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Several factors contribute to radiographic distortion whereby the anatomy examined is misrepresenting on the plain radiograph.
The x-ray beam originates from a point source within the x-ray tube. It is due to this point source nature that x-ray beams will all possess 'beam divergence'. As the beam exits the x-ray tube the beam will 'diverge' while the center point (central ray) of the beam will not suffer from any divergence. All regions peripheral to the central ray will then suffer from some degree of distortion. The amount of distortion can be limited via three factors.
Centering the anatomy to the central ray on the image receptor will limit beam distortion due to divergence. The point of the central ray is the focal spot on the x-ray tube; therefore one can angle the tube on a stationary axis and maintain the central ray at the level of the joint, seldom applied but useful for imaging that requires longer radiographs e.g. tibia/fibula x-ray.
Source image receptor distance
The source image receptor distance (SID), is the distance of the tube from the image receptor, affecting magnification. The greater the SID, the less magnification the image will suffer. Although 100 cm is the universally accepted SID, studies have shown that apart from improving distortion issues, increasing the SID from 100 cm to 122 cm decreases entrance skin dose despite any need for an increase in mAs 1.
Object image receptor distance
The object to image receptor distance (OID) is the distance between the object to the detector. The closer the anatomy is to the detector the less magnification/distortion. One can observe this when observing the shadow cast by the light source onto the anatomy.
- 1. John Lampignano, Leslie E. Kendrick. Bontrager's Textbook of Radiographic Positioning and Related Anatomy. (2017) ISBN: 9780323399661