Figure 2: Transverse spinous process view, when the transducer is positioned directly over the spinous process. Note: The acoustic shadow of the spinous process (arrow) and of the lamina (arrowhead). Erector spinae muscle (asterisk) is easily visualized. SP: Spinous process; L: lamina; ESM: erector spinae muscle. The images were obtained by means of a Sonoscape P50 Ultrasound (Sonoscape Europe s.r.l., Rome, Italy). The O2-O3 mixture was produced by means of a Multiossigen Medical 99IR generator (Multiossigen S.p.A., Gorle, Bergamo, Italy).