Quantifying ear-canal geometry with multiple computer-assisted tomographic scans

Abstract

Several audiological tests require knowledge of the sound-pressure spectrum at the eardrum. However, microphone readings are typically made at another, more-accessible position in the auditory canal. Recordings are then "adjusted" to the plane of the eardrum via mathematical models of the ear canal and eardrum. As bandwidths of audiological instruments have increased, ear-canal models have, by necessity, become more precise geometrically. Reported herein is a noninvasive procedure for acquiring geometry of the ear canal in fine detail. The method employs a computer-assisted tomographic (CAT) scanner in two steps to make radiographic images of parasagittal cross sections at uniform intervals along the lateral length of the canal. Accuracy was evaluated by comparing areas of cross sections appearing in radiographic images of a cadaver ear canal to cross sectional areas of corresponding michrotome slices of an injection mold of the same canal. Percent differences between these two areas had a mean value of 9.65% for 26 different cross sections of the one ear canal studied. Ear canal volume estimated from the CAT images was 6.12% different from the estimated volume of the injection mold: an improvement over the reported 39% maximum error of conventional acoustic volume measurements.

Inverse solution of ear-canal area function from reflectance

“…Comparison to the earlier studies of Stinson and Lawton (1989) showed only fair agreement, while comparison to Johansen (1975) showed good agreement. Although some agreement was demonstrated between the measurements of Johansen (1975), Stinson and Lawton (1989), and Egolf et al (1993), the disadvantage of these three studies is that the techniques are not suitable for application to living humans. Also, the shape of the canal may change during rigor mortis.…”

“…However, inter-subject variability was observed in the SWR. Egolf et al (1993) measured the area function of ear canals of human cadavers using a computer-assisted tomographic (CAT) scanner and silicone-injection molds method. They demonstrated good agreement between the two measurements.…”

The Shape of the Osseous External Auditory Canal and Its Relationship to Chronic External Otitis

“…The shape of the osseous external auditory canal (OEAC) has often been suggested in literature to be an etiologic factor in (C)OE (3,5Y9). Although several classifications of the shape of the OEAC have been proposed (10,11), no attempt to correlate a certain classification to pathology has been published to date. Clinical observations suggest that certain shapes are likely to be involved in the development of a chronic inflammation as they hamper proper cleaning of the OEAC.…”

Anthropometry of external auditory canal by non-contactable measurement

“…Zemplenyi et al (1985) developed a novel approach that uses the optical method to measure the length between the opening of the ear canal and the eardrum (Zemplenyi et al, 1985). Egolf et al (1993) used computer-aided tomography (CAT) to scan ear canals of cadavers to measure the volume of the external ear canal and compared the results with the those obtained using the injection measurement method, and observed a 6.12% difference between the two methods (Egolf et al, 1993). Shahnaz and Davies (2006) used tympanometry to measure tympanometric peak pressure and ear canal volume (ECV) and examine differences in the tympanum figure measurement values between Chinese people and Caucasians.…”