Density of lesions as measured by hounsfield units on chest CT is used to characterize the nature of thoracic lesions as solid versus cystic versus calcific. The aim of the study was to see if density of pleural effusion as measured by HU on chest CT could be used to differentiate transudative versus exudative pleural effusion; a non-invasive technique not being used for this purpose at this time.
38 patients with pleural effusion who had thoracentesis done and had chest CT were identified and evaluated retrospectively. Pleural fluid was classified as transudate or exudate using light’s criteria.Pleural fluid density using hounsfield units was calculated on CT chest as a mean of fluid density taken at four rectangular areas as shown in the figure.
17 patients had transudative effusion with mean(±SD) HU of 7.8(5.6). 21 had exudative effusion with mean(±SD) HU of 17.5(7.4). Based on the results 15 HU was used as cut off to differentiate between transudative and exudative effusion.
HU on CT of more than 15 can be used to characterize a pleural effusion as an exudate. Transudative effusions for the most part have HU less than 15. However HU <15 can be seen in exudative pleural effusions. Therefore this technique cannot be used to differentiate transudative from exudative pleural effusion when HU is <15.
In patients with HU >15 further work up is indicated and will include thoracentesis and possibly pleural biopsy. In patients with HU <15 thoracentesis is indicated only when the clinical picture indicates that the pleural effusion cannot be explained by disorders that cause transudative effusion.
Vijay Rupanagudi, None.