Information taken from a
booklet "X-Rays their discovery and Application" by Brian Bowers
Published by HMSO in 1970
and sold in the Science Museum in London. The Science Museum in South Kensington, London has an extensive and interesting collection of X-ray related equipment and is well worth a visit. Amongst other museums worth visiting for X-ray related information include The Thackray Museum in Leeds, and the National Photographic museum in Bradford. The background to
the discovery The German physicist Plucker concluded in 1859 that the fluorescence was due to something radiating from the negative electrode, or cathode. These `cathode rays' were closely studied during the remainder of the nineteenth century. The English physicist Crookes published the results of a series of researches on the subject in 1879. He showed that the cathode rays were emitted normally to the cathode surface and could be deflected by a magnet. When he focused the rays to a point sufficient heat was developed to melt glass or platinum foil. He concluded rightly that the rays consisted of negatively charged particles, but although his experimental results were indisputable the conclusions he drew from them were vigorously contested. Most German physicists thought that the cathode rays were a wave motion similar to light and that there was no propagation of matter. Hertz, working at Bonn, was seeking an experimental proof of Maxwell's theories of the nature of electricity and magnetism and he took up the study of discharges in a vacuum. He found that the cathode rays would pass through a thin film of gold or aluminium placed in its path. After his death in 1894 his pupil Lenard continued his work. Lenard made a tube with a thin aluminium window and succeeded in bringing cathode rays into the outside air. He found that they still produced fluorescence, but that the rays would not travel far through air at atmospheric pressure. Lenard said that the cathode rays passed through the hand. This was almost certainly an observation of X-rays produced where the cathode rays struck the window of this discharge tube, but he failed to notice that it was a different kind of ray. Sir William Crookes often made the observation that photographic plates which happened to be stored near his tubes became fogged and this must have been caused by X-rays. On one occasion he returned some plates to the manufacturer as unsatisfactory! Many workers must have produced X-rays accidentally while studying cathode rays, but only Roentgen observed their presence, and realised that he was dealing with a `new kind of rays'. W.C. Roentgen
Wilhelm Conrad Roentgen born on 27 March 1845 in Lennep in the German Rhineland. He was the only child of a cloth merchant and manufacturer, and may have been a distant relative of David Roentgen (1743-1807) who had achieved fame as a cabinet maker. When he was three years old his parents moved to Apeldoorn in Holland, the home of his mother's parents, and the family became Dutch instead of Prussian citizens. Roentgen went to a boarding school in Apeldoorn and later to Technical School in Utrecht. He was expelled from the Utrecht school after refusing to name the fellow student who drew a caricature of the teacher. At school he was not an exceptional pupil, though he showed an aptitude for mechanical things. He was a lover of nature and all his life liked to spend his holidays in the Alps or among the Lakes of North Italy. In later life he preferred traveling in a horse drawn carriage to using a motor car. Early in 1865 Roentgen attended the University of Utrecht, though not as a regular student since he lacked the required qualifications. He discovered that it was possible to enter the Zurich Polytechnical School in Switzerland by passing an entrance examination and without school references. He passed the entrance examination and in November 1865 he became a student of mechanical engineering in Zurich, and the stigma of his expulsion from school was left behind. For the rest of his life Roentgen retained happy memories of his year in Zurich and a sense of gratitude to Augustus Kundt, the professor of physics at the Polytechnic School, who inspired Roentgen to follow a career in physics. Roentgen became Kundt's assistant at Zurich and then at the University of Wurzburg when Kundt took a post there. While in Zurich Roentgen met Anna Bertha Ludwig, whom he married in I87?. They had no children of their own, but adopted Anna's niece. Despite all Kundt's efforts on his behalf Roentgen could not get an academic post at Wurzburg. In 1872 Kundt went to the newly founded University of Strasbourg (then in Germany) taking Roentgen with him. Two years later Roentgen became a `Privat Dozent' at