In mitral regurgitation (MR), blood enters the left atrium during ventricular systole due to inadequate closure of the mitral valve leaflets. MR can be functional (due to LV dilatation or papillary muscle dysfunction) or an anatomic abnormality of the mitral valve leaflets (1). The most common cause of MR (world wide) is due to rheumatic fever (1). In the western world, idiopathic mitral valve prolapse is the most common etiology (1).
Chest x-ray imaging findings for patients with chronic MR include both left atrial and left ventricular dilatation. The left ventricle enlarges both posteriorly and laterally.
The normal left atrium is positioned posteriorly. On the PA radiograph the an enlarged left atrial appendage can be seen as a “3rd mogul” on the left heart boarder, just under the shadow of the left main pulmonary artery. With moderate enlargement it and can be seen as a double density behind the right atrial shadow. In severe cases, the left atrium can expand to such an extent as to be termed “gigantic” (2). When this occurs, the boarder of the left atrium can be seen beyond the right atrial shadow. In extreme cases, the left atrial boarder can even extend to the right chest wall (2).
On the lateral image, as the left atrium enlarges and nears the anterior aspect of the thoracic spine, it displaces the left mainstem bronchus posteriorly and superiorly, increasing the angle between the right and left main bronchi (2).

Factoid Reviewed by Dr. Mihn Kenney (TAMC)