Objective: The objective of our study was to Evaluate the relationship between nature of effusions and quantity of pleural effusion Attenuation in chest CT scan in patients with pleural effusion referred to Firoozgar Hospital in Tehran. Materials and Methods: This is a retrospective cross-sectional study that was designed for patients with pleural effusion that came to firoozgar hospital in Tehran. After visiting, demographic information was recorded. Proper imaging was done and then results got registered. Results of the imaging were studied by two radiologists. Results: 113 patients that were eligible became included in this study. we recorded their information and did the proper imaging. Average patients' age was 64.47±18.16. 60.2% of the participants were male and 39.8% of them were female. 47 of the patients (41.6%) had exudative pleural effusion, 56 patients (49.6%) had transudative pleural effusion and 10 patients (8.8%) had empyema. CT scan with contrast was used in only 7 patients (6.2%). The mean Hounsfield scale in exudative effusions was 9.49 ± 5.25, minimum was 3 and maximum was 21. The mean Hounsfield scale in transudative effusions was 3.89. 6.43, minimum was 1 and maximum was 18. The average Hounsfield unit in empyema was 14.3 with a standard deviation of 7.51. also, minimum was 5 and maximum was 28. ROC analysis of Hounsfield scale showed that sensitivity rate is 59.6% and the specificity is 49.5%. Conclusion: this study shows that HU values may play a role in differentiating exudative pleural from transudative pleural effusion. Based on the results of our study, other CT scan parameters such as nodularity and pleural thickening are not reliable parameters for distinguishing between the different types of pleural effusion or the presence or absence of empyema. loculation in exudative pleural effusion was significantly higher than transudative pleural effusion. It seems that loculation can be a differentiating parameter for the type of pleural effusion.

CT scan, Attenuation, Hounsfield scale