Strasbourg and his parents moved there from Apeldoorn to be near their son. In 1875 he succeeded H.F. Weber, on Weber's recommendation, as Professor of Physics and Mathematics at the Hohenheim Agricultural Academy in Wurtemberg. He was not happy at Hohenheim and returned to Strasbourg a year later as Professor of Theoretical Physics. From 1879 to 1888 he held the chair of Physics at Giessen University, and because of his excellent work during these years was offered posts at Jena and Utrecht, which he declined. However, in 1888 he accepted an invitation to become Professor of Physics and Director of the Physical Institute of the University of Wurzburg. Thus he became a head of department in the University which had earlier refused him any academic position. At Wurzburg he turned his attention - as many scientists were then doing - to the study of cathode rays. It was his custom when taking up any new investigation to repeat experiments made previously by others working in the same field. For one of these experiments he had a Crookes tube covered with black card to mask the fluorescent glow which always exists in the glass in this kind of experiment. When the tube was connected up he noticed that some crystals of barium platino-cyanide which happened to be on a table nearby became fluorescent. The observation was made on the evening of Friday 8 November 1895 `at a late hour when assistants were no longer to be found in the laboratory'. Roentgen investigated and quickly satisfied himself that the tube was emitting some hitherto unknown kind of ray which produced the fluorescence. A screen coated with barium platino-cyanide and held near the tube fluoresced all over, but if a metallic object was placed between the tube and screen it cast a shadow. Roentgen told his friend Boveri, `I have discovered something interesting but I do not know whether or not my observations are correct', but with the exception of this one remark he told no one of his discovery for seven weeks. During that period he devoted himself to the study of the `something interesting', and prepared a paper setting out the main properties of X-rays. Roentgen himself used the term X-rays (`X-strahlen' in German), because the nature of the rays was uncertain. In Germany the name Roentgen-rays was adopted almost immediately and remains in use today. The discoverer's name has rarely been used in English-speaking countries, probably because the English speaker does not know how to pronounce `Roentgen'. The word `Skiagraph' (from the Greek for a shadow) was introduced in 1896 for a picture obtained by X-rays and was used for a few years. `Radiograph' was also introduced in 1896 with the same meaning, and in this country nowadays a picture obtained by means of Roentgen's rays is invariably called either an `X-ray' or a `radiograph'. The announcement of
Roentgen's discovery
Roentgen's paper was immediately printed and distributed. An English translation was published in Nature on 23 January 1896 and within a few weeks the news of the discovery had spread throughout the world. The scientific response to
Roentgen's discovery Few discoveries if any have captured the imagination and been announced around the world in so short a period of time as was the discovery of X-rays. But the impact of the news on the scientific world was enormously increased by the fact that every physical laboratory possessed the equipment necessary for generating the new rays. After the researches of Crookes and others, vacuum tubes and induction coils were standard equipment, and little more was needed to confirm the truth of Roentgen's results and to investigate further the properties of the new rays. In May 1896 the American Electrician began a three part series on how to make `A Roentgen Ray Outfit'. This described how to make an induction coil capable of producing a three-inch spark and how to make a rotary contact breaker. But it advised purchasing a Crookes' tube and recommended one with thin glass, and of German manufacture (because, it said, German glass contains less lead than American glass). A catalogue of `Roentgen Ray Apparatus' was issued by the American General Electric Company in the autumn of 1896. The manufacture of X-ray equipment was then established on a commercial scale, but it was to be several years before a user could purchase a complete X-ray equipment( in one unit, rather than a number of separate items - induction coil, contact breaker, tube, etc. - which had to be assembled and interconnected. One of the first to experiment with X-rays in America was T.A. Edison. For medical examinations he advocated the use of fluorescent screens rather than photographic plates so that the doctor could see fractures etc. immediately without having to wait for the plate to be developed. After testing every chemical available to him (nearly two thousand different substances) he decided that the material which fluoresced best under the influence of X-rays was calcium tungstate, which gave an image several times brighter than the barium platino-cyanide used by Roentgen. Roentgen's
