The Visible Bronchial Tree

During the early developmental period of chest roentgenology there was much discussion concerning the anatomical counterpart of the normal lung markings in the roentgenogram. It is now recognized that these normal lung markings are the shadows of vessels, mainly the branches of the pulmonary artery. This has been proved experimentally and is theoretically in harmony with the anatomical and physical facts. The blood-filled vascular tree absorbs more of the rays than the surrounding air-filled pulmonary parenchyma and casts a dark shadow in a well illuminated field. The bronchi are air-filled tubes with thin walls usually too thin to be visible on the film. Since air within the bronchi cannot contrast with air in the surrounding alveolar sacs, the bronchi are not visible. It may be assumed theoretically that in the presence of pneumonic parenchymal consolidation the air-filled bronchi may become visible as a ramifying system of radiolucent strands within an obscured field. This observation was first made in cases of cylindrical bronchiectasis within chronic atelectasis in children. With improved radiographic technic, the radiolucent bronchial tree was seen more frequently in various types of pneumonia, and this observation was reported in 1927 under the heading: “The Visible Bronchial Tree, a Diagnostic Roentgen Sign of Pneumonia” (1. See also 2 and 3). Although this observation probably has been made by others, it does not seem to be generally known. Since it is thought to be of diagnostic value, our recent experiences as well as an illustrative experiment will be reported. To elucidate this phenomenon, the following experiment was made. The inside of an open paper box was waterproofed with a thin coating of paraffin. Several paper drinking straws containing air were placed across the box in one direction. Another set of straws, filled with paraffin, was arranged perpendicular to the first set at almost the same depth (Fig. 1), paraffin being chosen because it absorbs x-rays to about the same degree as does water. A roentgenogram of the box was made. The box was then filled with water, to sufficient depth to cover both rows of straws, and another roentgenogram was made. Figure 2,a represents the roentgenogram of the box with both rows of straws, but otherwise empty, i.e., filled with air. The air-filled straws cannot be distinguished, although the original roentgenogram does permit recognition of the fine lines cast by the thin paper walls of the straws. The paraffin-filled straws, on the other hand, are clearly visible. Some of them betray the nature of their filling mass by small air bubbles included in the paraffin. Fig. 2,b, representing the second roentgenogram made of the water-filled box, shows that the air-containing straws stand out as radiolucent bands in the obscured field, while the paraffin-filled straws are invisible. Usually the phenomenon of the “visible bronchial tree” is easily recognized.