Chest

Chest

Volume 143, Issue 5, May 2013, Pages 1460-1471
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Special Features
Chronic Pulmonary Emboli and Radiologic Mimics on CT Pulmonary Angiography: A Diagnostic Challenge

https://doi.org/10.1378/chest.12-1384Get rights and content

Chronic pulmonary thromboembolism (CPE) is a challenging diagnosis for clinicians. It is an often-forgotten diagnosis and can be difficult to detect and easily misdiagnosed. The radiologic features on CT pulmonary angiography are subtle and can be further compounded by pathologic mimics and unusual findings observed with disease progression. Diagnosis is important because CPE can lead to progressive pulmonary hypertension, morbidity, and mortality. Moreover, chronic thromboembolic pulmonary hypertension is the only category of pulmonary hypertension with an effective curative treatment in the form of pulmonary endarterectomy. Therefore, CPE must be considered and recognized early. The features of chronic pulmonary emboli on CT scans can be categorized into vascular or parenchymal findings. Endoluminal signs include totally or partially occlusive thrombi and webs and bands. Parenchymal features such as mosaic attenuation and pulmonary infarction are also noted, in addition to features of pulmonary artery hypertension. Additional findings have been noted, including cavitation of infarcts, microbial colonization of cavities, and bronchopleural fistulae. As CPE can be diagnosed at different stages of its disease pathway, such findings may not necessarily arouse suspicion toward a causative diagnosis of chronic embolism. To aid diagnosis for clinicians, this article describes the characteristic vascular and parenchymal CT scan features of chronic emboli, as well as important ancillary findings. We also provide an illustrative case series focusing on CT pulmonary angiography specifically as an imaging modality to highlight the progressive nature of CPE and its sequelae, as well as important radiologic mimics to consider in the differential diagnosis.

Section snippets

Natural History

With treatment, most acute emboli undergo complete resolution. In some patients, for reasons that are unclear, the clot fails to resolve completely and may undergo organization and incorporation into the vessel wall. This may be due to underlying abnormalities in fibrinolysis or possibly recurrent embolism. The resulting thrombus is thought to trigger an arteriopathy in vessels distal to the occluded areas. A compensatory diversion of blood toward the patent vascular bed is also thought to

Alternative Imaging Techniques

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,

Filling Defects in the Arterial Bed

CPE can result in complete occlusion of the vessel lumen. The angiographic appearance has been described as a “pouch defect” because the contrast material has a convex margin within the affected vessel.21 Contraction of the thrombus can cause retraction of the vessel wall, thereby reducing its diameter. On CT scans, the features to look for are (1) sudden truncation of occluded vessels, (2) affected vessels appearing smaller than the patent vessels on the opposite side, and (3) an inability to

Pleural Signs

Pleural effusions are estimated to be present in up to 61% of patients with pulmonary emboli.39 They tend to be small, are commonly unilateral, and generally occupy less than one-third of the involved hemithorax. However, large or bilateral effusions may occur. The fluid is usually an exudate. Loculated effusions have also been described and in some cases have shown improvement with systemic anticoagulant therapy.40

Common Parenchymal Signs

Oligemia distal to occluded vessels causes a redistribution of blood away from the affected areas. These irregularities in perfusion are demonstrated on CT scans as a sharply demarcated, mosaic pattern of attenuation (Fig 6).4 Hypoperfused areas are of low attenuation, with increased attenuation where the vessels have become larger and more prominent. Mosaic attenuation is nonspecific and can be caused by a variety of other pulmonary conditions, including small airways disease and primary

Rare Parenchymal Findings

Aseptic cavitation of large pulmonary infarcts secondary to chronic thrombus (Fig 8) has been occasionally documented in the literature; it is rare and more typically occurs in preexisting areas of infarction.45 Necrosis of infarcted tissue to form a cavity is usually initially aseptic, but secondary bacterial infection or microbial colonization (Fig 9) may occur. Alternatively, infection may be present at the time of cavitation. Cavitation can also be complicated by the formation of a

Radiologic Mimics

There are a myriad of disorders that can mimic CPE radiologically, including a variety of types of thromboembolism (acute thromboembolism, tumor emboli, and in situ thrombus), and inflammatory (arteritis), malignant (primary sarcoma of the pulmonary artery), and developmental causes (proximal interruption of the pulmonary artery), as well as idiopathic pulmonary hypertension itself.

Treatment

The definitive treatment of thromboembolic hypertension is PEA, entailing removal (stripping) of the diseased intima in patients with proximal thromboembolic disease.57 Careful selection of patients for PEA is critical and requires multidisciplinary meetings with medical and surgical specialists and high-quality imaging.58 PEA is not suitable if there is distal or microvascular thromboembolic disease (usually in 20% to 40% of cases), or significant comorbidities, especially COPD or severe left

Conclusions

There are several vascular and parenchymal CT scan features that aid the diagnosis of CPE. Some are less common than others. A high index of suspicion needs to be maintained because the nonspecific clinical presentation and pathologic mimics can create diagnostic dilemmas, potentially delaying definitive treatment. While unusual findings like cavitation of infarcts with or without secondary colonization and bronchopleural fistulae can occur in many other disease pathologies, their presence

Acknowledgments

Financial/nonfinancial disclosures: The authors have reported to CHEST that no potential conflicts of interest exist with any companies/organizations whose products or services may be discussed in this article.

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