Ivo Dobrev

Digital holographic measurements of shape and three-dimensional sound-induced displacements of tympanic membrane

Abstract. Acoustically induced vibrations of the tympanic membrane (TM) play a primary role in the hearing process, in that these motions are the initial mechanical response of the ear to airborne sound. Characterization of the shape and three-dimensional (3-D) displacement patterns of the TM is a crucial step to a better understanding of the complicated mechanics of sound reception by the ear. Sound-induced 3-D displacements of the TM are estimated from shape and one-dimensional displacements measured in cadaveric chinchillas using a lensless dual-wavelength digital holography system (DWDHS). The DWDHS consists of laser delivery, optical head, and computing platform subsystems. Shape measurements are performed in double-exposure mode with the use of two wavelengths of a tunable laser, while nanometer-scale displacements are measured along a single sensitivity direction with a constant wavelength. Taking into consideration the geometrical and dimensional constrains imposed by the anatomy of the TM, we combine principles of thin-shell theory together with displacement measurements along a single sensitivity vector and TM surface shape to extract the three principal components of displacement in the full-field-of-view. We test, validate, and identify limitations of this approach via the application of finite element method to artificial geometries.

Full-field transient vibrometry of the human tympanic membrane by local phase correlation and high-speed holography

Abstract. Understanding the human hearing process would be helped by quantification of the transient mechanical response of the human ear, including the human tympanic membrane (TM or eardrum). We propose a new hybrid high-speed holographic system (HHS) for acquisition and quantification of the full-field nanometer transient (i.e., >10  kHz) displacement of the human TM. We have optimized and implemented a 2+1 frame local correlation (LC) based phase sampling method in combination with a high-speed (i.e., >40  K fps) camera acquisition system. To our knowledge, there is currently no existing system that provides such capabilities for the study of the human TM. The LC sampling method has a displacement difference of <11  nm relative to measurements obtained by a four-phase step algorithm. Comparisons between our high-speed acquisition system and a laser Doppler vibrometer indicate differences of <10  μs. The high temporal (i.e., >40  kHz) and spatial (i.e., >100  k data points) resolution of our HHS enables parallel measurements of all points on the surface of the TM, which allows quantification of spatially dependent motion parameters, such as modal frequencies and acoustic delays. Such capabilities could allow inferring local material properties across the surface of the TM.

Miniaturization as a key factor to the development and application of advanced metrology systems

Recent technological advances of miniaturization engineering are enabling the realization of components and systems with unprecedented capabilities. Such capabilities, which are significantly beneficial to scientific and engineering applications, are impacting the development and the application of optical metrology systems for investigations under complex boundary, loading, and operating conditions. In this paper, and overview of metrology systems that we are developing is presented. Systems are being developed and applied to high-speed and high-resolution measurements of shape and deformations under actual operating conditions for such applications as sustainability, health, medical diagnosis, security, and urban infrastructure. Systems take advantage of recent developments in light sources and modulators, detectors, microelectromechanical (MEMS) sensors and actuators, kinematic positioners, rapid prototyping fabrication technologies, as well as software engineering.

A MEMS Condenser Microphone-Based Intracochlear Acoustic Receiver

Goal: Intracochlear sound pressure (ICSP) measurements are limited by the small dimensions of the human inner ear and the requirements imposed by the liquid medium. A robust intracochlear acoustic receiver (ICAR) for repeated use with a simple data acquisition system that provides the required high sensitivity and small dimensions does not yet exist. The work described in this report aims to fill this gap and presents a new microelectromechanical systems (MEMS) condenser microphone (CMIC)-based ICAR concept suitable for ICSP measurements in human temporal bones. Methods: The ICAR head consisted of a passive protective diaphragm (PD) sealing the MEMS CMIC against the liquid medium, enabling insertion into the inner ear. The components of the MEMS CMIC-based ICAR were expressed by a lumped element model (LEM) and compared to the performance of successfully fabricated ICARs. Results: Good agreement was achieved between the LEM and the measurements with different sizes of the PD. The ICSP measurements in a human cadaver temporal bone yielded data in agreement with the literature. Conclusion: Our results confirm that the presented MEMS CMIC-based ICAR is a promising technology for measuring ICSP in human temporal bones in the audible frequency range. Significance: A sensor for evaluation of the biomechanical hearing process by quantification of ICSP is presented. The concept has potential as an acoustic receiver in totally implantable cochlear implants.

Intracranial Pressure and Promontory Vibration With Soft Tissue Stimulation in Cadaveric Human Whole Heads.

Hypothesis: Intracranial pressure and skull vibrations are correlated and depend on the stimulation position and frequency. Background: A hearing sensation can be elicited by vibratory stimulation on the skin covered skull, or by stimulation on soft tissue such as the neck. It is not fully understood whether different stimulation sites induce the skull vibrations responsible for the perception or whether other transmission pathways are dominant. The aim of this study was to assess the correlation between intracranial pressure and skull vibration measured on the promontory for stimulation to different sites on the head. Methods: Measurements were performed on four human cadaver heads. A bone conduction hearing aid was held in place with a 5-Newton steel headband at four locations (mastoid, forehead, eye, and neck). While stimulating in the frequency range of 0.3 to 10 kHz, acceleration of the cochlear promontory was measured with a Laser Doppler Vibrometer, and intracranial pressure at the center of the head with a hydrophone. Results: Promontory acceleration and intracranial pressure was measurable for all stimulation sites. The ratios were comparable between all stimulation sites for frequencies below 2 kHz. Conclusion: These findings indicate that both promontory acceleration and intracranial pressure are involved for stimulation on the sites investigated. The transmission pathway of sound energy is comparable for the four stimulation sites.

Influence of stimulation position on the sensitivity for bone conduction hearing aids without skin penetration.

