Lung field division using Modified Chest X-ray Scoring System on chest posteroanterior (PA) projection, The lung field is divided into six zones lower zone (1 or 2) is under the inferior wall of the lower right pulmonary vein (lung base), middle zones (3 or 4) is below the inferior wall of the aortic arch and above the inferior wall of the lower right pulmonary vein (ie, hilar structures), and upper zone (5 or 6) is above the inferior wall of the aortic arch.