Version August 2025
The longitudinal ('string of pearls') technique for identification of the cricothyroid membrane.
(A) The ultrasound transducer is placed transversely on the anterior neck cephalad to the suprasternal notch; a tracheal ring is recognized as a horseshoe-shaped dark structure with a posterior white line (highlighted in blue horseshoe on the right).
(B) The transducer is slid to the patient's right side towards the operator until the ultrasound image of the tracheal ring is truncated in half on the screen; at this point the border of the transducer is at the midline of the trachea.
(C) The free end of the transducer is rotated 90° degrees into the sagittal plane and the transducer is slid cephalad. A series of hypo-echoic rings ('string of pearls') are seen superficial to the hyper-echoic air-tissue border. On the right-hand side image, the anterior part of the cricoid cartilage appears as an elongated ring (dashed red circle) that is significantly larger and more anterior than the tracheal rings (solid light blue circles); the cricothyroid membrane is dashed green and the thyroid cartilage is dotted dark blue. The acoustic shadow from a needle placed on the skin is seen as a solid orange line.