The Latent Period Which Elapses Between the Onset of a Lesion and the Appearance of Radiographic Signs

A comparative macroscopic and radiographic study of such structures as the lungs or bones, the details of which appear to be so entire in the radiograph, readily reveals to the competent observer the fact that lesions need to have attained a sufficient size or have led to sufficient change before they will produce a contrast density which permits of their detection on the radiograph. Large lesions may be present, yet the radiograph will not reveal them. Obviously, considerable histologic change may occur before any macroscopic alteration is detected. In tissues other than bones and lungs, where the pathological changes produce no material increase in density, we are entirely dependent upon evidence of deformity of such contours of the affected or adjacent tissues as can be demonstrated by radiology. It should not be astonishing, therefore, to realise that it takes time for radiographic signs to become recognisable after the onset of any assault, whether it be on the patient or on the pathologic process by medicaments, etc. The latent period in association with infectious diseases, i.e., that period which elapses between infection and the appearance of clinical signs, is well recognised, but it is not appreciated as much as it should be that, while the clinical signs in the early stages of disease or following an injury may be very prominent, the radiograph may still fail to reveal any abnormality at the site. Perhaps even less is it recognised that, after the initial clinical signs have faded, radiographic evidence may still be absent or so insignificant that it may be missed by any but the experienced observer, and that, when radiographic signs are well marked, the clinical signs may be insignificant. In other words, the radiographic signs lag behind the clinical at the onset, throughout the duration, and during resolution. The duration of this lag shows considerable variation in different conditions. In some, as pneumonia, it is short; in others, as cysticercosis, it may be five or ten years. While we must, therefore, think of a negative latent radiographic period following onset, we must think also of that positive radiographic period which exists after the disappearance of clinical signs. In some conditions positive radiographic evidence persists throughout the remainder of an apparently healthy life. Obviously, radiographs during the latent radiographic period will reveal no evidence of the disease, though it may be present. Further radiographs must be taken after an interval depending on the condition, if we wish to obtain confirmatory evidence.