Tooth visualization in vowel production MR images for three-dimensional vocal tract modeling

Teeth are almost invisible in magnetic resonance imaging (MRI) because they lack free protons to magnetically react. In MRI-based studies on the vocal tract, the teeth must be visualized on the volume data obtained during articulation. To do so, varieties of techniques have been proposed, either covering the teeth by opaque materials or obtaining tooth images followed by their superimposition. In this article, a new method was proposed to visualize the teeth in vowel production MR images for the application of three-dimensional (3D) vocal tract modeling. 3D upper and lower jaw with the teeth was first extracted and reconstructed from static 3D-MRI data acquired during a simple 'tooth imaging' posture with minimal time and effort. The extracted 3D jaw with the teeth was superimposed onto the vowel production MRI volume three-dimensionally by using the dental pulps as volume-based landmarks to minimize fitting errors due to varied head positions across the scans. The effectiveness of the proposed method was demonstrated not only by the subjective opinions but also by the objective evaluation. The results show that the teeth are successfully and accurately superimposed onto the vowel production MR images. Also, the reconstructed 3D vocal tract models are observed with the bilateral interdental spaces after tooth superimposition. The proposed method solves the MRI-specific problem of the lack of tooth images and contributes to accurate 3D vocal tract measurement and reconstruction.