Ultrasound in peripheral pulmonary air-fluid lesions. Color Doppler imaging as an aid in differentiating empyema and abscess.

BACKGROUND: The aim of this study was to reevaluate the clinical significance of sonographic appearances, in particular the application of color Doppler ultrasound imaging, in discriminating peripheral air-fluid lung abscess from empyema.

METHODS: We retrospectively studied patients who had had peripheral air-fluid lesions due to empyema or lung abscess and who had undergone color Doppler ultrasound and grayscale ultrasound examinations between January 2003 and October 2007. A total of 34 patients with confirmed lung abscess and 30 patients with empyema were identified. The four sonographic characteristics observed and analyzed were the wall characteristics of the lesions (wall width, luminal margin, outer margin, and chest wall angle), split pleura sign, internal echogenicity (suspended microbubble sign, complex-septated effusions, and passive atelectasis), and identification of color Doppler ultrasound vessel signals in pericavitary lesions (consolidation or atelectasis).

RESULTS: Among the sonographic characteristics, complex-septated effusions and passive atelectasis were specific for empyema, but the sensitivity was only 40% (n = 12 of 30) and 47% (n = 14 of 30), respectively. The identification of color Doppler ultrasound vessel signals in pericavitary consolidation was the most useful and specific for identifying lung abscesses. In our series, if we define the identification of color Doppler ultrasound vessel signals in a pericavitary consolidation as a predictor for peripheral lung abscess, we can achieve sensitivity, specificity, positive predictive value, and negative predictive value of 94%, 100%, 100%, and 94%, respectively.

CONCLUSIONS: Color Doppler ultrasound is a powerful tool for differentiating the peripheral air-fluid abscess from empyema, with high specificity and without any risk.