subsequent work on X-rays Roentgen submitted a second paper on X-rays to the Wurzburg Physical Medical Society in March 1896. Most of this paper is an account of experiments showing that air (or any other gas) which has been exposed to X-rays could conduct electricity and would discharge an electrically charged body. He could not explain the phenomena he observed, but when J.J. Thomson's work on the electron became known it was possible to explain Roentgen's observations by saying that the radiation `ionised' the air (that is, stripped some electrons from the atoms) making it conductive. The ionising property of X-rays is exploited in some procedures for measuring the rays. In the same paper Roentgen says that it can be advantageous to include a Tesla apparatus in the circuit between the induction coil and the X-ray tube. More intense X-rays are obtained, and some tubes which are either too hard or too soft to be operated directly from an induction coil will work satisfactorily through the Tesla apparatus. This was an oscillatory circuit which causes the voltage across the X-ray tube to reach a higher peak value than would otherwise be the case. The Tesla principle was often exploited in early X-ray equipment, since then the induction coils did not have to withstand so great a voltage, and has occasionally been used in more recent equipment. His third and final paper was published in May I897, and contains an account of several further experiments and measurements. Roentgen found that any substance - even air - which was subjected to X-rays would itself emit X-rays. He showed that this phenomenon was truly the generation of fresh X-rays (`secondary radiation') and not mere scattering of the main beam. With a device similar to an optical photometer he compared the X-ray outputs of different tubes and also examined the distribution of the output from a single tube. He found that with soft X-rays the emission was uniform over a hemisphere centered on the target, though with hard X-rays and a thin target some radiation appeared from the back of the target. Finally he made a number of comparative measurements of the opacity to X-rays of different thicknesses of various substances. In his three papers Roentgen described most of the properties of the rays he had discovered, but the actual nature of the rays remained unknown at I times brighter than that time. The later history of X-rays is chiefly the story of technical improvements in the equipment and the refinement of techniques. Roentgen was very reticent by nature and the many honors bestowed on him were more of a burden than a pleasure. The University of Wurzburg gave him the honorary degree of Doctor of Medicine, and he accepted honorary citizenship of his native Lennep. He declined nearly all invitations to address scientific societies on his discovery. He declined the offer of nobility by the Prince Regent of Bavaria, who later bestowed the title `Excellency' on Roentgen. In 1901 he became the first Nobel laureate for physics and traveled to Stockholm to receive his rise though he did not give a Nobel Lecture. p In 1900 Roentgen left Wirzburg to take charge of the Physical Institute of the University of Munich, where he resumed his earlier work on the physical properties of crystals. After his retirement in 1920 he was given permission to use two rooms in the Institute. He continued to work there until a few days before his death on 10th February 1923. X-ray tubes Early X-ray tubes The essential components of an X-ray tube are an airtight vessel, usually of glass, and two electrodes sealed into it. In the early tubes the vessel is evacuated to a low pressure, but some gas molecules remain (for which reason these are known as `Gas-Tubes'). If such a tube were completely evacuated it would not function. The electric discharge when a high voltage is applied between the electrodes causes ionization of the gas atoms, and the positive ions are driven towards the cathode (negative electrode) by the electric potential across the tube. This bombardment of the cathode by the positive ions causes the emission of electrons which, on striking the target, generate X-rays. The target - sometimes called the anti-cathode - may be the wall of the vessel, the anode electrode or a separate metal insert connected to the anode.
Fig 11 Production of X-rays from a 'Crookes' Tube.
