Acute and Chronic Pulmonary Embolism: An In-depth Review for Radiologists Through the Use of Frequently Asked Questions Elena Pena, MD, and Carole Dennie, MD, FRCPC In this article, the authors review the role of the different imaging modalities in the diagnostic workup of patients with suspected acute or chronic pulmonary embolism (PE). The authors also discuss the current guidelines for the diagnosis of acute PE based on the pretest probability clinical assessment and outline the current recommendations for special patient populations. The recent guidelines from the American Thoracic Society/Society of Thoracic Radiology for the assessment of suspected PE in pregnancy are also reviewed. Finally, the imaging findings in acute and chronic PE are illustrated. Semin Ultrasound CT MRI 33:500-521 © 2012 Elsevier Inc. All rights reserved. V enous thromboembolism (VTE) refers to the pathologic formation of a thrombus within veins leading to 2 pos- sible clinical manifestations: deep vein thrombosis (DVT) and pulmonary embolism (PE). Acute PE is the third most common acute cardiovascular disease after myocardial in- farction and stroke. 1-3 The reported prevalence of PE is 0.4%. 4 Because PE is fatal in up to 30% of patients, prompt diagnosis and treatment is mandatory, reducing the mortality rate to 2%-10%. 5 Risk factors associated with PE include older age, a history of previous VTE, active cancer, recent surgery, prolonged bed rest or neurologic disease with extremity paresis, as well as congenital or acquired thrombophilia. 4 In up to 20% of patients, there are no recognizable risk factors, and this is termed as unprovoked PE. 4 How Should a Patient With Suspected PE Be Initially Managed? Before performing any laboratory or imaging tests, the clini- cian should begin with the assessment of the clinical pretest probability of acute PE using validated clinical prediction rules. 6-10 The most frequently used prediction rules are the Wells (Table 1) 11,12 and Geneva scores. 13 These allow classi- fication of patients into 3 categories: low, intermediate, and high clinical probability. What Is the Role of D-dimer in the Diagnostic Workup? D-dimer is a plasmin-derived fibrin degradation product. Levels in the blood are elevated in the presence of acute VTE. 14 The enzyme-linked immunosorbent assay (ELISA) has been established because the standard test and levels 500 ng/mL are considered abnormal. The strength of the D-dimer test is in its high sensitivity and high negative pre- dictive value (NPV) for VTE. 4 Specificity is much lower at 55%, 7,15-18 as many other common conditions lead to ele- vated D-dimer levels (Table 2). Hence, a positive test has a low positive predictive value (PPV) for VTE. 19 In outpatients, the D-dimer assay should be the first diag- nostic test performed in the presence of a low or moderate clinical pretest probability. 7,9,20-22 A negative ELISA D-dimer can safely exclude PE without further testing, 4,7,9,10,21 whereas a positive result mandates the use of imaging to rule out VTE. 7,9,23 The D-dimer test is not helpful in patients with a high clinical pretest probability because a negative result does not safely exclude PE. The clinician should move directly to im- aging studies as the initial diagnostic step. 7,9,20,23,24 Department of Medical Imaging, The Ottawa Hospital, University of Ottawa, Ottawa, Ontario, Canada. Address reprint requests to Carole Dennie, MD, FRCPC, Department of Medical Imaging, The Ottawa Hospital, University of Ottawa, 1053 Carling Ave, Ottawa, ON K1Y 4E9, Canada. E-mail: cdennie@ ottawahospital.on.ca 500 0887-2171/$-see front matter © 2012 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1053/j.sult.2012.06.001