Diagnosis

Septic pulmonary emboli

Findings

The chest radiograph demonstrates multiple rounded parenchymal nodules, several with central lucency suggesting cavitation (figure 1). There is no obvious pleural disease. The chest CT shows multiple parenchymal nodules that occur in a vascular distribution (figure 2). Some of these nodules demonstrate a feeding vessel; some lesions are cavitary. Multiple pleural-based wedge-shaped densities are noted that were not identified on the plain chest radiograph.

Discussion

Septic pulmonary emboli travel to the lungs from numerous sources, including infected heart valves, peripheral sites of septic thrombophlebitis, and infected indwelling venous catheters.¹ Other patients at risk are those with odontogenic infections.² In patients with a history of intravenous drug use, the most common source of septic pulmonary emboli is an infected tricuspid valve.¹ An echocardiogram can be obtained to evaluate for the presence of tricuspid valve vegetations. However, echocardiography may be limited technically and very small vegetations (less than 3 to 4 mm) may not be detected.³

Patients with septic pulmonary emboli who are diagnosed early and treated with broad-spectrum antibiotics have a better prognosis than patients with a delayed diagnosis. Establishing the diagnosis of septic pulmonary emboli is not always straightforward, as the radiographic findings may be non-specific. Blood cultures may be negative initially and there may not be a detectable heart murmur.¹

Documented CT findings in septic pulmonary emboli include multiple peripheral parenchymal nodules, a feeding vessel sign, cavitation, and wedge-shaped peripheral lesions abutting the pleura.¹ This case demonstrates all of these findings. Additional CT findings associated with septic pulmonary emboli include infiltrates,² air bronchograms within nodules, and extension into the pleural space.¹ When the chest radiograph is indeterminate, further evaluation with CT can be extremely valuable as it may disclose unsuspected pulmonary nodules and wedge-shaped subpleural densities.³ The extent of disease documented by CT is generally greater than that detected by radiography. There have been patients with negative chest radiographs in whom subsequent CT examination confirmed septic pulmonary emboli.¹

The diagnosis of septic pulmonary emboli is made on the basis of characteristic imaging abnormalities in association with one or more of the following criteria: positive blood cultures, tricuspid valve vegetations proven by echocardiography, a clinical course compatible with bacterial endocarditis (a new murmur which resolves after treatment), or other signs of septic embolization (splenomegaly, petechiae, or both).² In this case, the patient had Staphylococcus aureus bacteremia and a heart murmur.

Chest radiography remains the screening test of choice for septic pulmonary emboli.¹ Not every patient with suspicion of septic pulmonary emboli requires CT evaluation. However, studies have shown that the pattern of parenchymal involvement in septic pulmonary emboli is more recognizable and specific by CT than by radiography.¹

1. Kuhlman JE, Fishman EK, Teigen C. Pulmonary septic emboli: Diagnosis with CT. Radiology. 1990;174:211-213.

2. Iwasaki Y, Nagata K, Nakanishi M, et al. Spiral CT findings in septic pulmonary emboli. Eur J Radiol. 2001;37:190-194.

3. Huang R, Naidich D, Lubat E, et al. Septic pulmonary emboli: CT-radiographic correlation. AJR Am J Roentgenol. 1989;153:41-45.