When an x-ray is made, an x-ray beam leaves the x-ray tube, passes through the body and hits a phosphorus plate/detector. The whiteness (= density) depends on the amount of X-ray radiation passing through the tissue (fig. 2).
Figure 2. X-ray densities (= whiteness).
The more X-rays are obstructed (absorbed or scattered) and do not reach the phosphorus plate/detector, the denser (= whiter) the image. Highly absorbent materials, such as metal, will be imaged as dense. Another example: X-rays pass more easily through the air-filled lungs (black) than bone (white). The information received on the plate is converted into a digital image. Correctly imaged, an X-ray provides information on the ossal structures, fluid, air, soft tissue contours and prostheses/osteosynthetic material.
Figure 3. Effect of the divergent x-ray on the size of the heart (a = posterior-anterior technique, b = anterior-posterior technique).