You are in: eMedicine Specialties > Radiology > CHEST Acute Pulmonary Embolism (Helical CT)Article Last Updated: May 14, 2008AUTHOR AND EDITOR INFORMATIONSection 1 of 9
  Author: Kavita Garg, MD, Professor, Department of Radiology, University of Colorado Health Sciences Center Kavita Garg is a member of the following medical societies: American College of Radiology, American Roentgen Ray Society, Radiological Society of North America, and Society of Thoracic Radiology Editors: Judith K Amorosa, MD, FACR, Clinical Professor and Program Director, Department of Radiology, University of Medicine and Dentistry of New Jersey, Robert Wood Johnson Medical School; Consulting Staff, Department of Radiology, Robert Wood Johnson University Hospital; Bernard D Coombs, MB, ChB, PhD, Consulting Staff, Department of Specialist Rehabilitation Services, Hutt Valley District Health Board, New Zealand; Eric J Stern, MD, Professor of Radiology, Adjunct Professor of Medicine, Adjunct Professor of Medical Education and Biomedical Informatics, University of Washington School of Medicine; Director of Thoracic Imaging, Harborview Medical Center; Associate Medical Staff, Seattle Cancer Care Alliance; Robert M Krasny, MD, Consulting Staff, Department of Radiology, The Angeles Clinic and Research Institute; Eugene C Lin, MD, Clinical Assistant Professor of Radiology, University of Washington Medical School Author and Editor Disclosure Synonyms and related keywords: pulmonary thromboembolism, PE, deep vein thrombosis, deep venous thrombosis, DVT, thromboembolic disease, ventilation-perfusion scintigraphy, V/Q scintigraphy, V/Q scan, V-P scan, D-dimer assay, D-dimer INTRODUCTIONSection 2 of 9
  BackgroundPulmonary embolism (PE) was clinically described in the early 1800s, and von Virchow first described the connection between venous thrombosis and PE.1, 2 In 1922, Wharton and Pierson reported the first radiographic description of PE.3 Since that time, imaging has played an important role in the diagnosis of PE. For many years, ventilation-perfusion (V/Q) scintigraphy has been the main imaging modality for the evaluation of patients with suspected PE. However, with the advent of and the widespread availability of faster computed tomography (CT) scanners, CT scanning has emerged as another important diagnostic test for the evaluation of not only PE, but also deep venous thrombosis (DVT) in select patients. For excellent patient education resources, visit eMedicine's Lung and Airway Center and Circulatory Problems Center. Also, see eMedicine's patient education articles Pulmonary Embolism and Blood Clot in the Legs. PathophysiologyThree primary influences predispose a patient to thrombus formation; these form the so-called Virchow triad: (1) endothelial injury, (2) stasis or turbulence of blood flow, and (3) blood hypercoagulability.1, 2, 4 More than 90% of all PEs arise from thrombi within the large deep veins of the legs, typically the popliteal vein and the larger veins above it.1, 2, 4 The pathophysiologic consequences of thromboembolism in the lung largely depend on the cardiopulmonary status of the patient and on the size of the embolus, which, in turn, dictates the size of the occluded pulmonary artery. PE has 2 important consequences: (1) an increase in pulmonary artery pressure due to blockage of flow and, possibly, vasospasm caused by neurogenic mechanisms and/or release of mediators (eg, thromboxane A2 and serotonin) and (2) ischemia of the downstream pulmonary parenchyma. Thus, occlusion of major vessels or of more than 60% of the arterial bed suddenly increases the pulmonary artery pressures, diminishes cardiac output, and causes right-sided heart failure (acute cor pulmonale) or even death. Usually, hypoxemia develops as a result of multiple mechanisms. If smaller vessels are occluded, the result is less catastrophic, or the event may be even clinically silent. Conditions associated with an increased risk of thrombosis include the following:
