Thrombosis. Coloured X-ray angiogram showing thrombosis in the left brachial artery. The brachial arteries supply the arms with blood. The rib cage is seen at left of image with the shoulder joint at top right corner. The left brachial artery runs from top left to far right image. A thrombus appears as a narrowing of the artery at right of image. This means that blood flow to the arm is impaired. A thrombus is an abnormal blood clot in a diseased blood vessel. Fatal complications of thrombosis include heart attack and stroke.