Opacities in the Thorax
In the thorax structures are visible due to the high contrast between tissue types.
Air attenuates very little of the x-ray beam and appears black on radiographs. Therefore, all the air within the alveoli of the lung is normally black. This causes contrast against the adjacent soft tissue structures such as the blood vessels, cardiac silhouette and mediastinum.
Silhouetting
When two adjacent structures are the same opacity they silhouette with one another. This means that we are unable to distinguish them radiographically.
This radiographic finding occurs with heart failure, as we will see, where an accumulation of fluid in the lung (edema) silhouettes with the pulmonary vessels meaning that we can no longer distinguish them from the surrounding lung tissue.