Introduction and Objectives Several studies have described the typical imaging features of empyema on CT, including pleural thickening, pleural enhancement, septation and loculation. However, there are further CT features of empyema which are less well described, or have only been reported in small groups of patients. The aim of this study was to establish the prevalence of the different CT imaging features of empyema in a large, well-characterised cohort of empyema patients.

Methods This was a retrospective cohort study comprising a subgroup of patients from the MIST 2 trial, who underwent a pre-treatment thoracic CT at a single institution. Patients in this trial had confirmed empyema using consistent diagnostic criteria. To ensure consistent and reliable image interpretation, each CT examination was assessed for predefined imaging features by three independent Thoracic Radiologists using a proforma. These features are listed in Abstract S62 table 1. We explored the sensitivity of the presence of five “classic” features of empyema described by Kearney et al (2000): loculation, parietal pleural thickening and enhancement and increased extrapleural fat thickness and attenuation.

Results 97 patients were included in the study. Imaging features are summarised in Abstract S62 table 1. 98.5% (95% CI 90.9% to 99.9%) of patients had at least two of five classic CT features of empyema. Additionally, we noted a significant number of patients with visceral pleural enhancement (66%), with indrawing/tenting of the visceral pleural (30%) and with subvisceral oedema (34%). Parenchymal changes were seen in the majority of patients. Associated consolidation was seen in the ipsilateral lung in 63% of patients; which was adjacent to the empyema in 87% of cases.

Conclusion This study describes the prevalence of multiple CT features of empyema and highlights the frequency of associated visceral changes. Many of the typical features of empyema were highly prevalent in this population but we also noted a high frequency of several other, less well-reported features. Subvisceral oedema is a previously unreported feature of empyema. These features may have implications for both diagnosis and prognosis.