Fig. 4. A two-dimensional depth-scan image of the epidural space formed by 85 ultrasound A-mode lines recorded at different depths during the insertion of the epidural needle into the epidural space. Locations 1, 2, and 3 show that the tip of the needle transducer is 7.5–6 mm away from the dura mater. The right corner area shows the tip of the needle transducer is inserted into the epidural space.