Improved real-time tagged MRI using REALTAG

Abstract

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

Methods: 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.

Results: 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.

Conclusion: 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.

Keywords: real-time MRI; speech production; tag persistence; tagged MRI; tongue deformation.

Figure 1.

Phase sensitive reconstruction flowchart (left) and example images from intermediate steps (right). Low resolution and tag-line-free phase (B) was estimated for each time frame using a synthesized k-space center after gridding and coil combining (A). The lowest tag harmonic frequencies are located at ±2k_max/α, where α is the scaling factor between the tag spacing and the image resolution. The width of the low pass filter W = 2k_max/α was set as one half of this frequency. Example image (B) used W=20 for a 100-by-100 k-space with 2 mm image resolution and 1 cm tag spacing. Final images (D) were generated by taking the non-negative values from phase sensitive reconstruction (C) for better visualization. Note the bright spots existing in the intersection of tag lines in (D) due to double inversion.

Figure 2.

Simulated and measured tag CNR decay for TFA = 90° in the original implementation and with TFA = 180° with REALTAG in the proposed method. (Top) Solid lines show simulations; symbols and error bars show mean and standard deviation of the measurement, respectively. The in-vivo CNR measurement with 5 subjects is consistent with the simulation. Dashed horizontal line indicates CNR = 6. CNR drops below 6 around 600 ms for TFA = 90°; it reaches the same level around 1150 ms for TFA = 180° with REALTAG.

Figure 3.

Time threshold improvement by using REALTAG in 5 in-vivo scans with 1 cm tag spacing. The 2^nd, 3^rd and 4^th rows compare the two methods with CNR thresholds of 7, 6 and 5, respectively. For all 5 subjects, the proposed method significantly improved the persistence by a factor of >1.9x.

Figure 4.

Representative images during the speech stimuli production. (A), (B) and (C) show the subject speaking “a pie again,” “a poppy again,” and “a pop pip again,” respectively, progressing from the starting to the ending target vowel vocal tract constrictions. Arrows in the audio waveform indicate the timepoints of the selected images. The proposed method captures the tongue deformation of the starting and the ending vowels for all three stimuli (right: red, blue & gold), while CNR by the original method drops below threshold in the latter two cases and becomes obscure (left: blue and gold).