Ultrasound can aid in the diagnosis of pleural effusion. Based on the ultrasound features, it can also help to differentiate between a transudative and an exudative effusion. This in turn helps to narrow down the differentials for the cause of effusion.
A transudate is always anechoic in nature, while an exudate may show the following features:
1. May be anechoic or echogenic,
2. May show septae within
3. May show debris or other particulate matter within
4. May show loculations
5. May have associated pleural thickening [pleura >3mm]
6. May have associated pleural nodules
7. May have associated lung parenchymal lesions.

A few common causes of transudative pleural effusions are congestive heart failure, cirrhosis of liver, hypoalbuminemia, nephrotic syndrome, etc.
A few common causes of exudative pleural effusions are infections [bacterial, tuberculous], lung malignancy, metastases, lymphoma, collagen vascualr diseases [such as SLE, rheumatoid arthritis], drug-induced, etc.