A preliminary investigation of four-dimensional ultrasound for evaluation of middle ear ossicles: an in vitro study

Abstract

Objective: Temporal bone imaging in children has several inherent limitations. Computed tomography has the disadvantage of ionizing radiation, possible sedation, cost and accessibility. Magnetic resonance imaging has most of these disadvantages, with the exception of radiation, and provides bone images of limited resolution. Recent advances in ultrasound have led to its increased application in numerous medical fields. The purpose of this study was to investigate the ability of four-dimensional ultrasound (4DUS) to image middle ear ossicles in vitro and determine if this technology should be adapted for future clinical use.

Methods: Thirty cadaveric ossicles (10 malleus, 10 incus, and 10 stapes) were randomized and measured by two evaluators under a microscope. The ossicles were then immersed in a cold water bath and imaged, randomized, and measured using four-dimensional ultrasound by the same two evaluators. A separate cadaveric temporal bone, modified to allow the ultrasound probe to rest on the tympanic membrane, was imaged to visualize the ossicles in situ and evaluate whether or not the tympanic membrane and malleus would impede visualization of more medial structures.

Results: Microscopic measurements were: malleus (h=8.0 mm+/-0.32, w=2.7 mm+/-0.20), incus (h=6.8 mm+/-0.41, w=5.3 mm+/-0.46), stapes (h=3.5 mm+/-0.34, w=2.4 mm+/-0.17). Inter-rater reliability was 0.8. Measurements were in agreement with previously published values. Ultrasound measurements were: malleus (h=8.0 mm+/-0.51, w=2.9 mm+/-0.27), incus (h=6.8 mm+/-0.49, w=5.5 mm+/-0.42), stapes (h=3.6 mm+/-0.41, w=2.5 mm+/-0.19). Inter-rater reliability was 0.7. Mean intra-class correlation coefficient for microscopic and ultrasound measurements was 0.7. Images of the ossicular chain in continuity in the temporal bone specimen were not as clear as images of individual ossicles that were disarticulated and imaged under water.

Conclusions: 4DUS provides reasonable images of ossicles disarticulated and mounted in underwater medium. However, images of the intact ossicular chain in a modified cadaveric temporal bone were not as clear, making interpretation difficult. Further investigation into the development of a thinner ultrasound probe that can pass through the external auditory canal and into overcoming limitations of air in the middle ear cleft are warranted. This could allow for a clinically relevant, faster, lower cost and lower risk alternative to current imaging techniques.