or perhaps tracheal fluoroscopy.

This is about dogs but could apply to humans:
http://www.vin.com/proceedings/Proceedings.plx?CID=WSAVA2002...

The first step in diagnosing tracheal collapse is usually to perform inspiratory and expiratory lateral radiographs of the neck and thorax. Because the collapse is dynamic (the cervical trachea collapses on inspiration and the thoracic trachea on expiration), both inspiratory and expiratory films should be taken to evaluate the entire trachea. Radiographs are diagnostic in approximately 60% of patients with moderate to severe tracheal collapse. Fluoroscopy, though seldom available in veterinary practices, facilitates evaluation of the dynamic movement of the trachea and mainstem bronchi through all phases of respiration. Special attention should be paid to evaluating the mainstem bronchi because animals with mainstem bronchial collapse are unlikely to benefit from surgical repair of their collapsing cervical trachea. Presently, there is no clinically used method to stent collapsing mainstem bronchi; however, a recent publication investigated intraluminal stents for mainstem bronchial collapse and concluded that such a technique might be useful in affected dogs. In addition to evaluating the trachea on radiographs, thoracic radiographs should also be evaluated for cardiomegaly and pulmonary disease.



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Note added at 5 hrs (2008-10-22 09:31:14 GMT)
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German explanation and X rays here:
http://books.google.com/books?id=ma1-waw2rkcC&pg=RA4-PA231&l...

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Note added at 5 hrs (2008-10-22 09:47:17 GMT)
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tracheal views in inspiration and expiration