Phase-Contrast X-ray imaging

Virtually all X-rays imaging in a medical or biological context is performed by measuring differences in the attenuation of X-rays by the tissues. The material properties are inferred from the differences in the number of photons detected at the end of defined paths in the sample. Naturally then this form of X-ray imaging concentrates almost entirely on the particle side of X-ray behaviour. The main atomic process responsible for the attenuation and thus forming the image is the photo-electric effect. For the purposes of this summary the photo-electric effect can be considered a binary interaction which, if it occurs, stops the X-ray, if not the X-ray continues as before. The chance of the process occurring scales very strongly with atomic number and is only linearly dependent on density and thickness. As such when imaging anatomy with reasonable differences in atomic number and density such as bone to soft tissue it can provide excellent image contrast. However for thin tissues with very similar atomic number and only small density variation (such as is often the case in soft tissue imaging) the differences in absorption rates are very small hence the resultant image contrast is generally very poor.