Ultrasound imaging of the airway

Abstract

To the Editor, We read with interest the articles by Tsui et al. regarding ultrasound imaging of the airway using a sublingual scanning approach. A small footprint high frequency curved transducer placed intra-orally failed to capture images of the epiglottis and the laryngeal inlet due to posterior acoustic shadowing of the hyoid bone. Transcutaneous ultrasound scanning of the airway may overcome some of these challenges. In our pilot study, we were able to image airway structures in the transverse and parasagittal (1 cm away from midline) views using a linear 38 mm, 13–6 MHz transducer with the Sonosite M-Turbo ultrasound machine (Bothell, WA, USA). The epiglottis can be visualized (Fig. 1) through the transverse and parasagittal windows both above and below the hyoid bone. Because the hyoid bone casts an acoustic shadow posteriorly, part of the epiglottis is obscured. The epiglottis is best seen through the hyo-thyroid window between the hyoid bone and the thyroid cartilage. It appears as a hypoechoic structure with a curvilinear shape on parasagittal view and an inverted ‘C’ on transverse view. It is related anteriorly by the hyperechoic triangular preepiglottic space and lined posteriorly by a hyperechoic air–mucosa interface. Compared with the sublingual technique, the transcutaneous scan has several advantages. A high frequency transducer for transcutaneous scan captures images of higher resolution. A good transducer to skin contact can be maintained even at the level of the thyroid prominence with the linear transducer in the parasagittal position. Visualization of the epiglottis is possible without ultrasound beam interference due to hyoid acoustic shadow and attenuation by hypopharyngeal air (as seen with the sublingual scan). Furthermore, airway scanning at the level of the laryngeal inlet is possible and not impaired by restricted caudad transducer angulation due to the presence of upper incisors or the smaller capacity of the sublingual space (as seen with the sublingual scan). Both scanning approaches visualize tongue substance and musculature. Future studies will determine if the sublingual and transcutaneous approaches are complementary to each other. Ultrasound may prove to be an exciting evaluation tool for airway structures and ease of intubation.