The tube with which Roentgen discovered X-rays was probably a pear shaped `Crookes' tube' with the cathode at the pointed end and the anode to one side. The X-rays were generated in the area where the cathode rays struck the glass at the larger end of the tube. To get a sharp shadow picture the X-ray source must be as small as possible, and for the earliest X-ray pictures the end of the tube was covered with a lead sheet having a small hole in it. Only the rays passing through the hole could be utilised, so that the system was extremely inefficient and long exposure times were required. The `Focus' tube designed by Professor Herbert Jackson was designed to focus the electron beam on to a small target area in order to produce as small an X-ray source as possible. He achieved this by using a concave cathode. Since the electrons are emitted perpendicularly to the cathode surface and then travel in virtually straight lines (irrespective of the position of the anode) they converge at the center of curvature of the cathode, and the target is placed at this point. Crookes had used tubes with cathodes shaped in this way in the course of his studies of cathode rays, but Jackson seems to have been the first person to suggest using the idea in X-ray tubes, which he did in March 1896. In his first paper on X-rays Roentgen noted that X-rays were also generated if the cathode rays struck an aluminium insert instead of the glass of the tube. In the second paper, published in March 1896, he reported that every substance which he had tried would yield X-rays when used as a target and bombarded by cathode rays in a discharge tube. He found that there were qualitative differences in the X-rays generated using targets of different materials and that he could produce the most penetrating X-rays with a tube having a concave aluminium cathode and a platinum anode as a target inclined at 45" to the axis of the cathode. Many scientists in different countries experimented with the new rays during 1896. Tubes of different shapes and targets of various metals were tried. By the end of that year it was established that the shape of the tube did not matter, and that the best target metals were those of highest atomic weight. Tungsten (atomic weight 184) and uranium (at. wt. 238) were used experimentally, though platinum (at. wt. 195) was preferred because it is easier to work. Aluminium (at. wt. 27) was sometimes used, despite its low atomic weight, because it remains stable in a vacuum discharge whereas a tube with a platinum target tends to become coated inside with a thin layer of platinum black produced by the destructive effect of the discharge on the metal. The first X-ray tubes were of very low power by modern standards and exposures of long duration - at least several minutes - were required for most purposes. The penetrating power of the X-rays and the speed with which an adequate photograph could be obtained were found to increase as the voltage across the X-ray tube was increased. However, most of the energy in the electron beam is converted into heat where the beam strikes the target, and as increasing power was applied to X-ray tubes it became necessary to increase the mass of the target to prevent overheating. (The thin platinum targets used in some of the early tubes could easily be vapourised by the electron beam.) A large, solid, platinum target would be very expensive, and platinum plated nickel was adopted. This construction remained in general use until the introduction of tungsten, which is still the preferred metal for the target. Even with a tungsten target, however, the ability of the target to withstand the effect of the electron bombardment is a limiting factor in the rating of the tube. The working surface of the target develops fine cracks, with the result that a proportion of the electron beam generates X-rays at the sides or bottom of the cracks. The metal of the target screens such radiation and the useful radiation from the tube therefore falls as the surface of the target loses its initial smooth finish. Typical early X-ray tube with vacuum regulation
In early tubes the target was attached to a massive metal block which prevented excessive rise in temperature by absorbing the heat generated. Then the metal block was brought through the glass wall of the tube and attached to external radiating fins. In some tubes removable cooling tongs could be inserted in the block from outside the tube, and as soon as the target and tongs became too hot the operator could remove the tongs and replace them with a cool pair. Cooling water was circulated in a hollow target in some tubes. The arrangement was effective, but suffered from the disadvantage that the cooling water system including its external radiator had to be at the voltage of the target. A more practical arrangement was the boiling water tube, in which the target assembly and a reservoir above it contain water which boils steadily while the tube is operating. The temperature of the target remains steady and no connecting water pipes are required. The need for a small focal spot on the target means that intense heat is developed in a very small area and has to be conducted away through the target metal. The ability of the metal to conduct heat proved to be the factor which limited the power which could safely be applied without damaging the target, and hence limited the intensity of X-rays which could be produced. The `line-focus' tube was devised to alleviate this situation. In this type of tube the working area of the target is a narrow strip rather than a spot and the cathode is designed to emit a ribbon-like beam of electrons. The target strip is inclined at such an angle that, viewed in the direction in which the X-ray output is taken, the strip