16. Do you agree so far?
Two very similar projections.

Prone and standing up X-Ray
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What are they? What is the difference? How can it be explained?
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Pleural effusion: left = prone, right = standing up.
We  may also demonstrate those effects also  by means of sonography.

Pleural tumor
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Pleural process? How are the two pictures achieved?
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Difference in diaphragm position proves it is expiration (left) and inspiration (right). Partly encapsulated effusion considerably deforms with breathing.

In- and expiration
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Same case. What technique is applied here?
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Subtraction of in- and expiration. It demonstrates diaphragmal mobility and form instability of the mass lesion. This mobility is a good argument for "encapsulated effusion".

Subpulmonal effusion gastric perforation
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One form of pleural effusion often overlooked: the “subpulmonal” effusion, which (as an exception) we see here very clearly. Why?
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There is free air in the abdomen caused by gastric perforation. This thin layer of air causes a (caudal) silhouette of the diaphragm. Thus the thick layer between diaphragm and lung will be a subpulmonal effusion.

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Diagnosis? What do the black points show?
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A dark line, difficult to pick out, corresponds to paradiaphragmal fat. This gives us an idea of the diaphragm position and of the effusion on top of it:
basal = subpulmonal effusion.