- Published: Wednesday, 19 January 2011 00:00
Figure 1. One of Röntgen's early x-ray images of his wife's ring-bearing hand. The shadow cast by an object depends largely upon the electron density of a material.
X-ray imaging dates back to the discovery of X-rays by Wilhelm Conrad Röntgen on the 8th November 1895. Whilst examining the 'cathode rays' (now known to be electrons) produced in a partially evacuated glass tube, he observed a weak green light coming from a neighbouring fluorescent screen of barium platinocyanide.He deduced that this visible fluorescence was caused by previously unknown 'X-rays' emanating from a metallic target in the tube when it was struck by the cathode rays. Röntgen reported that these X-rays would pass through a 1000 page book or a thin lead foil, but that when he placed his hand in front of the screen he could form an image showing the outline of his bones. Since photographic plates were also affected by the X-rays, Röntgen was able to capture still radiographs and communicate his discovery to an excited scientific audience (Figure 1). The absorption of x-rays by an element is largely controlled by the density of the material, and increases dramatically with the atomic number. The metallic ring absorbs more x-rays than the calcium-rich bones, and the bones absorb more x-rays than the soft tissue.
Figure 2. A pulmonary (lung) angiogram observed by A.G. Fryett obtained using Pb3O4 as a radiocontrast agent.
A year later images of the circulatory system were generated by adding lead tetroxide to the blood to provide enhanced contrast in the radiographs (Figure. 2). This enhancement occurs because soft tissue is mainly composed of lighter elements like carbon, nitrogen, oxygen and hydrogen whereas heavy elements like lead generally absorb x-rays more strongly. Today less toxic iodine compounds are more commonly used as the "radiocontrast agent" in angiographies and venographies. Ingesting barium allows the digestive tract of a patient to be imaged by the same principles. Over the next fifteen years radiology became a valuable tool in medical diagnosis, allowing bone fractures to be detected and foreign objects such as bullets to be located in the body. Many of the early pioneers in this field developed severe skin burns which occasionally led to the loss of a hand or arm! This led to the introduction of improved protective screening to avoid unwanted exposure, and also inspired the use of x-rays to kill diseased body tissue in the treatment known as radiotherapy.
During the early years of x-ray imaging the source of x-rays remained similar to that used by Röntgen. Around 1913 Coolidge made an important development by using a high vacuum tube with a tungsten filament for a cathode. This modified x-ray source became known as the Coolidge tube, gaining use in many imaging systems. During this time Henry Moseley demonstrated empirically that the characteristic emission energies of x-rays emitted from a material and related to the atomic number of the element, which in turn reflected the exact number of positive charge in the central atomic nuclei of that element. This allowed Mendeleev's periodic table of the elements to be better understood, and offered a mechanism for future generations of radiographers to select appropriate x-ray energy for their specimen or patient's condition.
Figure 3. Examples of good and defective welds from radiography performed in 1942. From Sproull (1946)
In 1912 the first x-ray diffraction measurements using the crystal lattice of copper sulphate as a diffraction grating. Many more complex crystal structures have been revealed using x-ray diffraction, including the double-helical model of DNA as deduced by Crick and Watson. Progressing in parallel over the following century, the sister techniques of imaging and diffraction have occasionally intersected to produce methods such as x-ray topography or diffraction imaging. Over the last half century there have been several improvements in the resolution of imaging to allow x-ray microscopy. This has enabled x-ray imaging to find many uses in non-biological materials. These improvements in penetration and chemical sensitivity of x-rays allowed defects and voids to be detected in prototype machine parts. An important example was in weld analysis, where any weakness in the fusion of two metal surfaces can have huge ramifications (Figure 3). The resolution of x-ray microscopy lies between that of optical light and electron microscopes. Unlike electron microscopy, x-rays are generally non-destructive, can be used in air, do not cause significant charging in the sample, and allow biological specimens to be studied in their natural state. With the advent of synchrotron radiation sources designed for x-ray production from the 1970s onwards, the brilliance of x-rays and tunability of their energy and polarization enabled a range of new techniques, including "soft" x-ray microscopy which employs longer wavelength x-rays. Improved detector technologies and computational facilities for data analysis have allowed imaging to be extended from two spatial dimensions to three dimensions, the realm of x-ray tomography.
 Röntgen, W.C., Ann. Physik 64, 1 (1898) reprinted from (1895). Published in English as W. C. Röntgen, On a new kind of radiation, Nature 53, (1896) 274-276. Date of discovery quoted in Glasser, O., Wilhelm Conrad Röntgen and the Early History of the Roentgen Rays, Charles C. Thomas Publisher,
 Australasian Radiography 32 1 (1988) 12-17.
 Laue, M., Friedrich, W., and Knipping, P., Ann. Physik 41, 971 (1913).
 Watson, J.D., Crick, F.H., Molecular structure of nucleic acids; a structure for deoxyribose nucleic acid, Nature 171 (1953) 737–738.
 See for example: Tanner, B.K. and Humphreys, C.J., High resolution divergent-beam X-ray topography, J. Phys. D: Appl. Phys. 3 (1970) 1144-1146.
 Coolidge, W.D., A Powerful Röntgen Ray Tube with a Pure Electron Discharge, Phys. Rev. 2, (1913) 409
 Sproull, W.T., X-Rays In Practice,
 Moseley, H.G.J., The High Frequency Spectra of the Elements, Phil. Mag. 26 (1913) 1024 .
Bragg, W.L., The development of x-ray analysis, Bell, London (1975).
Sproull, W.T., X-Rays In Practice,