The lungs are diffusely aerated and appear radiographically radiolucent (i.e., black). A clear radiographic contrast exists between the radiolucent lungs and the soft tissue opacity of the lung interstitium and vessels. In pathological condition, on a thoracic radiograph, it is possible to detect an increase in opacity of the lung tissue, which needs to be further characterized by the radiologist in order to produce a list of differential diagnoses.
Four different patterns of increased lung opacity have been described in order to classify different disease processes:
Often, there will be a combination of the lung infiltrate (bronchoalveolar, bronchointerstitial, etc.), and it is very important to evaluate not only the type of lung patterns but also its distribution and features. An accurate description of what we see will help us in producing a correct list of differential diagnoses, which of course will be associated with the clinical examination of the patient.
How do we approach a radiographic study of the thorax?
It's very important to have a systematic approach of the thoracic structures in order to avoid undetected lesions, which could be essential for the clinical workup of the patient.
When evaluating the lungs, it can be easy to check the pulmonary vessels to understand if there is a pulmonary infiltrate (figure below).
The first diagnostic step is to define the infiltrate, and the second one is to describe it by using the radiographic signs relative to the lesion/s (number, shape, volume, margins, opacity and location). The goal of the radiographic interpretation is to produce a differential diagnoses list compatible with the lesions previously described.
Cellular or fluid types of infiltrate in the interstitial tissue of the lungs will correspond to an increase in soft tissue opacity on a thoracic radiograph, classified as interstitial pattern. This opacity can be diffuse and have a "foggy" appearance or be more "structured" and have a nodular appearance. The first radiographic sign compatible with an interstitial disease is the increase in lung opacity associated with loss of definition of the pulmonary vasculature. The vessels will still be visible but less clearly than normal and appear as "tree branches in a foggy forest." The interstitial infiltrate is the most aspecific pattern since artefactual condition such as expiratory projections, underexposed radiographs or obese patients can mimic pathological lung diseases. It's therefore essential to exclude any artefactual/technical condition before calling an interstitial pathology. Differential diagnoses for an interstitial infiltrate include: interstitial pneumonia, lung edema, pulmonary fibrosis, lung contusions and neoplastic diseases.
The bronchial pattern is caused by pathologies (mainly of inflammatory origin) that cause a thickening of the bronchial walls or by pathological infiltration of the peribronchial space. This pattern has on a radiograph a "reticular" aspect with clear evidence of the thickened bronchial walls that appear, depending from their section, as multiple "ring or donut shadows" or "railroad track shadows." The lung tissue still retains an almost normal radiolucency, and the vessels are usually quite well outlined.
The alveolar pattern is indicative of lack of air in the alveoli. This condition is caused by collapsed alveoli or infiltration (cellular or fluid types) of the alveolar lumen, which results in a consolidated increased opacity in the affected portion of the lungs. The key features of the alveolar pattern are the loss of definition of the lung vessels and the air bronchograms. The air bronchograms are radiographic signs caused by the air still present in the bronchi (appears radiolucent), surrounded by a homogeneous increase in soft tissue opacity, which masks and "covers" the bronchial wall and vessel definition. Differential diagnoses for alveolar infiltrate include pneumonia, edema, haemorrhage, contusion and neoplasia.
An overall increase in opacity of the lung fields can also be caused by an increase in the circulating pulmonary blood. The hyperperfusion of the lungs is radiographically detectable with evidence of increased size and numbers of normally visible vessels. The increased circulating blood will cause an overall increase in opacity even if the cardiac shadow and pulmonary lucency will often be conserved.
In those situations, it is important to evaluate the lung vessels with the cardiac silhouette in order to understand if there is an underlying cardiac disease causing the change in perfusion of the lungs.