is fore-shortened to a square of side equal to the width of the strip. The X-rays appear to come from a small square source, although the much greater area of the target strip is available to conduct away the heat generated . A further development is the rotating target tube in which the working portion of the target is continuously changing so that a still greater area of metal is effectively available to conduct away the heat. The line-focus principle is used, but the target strip is a continuously changing portion of the sloping side of a disc rotated at high speed by an induction motor. Most modern X-ray tubes use this principle. Until the 1920s most X-ray tubes were `gas' tubes, which depend on the presence of some residual gas in the tube for the electric discharge to take place. In operation some gas molecules become adsorbed in the glass of the tube, so that the vacuum tends to increase and the tube becomes `harder', requiring a higher operating voltage. It was recognised that in order to secure satisfactory and uniform working of these gas tubes the degree of vacuum had to be kept approximately constant. Hence numerous regulators were devised which controlled the vacuum by releasing small quantities of gas into the tube. The metal palladium has the property that when heated to incandescence hydrogen can pass through it. One system of vacuum regulation for X-ray tubes devised by Professor Villard in Paris in 1898 made use of this property. A palladium tube was sealed into the side of the glass bulb. To `soften' the X-ray tube the palladium tube was heated by a gas flame. Hydrogen in the f lame then passed through the palladium into the bulb, so reducing the vacuum. Other regulators employed substances which absorb gases, such as charcoal or mica. A small quantity of one of these substances was placed in a glass side tube which could be heated, when some of the absorbed gas would be driven off. An automatic vacuum regulating device was introduced about 1900. Mica containing absorbed gas was placed in a side tube and could be heated by a discharge between auxiliary electrodes connected in series with an external spark gap. As the tube hardened the supply voltage rose. When the supply voltage became sufficient to produce a spark across the external spark gap a discharge took place between the auxiliary electrodes. This heated the mica and gas was driven off to reduce the vacuum and soften the tube. Thermionic X-ray tubes The X-ray tubes described so far are all `gas' tubes, which depend for their operation on the presence of some residual gas in the tube. One disadvantage of the `gas' tube - the variation of the degree of vacuum over a period of time - has already been mentioned. Another disadvantage is that the voltage across the tube and the current through it are inter-dependent. The reason this matters is that the penetration of the X-rays depends on the voltage, and the intensity depends on the current. (The penetration is a measure of the distance the X-rays can pass through a body being radiographed; the intensity is a measure of the degree of blackening obtained on the photographic plate in a given time.) In. the gas tube a reduction in gas pressure (a `harder' tube) leads to a higher operating voltage and hence more penetrating X-rays. But, because there is less gas in the tube, the current falls and the intensity of the X-ray beam is reduced. In the thermionic X-ray tube, devised by W.D. Coolidge in the USA in 1913 the vacuum is as near perfect as possible, and the electrons are produced by `thermionic emission'. The cathode is no longer a cold metal block, but a tungsten filament heated to incandescence by a current passing through it. This filament is usually mounted within, and electrically connected to, a shaped metal block which focuses the electron beam on to the target. The intensity of the X-ray output depends only on the current in the electron beam, which is a function of the temperature of the filament, and this can be controlled by adjusting the current in the filament. The penetration of the X-rays produced depends - as with the gas tube - on the voltage between cathode and target. With this arrangement the intensity and penetration of the X-ray output are independently controlled by adjustment of the filament current and tube voltage, respectively. All modern X-ray tubes are of the thermionic type. Once it was realised that excessive exposure to X-rays was dangerous, steps were taken to screen the tube so that `stray' radiation was not emitted. Some early, low power, tubes are made mostly of lead glass, which is comparatively opaque to X-rays, but with a small `window' of lime glass through which the X-ray beam emerges. In this way all radiation except the mainbeam is screened from the operator or patient. In the I920s an entirely new design of X-ray tube - the `Metalix' tube - was developed in which the X-rays are generated inside a metal chamber forming the main part of the tube. This screens all radiation from the tube except the main X-ray beam which passes through a glass window. The metal is an alloy of chromium and iron which can be sealed directly on to the glass end pieces to form a good vacuum seal. Modern X-ray tubes are usually enclosed in an earthed metal housing which protects the user from electric shock and incorporates a layer of lead to prevent the emission of unwanted radiation. Accelerators of other designs X-ray tubes and equipment working as described above have been operated at up to a quarter of a million volts, but the practical difficulties are immense and for still more penetrating radiation other techniques are adopted. All X-ray tubes are `accelerators', devices which accelerate electrons to a high speed before they strike the target. The