Magnification and distortion

   Magnification and distortion

Magnification

In a projected image, magnification will always be present because the X-rays continue to diverge as they pass from the object to the image-acquisition device (henceforth referred to as a film for simplicity). The source of the X-rays is the X-ray tube focal spot. For a given focus-to-film distance (FFD), the greater the distance between the object and the film, the greater will be the magnification of the image. To minimize magnification, the object under examination should be positioned as close to the film as is reasonable to do so.

where FOD is the focus-to-object distance.

If the object-to-film distance has to be increased, e.g. in the case of a patient on a trolley, then the FFD can also be increased. This will then reduce the magnification caused by the above.
(NB: an increase in exposure will be needed in this case due to the effect of the inverse square law on the beam intensity).

Image distortion

A distorted image will be produced if not all parts of the image are  magnified by the same amount. Considering a thin, flat object, there will be constant magnification and thus no distortion when the film is parallel to the object. When possible, the part being radiographed should be placed parallel to the film to avoid distortion. If the object and film are not parallel to each other, then there is a difference in magnification of different parts of the object, leading to a distorted image.

In the diagrams below, the object and film are not parallel to each other. It can be seen that if the centre of the X-ray beam is directed at right-angles to the object but the object is not parallel to the image receptor,then a distorted, elongated image is produced. If the centre of the beam is directed at right-angles to the image receptor but is angled in relation to the object, then a distorted, fore-shortened image will be produced.

In cases when the object and film cannot be parallel to each other, a compromise can be made by directing the central ray at right-angles to an imaginary line bisecting the angle between the object and the film. Although distortion does occur, the net effect is neither elongation nor foreshortening of the image. This
technique may be required if a patient is unable to straighten a limb to bring it parallel to the film when imaging a long bone.