum) plays an important role in its function, such that variations in shape alter the acoustically induced motions of the TM. We present a method that precisely determines both shape and acoustically induced transient response of the entire TM using the same optics and maintaining the same coordinate system, where the TM transient displacements due to a broadband acoustic click excitation (50-μs impulse) and the shape are consecutively measured within <200 ms. Interferograms gathered with continuous high-speed (>2 kHz) optical phase sampling during a single 100-ms wavelength tuning ramp allow precise and rapid reconstructions of the TM shape at varied resolutions (50 to 200 μm). This rapid acquisition of full-field displacements and shape is immune to slow disturbances introduced by breathing or heartbeat of live subjects. Knowledge of TM shape and displacements enables the estimation of surface normal displacements regardless of the orientation of the TM within the measurement system. The proposed method helps better define TM mechanics and provides TM structure and function information useful for the diagnosis of ear disease.

High-Speed Holographic Shape and Full-Field Displacement Measurements of the Tympanic Membrane in Normal and Experimentally Simulated Pathological Ears

“…The MWHI method has been verified to reach an accuracy of 0.045 mm and exhibited repeatable measurements with 17 μm accuracy on a (National Institute of Standards and Technology) traceable ball and artificial samples [ 31 ]. This paper presents the results of TM shape measurements from three post-mortem human temporal bones (TB3, TB4, and TB5) under different manipulations.…”

“…As described in [31], the MWHI method has been designed to sweep the synthetic wavelength allowing multiple measurements of shape in a short time. A tunable laser operating in open loop mode is used as a coherent light source, and continual wavelength tuning is achieved by changing the laser cavity length while keeping the number of waves in the cavity constant [38].…”

“…We have previously developed and implemented the HDH method based on correlation interferometry that could “instantaneously” measure the full-field-of-view (>200,000 points at 67,200 camera frame rate) transient displacements of the TM in response to impulsive acoustical and mechanical excitations [ 24 , 28 , 30 ]. As described in [ 31 ], the HDH uses Pearson’s correlation to locate two reference frames I ref and I ref +π/2 with 0 and π/2 prior to the sample’s excitation based on Equation (3) . The zero-order terms of all correlated kernels are removed by summation of reference frames that are within one period of optical phase shifts.…”

“…The schematic diagram of the HDH system used in this study is shown in Figure 1 . The details of the HDH system and acoustic measurement setups can be found in our previous publication [ 31 ]. Three fresh non-fixed human postmortem temporal bones were used for the measurements.…”

“…The high-speed digital holographic system we developed measures the TM's shape and transient acoustic-induced displacement nearly instantaneously (<200 ms) with a spatial shape resolution of 50-200 µm and a displacement resolution of 15 nm at an acquisition rate of 67,200 Hz [24,28,[30][31][32]. This high-speed system takes into account the holographic-sensitivity vector variations due to the sample's shape and optical configuration to derive the true surface normal motion of the TM.…”