Depth of phrenic nerve from skin was measured by the shortest distance between the epineurium of phrenic nerve and the surface of skin (presented as solid line). The shortest distance between the epineurium of phrenic nerve and the anterior tubercle (AT) of the C6 transverse process was also measured (presented as dotted line). SCM, sternocleidomastoid muscle; AS, anterior scalene muscle; MS, middle scalene muscle; C6, C6 nerve root.