Exposure factors

There are only three interactions between x-ray and matter that is involved in medical imaging or what we call as radiography. These are photoelectric absorption, compton effect and coherent scattering. The key to these applications of x-ray interactions is the exposure factors used to generate the image. In most cases, the proper exposure of a radiograph is often a trial and error process, as there are many variables that affect the final radiograph. Some of the variables that affect the density of the radiograph include (NDT, 2007):

The spectrum of radiation produced by the x-ray generator. The voltage potential used to generate the x-rays (KeV). The amperage used to generate the x-rays (mA). The exposure time. The distance between the radiation source and the film. The material of the component being radiographed.  The thickness of the material that the radiation must travel through. The amount of scattered radiation reaching the film.  The film being used. The concentration of the film processing chemicals and the contact time.

Since the amount of radiation reaching the film is important, the properties of the three interactions of x-ray with matter affects the exposure factor. The current industrial practice is to develop a procedure that produces an acceptable density by trail for each specific x-ray generator involved. This process may begin using published exposure charts to determine a starting exposure, which usually requires some refinement. However, it is possible to calculate the density of a radiograph to a certain degree accuracy when the spectrum of an x-ray generator has been characterized.

The calculation cannot completely account for scattering but, otherwise, the relationship between many of the variables and their effect on film density is known. Therefore, the change in film density can be estimated for any given variable change. For example, from Newton’s Inverse Square Law, it is known that the intensity of the radiation varies inversely with distance from the source. It is also known that the intensity of the radiation transmitted through a material varies exponentially with the linear attenuation coefficient (m) and the thickness of the material

CONCLUSION

X-rays interacts with matter in five ways, photoelectric absorption, compton effect, coherent scattering, pair-production, and photodisintegration. Pair-production occurs only at very high photon energies therefore it is not used in medical imaging. Photodisintegration also is not involved in radiography because it occurs only in the development of nuclear fission. The three remaining x-ray interactions with matter define the principles used in radiography whereby the interactions are converted to images of the matters it has interacted with.

When a photon enters a slab of matter, it undergoes some level attenuation either it losses all its energy, or makes its way scattered in the forward direction with reduced energy. The quality of images produced in the use of x-rays is defined by many exposure factors. One important factor is the amount of radiation that reaches the film. This is related to the energies and attenuation involve in the different interactions. Photoelectric absorption contributes mainly to the non-penetration of x-rays while Compton effect and coherent scattering provides the level of x-rays that reaches the film.

References

Connolly, J. (2007). The Interaction of X-rays with Matter and Radiation Safety. Retrieved from the University of Mexico, Department of Energy and Planetary Science website: http://epswww. unm. edu/xrd/xrdclass/02-Rad-Safety. pdf , on February 10, 2007 Sprawls, P. (2006). Interaction of Radiation with Matter. Retrieved from The Physical Principle of Medical Imaging Online: http://www. sprawls. org/ppmi2/INTERACT/, on February 10, 2007