Abstract Objective: This study explores the influence of stimulation position on bone conduction (BC) hearing sensitivity with a BC transducer attached using a headband. Design: (1) The cochlear promontory motion was measured in cadaver heads using laser Doppler vibrometry while seven different positions around the pinna were stimulated using a bone anchored hearing aid transducer attached using a headband. (2) The BC hearing thresholds were measured in human subjects, with the bone vibrator Radioear B71 attached to the same seven stimulation positions. Study sample: Three cadaver heads and twenty participants. Results: Stimulation on a position superior-anterior to the pinna generated the largest promontory motion and the lowest BC thresholds. Stimulations on the positions superior to the pinna, the mastoid, and posterior-inferior to the pinna showed similar magnitudes of promontory motion and similar levels of BC thresholds. Conclusion: Stimulations on the regions superior to the pinna, the mastoid, and posterior-inferior to the pinna provide stable BC transmission, and are insensitive to small changes of the stimulation position. Therefore it is reliable to use the mastoid to determine BC thresholds in clinical audiometry. However, stimulation on a position superior-anterior to the pinna provides more efficient BC transmission than stimulation on the mastoid.

Implementation and Evaluation of Single Frame Recording Techniques for Holographic Measurements of the Tympanic Membrane In-Vivo

We are developing an advanced computer-controlled digital holographic system (DHS) with the ability to measure both shape and acoustically induced deformations of the tympanic membrane of several species, including humans. The DHS has been deployed in the clinic and is currently being optimized for in-vivo measurements.

Study of the Transient Response of Tympanic Membranes Under Acoustic Excitation

Characterization of the transient response of the human Tympanic Membrane (TM) subjected to impulse acoustic excitation is important in order to further understand the mechanics of hearing. In this paper, we present results of our initial investigations of the transient response of an artificial fully-constrained circular membrane as a simplified model of the TM. Two different optical methods used in our investigations are Laser Doppler Vibrometery (LDV) and Pulsed Double-Exposure Digital Holography (PDEDH) for single-point and full-field-of-view measurements of displacements, respectively. Applying Hilbert Transformation methods to the measured displacements allows determination of the transient characteristics of the membrane, including damping ratios and time constants, which are also computed and compared with corresponding FEM models. We expect to use this method in the investigation of the transient response of TM of specific species.

Design of a positioning system for a holographic otoscope

Current ear examination procedures provide mostly qualitative information which results in insufficient or erroneous description of the patients hearing. Much more quantitative and accurate results can be achieved with a holographic otoscope system currently under development. Various ways of accurate positioning and stabilization of the system in real-life conditions are being investigated by this project in an attempt to bring this new technology to the hospitals and clinics, in order to improve the quality of the treatments and operations of the human ear. The project is focused at developing a mechatronic system capable of positioning the holographic otoscope to the patients ear and maintaining its relative orientation during the examination. The system will be able to be guided by the examiner, but it will maintain the chosen position automatically. To achieve that, various trajectories are being measured for existing otoscopes being guided by doctors in real medical conditions. Based on that, various kinematic configurations are to be synthesized and their stability and accuracy will be simulated and optimized with FEA. For simplification, the mechanism will contain no actuators, but only adjustable friction elements in a haptic feedback control system. This renders the positioning system safe and easily applicable to current examination rooms. Other means of stabilization of the system are being investigated such as custom designed packaging of all of the otoscope subsystems, interferometrically compensating for the heartbeat induced vibration of the tympanic membrane as well as methods for monitoring and active response to the motion of the patients head.

Sound wave propagation on the human skull surface with bone conduction stimulation

Background Bone conduction (BC) is an alternative to air conduction to stimulate the inner ear. In general, the stimulation for BC occurs on a specific location directly on the skull bone or through the skin covering the skull bone. The stimulation propagates to the ipsilateral and contralateral cochlea, mainly via the skull bone and possibly via other skull contents. This study aims to investigate the wave propagation on the surface of the skull bone during BC stimulation at the forehead and at ipsilateral mastoid. Methods Measurements were performed in five human cadaveric whole heads. The electro‐magnetic transducer from a BCHA (bone conducting hearing aid), a Baha® Cordelle II transducer in particular, was attached to a percutaneously implanted screw or positioned with a 5‐Newton steel headband at the mastoid and forehead. The Baha transducer was driven directly with single tone signals in the frequency range of 0.25–8 kHz, while skull bone vibrations were measured at multiple points on the skull using a scanning laser Doppler vibrometer (SLDV) system and a 3D LDV system. The 3D velocity components, defined by the 3D LDV measurement coordinate system, have been transformed into tangent (in‐plane) and normal (out‐of‐plane) components in a local intrinsic coordinate system at each measurement point, which is based on the cadaver heads shape, estimated by the spatial locations of all measurement points. Results Rigid‐body‐like motion was dominant at low frequencies below 1 kHz, and clear transverse traveling waves were observed at high frequencies above 2 kHz for both measurement systems. The surface waves propagation speeds were approximately 450 m/s at 8 kHz, corresponding trans‐cranial time interval of 0.4 ms. The 3D velocity measurements confirmed the complex space and frequency dependent response of the cadaver heads indicated by the 1D data from the SLDV system. Comparison between the tangent and normal motion components, extracted by transforming the 3D velocity components into a local coordinate system, indicates that the normal component, with spatially varying phase, is dominant above 2 kHz, consistent with local bending vibration modes and traveling surface waves. Conclusion Both SLDV and 3D LDV data indicate that sound transmission in the skull bone causes rigid‐body‐like motion at low frequencies whereas transverse deformations and travelling waves were observed above 2 kHz, with propagation speeds of approximately of 450 m/s at 8 kHz.