Figure 2 - Ultrasonographic images of the esophagus (left) and lower esophageal sphincter (LES, right).

Images were obtained using a 12.5–MHz catheter-based transducer. Circle in the center (T) represents the ultrasound transducer. Note five layers of tissue in the esophagus and LES: MUC, mucosa; CM, circular muscle (intermuscular septum is between circular and longitudinal muscle); LM, longitudinal muscle (outside LM is adventitia); SP, spine; AO, aorta; A, anterior; P, posterior; L, left; R, right. Images were obtained in resting state. Note the asymmetry of the LES and esophagus. (Source: Liu et al.,⁴ with permission from American Physiological Society.)