Improved imaging of lingual articulation using real-time multislice MRI

Abstract

Purpose: To develop a real-time imaging technique that allows for simultaneous visualization of vocal tract shaping in multiple scan planes, and provides dynamic visualization of complex articulatory features.

Materials and methods: Simultaneous imaging of multiple slices was implemented using a custom real-time imaging platform. Midsagittal, coronal, and axial scan planes of the human upper airway were prescribed and imaged in real-time using a fast spiral gradient-echo pulse sequence. Two native speakers of English produced voiceless and voiced fricatives /f/-/v/, /θ/-/ð/, /s/-/z/, /∫/- in symmetrical maximally contrastive vocalic contexts /a_a/, /i_i/, and /u_u/. Vocal tract videos were synchronized with noise-cancelled audio recordings, facilitating the selection of frames associated with production of English fricatives.

Results: Coronal slices intersecting the postalveolar region of the vocal tract revealed tongue grooving to be most pronounced during fricative production in back vowel contexts, and more pronounced for sibilants /s/-/z/ than for /∫/-. The axial slice best revealed differences in dorsal and pharyngeal articulation; voiced fricatives were observed to be produced with a larger cross-sectional area in the pharyngeal airway. Partial saturation of spins provided accurate location of imaging planes with respect to each other.

Conclusion: Real-time MRI of multiple intersecting slices can provide valuable spatial and temporal information about vocal tract shaping, including details not observable from a single slice.

Figure 1

Illustration of the proposed acquisition and reconstruction scheme in real-time MRI of the upper airway during speech. (a) Slice and spiral-interleaf order is shown. Slice 1 (yellow), slice 2 (gray), and slice 3 (blue) are acquired at every TR. Spiral interleaf index is indicated by number within each block. (b) 4-interleaf spiral trajectory with the index at the end of the spiral interleaf. 13- and 9-interleaf spirals were used in the in-vivo experiments. (c) Example midsagittal, coronal, and axial slice locations for imaging of the vocal tract. (d) Frame reconstructions for slice 1, 2, and 3. Sliding window reconstruction is used to increase frame rates.

Figure 2

Spatial-temporal resolution tradeoff. Interleaved spiral trajectories were designed with a field-of-view (FOV) of 20 cm and readout duration of approximately 2.5 msec. Spatial and temporal resolutions indicated by the red and black dots were used for time-interleaved imaging of 2 and 3 slices, respectively.

Figure 3

Example midsagittal/axial frames captured using Protocol 1 for Subject 1 during (a) articulation of /ʃ/ in the front vowel context /i_i/ and (b) interspeech pause. The axial slice provides additional vocal tract information such as the cross-sectional area in the pharyngeal airway and grooving of the tongue (see the solid arrow in (a)). Partially saturated spins (see the hollow arrows in (b)) clearly indicate where one slice is located with respect to the other slice. See also Movie1 in the supplementary material.

Figure 4

Two-slice frames captured from Subject 1 using Protocol 1 during intervocalic productions of English sibilant fricatives. Top row: voiceless /ʃ/. Bottom row: voiced /ʒ/. The images from the axial slice clearly reveal increased areas in the pharyngeal cross-sectional airway in voiced fricative /ʒ/ (see arrows). For other voiceless/voiced fricative pairs, see Movie 1 in the supplementary material.

Figure 5

Three-slice frames captured from Subject 2 using Protocol 2 during intervocalic voiceless fricative productions. Top row: labiodental fricative; 2^nd row: dental fricative; 3^rd row: alveolar fricative; Bottom row: post-alveolar fricative. Each scan plane provides unique vocal tract shaping information that is not available from the other scan planes. For example, the midsagittal slice shows the constriction locations (see the dashed arrows), the coronal slice shows the degree of tongue grooving/doming (see the hollow arrows), and the axial slice shows the degree of aperture and shaping of the pharyngeal airway (see the solid arrows). See also Movie 2 in the supplementary material.

Figure 6

Representative midsagittal (left column), coronal (center), and axial (right) frames captured using Protocol 2 during the production /asa/ by Subject 2. Time evolution is from top to bottom.