Related Medscape topics: FrequencyUnited StatesDVT and PE are associated with approximately 300,000-600,000 hospitalizations each year, and as many as 50,000 individuals die each year as a result of PE.5 Mortality/MorbidityTreatment with anticoagulation or catheter-directed pharmacologic agents is highly effective but not without complications. Therefore, the diagnosis of PE, requires a high degree of certainty. Conversely, untreated PE can be fatal. Treatment reduces the mortality rate from 30% to less than 10%. RaceNo racial predispositions to thromboembolism are known. SexThe incidence of venous thromboembolic events in the older population is greater among men than women. In patients younger than 55 years, the incidence of PE is higher in females. The overall age- and sex-adjusted annual incidence of venous thromboembolism is reported to be 117 cases per 100,000 people (DVT, 48 cases per 100,000; PE, 69 cases per 100,000).6 AgePE is predominantly a disease in older individuals. The incidence of venous thromboembolic events in the elderly is more common among men than women. In patients younger than 55 years, the incidence of PE is higher in females. The overall age- and sex-adjusted annual incidence of venous thromboembolism is reported to be 117 cases per 100,000 people (DVT, 48 cases per 100,000; PE, 69 cases per 100,000).6 AnatomyKnowledge of bronchovascular anatomy is the key to the accurate interpretation of CT scans obtained for the evaluation of PE. A systematic approach in identifying all vessels is important. The bronchovascular anatomy has been described on the basis of the segmental anatomy of lungs. The segmental arteries are seen near the accompanying branches of the bronchial tree and are situated either medially (in the upper lobes) or laterally (in the lower lobes, lingula, and right middle lobe). Clinical DetailsPE should be considered whenever unexplained dyspnea occurs. Dyspnea with or without associated anxiety, pleuritic chest pain, and hemoptysis are common but nonspecific symptoms of PE. Any of these symptoms may also develop with other conditions, such as pneumonia, exacerbated chronic obstructive lung disease, congestive heart failure, and lung cancer. PE may cause light-headedness and syncope, but these may also be the result of other conditions that cause hypoxemia or hypotension. Clinical findings alone are not reliable in the diagnosis of PE. This fact is underscored by the high incidence of unsuspected PE in autopsy series. Physical examination of the patient with PE may reveal tachypnea, tachycardia, fever, and pleuritic rub, all of which are nonspecific findings. Hypoxemia is common in acute PE, but it is not universally present. Even the alveolar-arterial difference may be normal in rare cases of PE, particularly in younger patients without concomitant lung disease. Nonspecific electrocardiographic (ECG) abnormalities may develop in acute PE; these include T-wave changes, ST-segment abnormalities, and left- or right-axis deviation. Preferred ExaminationIn patients with possible PE, chest radiographic findings may indicate if lung scanning (V/Q) or helical CT scanning should be the next method of evaluation. If the chest radiograph is normal, V/Q findings may be diagnostic; if the chest radiograph is abnormal, helical CT should be performed.4, 5, 7 A quantitative D-dimer assay is reported to have high negative predictive value and may be effective for excluding the need for pulmonary CT angiography (CTA) in selected cases.8 Conventional pulmonary angiography is invasive, time consuming, and more expensive than other tests. The role of conventional angiography is limited to patients in whom other results are nondiagnostic or the clinical suspicion is high.6, 9, 10 In patients with suspected DVT, the workup should start with leg ultrasonography. Limitations of TechniquesV/Q findings may be nondiagnostic. Iodinated contrast agents are needed for helical CT pulmonary angiography, and their use may not be possible in patients with impaired renal function or in patients with a severe allergy to the contrast material. Small (subsegmental) emboli may be missed with CT angiography. Compared with CT scanning, conventional pulmonary angiography requires more expertise and support staff. Conventional angiography is also invasive, time consuming, more expensive, and less available. In addition, a chronic central mural thrombus that is easily seen with CT scanning may be missed at pulmonary angiography.11 DIFFERENTIALSSection 3 of 9
Aspiration Pneumonia Asthma Atelectasis, Lobar Extrinsic Allergic Alveolitis Lung, Arteriovenous Malformation Lung, Metastases Lymphangitic Carcinomatosis Myocardial Infarct, Acute Pneumonia, Atypical Bacterial Pneumonia, Pneumocystis Carinii Pneumonia, Viral Pneumothorax Pulmonary Edema, Noncardiogenic Pulmonary Hypertension Radiation Pneumonitis Superior Vena Cava Syndrome Other Problems to Be ConsideredAortic dissection RADIOGRAPHSection 4 of 9
