The needle is inserted in-plane from the cranial end of the probe until the tip of the needle penetrates the infero-lateral margin of the minor pectoral muscle and enters the lower part of the axilla between the Gerdy’s ligament and the serratus anterior muscle
Gerdy’s ligament (cyan arrow), minor pectoral muscle (green asterix), serrratus anterior muscle (magenta asterix), needle (red), rib III (r3), rib IV (r4)
