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1. 3D dynamic MRI of the vocal tract during natural speech

   https://doi.org/10.1002/mrm.27570  

   Lim, Yongwan ; Zhu, Yinghua ; Lingala, Sajan Goud ; Byrd, Dani ; Narayanan, Shrikanth ; Nayak, Krishna Shrinivas ( November 2018 , Magnetic Resonance in Medicine)

   To develop and evaluate a technique for 3D dynamic MRI of the full vocal tract at high temporal resolution during natural speech.

   We demonstrate 2.4 × 2.4 × 5.8 mm³spatial resolution, 61‐ms temporal resolution, and a 200 × 200 × 70 mm³FOV. The proposed method uses 3D gradient‐echo imaging with a custom upper‐airway coil, a minimum‐phase slab excitation, stack‐of‐spirals readout, pseudo golden‐angle view order ink_x‐k_y, linear Cartesian order alongk_z, and spatiotemporal finite difference constrained reconstruction, with 13‐fold acceleration. This technique is evaluated using in vivo vocal tract airway data from 2 healthy subjects acquired at 1.5T scanner, 1 with synchronized audio, with 2 tasks during production of natural speech, and via comparison with interleaved multislice 2D dynamic MRI.

   This technique captured known dynamics of vocal tract articulators during natural speech tasks including tongue gestures during the production of consonants “s” and “l” and of consonant–vowel syllables, and was additionally consistent with 2D dynamic MRI. Coordination of lingual (tongue) movements for consonants is demonstrated via volume‐of‐interest analysis. Vocal tract area function dynamics revealed critical lingual constriction events along the length of the vocal tract for consonants and vowels.

   We demonstrate feasibility of 3D dynamic MRI of the full vocal tract, with spatiotemporal resolution adequate to visualize lingual movements for consonants and vocal tact shaping during natural productions of consonant–vowel syllables, without requiring multiple repetitions.

    

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2. Intermittently tagged real‐time MRI reveals internal tongue motion during speech production

   https://doi.org/10.1002/mrm.27745  

   Chen, Weiyi ; Byrd, Dani ; Narayanan, Shrikanth ; Nayak, Krishna S. ( March 2019 , Magnetic Resonance in Medicine)

   To demonstrate a tagging method compatible with RT‐MRI for the study of speech production.

   Tagging is applied as a brief interruption to a continuous real‐time spiral acquisition. Tagging can be initiated manually by the operator, cued to the speech stimulus, or be automatically applied with a fixed frequency. We use a standard 2D 1‐3‐3‐1 binomial SPAtial Modulation of Magnetization (SPAMM) sequence with 1 cm spacing in both in‐plane directions. Tag persistence in tongue muscle is simulated and validated in vivo. The ability to capture internal tongue deformations is tested during speech production of American English diphthongs in native speakers.

   We achieved an imaging window of 650‐800 ms at 1.5T, with imaging signal to noise ratio ≥ 17 and tag contrast to noise ratio ≥ 5 in human tongue, providing 36 frames/s temporal resolution and 2 mm in‐plane spatial resolution with real‐time interactive acquisition and view‐sharing reconstruction. The proposed method was able to capture tongue motion patterns and their relative timing with adequate spatiotemporal resolution during the production of American English diphthongs and consonants.

   Intermittent tagging during real‐time MRI of speech production is able to reveal the internal deformations of the tongue. This capability will allow new investigations of valuable spatiotemporal information on the biomechanics of the lingual subsystems during speech without reliance on binning speech utterance repetition.

    

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3. Dynamic off‐resonance correction for spiral real‐time MRI of speech

   https://doi.org/10.1002/mrm.27373  

   Lim, Yongwan ; Lingala, Sajan Goud ; Narayanan, Shrikanth S. ; Nayak, Krishna S. ( July 2018 , Magnetic Resonance in Medicine)

   To improve the depiction and tracking of vocal tract articulators in spiral real‐time MRI (RT‐MRI) of speech production by estimating and correcting for dynamic changes in off‐resonance.

   The proposed method computes a dynamic field map from the phase of single‐TE dynamic images after a coil phase compensation where complex coil sensitivity maps are estimated from the single‐TE dynamic scan itself. This method is tested using simulations and in vivo data. The depiction of air–tissue boundaries is evaluated quantitatively using a sharpness metric and visual inspection.

