The value of CT in evaluating pleural fluid is that it can help differentiate the nature of the fluid even though it cannot tell us the cause. In this video, taken from our Chest CT Essentials course, we describe the differences between transudative, exudative, hemorrhagic, and chylous effusions on CT, and how knowing the difference can radically shift your management options.
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The value of CT evaluating pleural fluids is that it can help differentiate the nature of the fluid, even though it cannot tell us the cause. There are three kinds of pleural fluids of concern, transudates, exudates, and empyemas. The density of pleural fluid usually cannot be used to determine its nature, because different types of fluids may have the same densities.
The only time it can help is when there the hemothorax. A density measurement of 35 hounsfield units or above strongly suggests the flow to the blood, particularly if it's inhomogeneous and or if it shows a distinct fluid flow level, due to the separation of the clotting blood components. But, if we consider how pleural fluids are made, what their effect is on the pleura, and what we can see in the pleura, then we can begin to differentiate the different types.
Transudate result from a systemic factor that causes fluid to leak into the pleural space, and not from a pleural problem. It can result from increased hydrostatic pressure, as we see with congestive heart failure, or low blood protein, as we see with liver failure. Transudative fluids do not have cellular components, so they flow freely, layer dependently across in a crescent shape, and don't form loculations, which are fixed pockets in the pleural space bounded by fiber septa.
Because the pleura is normal and not thickened, it will not be visible. Exudates result from damage to the pleura due to infection, neoplasm or inflammation. The damaged pleura allows a fluid full of inflammatory cells and protein to leak into the pleural space. In the initial phase of exudate, the pleura doesn't thicken.
As the process progresses, fibrin deposition can lead to pleural thickening and loculations, which prevent the fluid from flowing to other areas of the pleural space. So, if there is nondependent fluid loculated in lens or crescent shaped collections with pleural thickening, think exudate. Empyemas are a common subgroup of exudates associated with infections, such as bacterial pneumonia. An empyema often starts as a sterile free flowing exudate with no pleural thickening.
As the infection spreads to the pleural space, pus collects and fibrin deposits on inflamed surfaces, causing the visible pleural thickening and loculations typical of empyemas. Putting all this information together, we get our two rules of pleural effusions. If pleural thickening is visible, the fluid is an exudate, and it may be an empyema, but it is not translate.
If no pleural thickening is visible, the fluid may be an exudate or a transudate, but it's not an empyema. One final tip is that CT can help differentiate between a loculated empyema and a lung abscess, which are both opacities filled with fluid or with an air fluid level. They differ in two ways, wall thickness and mass effect. An empyema has uniform wall thickness and an abscess has irregular wall thickness. An empyema is outside the lungs, so it exerts a mass effect that displaces blood vessels in bronchi, and abscesses within the lung, so it destroys lung tissue, but it's not displace it. It's important to distinguish between the two, as the treatments are very different. An empyema needs active drainage, while an abscess does not.