Answer to Question #11028 Submitted to "Ask the Experts"
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What is the definition of scattering ratio or scatter to primary ratio in medical or industrial radiography?
When x rays (photons) penetrate an object being imaged, there are three basic things that occur to the photons of the primary beam to form the two-dimensional image of the object.
- The photon penetrates the object in a straight line and exposes the film or digital imaging device, which produces the darker parts of the image.
- The photon is fully absorbed in the object, which produces the lighter parts of the image (bones appear white, while air filled lungs or gaps in a metal object appear black).
- The photon scatters within the object but still exposes the imaging device or film. This scattered radiation does not have any relation to the object’s different densities and only fogs the image with unnecessary and useless radiation.
If there was no scatter radiation the image would be as clear as possible. As the amount of scatter radiation increases, the image becomes more fogged and the image contrast is degraded.
The Scatter to Primary Ratio (SPR) is defined as the energy of the scattered radiation (S) divided by the energy of primary beam (P) striking the same point on the imaging device. If the SPR equals 1 there is as much scatter radiation as there is primary radiation forming the image of the object being radiographed. The SPR can be less than 1 or greater than 1 depending on a few factors. Thicker and denser objects have a higher SPR than thin and less dense objects. As the size of the primary beam (field diameter) increases or as the object being radiographed gets closer to the imaging device the SPR also increases. And, the energy of the primary radiation can affect the SPR.
There is some additional information on this topic at Scattered Radiation in Projection Radiographic Imaging or search “Scattered Radiation in Projection Radiographic Imaging.”
John Hageman, MS, CHP