The radiological differential diagnosis of unilateral total pulmonary veiling
An increase in density throughout one lung field seen on a postero-anterior radiograph of a chest is always relative, i.e. relative to the transradiancy of the opposite lung field. The increase may be real or apparent. A real increase in density may be due to a large number of causes, as discussed below. An apparent increase in density on one side may result from a relative decrease in density (i.e. increased translucency) on the opposite side.