On erect chest X-ray, free air under the right diaphragm is best identified as a crescentic lucency between the liver and diaphragm. Which is the MINIMUM amount of free peritoneal air that a properly exposed erect chest radiograph can reliably detect?
- A 1 mL
- B 10 mL ✓
- C 5 mL
- D 50 mL
Correct answer: B. 10 mL
Explanation
A properly exposed erect chest radiograph can reliably detect as little as 5–10 mL of free intraperitoneal air, with most published figures citing 10 mL as the reliable threshold. CT is far more sensitive, detecting as little as 1 mL. The left lateral decubitus view may detect slightly less air than the erect view. Knowing the detection threshold is important when ordering imaging in suspected perforation with equivocal erect CXR.
Reference: Grainger & Allison's Diagnostic Radiology, 7th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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