This sign, originally described by Benjamin Felson, notes that a mass or infiltrate which overlaps the hilum but through which the hilum is still seen, must be in front of or behind the hilum. If it were in the hilum, the hilar shadows would be obscured because of the principles of the silhouette sign. However, the majority of diseases which produce an interstitial radiographic pattern are in fact interstitial abnormalities pathologically and the majority of diseases which produce an alveolar radiographic pattern are histologically alveolar abnormalities. Once you determine whether the abnormality is interstitial or alveolar, then determine whether it is focal, multifocal or diffuse. These terms are relatively self explanatory.
Most difficulties arise in distinguishing widespread multifocal disease from diffuse disease.
The most common clinical situation in which this occurs is in differentiating widespread aspiration pneumonia from patchy pulmonary edema. In general, the more there are wide differences in appearance from location to location, the more likely it is to be multifocal and the more uniform the abnormality is, the more likely it is to be diffuse.