Which is better test to Dx mediastititis caused by potential tear in esophagus caused by impact of car collison? CT, MRI, or barium swallow test?

Barium Swallow. The first line test is a barium swallow with water soluble contrast. Depending on the results from that exam at Chest CT is the next line exam.
Tear/leak in. esophagus is probably best demonstrated by contrast esophagram. Mediastinitis is probably best demonstrated by chest CT. But diagnostic tests are never 100%. False negatives occur with finite incidence and false postives too. That is why one needs to get diagnostic tests only when they are clinically indicated.

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Which is better test to Dx small tear in esophagus due to impact? CT, MRI, or barium swallow test?

I prefer barium swal. I was trained to use barium swallow for esophageal tears. However, it is bad to use if the tear penetrates through full thickness of the wall. For this we may use a water soluble contrast. CT is an excellent tool for this but uses a lot of radiation. I would use this if there are complications to worry about or the initial swallow is non diagnostic. MRI is not good. Read more...