Construction of rib-vertebral angle (RVA) and rib–vertebral angle difference (RVAD). (1) Draw line parallel to bottom of apical vertebra (apical vertebral endplate). (2) Draw line perpendicular to line drawn in Step 1. (3) Find midpoint of head of rib. Find midpoint of neck of rib. These landmarks are estimated and mental note is taken. (4) Draw line from midpoint of head of rib to midpoint of neck of rib to line from Step 2. (5) Resulting angle is RVA for one side. (6) To calculate RVAD, calculate RVA for other side. Use lines created in Steps 1 and 2, and repeat Steps 3-5 for other side.