Atelectasis (Compressive/Passive)

This radiograph shows a left layering pleural effusion. This appearance is the result of pleural fluid layering in the basal and posterior pleural space due to gravity in a semi-recumbent position. Despite the image being labeled as “UPR” (upright), portable exams performed with the patient in a bed or gurney are rarely fully upright. As the x-ray photons pass through the effusion, they create a gradient effect in the craniocaudal direction as photons pass through more and more fluid relative to air the the more inferior they are relative to the patient. There is often some degree of compressive atelectasis associated with effusions, which contribute to this effect. This implies that the fluid is not loculated.

If the combined effusion and atelectasis is large enough to cover the entire hemidiaphragm, as in the case, there is “silhouetting” of the diaphragm and it is no longer distinguishable on radiography. The heart borders can also be silhouetted in the same way, but the effusion must be large enough to cover the entire heart.