penetrating power of the X-rays generated depends on the speed with which the electrons strike the target; this speed is determined solely by the voltage across the tube. For some applications, such as the radiographic examination of thick metal structures, X-rays of very high penetration are required. By using machines such as the linear accelerator or the betatron which have been developed for research into atomic physics, electrons may be accelerated to speeds which would require several million volts across a conventional X-ray tube. In these machines electrons are repeatedly accelerated across the same potential drop so that a voltage multiplying effect is obtained before they strike the target. In many applications, both in medicine and industry, very high energy X-rays are being replaced by gamma rays from radio-active isotopes. Gamma rays occupy the part of the electro-magnetic spectrum adjacent to X-rays and may be regarded as very hard X-rays. The isotopes used, such as cobalt 60, need careful shielding, but this may be less of a problem than the elaborate equipment needed for very high voltage X-rays. Gamma ray apparatus has the disadvantage that the source area is usually greater than the effective source area of the alternative X-ray equipment, so that photographs obtained by gamma radiography are not so sharp as X-radiographs. Medical applications Medical diagnosis The use of X-rays in medical diagnostic work was foreshadowed in Roentgen's first paper. Within a few months of the discovery many X-ray photographs - radiographs - had been taken showing broken bones and the exact location of foreign bodies such as needles, pieces of glass and bullets embedded in patients. Damage to the joints in cases of severe arthritis, and abnormal bony growths were shown. By April 1896 several experimenters were seeking the best way of obtaining pictures of the human stomach and intestines by filling them with a suitable liquid containing X-ray opaque elements. At first the physician employed a physicist to take radiographs for him, and some physicists set up laboratories for the purpose. The first in this country was established by A.A. Campbell Swinton in March 1896. One of Swinton's first clients was a man with a bullet in his head. The bullet was successfully located but the patient’s hair began to fall out and he threatened to sue Swinton. However, the man's hair soon grew again and nothing came of the threat. The Electrical World! of June 1896 carried the suggestion that X-rays might be used as a substitute for shaving! The possible dangers involved in the use of X-rays were not appreciated at that time. Lord Salisbury, the Prime Minister of the day, visited Swinton's laboratory and had a photograph taken of the bones of his hand. Lord Salisbury was very pleased with it and wrote to say that Lady Salisbury would like to see a photograph of the bones of her hand. Swinton duly obliged! As soon as the practical value of radiography in medicine was established, doctors began to set up and operate their own X-ray equipment. The Lancet, which seems to have approached the first reports of X-rays with some skepticism, announced with astonishment on 1 February 1896 that the Belgians had decided to bring X-rays into practical use in hospitals throughout the country. As X-ray techniques developed it became possible to improve the 'contrast' achieved in radiographs, and to distinguish organs whose X-ray opacity was only slightly different from that of the rest of the body. Tumours of the stomach and other organs were first shown radiographically in June 1896.
Line focus tube from a dental x-ray machine (Tube housing removed)
Medical radiographs were soon used as evidence in legal proceedings actress who fell and injured a foot in a Nottingham theatre brought an action for damages which was heard in March 1896. She produced X-ray photographs of both feet which convinced the judge and jury that a bone was out of place in the injured foot and that real injury had been sustained. In a number of similar cases Counsel argued at length a5out the admissibility in evidence of X-ray photographs, but they quickly gained. acceptance by the courts. Dental X-ray pictures were taken at least as early as April I89b, but X-rays were not generally applied in dentistry until about 1916. X-ray examination can reveal impacted teeth and caries which are otherwise not detectable, but because the details which need to be seen are so small, an X-ray tube with a very small X-ray source area is essential. Also the tube has to be very close to the patient's head, with consequent risk of severe electric shock. The first equipment designed specially for dental radiography appeared about 1923. This had an exposed high voltage wire from the transformer to the X-ray tube so that the patient was not entirely safe from the risk of shock. The next development was the Philips `Metalix' set in which the tube and high voltage connection to the transformer were totally enclosed. The modern dental unit in which X-ray tube and high voltage transformer are both contained in a single, comparatively small, housing was introduced in 1933. The modern diagnostic X-ray equipment uses a rotating anode tube which may have a power rating of fifty kilowatts or more, but the exposure times required are only a small fraction of a second. This may be contrasted with the long exposures, sometimes half an hour or more, required in the early months of radiography. Fig 21 Rotating anode tube as used in medical radiography. The Anode is the central disc and its inclined edge forms the target area. The cathode is to the right of the anode and just above its centre line. The motor driving the anode is on its right. The sockets at each end are for the high voltage connections.
