SSRL: Synchrotron Radiation Interactions with Matter
There are two types of x-ray interactions that give structural information about chemical or biological materials -- scattering and absorption.
Scattering
An x-ray beam is directed at a target, and the pattern of "scattered" radiation (x-rays bouncing off the target) is observed. The pattern contains information about the spatial structure of the scattering object. Since scattering is most informative when the wavelength is somewhat less than the size of the scattering object, x-rays with short wavelengths near one angstrom are ideal for investigating the positions of atoms in a structure, whereas x-rays with longer wavelengths are more appropriate for larger features.
Absorption
An x-ray beam is directed through a target and the fraction of the beam energy that is absorbed is measured for a range of beam x-ray wavelengths. X-ray absorption provides a way to study electron-level structure because the energy range of the x-ray photons nicely matches that needed to excite electrons from one energy state to another.
These excitations are wavelength dependent. If a specimen strongly absorbs x-rays of one wavelength but does not absorb x-rays of another wavelength, then comparing the two-dimensional absorption patterns of a specimen at the two different wavelengths gives rise to contrast that can be used to image that specimen.
A good example of the application of synchrotron radiation in the medical field is the development of non-invasive coronary angiography. Most patients undergoing tests for arterial restrictions or blockages today are treated by invasive angiography. This involves using a catheter to inject a form of dye into an artery, an inherently dangerous procedure. The high contrast of the image obtained by synchrotron radiation allows for intravenous rather than arterial injection, eliminating much of the risk in the procedure.
