Applied Radiology

Dr. Reittner and Dr. Müller are with the Department of Radiology, Vancouver General Hospital, University of British Columbia, Vancouver, BC, Canada.

P ulmonary embolism (PE) is a frequent cause of morbidity and mortality. ¹ It has been estimated that 650,000 cases of PE occur each year in the United States, with approximately 50,000 to 100,000 deaths resulting. ² Clinically, the diagnosis of PE is often difficult to establish, because the symptoms are nonspecific and may be confused with a variety of other cardiopulmonary disorders. ^1,2 Goldhaber et al ³ reported that only 16 (30%) of 54 patients with PE at autopsy had a correct antemortem diagnosis.

For many years, ventilation-perfusion (V/Q) scintigraphy has been the main imaging modality used in the evaluation of patients with suspected PE. ^1,4,5 A high probability V/Q scan provides sufficient certainty to confirm the diagnosis of PE, while a normal or near normal scan reliably excludes the diagnosis. ⁵ However, in the PIOPED (Prospective Investigation of Pulmonary Embolism Diagnosis) study, indeterminate scans, present in 39% (364 of 931) of patients, showed a 30% incidence of PE; and low-probability scans, seen in 34% (312 of 931) of patients, had a 14% incidence. Based on these data, the authors concluded that indeterminate and low probability lung scans (i.e., two-thirds of V/Q scans in the PIOPED study) were not useful in establishing or excluding PE. ⁵ Furthermore, although there was good interobserver agreement for high-probability and normal V/Q scans, there was a 25% to 30% disagreement between observers in the interpretation of intermediate and low-probability scans. ⁵

Currently, pulmonary angiography is considered the gold standard for diagnosing PE. ¹ Pulmonary angiography allows direct visualization of the pulmonary arterial tree and detection of filling defects, typical for PE. However, it is an invasive test with associated morbidity (6%) and mortality (0.5%) and is underutilized. ⁶ It has been estimated that even in academic centers only 12% to 14% of patients with nondiagnostic V/Q scan undergo pulmonary angiography. ^6,7

Although incidental detection of PE has been described with conventional CT, slow data acquisition time precluded the inclusion of conventional CT in diagnostic algorithms for the diagnosis of PE. ^8,9 With the introduction of spiral CT technology, it is now possible to image the entire chest in a short period of time and analyze the pulmonary arteries during the peak of contrast enhancement. Several studies have demonstrated that spiral CT has a high sensitivity and specificity for the diagnosis of PE. ^9-15

The aim of this article is to review a practical approach for the use of spiral CT in diagnosing pulmonary embolism and to illustrate the characteristic imaging features.

Technique

Optimal assessment of the pulmonary vessels on spiral CT requires careful attention to several parameters including scan collimation, imaging volume, and contrast enhancement. We currently use the following protocol: spiral CT during a 20 to 30 second breath-hold using 3-mm collimation, a table speed of 5 to 6 mm/sec, pitch 1.7 to 2.0, 120 kVp, and 180 to 320 mA. Images are reconstructed at 1.5-mm intervals and a field-of-view appropriate for the size of the patient.

The lung volume scanned should be large enough to include all segmental and subsegmental pulmonary arteries. This can be achieved by scanning from the top of the aortic arch to the dome of the diaphragm. Although the scans can be performed in the craniocaudal direction, it has been shown that scanning caudocranially helps to minimize motion artifacts, particularly in patients unable to hold their breath for the duration of the scan. ^16,17

Non-ionic iodinated contrast material is administered through an antecubital venous access or a central line using a power injector. ^16,17 Injection rates from 2 to 7 mL/ sec have been reported. ⁴ We use 120 to 150 mL of 30% iodinated contrast material, injected at a rate of 4 mL/sec. In hemodynamically stable patients, a 10- to 15-second scan delay provides optimal contrast enhancement of the pulmonary arteries. This delay may need to be increased in patients with severe pulmonary hypertension or right-sided heart failure. In order to determine the optimal time delay, we perform a test injection to assess the circulation time. A total of 20 mL of contrast material is injected at a rate of 4mL/sec and serial scans are performed at 3 to 5 second intervals for 20 seconds at the level of the main pulmonary artery. To ensure opacification of the peripheral arteries during the diagnostic study, 5 seconds are added to the time to peak enhancement of the main pulmonary artery. ^16,17

Images are viewed at settings for pulmonary vasculature (window width, 250 HU; window level, 35 HU) and lung parenchyma (window width, 1,500 HU; window level, -700 HU). In selected cases, multiplanar reformatting may be helpful in demonstrating the extent of the pulmonary emboli. ¹⁷

Image interpretation

Knowledge of the bronchovascular anatomy is essential for correct assessment of the images. The segmental arteries are located adjacent to the corresponding branches of the bronchial tree and are situated either medially (upper lobes) or laterally (lower lobes, lingula, middle lobe) to the bronchi. ^18,19

Characteristic findings of acute PE are: 1) partial central or marginal filling defect surrounded by a thin rim of contrast material (figures 1 and 2); or 2) complete filling defect with obstruction of an entire vessel section ("vessel cut-off sign," figure 3). Pulmonary arteries completely obstructed by an acute embolus usually have an increased diameter. Diagnosis of acute PE requires assessment of both the vascular and parenchymal findings. Assessment of the lung windows is important, not only to identify the pulmonary arteries by their proximity to the bronchi, but also to assess for the presence of ancillary signs that may be helpful in suggesting the presence of pulmonary embolism. ²⁰ The most helpful ancillary sign is the presence of a non-enhancing pleural-based wedge-shaped pulmonary opacity. ²⁰ Linear opacities, presumably representing plate-like atelectasis, are also seen with increased frequency on CT in patients with acute PE. ²⁰ In a study of 98 patients assessed due to clinical suspicion of acute PE, non-enhancing pleural-based wedge-shaped opacities were seen on CT in 62% of patients with pulmonary embolism compared to 27% of patients without PE, and linear opacities were seen in 46% and 21% of patients, respectively. ²⁰ Other findings, such as areas of decreased attenuation and pleural effusion, were found with equal frequency in patients with and without acute PE.

