RADIOLOGY, that branch of science which deals with radi ant energy and its physical laws. The term radiology is however often used to denote the application of radiant energy in medicine and that will be its reference here.
Classes of Radiations.—The association of various kinds of radiation with the science and practice of medicine has been a very gradual one, quickened in the last 30 odd years by the discovery of kinds of radiation of which mankind was ignorant, although they are all emitted by the sun. Five groups of aether disturbance are of interest in medicine : a region of radiant heat used in thermo-therapy, the region of light for heliotherapy, that of ultra-violet rays in actino-therapy, the group of radiations known as X-rays used in radio-therapy, but often called Röntgen therapy in honour of the discoverer of the rays, and finally a group known as gamma rays which are the principal, though not the only, agent in radium-therapy (q.v.) ; this is often spoken of in France as Curie-therapy in honour of the discoverer of radium.
Though differing in many characteristics, these groups of radi ation have the common feature of travelling with the speed of light. They are periodic electromagnetic disturbances in the aether, not mechanical vibrations in any sense of the term.
More use is made in actual practice of measured values of the wave-lengths of these radiations than of their frequencies. Meas urements of the wave-lengths of light were first made just over a century ago by Fraunhofer; he estimated the wave-length of the yellow light given out by a sodium flame to be .0005888 mm. which was afterwards proved to be remarkably accurate. It is usual now to express measurements of wave-lengths in Angstrom units (A.U.) one of these being equal to one hundred millionth of a centimetre. The bigger the value in Angstrom units, the longer the wave-length. The limits of vision for people of normal colour perception are generally held to extend over wave-lengths ranging from about 7,600-3,800 A.U., waves longer than this extend more or less indefinitely but the region adjoining the red rays is called Radiant Heat ; on the shorter wave-length side is ultra-violet radiation, the part of medical interest being limited to the wave-lengths between 3,80o and 2,500 A.U. Waves of shorter wave-length than this are very easily absorbed by prac tically all forms of matter except when the wave-length becomes very much shorter, then they become more and more able to penetrate matter; the waves from X-rays come in this division.
Medical use is made of the rays which extend between the ap proximate limits 1--0.15 A.U. Finally the group of gamma rays emitted by radium and other radio-active bodies extends between the limits of 0.04-0.07 A.U., overlapping the short wave-length X rays.
Importance in Medicine.—One of the first photographs sent out by Professor Rontgen in 1895 to scientific laboratories an nouncing his discovery of the X-rays was a picture of a hand showing that, by the help of the rays, one could literally see through the tissues; the soft tissues are less opaque to these rays than bone, so that if the hand is put between the X-rays and a photographic plate, a shadowgraph is formed on the plate. Advances in Technique.—Some idea of the advances in the technique of radio-diagnosis may be got from the fact that in the early days of X-ray work, for a radiograph of the kidney for a suspected stone, an exposure of about 3o minutes was necessary; for such radiography now, exposures of less than one second are given. This extraordinary shortening of the time has mainly been brought about by three improvements in technique. The most important of these is an increased output of radiation from the X-ray tube; this has been achieved by the construction of more massive X-ray tubes which deal effectively with the large amount of heat generated when X-rays are produced, and by the design of much more effective transformers for the production and maintenance of the high voltages needed. The second and third improvements are on the purely photographic side. The thickness of the coating on the plate or film has been increased, and double-coated celluloid films are now very often used; and intensifying screens are used in close contact with the film of the photographic plate. When X-rays go through the object exposed, they excite these fluorescent intensifying screens, so that they give out light which acts directly on the photographic plate, which of course also receives the transmitted X-ray beam. By means of these technical developments successful cinematographic X-ray records of the movements of the limbs have been produced, but X-ray cinematography does not yet form part of the normal radiographic equipment of a hospital.