Sample Composition: High-Z Materials

High-Z materials and their interaction with X-rays

If a sample has dense concentrations of high atomic number (high-Z) material, it can be difficult or even impossible to generate a useful microCT image. As stated before, the higher the atomic number and/or density of the material, the more x-rays are attenuated through the sample. This characteristic of X-rays can cause a variety of issues for the quality of microCT data. These issues are listed below with possible solutions to mitigate the risk of degraded or failed image collection.

• Photon Starvation
  • All the x-rays can be attenuated by the sample if it is dense enough or the atomic number of the material is high enough. If no x-rays pass through the sample, the detector will not have information to provide to the 3D reconstruction algorithm over that x-ray path. This is called photon starvation and will cause large streaking and shadow artifacts in the reconstructed image.
• X-ray Scattering
  • X-ray scattering is highly dependent on the sample and scanning parameters. At higher X-ray energies, X-ray scattering becomes more relevant and can cause a number of issues in reconstructed images. The primary effect is the increase noise within the image and the propagation of some streaking artifacts around highly attenuating areas.
• Beam Hardening
  • Beam hardening is one of the most prevalent image artifacts of polychromatic X-ray sources. A polychromatic beam by definition has a distribution of X-ray energies and this distribution can change as it propagates through the sample and gets attenuated. Generally, the low energy X-rays get attenuated first which leads to a "harder" beam or a beam that has shifted to a higher energy spectrum. This shift in energy through the sample causes a cupping effect in the reconstructed image where the brightness (i.e. attenuation) of the sample is artificially higher near the outside edges of the sample.

Possible Solutions: 

1. The most efficient way to compensate for high-Z materials is usually to increase the scanning energy. Due to the nature of X-rays, higher energy X-rays interact less with matter and therefore more x-rays will pass through the sample and potentially reach the detector. This will help with photon starvation, but increase x-ray scattering and beam hardening effects.
2. When increasing scanning energy, it is also usually best to also put a larger in-line filter on the X-ray source to reduce the low energy X-rays that are being readily attenuated by the sample and providing minimal benefit to image quality. This will help the issue of beam hardening at the expense of temporal resolution.
3. Many of the problems of high-Z materials can be mitigated during the reconstruction of the microCT dataset. Many reconstruction algorithms include corrections for beam hardening and x-ray scattering. These are usually effective when other physical countermeasures cannot be used.
4. If all else fails there are usually two less ideal options:
   • Reconsider the preparation of the sample. This usually means that the diameter of the sample needs to be reduced so that the X-rays beam can fully penetrate the sample.
   • Use a different CT system or imaging technology. High concentrations of high-Z materials can be difficult to manage for many conventional microCT systems. Some industrial CT systems might have a better chance of getting usable data, usually at the cost of image resolution. Or other imaging technologies may need to be used for the sample, for example neutron imaging.