Advanced otoacoustic emission detection techniques and clinical diagnostics applications

Transcription

1 Advanced otoacoustic emission detection techniques and clinical diagnostics applications Arturo Moleti Physics Department, University of Roma Tor Vergata, Roma, ITALY

2 Towards objective diagnostics of human hearing Otoacoustic emissions (OAEs) provide fast, objective, non-invasive hearing tests but Generation mechanisms are complicated clinical interpretation is difficult Ear canal acoustics must be accounted for calibration of stimulus and response is needed Technical advances help solving both problems

3 Cochlear Mechanics Greenwood tonotopic map p U Analogue of an electrical nonlinear transmission line (P V and d /dt I) Each frequency resonates at a different longitudinal place x

4 Cochlear Mechanics Cochlear mechanics equations predict forward and backward propagation of slow o(10m/s) transverse waves (traveling waves, or TW), coupling the transverse BM vibration to the differential fluid pressure Each frequency component is amplified approaching its resonant place, and locally absorbed, giving rise to frequency-selective acoustical perception.

5 Backward waves and OAEs Cochlear models also predict the generation of backward waves along the BM. These backward waves are transmitted through the middle ear and eventually measured in the ear canal as otoacoustic emissions (OAEs) OAEs are therefore a by-product of the cochlear activity, sensitive to the level of the local (at the OAE generation place) BM displacement

6 OAE generation mechanisms Nonlinear distortion Flat phase-frequency relation (in scale-invariant models) Reflection from cochlear roughness (randomly distributed fluctuations of the BM impedance) Phase is a rapidly-rotating function of frequency

7 OAE generation mechanisms DPOAE: primary generation mechanism is distortion (null delay). Reflection of the forward DP wave yields a rotating-phase component. SFOAE and TEOAE: reflections from different cochlear places yield different-delay components.

8 OAE acquisition techniques TEOAE evoked by transient stimulus (click or TB) DPOAE evoked by two tones f 1 and f 2 at their main intermodulation distortion frequency 2f 1 -f 2 SFOAE evoked by a single tone at the same frequency

9 Clinical relevance of OAEs (hearing sensitivity and OAE level) OAE levels are directly related to local BM displacement, and, consequently, to auditory perception level. Therefore, OAE levels are correlated with audiometric threshold. BUT Interference between components of different delay causes spectral oscillations affecting correlation with hearing threshold

10 Clinical relevance of OAEs (freq. resolution and OAE delay) OAE group delay (phase slope) is related to the frequency resolution of hearing (cochlear tuning). Therefore, OAE delay measurements yield objective tuning estimates BUT The presence of OAE components of different delay generated by different mechanisms (or at different places) affects tuning estimates

11 OAE stimulus and response calibration Conventional OAE probes measure only the pressure component of the acoustic field at the ear canal entrance for both stimulus and OAE response BUT we need to know: 1) the stimulus transmitted through the middle ear 2) the OAE level emitted by the eardrum These quantities are related to the pressure levels measured by the probe by unknown ear canal impedances. Due to the resonant nature of the ear canal (closed, almost cylindrical tube), the frequency dependence of these impedances is very sharp: large systematic errors (up to 20dB)

12 Solving the problems: 1) Time-frequency filtering Wavelet transform: representation of the OAE response in the timefrequency domain, where cochlear approximate scaling symmetry manifests itself (same physics along the lines, same place along the arrows) Delay and frequency are not the right coordinates for a scaling symmetric system, so filtering must follow the right ones

13 DPOAE wavelet unmixing Hyperbolic filtering regions following the delay-frequency relation implied by scaling symmetry are most effective for unmixing the two DPOAE components over a wide frequency range. Distortion 6ms IFT w 2ms IFT w

14 Solving the problems: 2) Intensimetric OAE detection p-v miniaturized probes are available (Microflown) that allow one to measure simultaneously and at the same place (typically the ear canal entrance) both pressure and velocity of the acoustic field. From p-v measurements, intensimetric quantities are easily computed (active intensity and power density) which directly provide the transmitted stimulus level and the OAE response level at the eardrum (Sisto et al, 2017) 6ms IFT window 2ms IFT window

15 Intensimetric OAE detection Measuring both pressure and velocity of the acoustic field allows one to calibrate the stimulus intensity transmitted to the middle ear and convert the OAE signal to that emitted by the middle ear 6ms IFT window 2ms IFT window

16 DPOAE Level (db) Intensimetric OAE detection An alternative ear canal calibration method (Charaziak and Shera, 2017), requiring accurate previous measurements of the Thèvenin parameters of the sound source, gives very similar results. Unfortunately, currently available p-v sources have high noise floor, so technical advances would be necessary "a) pv" "b) EPL Th" noise pv noise p 6ms IFT window 2ms IFT window Frequency (Hz)

17 Conclusions Objective estimates of the sensitivity and frequency resolution of human hearing can be obtained, based on accurate measurements and appropriate analysis of the level and phase of the OAE response. The accuracy of these OAE methods is improved by recent technological advances: time-frequency analysis techniques (unmixing OAE components associated with different physical phenomena) p-v acquisition systems (automatic calibration in the ear canal)