chest radiograph

A chest radiograph is the everyday photograph of the chest made with X-rays. You stand against a flat detector, hold a deep breath, and in a fraction of a second a shadow-picture of your lungs, heart, ribs and the spaces between them is captured. It is fast, cheap, low in radiation and available almost everywhere, which is why it is usually the very first image a doctor reaches for when something seems wrong inside the chest.

X-rays pass through air easily but are blocked by denser tissue, so on the resulting image air-filled lung appears dark, bone appears bright white, and soft tissue and fluid appear in shades of grey. A reader interprets the pattern of light and dark — looking at lung fields, the size and shape of the heart, the position of the diaphragm, the bones, and any abnormal whiteness (an opacity) or blackness (suggesting trapped air or a collapsed lung).

Its great strength is screening: it quickly answers broad questions such as whether there is pneumonia, a large pleural effusion, a pneumothorax, an obviously enlarged heart, or a sizeable mass. Its limitation is that it flattens a three-dimensional chest into a single two-dimensional shadow, so structures overlap and small or subtle lesions are easily hidden. When the plain film raises a question it cannot answer, the next step is often cross-sectional imaging such as a CT scan.