What is cephalization?

Normally, on a frontal upright chest radiograph, vessels in the lower lung zones are larger than vessels in the upper lung zones as a result of gravity. However, when vessels in the upper lung zones appear larger than those in the lower lung zones, this is known as cephalization. This occurs when pulmonary venous pressure is elevated, typically in patients with chronic left ventricular failure, and reflects a redistribution of blood flow to the upper lung zones.