This is right middle lobe collapse. It is difficult to appreciate the volume loss on the PA radiograph. The horizontal fissure is not seen and the lateral radiogrpah shows a thin wedge that is the collapsed middle lobe. You are alerted to pathology in the right middle lobe by loss of the right heart border on the PA radiograph. On the lateral radiograph the horizontal fissure has moved downwards slightly and the oblique fissure has moved up to meet it creating a high density, thin wedge that is all that is left of the right middle lobe. Refer to the article Patterns of lobar collapse for more information.