The [[Röntgen equivalent man|rem]] is the traditional unit of dose equivalent. This describes the energy delivered by [[gamma|<math>\gamma</math>]] or X-radiation (indirectly ionizing radiation) for humans. The SI counterpart is the [[sievert]] (Sv). One sievert is equal to 100 rem. Because the rem is a relatively large unit, typical equivalent dose is measured in millirem (mrem) - 1/1000 rem, or in microsievert (μSv) - 1/1000000 Sv -, whereby 1 mrem equals 10 μSv.
The average person living in the [[United States]] is exposed to approximately 150 mrem annually from [[background radiation|background sources]] alone.
Reported dosage due to dental X-rays seems to vary significantly. Depending on the source, a typical dental X-ray of a human results in an exposure of perhaps, 3<ref>http://www.doctorspiller.com/Dental%20_X-Rays.htm and http://www.dentalgentlecare.com/x-ray_safety.htm</ref>, 40<ref>http://hss.energy.gov/NuclearSafety/NSEA/fire/trainingdocs/radem3.pdf</ref>, 300<ref>http://www.hawkhill.com/114s.html</ref>, or as many as 900<ref>http://www.solarstorms.org/SWChapter8.html and http://www.powerattunements.com/x-ray.html</ref> mrems (30 to 9,000 [[sieverts|μSv]]).
==Physics==
When medical X-rays are being produced, a thin metallic sheet is placed between the emitter and the target, effectively filtering out the lower energy (soft) X-rays. This is often placed close to the window of the [[X-ray tube]]. The resultant X-ray is said to be ''hard.'' Soft X-rays overlap the range of [[extreme ultraviolet]]. The frequency of hard X-rays is higher than that of soft X-rays, and the wavelength is shorter. Hard X-rays overlap the range of "long"-wavelength (lower energy) [[gamma rays]], however the distinction between the two terms depends on the source of the radiation, not its wavelength; X-ray [[photon]]s are generated by energetic [[electron]] processes, gamma rays by transitions within [[atomic nucleus|atomic nuclei]].
|+ X-ray K-series spectral line wavelengths (nm) for some common target materials.<ref>{{citebook | year=| title=CRC Handbook of Chemistry and Physics 75th edition | editor=David R. Lide | others= | pages=10–227 | publisher=CRC Press | id=ISBN 0-8493-0475-X | authorlink= }}</ref>
! Target
! Kβ₁
! Kβ₂
! [[K-alpha|Kα₁]]
! Kα₂
|-00
! Fe
| 0.17566
| 0.17442
| 0.193604
| 0.193998
|-
! Ni
| 0.15001
| 0.14886
| 0.165791
| 0.166175
|-
! Cu
| 0.139222
| 0.138109
| 0.154056
| 0.154439
|-
! Zr
| 0.070173
| 0.068993
| 0.078593
| 0.079015
|-
! Mo
| 0.063229
| 0.062099
| 0.070930
| 0.071359
|}
The basic production of X-rays is by accelerating electrons in order to collide with a metal target. (In medical applications, this is usually [[tungsten]] or a more crack resistant alloy of [[rhenium]] (5%) and tungsten (95%), but sometimes [[molybdenum]] for more specialized applications, such as when soft X-rays are needed as in mammography. In crystallography, a [[copper]] target is most common, with [[cobalt]] often being used when fluorescence from [[iron]] content in the sample might otherwise present a problem). Here the electrons suddenly decelerate upon colliding with the metal target and if enough energy is contained within the electron it is able to knock out an electron from the inner [[Electron shell|shell]] of the metal atom and as a result electrons from higher energy levels then fill up the vacancy and X-ray photons are emitted. This process is extremely inefficient (~0.1%) and thus to produce reasonable flux of X-rays plenty of energy has to be wasted into heat which has to be removed.
The spectral lines generated depends on the target (anode) element used and thus are called characteristic lines. Usually these are transitions from upper shells into K shell (called [[K-alpha|K lines]]), into L shell (called L lines) and so on. There is also a continuum ''[[Bremsstrahlung]]'' radiation given off by the electrons as they are scattered by the strong electric field near the high-''Z'' ([[proton]] number) nuclei.
Radiographs obtained using x-rays can be used to identify a wide spectra of pathology. Due to their short wavelength, in medical applications X-rays act more like a particle than a wave. This is in contrast to their application in crystallography, where their wave-like nature is most important.
For most modern non-medical applications, X-ray production is achieved by [[synchrotron]]s (see [[synchrotron light]]).
To generate an image of the cardiovascular system, including the arteries and veins ([[angiography]]) an initial image is taken of the anatomical region of interest. A second image is then taken of the same region after iodinated contrast material has been injected into the blood vessels within this area. These two images are then digitally altered, leaving an image of only the iodinated contrast outlining the blood vessels. The doctor (Radiologist) or surgeon then compares the image obtained to normal anatomical images to determine if there is any damage or blockage of the vessel.
