Acute Pulmonary Embolism (Helical CT)

 

INTRODUCTION

Background: Pulmonary embolism (PE) was clinically described in the early 1800s, and von Virchow first described the connection between venous thrombosis and PE. In 1922, Wharton and Pierson reported the first radiographic description of PE.

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. More recently, with the widespread availability of faster CT scanners, CT has emerged as another important diagnostic test for the evaluation of not only PE but also deep venous thrombosis (DVT) in select patients.

 

Pathophysiology: Three 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.

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. 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:

 

  • Primary (genetic) conditions

     

    • Mutations in factor V

       

    • Antithrombin III deficiency

       

    • Protein C or protein S deficiency

       

    • Fibrinolysis defects

     

  • Secondary (acquired) conditions

     

    • High risk for thrombosis

       

    • Prolonged bed rest or immobilization (many hours of travel)

       

    • Myocardial infarction

       

    • Tissue damage (surgery, fracture, burns)

       

    • Cancer

       

    • Prosthetic cardiac valves

       

    • Disseminated intravascular coagulation

       

    • Lupus anticoagulant

       

    • Atrial fibrillation

       

    • Cardiomyopathy

       

    • Nephrotic syndrome

       

    • Hyperestrogenic states

       

    • Oral contraceptive use

       

    • Sickle cell anemia

       

    • Smoking

 

Frequency:

  • In the US: In the United States, DVT 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.

Mortality/Morbidity: Treatment 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%.

Race: No racial predispositions to thromboembolism are known.

Sex: The 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).

Age: PE 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).

Anatomy: Knowledge 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 Details: PE 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 lightheadedness 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 abnormalities may develop in acute PE; these include T-wave changes, ST-segment abnormalities, and left- or right-axis deviation.

Preferred Examination: In patients with suspected PE, chest radiographic findings may indicate if lung scanning (V/Q) or helical CT 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.

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. In patients with suspected DVT, the workup should start with leg sonography.

Limitations of Techniques: V/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 to patients with severe allergy to the contrast material.

Small (subsegmental) emboli may be missed with CT angiography. Compared with CT, 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 may be missed at pulmonary angiography.

DIFFERENTIALS

Aspiration Pneumonia
Asthma
Atelectasis, Lobar
Extrinsic Allergic Alveolitis (Hypersensitivity Pneumonitis)
Lung, Arteriovenous Malformation
Lung, Metastases
Lung, Trauma
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 Considered:

Aortic dissection
Mediastinitis, acute
Pneumomediastinum

 

X-RAY

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, a wedge-shaped, pleura-based triangular opacity with an apex pointing toward the hilus (Hampton hump). This finding is suggestive of PE but infrequently observed.

Degree of Confidence: Prominent central pulmonary arteries, cardiomegaly (especially on the right side of the heart) and pulmonary edema are other findings. In the appropriate clinical setting, they could be consistent with acute cor pulmonale. A normal-appearing chest radiograph in a patient with severe dyspnea and hypoxemia 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.


CAT SCAN

Findings: Recent technical advances in CT, including the development of multidetector-array scanners, have led to the emergence of CT as an important diagnostic technique in suspected PE. Contrast-enhanced CT 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.

CT show emboli directly, as does pulmonary angiography, and it is also noninvasive, cheaper, and widely available. CT is the only test that can provide significant additional information related to alternate diagnoses; this is a clear advantage of CT compared with either pulmonary angiography or scintigraphy.

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. 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.

When 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 the following criteria are met: (1) It is eccentric and contiguous with 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.

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 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 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 a recent study. 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, 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%.

Recently, investigators have reported uneventful clinical outcomes in patients (with a negative predictive value [NPV] 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, the subsequent morbidity rate with PE does not appear to be high.

The new multidetector-row 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, and their findings are easier to interpret with this technique.

Accuracy of helical CT pulmonary angiography

 
Reference No. of Patients Sensitivity, % Specificity, % Collimation and Anatomic Level

 

Remy-Jardin et al, 1992 42 100 96 5 mm, segmental
Goodman et al, 1995 20 86 92 5 mm, segmental
Remy-Jardin et al, 1996 75 91 78 3 and 5 mm, segmental
Mayo et al, 1997 142 87 95 5 mm, segmental
Garg et al, 1998 54 67 100 3 mm, subsegmental
Drucker et al, 1998 47 53-60 81-97 5mm, segmental

False Positives/Negatives: The pitfalls, 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 3).

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 4).

MRI

Findings: Few investigators have reported the feasibility of MRI. 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. Newer blood-pool contrast agents and respiratory navigators may enhance the role of MRI in the diagnosis of PE.

NUCLEAR MEDICINE

Findings: In 1990, the PIOPED trial, a multiinstitutional study of V/Q scanning and pulmonary angiography, 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, ie, 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 rather than scintigraphy as the primary screening test is reasonable.

PICTURES
Caption: Picture 1. Acute pulmonary embolism (helical CT). Acute pulmonary embolism in a 53-year-old man. CT angiogram shows an intraluminal filling defect that occludes the anterior basal segmental artery of the right lower lobe. Also present is an infraction of the corresponding lung, which is indicated by a triangular pleura-based consolidation (Hampton hump).
Picture Type: CT
Caption: Picture 2. Acute pulmonary embolism (helical CT). This young man experienced acute chest pain and shortness of breath after a transcontinental flight. CT angiography 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).
Picture Type: CT
Caption: Picture 3. Acute pulmonary embolism (helical CT). Chronic pulmonary embolism in a 69-year-old man with known pulmonary arterial hypertension. CT angiogram shows an eccentric mural thrombus with punctate calcification along the anterior wall of the right lower interlobar artery.
Picture Type: CT
Caption: Picture 4. Acute pulmonary embolism (helical CT). Perivascular segmental enlarged lymph nodes in a 55-year-old man with suspected pulmonary embolism. CT angiogram obtained at the level of the lower lobes level shows prominent extraluminal soft tissue interposed between the artery and the bronchus.
Picture Type: CT
Caption: Picture 5. Acute pulmonary embolism (helical CT). Acute deep venous thrombosis in a 65-year-old man with suspected pulmonary embolism. CT venograms show intraluminal filling defects in the bilateral superficial femoral veins.
Picture Type: CT