The Intensity of the beam

• The beam obeys the inverse square law i.e. for a given exposure the amount of radiation falling on a particular area (e.g. the radiographic film). will vary inversely as the square of the distance of that area from the source of irradiation (the focal spot).

• The significance of this is that if the film-local distance is doubled the m A-s output of the X-ray machine has to be quadrupled to compensate for this. therefore low output X-ray units can only be used at comparatively short film-focal distances (usually not more than 75 cm).

Anode-Film Distance and magnification

• An anode film distance of 100 cm was chosen for the radiography of the equine lumber spine above for two reasons:

• The average focused stationary grid operates at this distance.

• A greater distance would require an increased exposure which would require an increased exposure which would be excessive, even for s high – power unit.

• In order to reduce the magnification it is theoretically desirable to increase the anode-film distance to 180 cm because at this distance the rays reaching the film are virtually parallel.

• Conversely, a reduction of the anode-film distance to 75 cm (the usual distance for small animal radiography) increases the magnification and loss of detail when the part under examination is 30 cm from the film.