Findings suggestive of chronic PE include: 1) filling defects with obtuse angles to the vessel wall (figure 4); 2) irregularity and narrowing of the arteries; 3) evidence of recanalization of the thrombus; and 4) presence of linear filling defects ("arterial webs," figure 5). In patients with chronic PE, occlusion of small pulmonary arteries leads to blood flow redistribution to uninvolved vessels and a mosaic pattern of attenuation and perfusion on CT (figure 6).

Interpretive pitfalls and artifacts

A number of technical, anatomical, and patient-related pitfalls may lead to misinterpretation of the CT images. Technical failures occur in 1% to 5% of scans, and usually are due to motion artifacts in dyspneic patients or insufficient vascular enhancement. ¹⁰ In patients with severe dyspnea, motion artifacts can produce respiratory misregistration and inadequate sampling of the pulmonary vessels, resulting in focal areas of decreased attenuation which can mimic a clot.

Streak artifacts originating from dense intravenous contrast within the superior vena cava may obscure the right main and upper lobe pulmonary arteries. These streak artifacts are frequently observed when using a craniocaudal image acquisition and highly concentrated contrast material. These artifacts can be minimized or eliminated by reducing the iodine concentration or scanning caudocranially. ^16,21

The lymphatic and connective tissue located adjacent to the pulmonary arteries may mimic the appearance of pulmonary emboli. This pitfall can be minimized by careful review of the images and the use of additional imaging rendering tools such as cine-viewing (which we use routinely) and multiplanar reconstructions.

Diagnostic accuracy of spiral CT

The reported diagnostic accuracy of spiral CT has varied depending on the technique used, the patient population, and whether the authors have limited the analysis to the central pulmonary arteries down the level of the segmental vessels or have included subsegmental arteries. ^9-15 The results of the various studies are summarized in Table 1. In a study of 139 patients in whom the CT scans were assessed prospectively, the sensitivity of spiral CT for the detection of pulmonary embolism was 87% and the specificity was 95%. ¹³ Isolated subsegmental pulmonary emboli accounted for two of the six false-negative spiral CT scan interpretations. Recently, a presentation of preliminary data from
the prospective European Multicentre Study (ESTIPEP), which included 391 patients, demonstrated a sensitivity of spiral CT of 95% and a specificity of 97%. ¹⁵ Other investigators have reported lower sensitivities for spiral CT, however, particularly in studies in which CT scans were performed only in patients with indeterminate V/Q scans. In one study, the authors compared spiral CT with pulmonary angiography in 20 patients with indeterminate V/Q scans. ¹⁰ When only the central arteries were analyzed, the sensitivity of CT was 86% and the specificity 92%. When subsegmental vessels were included, the sensitivity of CT decreased to 63%. Similar results were reported in a study of 54 patients with indeterminate V/Q scans. ¹⁴ These results indicate that although spiral CT has a high sensitivity in the detection of central emboli, it is of limited value in the diagnosis of subsegmental emboli. It should be noted, however, that the clinical significance of isolated subsegmental emboli, especially in patients with no underlying disease, is controversial. ⁵ Furthermore, it has been shown that even though pulmonary angiography is considered the gold standard for the diagnosis of pulmonary embolism, the interobserver agreement for the diagnosis of subsegmental emboli on angiography is only 66%. ⁵ Preliminary results suggest that the accuracy of spiral CT in the diagnosis of subsegmental emboli will be improved with the use of thinner sections such as 1 or 2 mm collimation. ¹⁷

Diagnostic algorithm

Given the data in the literature, the following algorithm is recommended for the evaluation of patients suspected of having acute pulmonary embolism ²² : 1) All patients should have a chest radiograph, the main role of which is to exclude abnormalities, such as acute pneumonia, that may mimic pulmonary embolism clinically; 2) Patients with symptoms or signs of deep vein thrombosis should undergo evaluation of the leg veins, the most recommended technique being Doppler ultrasound. If Doppler is positive, the patient can be considered to have acute pulmonary embolism and usually does not require further investigation; 3) Patients who have no symptoms or signs of deep vein thrombosis and symptomatic patients who have a negative Doppler ultrasound examination, and who do not have extensive underlying parenchymal lung disease or COPD, should undergo ventilation-perfusion scintigraphy. A high-probability or normal V/Q scan can
be considered diagnostic. All other patients should undergo further evaluation with contrast enhanced spiral CT; 4) Patients who have extensive pulmonary parenchymal disease or COPD and patients who have non-diagnostic V/Q scan should undergo contrast-enhanced spiral CT; and 5) Patients in whom the CT scans are suboptimal and patients in whom the CT scan results are negative, but who have a high clinical index of suspicion for acute pulmonary embolism, should undergo pulmonary angiography.

Conclusion

Several studies have shown that spiral CT can play a major role in the diagnosis of pulmonary embolism. Optimal assessment requires knowledge of anatomy, careful attention to technique, and awareness of potential diagnostic pitfalls. AR

References

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