Automatic tongue surface extraction from three-dimensional ultrasound vocal tract images

Three-dimensional (3D/4D) ultrasound (US) imaging of the tongue has emerged as a useful instrument for articulatory studies. However, extracting quantitative measurements of the shape of the tongue surface remains challenging and time-consuming. In response to these challenges, this paper documents and evaluates the first automated method for extracting tongue surfaces from 3D/4D US data. The method draws on established methods in computer vision, and combines image phase symmetry measurements, eigen-analysis of the image Hessian matrix, and a fast marching method for surface evolution towards the automatic detection of the sheet-like surface of the tongue amidst noisy US data. The method was tested on US recordings from eight speakers and the resulting automatically extracted tongue surfaces were generally found to lie within 1 to 2 mm from their corresponding manually delineated surfaces in terms of mean-sum-of-distances error. Further experiments demonstrate that the accuracy of 2D midsagittal tongue contour extraction is also improved using 3D data and methods. This is likely because the additional information afforded by 3D US compared to 2D US images strongly constrains the possible location of the midsagittal contour. Thus, the proposed method seems appropriate for immediate practical use in the analysis of 3D/4D US recordings of the tongue.