The X-ray Shadows of Secondary Infection in Lung Tuberculosis

Abstract

ON account of the physical conditions in the lungs, their radiologic densities are susceptible of finer analysis than are those of any other part of the body. Further, nothing in this field is of more interest and value than the differentiation, whenever possible, of lesions caused by tuberculosis from those produced by other infections. It is well recognized that there are no pathognomonic physical signs of lung tuberculosis; however, radiologists believe that their particular contribution to the physical examination of the chest, when skillfully interpreted, will more accurately indicate the nature of a lesion, as well as its location and extent, than will any other part of that examination. Since the criteria which the clinician accepts in physical findings as suggesting tuberculosis and those which point the radiologist to this conclusion are based on the same fundamental anatomy and pathology, there should be no antagonism. The radiologist must always remember that his contribution to the physical examination is supplementary, even on those occasions when the supplement, like a woman's postscript, furnishes the real meat of the examination. The fundamental investigations of Miller and Dunham, many years ago, regarding the architectural structure of the lung, the method of invasion by tubercle bacilli and the pathology produced pointed the way to our subsequent observations that tuberculous infection and infection by other bacterial invaders produce different kinds of shadows in the lung field. If we will base our interpretations upon the fundamental investigations of Miller and Dunham, to which there have been no material additions, we will be in position to differentiate etiological factors with a reasonable degree of certainty. It is our belief that wherever we have departed from the fundamentals mentioned and interpreted shadows as tuberculosis, without proper regard for the underlying pathology which must produce such densities, we have been out on the thin ice of conjecture and have too often been wrong in our conclusions. We know that tuberculosis invades lung tissue through the lymph channels; therefore, that the infection first surrounds the lobules, because the lymphatic vessels enclose each anatomical lung unit very much as a fishnet holds its contents. The smallest visible tuberculous lesion, therefore, is the dim outline of a secondary lobule, with its congested lymphatics and beginning granulomatous consolidation. Such a lesion is essentially parenchymatous, and since the lung parenchyma fills all the space between the hilus and the pleura, the tuberculous density may occur anywhere in this lung field. The lesion has a definite relation to the bronchial tree, and this relation can be traced by the striation densities which mark the blood vessels accompanying the bronchi.