Interaction of X-Rays with Matter

In order to understand how microCT works, one must first understand the basics of how X-rays interact with matter. For most conventional microCT systems, there are two main interactions with matter at the energies common to those systems: photoelectric absorption and Compton scattering. Both of these processes attenuate, or reduce the intensity of, the X-ray beam. This attenuation is based on a number of factors. 

• Width of the material - as the width of the material increases the attenuation of the X-ray beam increases. In other words, the longer the path an X-ray needs to travel through a sample, the larger the chance there is for the X-ray to be absorbed or scattered by an atom within the sample.
• Density of  the material - as the density of the material increases the attenuation of the X-ray beam increases. Similar to the width of material, if the material is more dense there is a higher chance that an X-ray will be absorbed or scattered by an atom within the sample.
• Atomic number (Z) of the material - as the atomic number of the material increases the attenuation of the X-ray beam increases. This dependence is variable depending on the X-ray energy.

The attenuated X-ray intensity (I) through a material can be measured by the Beer-Lambert Law:

I = I₀exp[-µx]

where I_o is the initial X-ray intensity, µ is the attenuation coefficient of the material and x is the length of the X-ray path through the material. This equation does make a number of assumptions of the system, but the above formulation is sufficient to gain a basic understanding of how X-rays are attenuated in a material.

For those interested, two main assumptions are made in the Beer-Lambert Law:

1. The first assumption is that the material is homogeneous. The vast majority of samples will consist of multiple materials and therefore the attenuated X-ray intensity would need to be summed for each material along the X-ray path. 
2. The second assumption is that the X-ray beam consists of X-rays all of the same energy. This can be the case of some sources that produce a monochromatic beam (i.e. an X-ray beam with essentially only one X-ray energy), but most laboratory microCT systems produce a polychromatic beam (i.e. an X-ray beam with a large distribution of X-ray energies). This is an important distinction because the attenuation coefficient is highly dependent on the X-ray energy and therefore more expansions to the Beer-Lambert Law would be needed in practice.