Therapeutic X-rays The early pioneers of X-rays had no reason to expect that the new rays would have any physiological effect, and consequently there was no reason for protecting themselves from the rays. Rontgen himself made all his experiments in a large zinc box with the tube outside, but the reason he adopted this arrangement was probably that he wanted to obtain a clearly defined beam of X-rays with no stray radiation to fog his photographic plates. However, these experimental precautions must have protected him from the rays. A few other workers protected themselves in this way. Some may have felt intuitively that rays of such penetrating power might be dangerous, but many saw no need for caution and received severe burns. Edison was one of the first to notice physiological effects from X-rays: he experienced severe pain in his eyes after several hours work. Mention has already been made of loss of hair after radiography. Reports of more serious skin damage after exposure to X-rays appeared from time to time during 1896, and a writer in The Lancet of October that year described the effect as being like sunburn, from which he concluded that X-rays must be present in sunlight! During the year some investigators studied the effect of X-rays on bacteria and most reported that they could find no effect at all, although some claimed to have observed a germicidal effect. Probably the first successful therapeutic use of X-rays was the removal of a hairy birthmark on a child by Dr L. Freund of Vienna, towards the end of 1896. Several physicians attempted to treat tuberculosis by X-rays through the lungs and a number of successes were claimed, though it is doubtful whether rays of the low intensity attainable in 1896 could have achieved the results claimed. The treatment of cancer by radiation quickly led to encouraging results. It was found that tumours were decreased in size and pain diminished, and a French physician suggested that X-rays could be a better anaesthetic than morphine. Despite the early appreciation of the beneficial effects of X-rays on malignant tumors, it is only since the Second World War that radiotherapy has been firmly established as a distinct clinical specialty. Initially, treatment was of necessity an art rather than an exact science. The basic guide to dose was the reaction of the patient's skin. It took years' of experience after this to define the value of radiotherapy in the various types of cancers. Even today, the radiosensitivity of some rare tumors is uncertain. The growth of therapeutic X-ray technology, including the development of very high voltage X-rays, has given rise to the profession of the medical physicist. The physician is no longer pre-occupied with the apparatus and can concentrate on the patient. This is an interesting reminder of the situation in the early days of X-rays when the physicist provided and operated the equipment and the physician took his patient to the physicist for radiography. Today surgery and radiotherapy are the major curative treatments for cancer, though some drugs are also useful. Radiotherapy also has an important role in the relief of painful and distressing symptoms occurring in the course of some common cancers. An unfortunate and frequent sequel to radiotherapy used to be breakdown of the skin. This is seldom seen today unless the skin itself has to be treated, since the very penetrating rays used pass through the skin without affecting it. With X-rays generated above two megavolts or the gamma rays from cobalt-60 the maximum dose is absorbed 0·3 to 2 cm. beneath the skin. The limit to the total dose of radiation that is given to a tumour is determined by the tolerance of the adjacent normal tissues since these cannot be spared in the way other areas of tissue are, by using multiple X-ray beams interesting at the place it is desired to treat. The damage done by X-rays is not selective for malignant cells. A proportion of both normal and malignant cells are killed but normal tissues are reconstituted by the orderly division of a population of reserve cells which is not present in the tumour. The chief factor determining normal tissue tolerance appears to be irreversible damage to the blood vessels which progressively worsens as time passes. Until improved drugs are developed, radiotherapy will retain its leading role in the treatment of cancer. Potted History! History and development (Taken from http://library.thinkquest.org/27314/mainframe.html)1895
Discovery x-rays (W.C. Roentgen) MAJOR ADVANCE IN COMPUTED TOMOGRAPHY
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