The left lower lobe collapses medially and posteriorly to lie behind the heart. It classically displays a triangular opacity, which may be visible through the cardiac shadow or may overlie it, giving the heart an unusually straight lateral border. Silhouetting of the left hemidiaphragm usually occurs, which may be associated with an air bronchogram. It is also easily missed, especially on an underpenetrated film, where no detail is seen behind the heart