Pulmonary Embolism

Pulmonary 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.

Image 1 :Computed tomography angiogram in a 53-year-old man with acute pulmonary embolism. This image shows an intraluminal filling defect that occludes the anterior basal segmental artery of the right lower lobe. Also present is an infarction of the corresponding lung, which is indicated by a triangular, pleura-based consolidation (Hampton hump).

Image 2 :Computed tomography angiography in a young man who experienced acute chest pain and shortness of breath after a transcontinental flight. This image demonstrates a clot in the anterior segmental artery in the left upper lung (LA2) and a clot in the anterior segmental artery in the right upper lung (RA2).

Image 3 :Computed tomography angiogram in a 69-year-old man with known pulmonary arterial hypertension and a history of chronic pulmonary embolism. This image shows an eccentric mural thrombus with punctate calcification along the anterior wall of the right lower interlobar artery


Image 4 :Computed tomography angiogram in a 55-year-old man with possible pulmonary embolism. This image was obtained at the level of the lower lobes and shows perivascular segmental enlarged lymph nodes as well as prominent extraluminal soft tissue interposed between the artery and the bronchus.



Image 5 :Computed tomography venograms in a 65-year-old man with possible pulmonary embolism. This image shows acute deep venous thrombosis with intraluminal filling defects in the bilateral superficial femoral veins.