FindingsChest radiographs are abnormal in most cases of PE, but the findings are nonspecific. Common radiographic abnormalities include atelectasis, pleural effusion, parenchymal opacities, and elevation of a hemidiaphragm. The classic radiographic findings of pulmonary infarction include a wedge-shaped, pleura-based triangular opacity with an apex pointing toward the hilus (Hampton hump) or decreased vascularity (Westermark sign). These findings are suggestive of PE but are infrequently observed. Degree of ConfidenceA prominent central pulmonary artery (knuckle sign), cardiomegaly (especially on the right side of the heart), and pulmonary edema are other findings. In the appropriate clinical setting, these findings could be consistent with acute cor pulmonale. A normal-appearing chest radiograph in a patient with severe dyspnea and hypoxemia but without evidence of bronchospasm or a cardiac shunt is strongly suggestive of PE. Generally, chest radiographs cannot be used to conclusively prove or exclude PE; however, radiography and electrocardiography may be useful for establishing alternative diagnoses (see Differentials and Other Problems to Be Considered). CT SCANSection 5 of 9
FindingsTechnical advances in CT scanning, including the development of multidetector-array scanners, have led to the emergence of CT scanning as an important diagnostic technique in suspected PE.12, 13 Contrast-enhanced CT scanning is increasingly used as the initial radiologic study in the diagnosis of PE, especially in patients with abnormal chest radiographs in whom scintigraphic results are more likely to be nondiagnostic.4, 5, 7 CT scanning shows emboli directly, as does pulmonary angiography, and it is also noninvasive, cheaper, and widely available. CT scanning is the only test that can provide significant additional information related to alternate diagnoses; this is a clear advantage of CT scanning compared with either pulmonary angiography or scintigraphy.14 Because DVT and PE are part of the same disease process, CT venography can easily be performed after CT pulmonary angiography, without the administration of additional contrast material.10, 15, 16 This study requires only a few extra minutes and allows "one-stop imaging" for both PE and DVT. The technique for CT pulmonary angiography with single-section helical CT involves the following parameters: 3-mm collimation, 2-mm reconstruction interval, pitch of 2, and an average acquisition time of 24 seconds. Iodinated contrast medium is administered as a bolus with an automated injector. Generally, a large volume (100-150 mL) of contrast material is administered at a high flow rate (4 mL/s) for good-quality diagnostic opacification of vessels. When a PE is identified, it is characterized as acute or chronic. An embolus is acute if it is situated centrally within the vascular lumen or if it occludes a vessel (vessel cutoff sign) (see Image 1). Acute PE commonly causes distention of the involved vessel. An embolus is chronic if (1) it is eccentric and contiguous with the vessel wall (see Image 2), (2) it reduces the arterial diameter by more than 50%, (3) evidence of recanalization within the thrombus is present, and (4) an arterial web is present. A PE is further characterized as central or peripheral, depending on the location or the arterial branch involved. Central vascular zones include the main pulmonary artery, the left and right main pulmonary arteries, the anterior trunk, the right and left interlobar arteries, the left upper lobe trunk, the right middle lobe artery, and the right and left lower lobe arteries. Peripheral vascular zones include the segmental and subsegmental arteries of the right upper lobe, the right middle lobe, the right lower lobe, the left upper lobe, the lingula, and the left lower lobe. Degree of ConfidenceIn most cases, when spiral CT scan findings are positive for PE, the emboli are multiple, with intraluminal filling defects observed in the larger central arteries and in the segmental and subsegmental vessels. An apparent filling defect in a single segmental or (especially) subsegmental vessel can be challenging. One should consider all the pitfalls, especially those related to volume-averaging artifacts before diagnosing an isolated subsegmental embolus. The emboli are often bilateral and more common in the arteries to the lower lobes. The sensitivity of spiral CT scanning in the evaluation of central PE is as high as 100%. However, it is reportedly variable and lower (see Table). Also, the reported incidence of isolated subsegmental PE varies from 5% in the Prospective Investigation