Increased Penetrability of X-Rays Through Normal Lung and Other Air-Infiltrated Substances

Abstract

X-rays are absorbed by a substance through which they pass in proportion, first, to a power of the atomic weight and, second, to the mass of the substance penetrated. This principle is fundamentally concerned in clinical roentgenology, for it permits certain tissues to be distinguished from others on the roentgen-screen and film. The normal lung is a case especially in point. Its average atomic weight is less than that of bone and its mass per unit thickness is much less than that of the neighboring soft tissues, owing to its infiltration with air. The organ is, therefore, more radiolucent than its environs. When air leaves the lung completely and the tissues shrink together, as in atelectasis, the mass per unit thickness becomes approximately the same as that of other soft tissues and the organ may not be distinguishable by x-ray examination from them. However, during certain roentgenologic observations by one of us on specimens of atelectatic lungs, the contrast between the density of atelectatic and air-containing lobes seemed greater than it should be, if atomic weight and mass alone were responsible. The matter was put to the following tests:

A dog was operated upon surgically and the main bronchus of the accessory lobe was ligated. The animal was sacrificed 24 hours later. The lobe was removed and found to be totally airless and collapsed. It was packed into one end of a cardboard cylinder, 3.7 cm. in diameter and 12 cm. in length, and the cylinder was placed upon one quadrant of a roentgenographic film-casset with the lobes next to the casset. The other quadrants were covered with a lead sheet. The ensemble was brought beneath, and 65 cm. away from, a linical x-ray tube, so that the rays could pass longitudinally through the clinder onto the film, and an exposure was made.