Radiography
Findings
Chest 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 Confidence
A 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
Computed Tomography
Technical 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.CT venograms can be acquired 3-4 minutes after the start of the administration of contrast material. The new multidetector-row CT (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). With introduction of dual-source CT technology, ECG-gated CTA of the chest may become practical and help provide clinicians with cardiac functional information.There is ongoing research in the field of postprocessing of CT data for acute PE, one dealing with the detection of perfusion defects as an adjunct to transverse CT scans for detection of small peripheral PE and another focusing on the automatic computer-aided detection of endoluminal clots. Efforts should be made to minimize the radiation dose by using all available equipment-specific dose reduction techniques.
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 first Image below and Image 1 in Multimedia). 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 second Image below and Image 2 in Multimedia), (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 Confidence
In 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.
False Positives/Negatives
The 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 below and Image 4 in Multimedia).
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 below and Image 5 in Multimedia).
Magnetic Resonance Imaging
Findings
Few 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 Imaging
Findings
The 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.A second group of PIOPED investigators (PIOPED II) formulated recommendations based on their own studies as well as others.7 The authors included information regarding clinical probability assessment scoring indexes as well as empirical assessment, flowcharts, and various patient scenarios (eg, patients with impaired renal function, pregnant patients, etc).
References
Boyden EA. Segmental Anatomy of the Lungs: Study of the Patterns of the Segmental Bronchi and Related Pulmonary Vessels. New York, NY: McGraw-Hill; 1955:23-32.
Mitchell RN, Kumar V. Hemodynamic disorders, thrombosis, and shock. In: Kumar V, Cotran RS, Robbins SL, eds. Basic Pathology. 6th ed. Philadelphia, Pa: WB Saunders; 1997:60-80.
Wharton LR, Pierson JW. Minor forms of pulmonary embolism after abdominal operations. JAMA. 1922;79:1904-10.
Remy-Jardin M, Pistolesi M, Goodman LR, et al. Management of suspected acute pulmonary embolism in the era of CT angiography: a statement from the Fleischner Society. Radiology. Nov 2007;245(2):315-29. [Medline]. [Full Text].
Patel S, Kazerooni EA. Helical CT for the evaluation of acute pulmonary embolism. AJR Am J Roentgenol. Jul 2005;185(1):135-49. [Medline]. [Full Text].
Silverstein MD, Heit JA, Mohr DN, et al. Trends in the incidence of deep vein thrombosis and pulmonary embolism: a 25-year population-based study. Arch Intern Med. Mar 23 1998;158(6):585-93. [Medline]. [Full Text].
Stein PD, Woodard PK, Weg JG, et al. Diagnostic pathways in acute pulmonary embolism: recommendations of the PIOPED II Investigators. Radiology. Jan 2007;242(1):15-21. [Medline]. [Full Text].
Abcarian PW, Sweet JD, Watabe JT, Yoon HC. Role of a quantitative D-dimer assay in determining the need for CT angiography of acute pulmonary embolism. AJR Am J Roentgenol. Jun 2004;182(6):1377-81. [Medline]. [Full Text].
Doshi SK, Negus IS, Oduko JM. Fetal radiation dose from CT pulmonary angiography in late pregnancy: a phantom study. Br J Radiol. Apr 14 2008;[Medline].
Yankelevitz DF, Gamsu G, Shah A, et al. Optimization of combined CT pulmonary angiography with lower extremity CT venography. AJR Am J Roentgenol. Jan 2000;174(1):67-9. [Medline]. [Full Text].
Oser RF, Zuckerman DA, Gutierrez FR, Brink JA. Anatomic distribution of pulmonary emboli at pulmonary angiography: implications for cross-sectional imaging. Radiology. Apr 1996;199(1):31-5. [Medline]. [Full Text].
Yasui T, Tanabe N, Terada J, et al. Multidetector-row computed tomography management of acute pulmonary embolism. Circ J. Dec 2007;71(12):1948-54. [Medline]. [Full Text].
Patel S, Kazerooni EA, Cascade PN. Pulmonary embolism: optimization of small pulmonary artery visualization at multi-detector row CT. Radiology. May 2003;227(2):455-60. [Medline].
Remy-Jardin M, Remy J, Deschildre F, et al. Diagnosis of pulmonary embolism with spiral CT: comparison with pulmonary angiography and scintigraphy. Radiology. Sep 1996;200(3):699-706. [Medline]. [Full Text].
Loud PA, Katz DS, Klippenstein DL, Shah RD, Grossman ZD. Combined CT venography and pulmonary angiography in suspected thromboembolic disease: diagnostic accuracy for deep venous evaluation. AJR Am J Roentgenol. Jan 2000;174(1):61-5. [Medline]. [Full Text].
Garg K, Kemp JL, Wojcik D, et al. Thromboembolic disease: comparison of combined CT pulmonary angiography and venography with bilateral leg sonography in 70 patients. AJR Am J Roentgenol. Oct 2000;175(4):997-1001. [Medline]. [Full Text].
Remy-Jardin M, Remy J, Wattinne L, Giraud F. Central pulmonary thromboembolism: diagnosis with spiral volumetric CT with the single-breath-hold technique--comparison with pulmonary angiography. Radiology. Nov 1992;185(2):381-7. [Medline]. [Full Text].
Goodman LR, Curtin JJ, Mewissen MW, et al. Detection of pulmonary embolism in patients with unresolved clinical and scintigraphic diagnosis: helical CT versus angiography. AJR Am J Roentgenol. Jun 1995;164(6):1369-74. [Medline]. [Full Text].
Mayo JR, Remy-Jardin M, Müller NL, et al. Pulmonary embolism: prospective comparison of spiral CT with ventilation-perfusion scintigraphy. Radiology. Nov 1997;205(2):447-52. [Medline]. [Full Text].
Garg K, Welsh CH, Feyerabend AJ, et al. Pulmonary embolism: diagnosis with spiral CT and ventilation-perfusion scanning--correlation with pulmonary angiographic results or clinical outcome. Radiology. Jul 1998;208(1):201-8. [Medline]. [Full Text].
Drucker EA, Rivitz SM, Shepard JA, Boiselle PM, et al. Acute pulmonary embolism: assessment of helical CT for diagnosis. Radiology. Oct 1998;209(1):235-41. [Medline]. [Full Text].
Gupta A, Frazer CK, Ferguson JM, et al. Acute pulmonary embolism: diagnosis with MR angiography. Radiology. Feb 1999;210(2):353-9. [Medline]. [Full Text].
Meaney JF, Weg JG, Chenevert TL, et al. Diagnosis of pulmonary embolism with magnetic resonance angiography. N Engl J Med. May 15 1997;336(20):1422-7. [Medline]. [Full Text].
The PIOPED Investigators. Value of the ventilation/perfusion scan in acute pulmonary embolism. Results of the prospective investigation of pulmonary embolism diagnosis (PIOPED). JAMA. May 23-30 1990;263(20):2753-9. [Medline].
Diffin DC, Leyendecker JR, Johnson SP, Zucker RJ, Grebe PJ. Effect of anatomic distribution of pulmonary emboli on interobserver agreement in the interpretation of pulmonary angiography. AJR Am J Roentgenol. Oct 1998;171(4):1085-9. [Medline]. [Full Text].
Garg K, Sieler H, Welsh CH, Johnston RJ, Russ PD. Clinical validity of helical CT being interpreted as negative for pulmonary embolism: implications for patient treatment. AJR Am J Roentgenol. Jun 1999;172(6):1627-31. [Medline]. [Full Text].
Goodman LR, Lipchik RJ, Kuzo RS, et al. Subsequent pulmonary embolism: risk after a negative helical CT pulmonary angiogram--prospective comparison with scintigraphy. Radiology. May 2000;215(2):535-42. [Medline]. [Full Text].
Jackson CL, Franklin J. With a system of nomenclature the bronchial tree and lungs correlated. Chest. 1943;9:319-26. [Full Text].
Novelline RA, Baltarowich OH, Athanasoulis CA, et al. The clinical course of patients with suspected pulmonary embolism and a negative pulmonary arteriogram. Radiology. Mar 1978;126(3):561-7. [Medline].
Piasecki DP, Poynton AR, Mintz DN, et al. Thromboembolic disease after combined anterior/posterior reconstruction for adult spinal deformity: a prospective cohort study using magnetic resonance venography. Spine. Mar 15 2008;33(6):668-72. [Medline].
Schoepf UJ, Costello P. CT angiography for diagnosis of pulmonary embolism: state of the art. Radiology. Feb 2004;230(2):329-37. [Medline]. [Full Text].