Abstract
Objective: When evaluating swallowing function using MRI, sequences with an extremely short repetition time using an ultra-high-speed applied voltage pulse are required, such as steady-state free precession sequences (SSFP) . In SSFP sequences, the flip angle used determines the level of noise and contrast in various tissues, which in turn determine the quality of the dynamic image of the swallowing process. Thus, we investigated the effects of flip angle on signal intensity and potential for visual evaluation of the tissues involved in swallowing during the resting state. We conclude by identifying an optimal flip angle for evaluation of swallowing function.
Methods: Seven dentulous adults (average age 30.1 years) without any abnormality in stomatognathic function participated in this study. Images of the subjects were obtained with a 1.5-Tesla superconducting MRI unit using SSFP pulse sequences. To capture the swallowing motion using this ultra-high-speed imaging method, extremely short repetition time (TR) and echo time (TE) values are essential. However, to enhance the accuracy of signal intensity measurements, in the present study we used the same voltage pulse parameters to capture resting state images of the neck. The flip angles ranged between 10°and 100°at intervals of 10° (i.e. a total 10 different flip angles were used) . For images obtained with each flip angle, we assessed signal intensity and potential for visual evaluation of the tissues involved in swallowing (i.e. the soft palate, lingual muscles, epiglottis, thyroid cartilage, geniohyoid muscle, mandibular bone and hyoid bone) .
Results: The signal intensities of the tissues changed significantly depending on the flip angle. The greatest differences in the signal intensities of the various tissues, and thus the highest overall contrast, were observed at flip angles of 10°, then 20°and 30° (in that order) . Flip angles of 40-100°yielded the same signal intensities, which were lower than values for the other flip angles. Only flip angles of 20°, 30°and 40°maintained the overall signal to noise ratio (SNR) at a high level. Flip angles of 30°and 40°produced sharp images that permitted clear visual evaluation of the tissues involved in swallowing.
Conclusion: Based on the potential for visual evaluation of an image and differences in signal intensity between the tissues involved in swallowing, when obtaining images using ultra-high-speed MRI with SSFP sequences, a flip angle of 30°is optimal. This information may assist clinicians to clearly visualize anatomical structures during dynamic observations of swallowing function.
