Describing the geometry of the ear canal from CT scans

Description of the ear canal’s geometry is important in understanding how sound flows from the external ear through the auditory processing system. However, there are relatively few studies that directly describe ear canal geometry, and most measures come from cadaver and not live ears. Wideband acoustic immittance (WAI) measures are a potential non-invasive diagnostic tool for auditory pathologies. The two FDA approved systems for WAI measures (Mimosa Acoustics HearID and Interacoustics Titan) use an industry standard for ear canal area at the probe’s location (44 mm2 and 50 mm2 , respectively); these areas are substantially smaller than the average of recently measured values. In this work, a method is developed to measure the ear canal’s length and cross-sectional area from CT scans of 5 ears (3 subjects). Using the advanced multidimensional medical imaging application OsiriX, multi-planar reconstruction of CT images was used to measure length as a spline mapped through the center of the canal. Cross-sectional area was measured perpendicular to the canal, along its length. Novel and clear definitions were developed for the canal’s entrance and termination, and area measurements were made along the length in 1-2 mm increments. Large variations in area were found between subjects; left and right ears from the same subject were far more similar than ears from different subjects. In all ears, area generally increased in the lateral direction from the first bend to the entrance of the canal. At the first bend, three of the five ears measured had cross-sectional areas larger than the industry standard set by both HearID and Titan (85.6 mm2 , 68.7 mm2 , 76.6 mm2 , 37.5 mm2 , and 43.4 mm2 ). A better description of ear canal geometry can assist in determining the scale in which WAI measures are affected by intersubject variations in ear geometry. This work also developed a process for the conversion of ear canal airspace within CT images to 3D printable models that can be used in the future to make acoustical measurements to determine the effects of different ear canal geometries. The method described in this work has the potential for further development as a noninvasive procedure to understand the range of ear canal geometry and its effect on WAI measurements