Expansion
Loss of a normal border means the abnormality is in contact with that structure
Expansion
A border is visible on a radiograph only where two tissues of different density touch. Air-filled lung against soft-tissue heart gives a sharp line. Replace that air with fluid or consolidation of the same density and the border disappears.
- Right heart border lost - right middle lobe
- Left heart border lost - lingula
- Right hemidiaphragm lost - right lower lobe
- Left hemidiaphragm lost - left lower lobe
- Aortic knuckle lost - left upper lobe, apicoposterior segment
- Ascending aorta or superior vena cava lost - right upper lobe, anterior segment
The logic runs both ways. A dense opacity that does not obliterate the heart border must lie posteriorly, in a lower lobe, not adjacent to the heart, which is how a single frontal film can give depth information.