Faster dynamic imaging of speech with field inhomogeneity corrected spiral fast low angle shot (FLASH) at 3 T

Abstract

Purpose: To evaluate the impact of magnetic field inhomogeneity correction on achievable imaging speeds for magnetic resonance imaging (MRI) of articulating oropharyngeal structures during speech and to determine if sufficient acquisition speed is available for visualizing speech structures with real-time MRI. Materials and Methods:We designed a spiral fast low angle shot (FLASH) sequence that combines several acquisition techniques with an advanced image reconstruction approach that includes magnetic field inhomogeneity correction. A simulation study was performed to examine the interaction between imaging speed, image quality, number of spiral shots, and field inhomogeneity correction. Six volunteer subjects were scanned to demonstrate adequate visualization of articulating structures during simple speech samples. Results:The simulation study confirmed that magnetic field inhomogeneity correction improves the available tradeoff between image quality and speed. Our optimized sequence co-acquires magnetic field maps for image correction and achieves a dynamic imaging rate of 21.4 frames per second, significantly faster than previous studies. Improved visualization of anatomical structures, such as the soft palate, was also seen from the field-corrected reconstructions in data acquired on volunteer subjects producing simple speech samples.Conclusion: Adequate temporal resolution of articulating oropharyngeal structures during speech can be obtained by combining outer volume suppression, multishot spiral imaging, and magnetic field corrected image reconstruction. Correcting for the large, dynamic magnetic field variation in the oropharyngeal cavity improves image quality and allows for higher temporal resolution. MAGNETIC RESONANCE IMAGING (MRI) has long been considered as a tool with great potential for examining soft tissue movements associated with speech. Neither x-ray video fluoroscopy nor nasoendoscopy are able to provide unobstructed visualization of the moving soft tissues, including underlying musculature, whereas MRI can provide dynamic images of soft tissues and underlying musculature in any arbitrary plane. The early applications of MRI to speech research were restricted by long image acquisitions that were not fast enough to capture the natural movements of vocal tract articulators (1-4). As a result, many researchers focused on sustained vowels and fricatives that keep the vocal tract still for up to several seconds to compensate for the low temporal resolution. Examples of early research include extraction of vocal tract transfer functions (5) and evaluation of velopharyngeal insufficiencies in patients with motor dysfunctions (3) or in patients who had surgery to repair cleft palate (6).In order to resolve faster articulatory movements during speech, later studies took advantage of MRI techniques that divide an image acquisition over as many as 128 repetitions of the motion of interest (7). These gated acquisitions acquire only part of k-space data for an image during a single repetition of a speec...

Towards clinical assessment of velopharyngeal closure using MRI: evaluation of real-time MRI sequences at 1.5 and 3 T

“…An increasing number of research studies have used MRI to image the soft palate [10][11][12][13] and upper vocal tract [14][15][16][17]. In contrast to X-ray videofluoroscopy and nasendoscopy, MRI provides tomographic images in any plane with flexible tissue contrast.…”

“…As a result, MRI has been used to obtain images of the musculature of the palate at rest and during sustained phonation [10,18,19]. It has also been used to image the whole vocal tract at rest or during sustained phonation [20][21][22][23][24][25][26][27] and with a single mid-sagittal image dynamically during speech [13,[15][16][17][28][29][30][31][32][33][34][35].…”

“…Dynamic MRI of the vocal tract has been performed using both segmented [17,33,37] and real-time acquisitions [13,15,16,28,31,38]. Segmented acquisitions [39] acquire only a fraction of the k-space data required for each image during one repetition of the test phrase and, hence, require multiple identical repetitions.…”

“…However, for real-time speech imaging, the highest acquired frame rates have been achieved by sampling k-space along a spiral trajectory [15,16,30,49]. While spiral imaging is an efficient way to sample k-space and is motionresilient, it is prone to artefacts, particularly blurring caused by magnetic field inhomogeneities and offresonance protons (i.e.…”

“…fat) [50]. Recently, one group successfully used spiral imaging with multiple saturation bands and an alternating echo time (TE) to achieve an acquired real-time frame rate of 22 fps [13,16]. The saturation bands were used to allow a small field of view to be imaged without aliasing artefacts.…”