   Simulations demonstrate that the proposed method provides robust off‐resonance correction for spiral readout durations up to 5 ms at 1.5T. In ‐vivo experiments during human speech production demonstrate that image sharpness is improved in a majority of data sets at air–tissue boundaries including the upper lip, hard palate, soft palate, and tongue boundaries, whereas the lower lip shows little improvement in the edge sharpness after correction.

   Dynamic off‐resonance correction is feasible from single‐TE spiral RT‐MRI data, and provides a practical performance improvement in articulator sharpness when applied to speech production imaging.

    

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4. Deblurring for spiral real‐time MRI using convolutional neural networks

   https://doi.org/10.1002/mrm.28393  

   Lim, Yongwan ; Bliesener, Yannick ; Narayanan, Shrikanth ; Nayak, Krishna S. ( July 2020 , Magnetic Resonance in Medicine)

   To develop and evaluate a fast and effective method for deblurring spiral real‐time MRI (RT‐MRI) using convolutional neural networks.

   We demonstrate a 3‐layer residual convolutional neural networks to correct image domain off‐resonance artifacts in speech production spiral RT‐MRI without the knowledge of field maps. The architecture is motivated by the traditional deblurring approaches. Spatially varying off‐resonance blur is synthetically generated by using discrete object approximation and field maps with data augmentation from a large database of 2D human speech production RT‐MRI. The effect of off‐resonance range, shift‐invariance of blur, and readout durations on deblurring performance are investigated. The proposed method is validated using synthetic and real data with longer readouts, quantitatively using image quality metrics and qualitatively via visual inspection, and with a comparison to conventional deblurring methods.

   Deblurring performance was found superior to a current autocalibrated method for in vivo data and only slightly worse than an ideal reconstruction with perfect knowledge of the field map for synthetic test data. Convolutional neural networks deblurring made it possible to visualize articulator boundaries with readouts up to 8 ms at 1.5 T, which is 3‐fold longer than the current standard practice. The computation time was 12.3 ± 2.2 ms per frame, enabling low‐latency processing for RT‐MRI applications.

   Convolutional neural networks deblurring is a practical, efficient, and field map‐free approach for the deblurring of spiral RT‐MRI. In the context of speech production imaging, this can enable 1.7‐fold improvement in scan efficiency and the use of spiral readouts at higher field strengths such as 3 T.

    

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5. Improved 3D real‐time MRI of speech production

   https://doi.org/10.1002/mrm.28651  

   Zhao, Ziwei ; Lim, Yongwan ; Byrd, Dani ; Narayanan, Shrikanth ; Nayak, Krishna S. ( January 2021 , Magnetic Resonance in Medicine)

   To provide 3D real‐time MRI of speech production with improved spatio‐temporal sharpness using randomized, variable‐density, stack‐of‐spiral sampling combined with a 3D spatio‐temporally constrained reconstruction.

   We evaluated five candidate (k,t) sampling strategies using a previously proposed gradient‐echo stack‐of‐spiral sequence and a 3D constrained reconstruction with spatial and temporal penalties. Regularization parameters were chosen by expert readers based on qualitative assessment. We experimentally determined the effect of spiral angle increment andk_ztemporal order. The strategy yielding highest image quality was chosen as the proposed method. We evaluated the proposed and original 3D real‐time MRI methods in 2 healthy subjects performing speech production tasks that invoke rapid movements of articulators seen in multiple planes, using interleaved 2D real‐time MRI as the reference. We quantitatively evaluated tongue boundary sharpness in three locations at two speech rates.

   The proposed data‐sampling scheme uses a golden‐angle spiral increment in thek_x−k_yplane and variable‐density, randomized encoding alongk_z. It provided a statistically significant improvement in tongue boundary sharpness score (P < .001) in the blade, body, and root of the tongue during normal and 1.5‐times speeded speech. Qualitative improvements were substantial during natural speech tasks of alternating high, low tongue postures during vowels. The proposed method was also able to capture complex tongue shapes during fast alveolar consonant segments. Furthermore, the proposed scheme allows flexible retrospective selection of temporal resolution.

   We have demonstrated improved 3D real‐time MRI of speech production using randomized, variable‐density, stack‐of‐spiral sampling with a 3D spatio‐temporally constrained reconstruction.

    

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