Imagerie par Résonance Magnétique
(Thierry Metens, Alain Soquet et Didier Demolin)

MRI of the vocal tract can be used for different purposes:

• Imaging of the vocal tract during phonation. An important step in the study of the relation between vocal tract geometry and speech is provided by area functions. Tongue, velum, lips and larynx movements are of crucial interest.
• Study of the pharyngeal airway in obstructive sleep apnea.
• Study of patients with deficit of speech function (native deficit, recovery after surgery).

MRI of the vocal tract remains a challenge:

• Respiration, swallowing, speech movement demand motion insensitive imaging;
• Sequences must be less sensitive to susceptibility artifacts;
• Imaging during speech processes or non-cooperative patients (asleep!) demands ultrafast imaging.

1. Static study of the vocal tract

   The first studies provided soft tissue contrast and geometrical information with low resolution during vowel phonation sustained for a quite long time, 20 seconds up to 3 minutes!

   • T. Baer et al Analysis of vocal tract shape and dimension uisng MRI: vowels. JASA 90,799-828; SE T1 8mm 3minutes
   • D.Demolin, J.M. Hombert, V.Lecuit, A.Soquet, C. Segebarth. An MRI study of French vowels, Eurospeech, Madrid, 2235-2238. SAG Single slice 8mm, resolution 2x1mm, SE T1: TR=200, TE=25, 12 seconds
   • An MRI-based study of pharyngeal volume contrasts in Akan and English. M. Tiede, Journ of Phonetics 1996, 2: 399-421 3D, 28 slices, 5mm, Steady state phonation 3minutes

   With recent MR scanners, more realistic experiments are possible: fast imaging during short steady state phonation of a single vowel, with high spatial resolution.

   It involves fast imaging i.e.: Gradient echo sequences (prone to susceptibility artifacts) Turbo spin echo sequences (less sensitive to susceptibility artifacts)

   In recent work we have been studying the entire oral tract with MRI during vowel phonation, in order to determine accurate area functions. 18 parallel contiguous slices of 1x1 mm resolution during sustained phonation of 14 sec were acquired in TSE.

   A multi-oblique stack of 14 independently positioned and orientated slices of 1x1 mm resolution was acquired during sustained phonation of 12 sec (D.Demolin, M.George, V.Lecuit, T.Metens, A.Soquet. submitted to Jour. Speech and Hearing Research, 1999)

   Multistack simultaneous acquisition of 14 slices in 12 sec

   

   Saturation bands!

2. Dynamic study of the vocal tract: TSE Zoom Imaging

   Rapid changes in the respective positions of anatomic components of the vocal tract during transitions, i.e. dynamic changes during phonation, represent an extreme challenge and can only be studied with subsecond imaging.

   Ultrafast Gradient Echos

   Dynamic study of the vocal tract: phonation, fast and ultrafast imaging

   General trade off between spatial resolution and acquisition time.

   • Dynamic study of the upper airway with ultrafast Spoiled Grass MRI & Occlusion and narrowing of the pharyngeal airway in obstructive sleep apnea evaluation by ultrafast spoiled grass MRI. Shellock F. et al: JMRI,1992,2,103 & AJR,1992,(158):1019-1024 SAG Single slice 5mm, resolution 2x1.5mm, GE T1: TR=8.4, TE=2.9, 1sl/sec
   • Evaluation of of the pharyngeal airway in patients with sleep apnea value of ultrafast MRI. Suto Y. et al AJR,1993,160:311-314 radiology 1996,201: 393-398. SAG Singleslice 10mm, resolution 2x2 mm, TurboFlash T1: TR=6.5, TE=3.5 0.5 sl/sec ( acquisition 1.1 sec+relaxation 1sec)
   • Dynamic MRI in the study of vocal tract configuration. M.Crary et al, Journ of Voice 1996,4: 378-388 SAG Single slice 8mm, resolution 6x3mm, GE T1, TR=5, TE=2, 3/sec
   • Foucart M et al Kinetic MRI analysis of swallowing: new approach to pharyngeal function. SAG Single slice Single slice 10mm, resolution 3x3mm Turbo Flash T1: TR=55, TE=1.2, 5/Sec

   All are of Gradient Echo type!

   Advanced TSE : TSE Zoom Imaging

   The TSE Zoom sequence is designed such that the initial 90¡ and the subsequent 180¡ refocusing pulses excite perpendicular slabs, resulting in an intersecting slice, free of foldover artifacts. The delay between echos in the train is short and the field of view is rectangular. TSE Zoom provides ultrafast MRI without compromising the spatial resolution and without susceptibility artifacts. Unlike EPI or Ultrafast gradient echo sequences, TSE Zoom can be implemented on a 17mT/m/ms, 15 mT/m MR scanner.

   One sagittal T1-weighted section of 6 mm thickness was continuously acquired during at least 20 sec, using a quadrature neck coil at 1.5T (Philips Gyroscan ACS NT PT1000, Best, The Netherlands). TR=250 ms, TE=30 ms, _=60¡, ESP=7.8ms, ETL=19 and 60% Partial Fourier acquisition, Field of View=300 x150mm with a 32 x 128 Matrix.

   TSE Zoom implemented on a 100 mT/m/ms 21 mT/m scannner. Acquisition of 4-6 images /seconds are possible with a shorter TE, i.e. a better T1 weighting and a further reduced sensitivity to susceptibility effects. The FOV is a vertical rectangle with a spatial resolution of 2x2 mm (TR=169, TE =15) One sagittal T1-weighted section of 6 mm thickness was continuously acquired during at least 6 sec, using a quadrature neck coil at 1.5T (Philips Gyroscan ACS NT PT6000, Best, The Netherlands).

   • Tongue movements

     Dynamic MRI of the vocal tract allows observation of different tongue movements.

     • The lowering and the raising of the tongue body in speech production.
     • The front-back movements of the tongue body in speech production.
     • Constriction and closure gestures of tongue tip and tongue body.

   • Larynx movements

     MRI movements of laryngeal inner structures such as the vocal folds remain a challenge but : Vertical movements of the larynx can be observed during the production of vowels and other speech sounds

   • Velum movements

     • Raising and lowering of the velum is easily observed while breathing and during the production of nasal sounds (nasal vowels and consonants).
     • The timing of the velum gestures and the coordination with tongue and lips movements can also be described.

     
     

   • Lips movements

     • Rounding and protrusion.
     • Spreading.
     • Coarticulation between lips and other speech articulators such as tongue tip, tongue body and velum.

     
     

   • Mandible oscillation

     Movements of the lower jaw and the coordination with other gestures involved in speech is possible with dynamic MRI.

     • Jaw retraction and tongue root advancing.
     • Jaw retraction and larynx lowering.
     • Coordination between jaw movements and the shaping of pharyngeal volume.

   • Compensation movements

     Dynamic MRI of the vocal tract is a promising technique for studying compensatory movements of different articulators.

     'Bite block' experiments, made when biting a small plastic bottle containing water, showed that :

     • When the jaw is fixed there is a reduced movement of the tongue root as well as a reduced movement of the larynx.
     • The tongue body moves differently in order to shape the two cavities necessary to produce the correct acoustic resonances.

   • 3D reconstruction of the vocal tract

     Mathematical tools allow the modelling of both the oral and nasal cavities. From MRI images it is possible to reconstruct and to visualize :

     • Pharyngeal shape and volume.
     • The shape and the dimension of the nasal tract when the velum is lowered.
     • The shape of the oral and nasal cavities.