Mitral regurgitation. Four axial coloured magnetic resonance imaging (MRI) scans through the chest of a patient with mitral regurgitation (MR). The mitral valve is between the heart's left atrium (upper chamber) and the left ventricle (lower chamber). If it is damaged blood flows back through the heart, mitral regurgitation, resulting in decreased blood flow to the rest of the body and increased work for the heart. The spine is at lower centre. The small bright mass near centre is the aorta, the main artery of the body. Immediately above the aorta is the left side of the heart, and above that the right. MR may be due to injury after a heart attack, or an infection.