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1. Improved real‐time tagged MRI using REALTAG

   https://doi.org/10.1002/mrm.28144  

   Chen, Weiyi ; Lee, Nam Gyun ; Byrd, Dani ; Narayanan, Shrikanth ; Nayak, Krishna S. ( December 2019 , Magnetic Resonance in Medicine)

   To evaluate a novel method for real‐time tagged MRI with increased tag persistence using phase sensitive tagging (REALTAG), demonstrated for speech imaging.

   Tagging is applied as a brief interruption to a continuous real‐time spiral acquisition. REALTAG is implemented using a total tagging flip angle of 180° and a novel frame‐by‐frame phase sensitive reconstruction to remove smooth background phase while preserving the sign of the tag lines. Tag contrast‐to‐noise ratio of REALTAG and conventional tagging (total flip angle of 90°) is simulated and evaluated in vivo. The ability to extend tag persistence is tested during the production of vowel‐to‐vowel transitions by American English speakers.

   REALTAG resulted in a doubling of contrast‐to‐noise ratio at each time point and increased tag persistence by more than 1.9‐fold. The tag persistence was 1150 ms with contrast‐to‐noise ratio >6 at 1.5T, providing 2 mm in‐plane resolution, 179 frames/s, with 72.6 ms temporal window width, and phase sensitive reconstruction. The new imaging window is able to capture internal tongue deformation over word‐to‐word transitions in natural speech production.

   Tag persistence is substantially increased in intermittently tagged real‐time MRI by using the improved REALTAG method. This makes it possible to capture longer motion patterns in the tongue, such as cross‐word vowel‐to‐vowel transitions, and provides a powerful new window to study tongue biomechanics.

    

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2. 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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3. 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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4. 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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5. MR method for measuring microscopic histologic soft tissue textures

   https://doi.org/10.1002/mrm.28731  

   Sonn, Geoffrey A. ; Fan, Richard E. ; Kunder, Christian A. ; Gold, Garry E. ; James, Kristin M. ; Parker, Ian D. ; Carlson, Jean M. ; Cannizzaro, Scott M. ; James, Timothy W. ( February 2021 , Magnetic Resonance in Medicine)

   Provide a direct, non‐invasive diagnostic measure of microscopic tissue texture in the size scale between tens of microns and the much larger scale measurable by clinical imaging. This paper presents a method and data demonstrating the ability to measure these microscopic pathologic tissue textures (histology) in the presence of subject motion in an MR scanner. This size range is vital to diagnosing a wide range of diseases.

   MR micro‐Texture (MRµT) resolves these textures by a combination of measuring a targeted set of k‐values to characterize texture—as in diffraction analysis of materials, performing a selective internal excitation to isolate a volume of interest (VOI), applying a high k‐value phase encode to the excited spins in the VOI, and acquiring each individual k‐value data point in a single excitation—providing motion immunity and extended acquisition time for maximizing signal‐to‐noise ratio. Additional k‐value measurements from the same tissue can be made to characterize the tissue texture in the VOI—there is no need for these additional measurements to be spatially coherent as there is no image to be reconstructed. This method was applied to phantoms and tissue specimens including human prostate tissue.

   Data demonstrating resolution <50 µm, motion immunity, and clearly differentiating between normal and cancerous tissue textures are presented.

   The data reveal textural differences not resolvable by standard MR imaging. As MRµT is a pulse sequence, it is directly translatable to MRI scanners currently in clinical practice to meet the need for further improvement in cancer imaging.

    

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