Blood clot in the heart. Sequence of magnetic resonance imaging (MRI) scans showing thrombosis or embolism in the left ventricle of the heart. A thrombus or embolus is caused by a clot of blood factors, such as platelets. A thrombus forms at the site it is blocking, while a embolus forms elsewhere and is carried to the blockage site by the blood flow. The blockage can be fatal if left untreated. Treatment is with thrombolytic drugs to breakdown the clot and anticoagulants to prevent further clotting. In some cases surgery is necessary to remove the clot.