of Pulmonary Embolism Diagnosis (PIOPED) study to 36% in another study.7 Moreover, the true significance of small emboli has not been proven conclusively. Small thromboemboli may have clinical significance in patients with limited cardiopulmonary reserve. Pulmonary angiography demonstrates subsegmental vessels in more detail than CT scanning, although the superimposition of the small vessels remains a limiting factor. As a result, the interobserver agreement rate for isolated subsegmental PE is only 45%. In recent years, investigators have reported uneventful clinical outcomes in patients (with a negative predictive value of 99%) in whom CT scans were interpreted as negative for PE and who were not treated with anticoagulation or catheter-directed pharmacologic thrombolysis. The outcome was similar to those of patients with clinically suspected PE but without emboli on pulmonary arteriograms. This finding indicates that, although some small emboli may be missed at helical CT scanning, the subsequent morbidity rate with PE does not appear to be high. The new MDCT scanners are considerably faster, allowing the performance of thin-section (1.25-mm) helical CT pulmonary angiography during a shorter breath hold (15-17 seconds). The segmental and subsegmental vessels are better demonstrated, findings are easier to interpret, and interobserver agreement is improved with this technique. MDCT increases diagnostic capabilities; however, the large amount of data (a thin-section 16-detector row CT pulmonary angiography results in 500-600 axial slices) generated puts a substantial strain on any image analysis and archiving system. Development of dedicated algorithms for computer-aided detection and greater use of maximum intensity projection reconstruction techniques may be helpful in the future for identification of pulmonary emboli in large-volume MDCT data sets. Accuracy of helical CT* pulmonary angiography
*Single-slice CT scanners. The accuracy with newer MDCT scanners is reported to be higher. False Positives/NegativesThe pitfalls of CT scanning, especially those related to volume averaging of perivascular tissue, branching points, and nonvertical vessels, can be limited by using a trackball on a workstation and by knowing the vascular anatomy. The lymphatic and connective tissue, more commonly adjacent to central vessels, are located between the artery and the bronchus (see Image 4). Flow-related and motion artifacts can result in pseudofilling defects and should be kept in mind when the quality of study is evaluated and when the image is interpreted. Flow-related pseudofilling defects can also result in false-positive findings on the CT venogram. Overall, findings in 2-4% of CT pulmonary angiographic examinations are nondiagnostic because of severe motion artifacts (severe dyspnea) or poor venous access. In 8-10% of examinations, the scans are suboptimal in quality; these allow for confident evaluation of only the central pulmonary arteries. In addition to CT pulmonary angiograms, CT venograms obtained may be useful in patients with a nondiagnostic angiogram, particularly if it is positive for DVT (see Image 5). MRISection 6 of 9
FindingsFew investigators have reported the feasibility of MRI in the evaluation of PE. However, the role of MRI is mostly limited to the evaluation of patients who have impaired renal function or other contraindications for the use of iodinated contrast material.22, 23 Newer blood-pool contrast agents and respiratory navigators may enhance the role of MRI in the diagnosis of PE. NUCLEAR MEDICINESection 7 of 9
FindingsThe 1990 PIOPED trial was a multi-institutional study of V/Q scanning and pulmonary angiography.24 The investigators revealed that a V/Q scan with normal findings virtually excludes PE and that a scan with high-probability findings is virtually diagnostic for the disease. However, the diagnosis was established or excluded in only 174 (24.4%) of 713 patients—that is, those with scans showing clear and concordant clinical and lung findings. Most patients, including those with underlying cardiopulmonary disease, had indeterminate or nondiagnostic V/Q findings and required additional imaging. Therefore, in patients with abnormal chest radiographs, the use of helical CT scanning rather than scintigraphy as the primary screening test is reasonable. MULTIMEDIASection 8 of 9
REFERENCESSection 9 of 9
Acute Pulmonary Embolism (Helical CT) excerpt Article Last Updated: May 14, 2008 | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
