TABLE 1.
Recommended RT-MRI Protocols for Speech Imaging
| Recommended protocols | Protocol 1 | Protocol 2 | Protocol 3 | Protocol 4 |
|---|---|---|---|---|
| Accessibility | Widely available on most commercial scanners | Custom implementation | Custom implementation | Custom implementation |
| Key advantages | Compatible with linear reconstruction schemes, which enables on-the-fly reconstruction, a feature beneficial for frequently updating scan planes, and diagnosis, & correction for artifacts on the fly | |||
| Motion robust spiral readouts | Motion robust radial readouts | High temporal resolution | ||
| Compatible with iterative offline reconstruction schemes Enables high spatiotemporal resolutions (eg, up to 1.5–2.5mm2; 10–50 ms/frame) |
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| Speech tasks On-line visualization (also see Fig 1 for spatiotemporal resolutions requirements) |
Sustained sounds Velo-pharyngeal closure Bulk tongue movements |
Sustained sounds Velopharyngeal closure Bulk tongue and few rapid tongue movements (eg, consonant to vowel) Parts of consonant constriction sounds |
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| Speech tasks: Off-line visualization (also see Fig 1 for spatiotemporal resolutions requirements) |
Sustained sounds Velopharyngeal closure Most rapid tongue movements Consonant constriction sounds Coarticulation events |
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| Receiver coil | In order of preference (Also see Fig. 2): Custom upper airway coil Head and neck coil Neurovascular coil Head coil |
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| FOV | 20 × 20 to 30 × 30 cm (dependent on the coil geometry) | |||
| Orientation | Dependent on speech task; Mid-sagittal for most tasks | |||
| Slice thickness | 5–10 mm | |||
| Field strength | 1.5 T or 3T | 1.5 T | 1.5 T (preferred) 3T |
1.5 T 3 T (preferred) |
| Sampling | Cartesian | Spiral | Radial | Specialized (Spiral + Cartesian) |
| Sequence | GRE or SSFP | GRE | GRE | GRE |
| Example sequence parameters |
SSFP 3T FA: 15°, TE/TR: 1.1/2.3 msec, GRE3T FA: 30°, TE/TR: 1.0/2.2 msec |
TR = 6 msec, FA = 15°, spiral readout length = 2.4 msec |
1.5 T TE/TR: 1.44/2.2 ms, FA = 5° |
At3T Spiral FLASH TE/TR = 0.85/9.8 ms Cartesian FLASH TE/TR = 2.3/9.8 ms |
| Specifications of the sampling pattern | Cartesian subsampling (2–3 fold) | Incoherent repetition of short spiral interleaves along time | Incoherent repetition of radial readouts along time | Temporal navigation by short spiral readouts. Randomized Cartesian sub-sampling for imaging data |
| On-the-fly reconstruction Parallel Imaging, Partial Fourier Gridding combined with sliding window | ✓ | ✓ | ✓ | (x) |
| Offline reconstruction Iterative constrained reconstruction Eg, spatial-spectral sparsity, spatiotemporal finite difference sparsity, low rank constraints | (x) | ✓ | ✓ | ✓ |
| Figures demonstrating quality and expected artifacts | Figs. 3–5 | Figs. 5–9 | Figs. (5 and 10) | Fig. 11 |
The various speech tasks that can be captured by the recommended four protocols are classified based on the latency in visualization (on-the-fly versus off-line). Protocol 1 can be enabled by Cartesian sequences, which are widely available on commercial scanners. Protocols 2, 3 are based on non-Cartesian sampling trajectories, and provide options for both on-the-fly and off-line reconstructions. Protocol 4 relies on special sampling schemes, and is based on offline reconstruction. On-the-fly reconstructions allow for real-time visualization of the speech events, and provide flexibility in prescription of planes, diagnosis of artifacts. In comparison to on-the-fly reconstructions, off-line iterative constrained reconstructions offer capabilities of imaging at high spatiotemporal resolutions. As discussed in the text, careful prescriptions of these protocols are required to avoid some common artifacts.