Mouse middle-ear forward and reverse acoustics

The influence of tympanic-membrane orientation on acoustic ear-canal quantities: A finite-element analysis

Assuming plane waves, ear-canal acoustic quantities, collectively known as wideband acoustic immittance (WAI), are frequently used in research and in the clinic to assess the conductive status of the middle ear. Secondary applications include compensating for the ear-canal acoustics when delivering stimuli to the ear and measuring otoacoustic emissions. However, the ear canal is inherently non-uniform and terminated at an oblique angle by the conical-shaped tympanic membrane (TM), thus potentially confounding the ability of WAI quantities in characterizing the middle-ear status. This paper studies the isolated possible confounding effects of TM orientation and shape on characterizing the middle ear using WAI in human ears. That is, the non-uniform geometry of the ear canal is not considered except for that resulting from the TM orientation and shape. This is achieved using finite-element models of uniform ear canals terminated by both lumped-element and finite-element middle-ear models. In addition, the effects on stimulation and reverse-transmission quantities are investigated, including the physical significance of quantities seeking to approximate the sound pressure at the TM. The results show a relatively small effect of the TM orientation on WAI quantities, except for a distinct delay above 10 kHz, further affecting some stimulation and reverse-transmission quantities.

Finite-Element Modelling Based on Optical Coherence Tomography and Corresponding X-ray MicroCT Data for Three Human Middle Ears

PURPOSE

Optical coherence tomography (OCT) is an emerging imaging modality which is non-invasive, can be employed in vivo, and can record both anatomy and vibrations. The purpose here is to explore the application of finite-element (FE) modelling to OCT data.

METHODS

We recorded vibrations for three human cadaver middle ears using OCT. We also have X-ray microCT images from the same ears. Three FE models were built based on geometries obtained from the microCT images. The material properties and boundary conditions of the models were obtained from previously reported studies.

RESULTS

Tympanic-membrane (TM) vibration patterns were computed for the three models and compared with the patterns measured using OCT. Frequency responses were also computed for all three models for several locations in the middle ear and compared with the OCT displacements and with the literature. The three models were compared with each other in terms of geometry and function. Parameter sensitivity analyses were done and the results were compared among the models and with the literature. The simulated TM displacement patterns are qualitatively similar to the OCT results. The simulated displacements are closer to the OCT results for 500 Hz and 1 kHz but the differences are greater at 2 kHz.

CONCLUSION

This study provides an initial look at the combined use of OCT measurements and FE modelling based on subject-specific anatomy. The geometries and parameters of the existing FE models could be modified for individual patients in the future to help identify abnormalities in the middle ear.

Stimulus-frequency otoacoustic emissions and middle-ear pressure gains in a finite-element mouse model

For evoked otoacoustic emissions (OAEs), the stimulus and emission signals traverse the middle ear (ME) in forward and reverse directions, respectively. In this study, a fully coupled three-dimensional finite-element model of the mouse ear canal (EC), ME, and cochlea was used to calculate ME pressure gains, impedances, and reflectances at the EC-entrance and stapes-footplate-cochlear-fluid interfaces. The cochlear model incorporates a series of interdigitated Y-shaped structures sandwiched between the basilar membrane and reticular lamina, each comprised of a Deiters' cell, its phalangeal-process extension, and an outer hair cell (OHC). By introducing random perturbations to the OHC gains, stimulation-frequency otoacoustic emissions (SFOAEs) were generated. Raising the perturbation magnitude from 10% to 80% increased the SFOAE magnitude by up to 24 dB in the 10-30 kHz frequency range. Increasing or decreasing the stiffness of the stapes annular ligament and eardrum by a factor of 8 changed the SFOAEs by up to 30 dB, but the round-trip ME gain as measured could not account for this. A modified round-trip ME gain, with reflections removed at the EC-entrance and stapes-cochlea boundaries, eliminated a ±10 dB discrepancy and allowed ME changes to be quantitatively associated with changes in measured OAEs.