The quality of a material used for beam hardening filtration can be very important. Many materials are not homogenous and therefore create a variation in X-ray attenuation. This variation is easily calibrated out when acquiring detector corrections, however as the filter moves slightly or the focal spot shifts, this can cause significant image mottling that will show up in an image.
An easy way to test a material for homogeneity is as follows:
Place the candidate material in a fixture and place it near the face of the tube and acquire a detector gain correction through the filter material.
- Acquire an image through the filter with the new gain correction applied.
- Measure the signal to noise ratio of the new image.
- Move the filter slightly and acquire another image through the filter.
- Compare the Signal to Noise ratio of the two images and window/level the histogram to aggressively enhance the contrast.
- If the material is homogenous there will be very little difference in the SNR and the two images will look similar.
- If the material is not homogenous there will be a larger difference in the SNR and the second image will have a distinct mottling pattern to it.
Note: The thinner the filter material the higher the subject contrast therefore the more visible a lack of material homogeneity will be. This test will also highlight scratches and foreign material on the surface of the filter.