Thrombosis of deep calf veins is not considered to be a source of pulmonary embolism (PE). However, it is associated with thrombosis of the popliteal and deep femoral veins, which together account for 70–90% of PE. Shortness of breath is the most common symptom, with chest pain almost as common; tachypnea is the most frequent sign. Syncope is an uncommon but important sign. Patients with PE nearly always have an identifiable risk factor. The risk factors of Virchow's triad are: stasis (heart disease, immobilization), hypercoagulability (contraceptives, pregnancy, malignancy, polycythemia, postoperation), and vessel wall trauma. A normal ventilation–perfusion scan virtually excludes the diagnosis. Multibeam helical computerized tomographic pulmonary angiography (HCTPA) is replacing standard pulmonary angiography as the study of choice, although the literature is still unclear about how accurate this study may be for small distal clots, or the significance of missing those clots.