When patients lie directly on the radiographic cassette as they might for a portable, supine radiograph, a fold of the patient's skin may become trapped between the patient's back and the surface of the cassette. This can produce an edge (dotted white arrow) in the expected position of a pneumothorax, and that edge may parallel the chest wall just as you would expect a pneumothorax to do (A). Unlike the thin, white line of the visceral pleura in a different patient with a pneumothorax (solid white arrow-in B), skin folds produce relatively thick, white bands of density. A skin fold is an edge; the visceral pleura produces a line.
cassette /kə'sɛt/ n. 盒
Several pitfalls can lead to the mistaken diagnosis of a pneumothorax.
Pitfall 1: Absence of lung markings mistaken for a pneumothorax.
The simple absence of lung markings is not sufficient to warrant the diagnosis of a pneumothorax as other diseases produce such a finding.
These diseases include:
• Bullous disease of the lung
• Large cysts in the lung
• Pulmonary embolism, which can lead to a lack of perfusion and hence a decrease in the number of vessels visible in a particular part of the lung (Westermark sign of oligemia).
• In none of these diseases would the treatment ordinarily include the insertion of a chest tube. In fact, insertion of a chest tube into a bulla might actually produce an intractable pneumothorax.
• Solution: Look at the contour of the structure you believe is the visceral pleural line. Unlike the margin of a bulla, the visceral pleural line will be convex outward toward the chest wall and will parallel the curve of the chest wall.
Pitfall 2: Mistaking a skin fold for a pneumothorax.
When the patient lies directly on the radiographic cassette (as for a portable supine radiograph), a fold of the patient’s skin may become trapped between the patient’s back and the surface of the cassette.
• This can produce an edge in the expected position of the visceral pleural line which may, in fact, parallel the chest wall just as you would expect the visceral pleural line in a pneumothorax.
• Solution: Unlike the thin, white line of the visceral pleura, skin folds produce a relatively thick, white band of density.
Pitfall 3: Mistaking the medial border of the scapula for a pneumothorax.
Ordinarily, the patient is positioned for an upright frontal chest radiograph in such a way that the scapulae are retracted lateral to the outer margin of the rib cage, thus preventing the medial borders of the scapulae from overlapping the lung fields.
• With supine radiographs, the medial borders of the scapulae may superimpose on the upper lobes and mimic the visceral pleural line of a pneumothorax .
• Solution: Before you diagnose a pneumothorax because you think you see the visceral pleural line, make sure you can trace the outlines of the scapula on the side in question and identify its medial border as being separate from the suspected pneumothorax.
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