Varying amounts of saline and contrast material were injected into the pleural space of cadavers to determine the minimum amount of fluid detectable on lateral decubitus radiographs. As the amount of fluid injected increased, a predictable stepladder increase in density was observed; however, as little as 5 ml of fluid was visible on properly exposed radiographs. INDEX TERM: Pleura, fluid Radiology 109:33-35, October 1973 Diagnostic RadiologyT H E PLEURA forms a potential space lined by a surface membrane which norinally secretes a small amount of lubricating fluid. The amount of pleural fluid necessary to be visible on a radiograph has engendered great diversity of opinion. Free fluid initially gravitates to the lowest port.ion of the thoracic cavity and into the posterior gutter where amounts of 300 to 400 ml may be present but not visible on routine erect posteroanterior and lateral radiographs. The best radiographic method for the visualization of small quantities of pleural fluid has been the lateral decubitus film. Numerous authors have called attention to this technique for the visualization of small effusions; however, no definitive study has been performed to determine the minimum quantity visible on a radiograph. METHODIn an attempt to answer the above question; cadavers were obtained through the cooperation of the medical examiner's office. Only individuals who had died without significant cardiopulmonary disease were studied. They were placed in a right lateral decubitus position and a scout film obtained which was checked for free fluid or other pulmonary pathology. A small polyethylene catheter with a three-way stopcock, to insure an air-tight seal, was then inserted into the pleural space, generally between the eighth and ninth ribs. In the initial group of experiments, small quantities of fluid mixed with contrast medium were instilled in order to facilitate visualization ( Figs. 1 and 2). Five milliliters of fluid was visible, and increasing amounts of fluid produced a predictable "stepladder" increase in density. A second group of cadavers was then . studied without injection of contrast medium, using instead saline or plasma which produced identical and visible layers 011 the lateral decubitus films with 5 ml of fluid. Ten milliliters was easily visible, and increasing amounts again demonstrated the stepladder increase in density (Figs. 3-8).Examinations were performed on all cadavers after the conclusion of the experiment, and the fluid in the pleural space was carefully measured. Within small limits explained by the difficulty in collecting all of the fluid at postmortem examination, close correlation with the amount of fluid injected was obtained. DISCUSSION

Specific guidelines can effect change in the location and number of radiologic examinations performed, with an improvement in the quality of the studies and a decrease in radiation dose and cost. No decline in quality of care appears to result, despite claims by opponents to such changes that widespread serious quality impairment would occur.

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A patient with a new mediastinal mass 25 years after resection of a mediastinal bronchogenic cyst is presented. Computed tomography and subsequent thoracotomy revealed a recurrent bronchogenic cyst. This case illustrates that incompletely resected mediastinal bronchogenic cysts may recur many years later.

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