X-ray

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).

X-ray densities

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.

 

Comments:

  • Each X-ray is evaluated as if you are standing in front of the patient; so the right side of the image is the patient’s left side and vice versa.
  • Importantly, the X-ray beam has a divergent property. This means it widens as the distance to the X-ray tube increases. A drawback of this phenomenon is that tissues/structures farther from the plate are imaged larger. This is relevant when evaluating the size of the heart in a chest x-ray (fig. 3).
Divergent property of a X-ray beam.
Divergent property of a X-ray beam.

Figure 3. Effect of the divergent x-ray on the size of the heart (a = posterior-anterior technique, b = anterior-posterior technique).