Method for determining CTR from chest radiography. The midline was defined as a vertical line drawn through the spinous processes. (A) The maximum distance from the midline to the right cardiac border was added to (B) the maximum distance from the midline to the left cardiac border. Transverse diameter of the cardiac silhouette = A+B. (C) The distance between the internal margins of the ribs at the level of the right hemidiaphragm. CTR = (A+B)/C. CTR, cardiothoracic ratio.