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S25 Radiology
The accuracy of the cone beam CT in evaluating the size of the facial recess
Hiraumi H. , Suzuki R. , Yamamoto N. , Sakamoto T. , Ito J.
Kyoto University Graduate School of Medicine, Otolaryngology, Head and Neck Surgery, Kyoto, Japan
Introduction: The facial recess approach is the standard approach in the cochlear implantation surgery. The
size of the facial recess depends on the course of the chorda tympani. The chorda tympani branches from the
mastoid portion of the facial nerve, and the position of the bifurcation varies from case to case. In some cases,
the chorda tympani branches from the facial nerve at very superior portion. In that case, the posterior
tympanotomy can be challenging. The chorda tympani runs through a bony canal (posterior canaliculus), and the
posterior canaliculus can be detected using an X-ray microtomography (McManus et al., 2012), but the machine
is not for clinical use. In clinically available conventional computed tomography (CT), however, the posterior
canaliculus is not always detectable (Parlier-Cuau et al., 1998). Recently, cone beam CT has been used in a
clinical setting. The cone beam CT has higher resolution than the conventional high resolution CT, and multi
planar reconstruction can be obtained with ease. In this study, we explored the detectability and accuracy of
cone beam CT in the evaluation of the course of the chorda tympani.
Material and methods: A total of 12 human temporal bones are included in this study. All the temporal bone
received cone beam CT (Accuitomo, Morita, Japan). The acquired image was 3D reconstructed. First, the
mastoid portion of the facial nerve was detected, and the three dimensionally reconstructed image was rotated
around the mastoid portion of the facial nerve. With this technique, even the smallest branches are easily
detected. The branch was diagnosed as the posterior canaliculus when it can be followed to the pyramidal
process. The distance between the bifurcation and the tip of the short crus of the incus was measured. The
temporal bone was dissected afterwards, and the actual distance between the bifurcation and the tip of the short
crus of the incus was measured.
Results: In all the 12 temporal bones, the posterior canaliculus was detected with the above described
technique. In all specimens, the chorda tympani branched from the facial nerve within the temporal bone. In the
CT-based evaluation, the average distance from the bifurcation and the incus short crus was 12.7 mm (8.3 - 15.8
mm). The actual distance after dissection was 12.6 mm (8.2 - 16.4 mm). The largest difference between the
distances evaluated with the two procedures was 1.1 mm.
Conclusion: The cone beam CT is very useful in detecting the course of the chorda tympani within the temporal
bone. The measured distance is accurate enough to be used in the clinical setting.