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Röntgen's discovery unleashed an enormous amount of work on the new mystery rays.

On the right are two radiographs of a hand. These images are separated in time and technology by 100 years. The most remarkable thing about these images is that they are not so very different. The modern one is sharper and displays some rather poor soft tissue contrast. Nevertheless the improvement is rather poor for 100 years development. Remember that it was only 66 years from the first powered flight in 1903 to landing a man on the moon in 1969!

Plane Radiography

Radiographs like the ones above are simply shadowgrams. They are formed simply because the X-rays either pass straight through or are stopped by the object. The diagram on the left illustrates the principle and shows a perfect shadow.n reality, things are not like this. A large fraction of the X-rays are not simply absorbed or transmitted by the object but are scattered. The diagram on the right illustrates this effect and illustrates the fuzzy edge of the object that is produced in the image by the scattered X-rays. The scatter is due to a number of effects.

X-Ray Diffraction

The first kind of scatter process to be recognised was discovered by Max von Laue who was awarded the Nobel prize for physics in 1914 "for his discovery of the diffraction of X-rays by crystals". His collaborators Walter Friedrich and Paul Knipping took the picture on the right in 1912. It shows how a beam of X-rays is scattered into a characteristic pattern by a crystal. In this case it is copper sulphate.

Bragg's Law

The very next year 1915, the father and son team of Sir William Henry and William Lawrence Bragg were awarded the Nobel prize for physics "for their services in the analysis of crystal structure by means of Xrays". These gentlemen were responsible for the famous Bragg Law which describes the mechanism by which X-ray diffraction occurs and is illustratedin the following diagram.

Bragg's law was an extremely important discovery and formed the basis for the whole of what is now known as crystallography. This technique is one of the most widely used structural analysis techniques and plays a major role in fields as diverse as structural biology and materials science. Nevertheless the effect has been ignored in the application of X-rays to medical imaging.

The most important effect in medical radiography, the photoelectric effect, was not understood until somewhat after the understanding of X-ray diffraction. Albert Einstein was awarded the Noble prize for physics in 1921 "for his services to Theoretical Physics, and especially for his discovery of the law of the photoelectric effect". It is photoelectric absorption that is responsible for most of the absorption in a mammogram that creates the contrast in the image.

Another important effect was discovered by Arthur Holly Compton who was awarded the Nobel prize for physics in 1927 "for his discovery of the effect named after him".

The Compton effect is considered to be responsible for the bulk of scattering effects in radiography.