Elucidating ultrasound imaging of /ɹ/ tongue shapes using full-wave numerical simulations

Midsagittal ultrasound images depict much of the tongue surface from root to tip, providing important articulatory information about tongue shape. However, portions of the tip can be obscured due to shadowing from the mandible bone and sublingual airspace. Also, tongue curvature or grooving within the elevational beam width can cause ambiguous or double-edge artifacts that obfuscate the midsagittal tongue surface. Because ensuing misinterpretations of tongue shape may negatively affect clinical and scientific assessments of speakers' articulation, this study characterizes causes of imaging artifacts and validates best practices for interpretation. Segmentations of /ɹ/ tongue shapes from magnetic resonance images (MRI) were compared to simulated ultrasound images, obtained by modeling full-wave acoustic wave propagation through segmented tissue models using the k-Wave toolbox. Acoustic parameters were adjusted to allow for accurate yet efficient simulations. The extent of tongue tip visibly missing from simulated images varied significantly across categories of /ɹ/ tongue shapes. Ambiguous edge artifacts occurred in 85% of parasagittal images, supporting the guideline to check midsagittal transducer placement when ambiguous edges are observed. The ground-truth tongue surface corresponded to local brightness maxima within the visible tongue contour. For 92% of ambiguous edges, the most proximal brightness maximum corresponded to the midsagittal tongue surface.