The pros and cons of the two commonly used modalities, CTPA and perfusion scanning, are given in Table 1. Perfusion scanning is extremely useful when completely normal, as this excludes CPE by virtue of its high negative predictive value. As an initial test, perfusion scanning has a greater sensitivity than CTPA in diagnosing CPE (96% vs 51%10) with a very good, albeit slightly lower, specificity (90-95% vs 99%10). Of course, perfusion scanning has its own problems with false-positive scans, indeterminate findings, and an underestimation of clot burden, especially in large-vessel disease. More importantly, in the presence of effusions, atelectasis, or other common radiographic changes, perfusion scanning becomes indeterminate and unhelpful in this context. In addition, unlike CT scan, perfusion scan fails to detect other findings that are helpful in the diagnosis of CPE. This reason (and the inability to directly image the vessels and heart) probably accounts for the reason CTPA has replaced perfusion scanning for detection of CPE in most institutions. Diagnosis of acute thrombus in the setting of CPE is also superior if the first test is CTPA rather than perfusion scanning. CTPA is often easier to organize and is more accessible in many radiology departments than perfusion scanning. Finally, CTPA can detect other causes of dyspnea if there is no evidence of CPE, such as pericardial effusion, myocardial infarction, mucus plugging, and other underlying lung, pleural, or mediastinal disease.