Optimization of a lensless digital holographic otoscope system for transient measurements of the human tympanic membrane

Abstract

In this paper, we propose a multi-pulsed double exposure (MPDE) acquisition method to quantify in full-field-of-view the transient (i.e., >10 kHz) acoustically induced nanometer scale displacements of the human tympanic membrane (TM or eardrum). The method takes advantage of the geometrical linearity and repeatability of the TM displacements to enable high-speed measurements with a conventional camera (i.e., <20 fps). The MPDE is implemented on a previously developed digital holographic system (DHS) to enhance its measurement capabilities, at a minimum cost, while avoiding constraints imposed by the spatial resolutions and dimensions of high-speed (i.e., >50 kfps) cameras. To our knowledge, there is currently no existing system to provide such capabilities for the study of the human TM. The combination of high temporal (i.e., >50 kHz) and spatial (i.e., >500k data points) resolutions enables measurements of the temporal and frequency response of all points across the surface of the TM simultaneously. The repeatability and accuracy of the MPDE method are verified against a Laser Doppler Vibrometer (LDV) on both artificial membranes and ex-vivo human TMs that are acoustically excited with a sharp (i.e., <100 μs duration) click. The measuring capabilities of the DHS, enhanced by the MPDE acquisition method, allow for quantification of spatially dependent motion parameters of the TM, such as modal frequencies, time constants, as well as inferring local material properties.

Keywords: Acoustic-solid interaction; High-speed digital holography; Miniaturization engineering; Otology; Transient response; Tympanic membrane.

Fig. 1

Overview of the existingdigital holographic system (DHS)[1, 2].The DHS is composed of: (1) laser delivery system with temporal phase stepping,stroboscopic illuminationand fiber coupled output capabilities; (2)otoscope head (OH) forlensless digital holography and sound presentation capabilities;(3) a mechatronic otoscope positioner (MOP) that provides support for the OH during measurements; (4)and (5) control software and a I/O modulesfor user control of the excitation, illumination, and frame acquisition parameters

Fig. 2

Schematic showing the major modules of the stroboscopic acquisition hardware and softwareof the origina DHS, shown in Fig. 1. The components with in the I/O control module communicate with the sound presentation, laser delivery, and camera modules that are synchronized by the stimulus generator component serving as the master

Fig. 3

Timing diagrams of the acquisition of aset of double exposure holograms with the MPDE method: (a) synchronization between the reference hologram, acoustic excitationexcitation, and deformed hologram; (b) the minimumtime between exposures is achieved by setting the exposuretime of the camera to the minimum inter-frameintertime. Pulsed illumination for the reference state, Pnd of the first camera exposurewhile a pulsed illumination for the deformed reference state, P_REF, is applied at the end of the first camers exposure while a pulsed illumination for the deformed state, P_DEF, is applied at the beginning of the second exposure

Fig. 4

Timing diagrams of the acquisition of a set of multiple double exposure holograms with the MPDE method.The same procedure as the one depicted in Fig. 3 is applied multiple times. This allows for a sampling rate Representative values of major parameters are indicated independent of the camera frame rate. Representative values of major parameters are indicated

Fig. 5

Schematic showing the major modules of the MPDE acquisition hardware and software.The components within the I/O control module communicate with the sound presentation, laser delivery and, and camera modules that are synchronized by the digital output component serving as the master clock. Changes in the new control architecture of the MPDE relative to the original, Fig. 2, include the addition of adigital output component connected as a master clock to the camera interface, signal generator, analog input, and laser delivery module

Fig. 6

Experimental setup andsamplesutilized for validation of the MPDE: (a)schematic of the DHS setup thatincludes otoscopehead (OH) and sound presentation modulemodule;(b) cadaveric human TM sample;(c) circular latexmembrane.The manubrium of the TMin (b) is outlined with solid line also indicating theseparation betweenparstensa and pars flaccida

Fig. 7

Representative LDVmeasurementsmeasurements,in the time and frequency domains,demonstrating the repeatability oftheacoustically induced transient response ofthe latex membrane and the human TM.The graphs indicate the timewaveform and frequency domain transfer functionwith the maximum and minimum responseacrossthe surface ofeach sample at twoconsecutive measurements (solid and dotted line).Measured time waveformsshow >99 %correlation, and frequency domain transfer functionnd frequency domain transfer functions show <3 dB variation.Maximum instantaneous sound pressurewas 124dB SPL for the human TM and 108dB SPL for latex sample.The microphoneresponse(Mic.) is marked with a dashed line

Fig. 8

Representative transient measurements, in the time and frequency domains, demonstrating the repeatability of the MPDE acquisition in the DHS.The graphs indicate the time waveform and frequency domain trans ferfunction(TF) with the maximum and minimum responseacross the surface of each sample for three consecutive measurements (dotted lines) and their average (solid line). Measured time waveforms show >99% correlation, and frequency domain transfer functions show <5 dB variation. Maximum instant aneous soundpressure was 124 dBSPL for the human TM and 108dB SPL for latex sample

Fig. 9

Representative transient measurements demonstrating the accuracy of MPDE acquisition in the DHS versusLDV. Correlation of the velocity and displacementtime waveforms between DHS(solid line) and LDV (dotted line) is >96 %. Maximum instantaneous sound pressure level (SPL) was 124 dB SPL for the human TM and 108 dBSPL for latex sample. The microphone response (Mic.) is marked with a dashed line

Fig. 10

Representative displacement maps of the response of the human TM due to a 100 μs click at 124 dB SPL, recorded with the MPDE at 0.3-1.26ms (a-ad) after the beginning of the acoustic excitation. The outlines of the membrane and the manubrium are indicated with solid black linealso indicating the separation between parstensa and pars flaccida of the TM. The data indicate a 0.62μm p-p maximum displacement across the surface throughoutthe full duration of the transient response

Fig. 11

Spatial dependence of the motion parameters across the surface of the TM: (a) quantification of the time constant based on automatic decay envelope estimation and exponential fitting of the time waveform of the transient response at the umbo; (b) map of the time constant at each point on the surface of the TM indicatinga mean of 0.71ms; (c) power spectrum of the displacement transfer function (TF) at the umbo measured with DH (blue line) andLDV (red line); and(d) map of the dominant frequency at each point on the surface of the TM. Outline of the manubrium in (b) and (d)is indicated with a dashed line. Dashed lines in (c)refer to automatically determined modal frequencies indicating <5% difference between DHS and LDV