To take an X-ray of the bones, short X-ray pulses are shot through a body with radiographic film behind. The bones absorb the most photons by the [[photoelectric]] process, because they are more electron dense. The X-rays that do not get absorbed turn the photographic film from white to black, leaving a white shadow of bones on the film.
==Detectors==
===Photographic plate===
The detection of X-rays is based on various methods. The most commonly known methods are a [[photographic plate]], X-ray [[photographic film|film]] in a cassette, and [[Rare earth element|rare earth]] screens. Regardless of what is "catching" the image, they are all categorized as "Image Receptors(IR)".
Before computers and before digital imaging, a [[photographic plate]] was used to produce radiographic images. The images were produced right on the glass plates. However, film replaced these plates and was used in [[hospital]]s to produce images. However, computed & digital radiography has started to replace film. Although, film technology is still used in industrial radiography processes (to inspect welded seams). Photographic plates are a thing of history, and their replacement (intensifying screens) is now becoming part of that same history. Silver (necessary to the radiographic & photographic industry) is a non-renewable resource, that has now been replaced by digital (DR) and computed (CR) technology. Where film required wet processing facilities on site, these new technologies do not. Archiving of these new technologies is also space saving for facilities.
Regardless of whether the image receptor technology is plate, film or CR/DR Since photographic plates were sensitive to X-rays, they provide a convenient and easy means of recording the image, but they required a lot of exposure (to the patient). This is where intensifying screens came into the picture. The use of such, allowed for a lower dose to the patient – because the screens took the X-ray information and "intensified" it so that it could be recorded on the film lying next to the intensifying screen.
The part of the patient to be X-rayed is placed between the X-ray source and the image receptor to produce what is a shadow of all the internal structure of that particular part of the body being X-rayed. X-rays are somewhat blocked ("attenuated") by dense tissues such as bone, and pass more easily through soft tissues. Those areas where the X-rays strike the image receptor will produce photographic density (ie. it will turn black when developed). So where the X-rays pass through "soft" parts of the body such as organs, muscle, and skin, the plate or film turns black.
Contrast compounds containing [[barium]] or [[iodine]], which are [[radiopaque]], can be ingested in the gastrointestinal tract (barium) or injected in the artery or veins to highlight these vessels. The contrast compounds have high atomic numbered elements in them that (like bone) essentially block the X-rays and hence the once hollow organ or vessel can be more readily seen. In the pursuit of a non-toxic contrast material, many types of high atomic number elements were experimented with. For example, the first time the forefathers used contrast it was chalk, and was used on a cadaver's vessels. Unfortunately, some elements chosen proved to be harmful – for example, many years ago [[thorium]] was used as a contrast medium (Thorotrast) – which turned out to be toxic in some cases (causing injury and occasionally death from the effects of thorium poisioning). Contrast material used today has come a long way, and while there is no way to determine who may have a sensitivity to the contrast – the occasions of having an "allergic-type reaction" are very low. (The risk is compared to that associated with penicillin ... that is, just as many people are allergic to penicillin as they are to radiographic contrast material.)
===Photostimulable phosphors (PSPs)===
An increasingly common method of detecting X-rays is the use of Photostimulable Luminescence (PSL), pioneered by Fuji in the 1980s. In modern hospitals a PSP plate is used in place of the photographic plate. After the plate is X-rayed, excited electrons in the phosphor material remain 'trapped' in 'colour centres' in the crystal lattice until stimulated by a laser beam passed over the plate surface. The light given off during laser stimulation is collected by a photomultiplier tube and the resulting signal is converted into a digital image by computer technology, which gives this process its common name, [[computed radiography]] (also referred to as '''digital radiography'''). The PSP plate can be used over and over again, and existing X-ray equipment requires no modification to use them.
===Geiger counter===
Initially, most common detection methods were based on the [[plasma (physics)|ionization of gases]], as in the [[Geiger counter|Geiger-Müller counter]]: a sealed volume, usually a cylinder, with a mica, polymer or thin metal window contains a gas, and a wire, and a high voltage is applied between the cylinder ([[cathode]]) and the wire ([[anode]]). When an X-ray photon enters the cylinder, it ionizes the gas and forms ions and electrons. Electrons accelerate toward the anode, in the process causing further ionization along their trajectory. This process, known as an avalanche, is detected as a sudden flow of current, called a "count" or "event".
Ultimately, the electrons form a virtual cathode around the anode wire drastically reducing the electric field in the outer portions of the tube. This halts the collisional ionizations and limits further growth of avalanches. As a result, all "counts" on a Geiger counter are the same size and it can give no indication as to the particle energy of the radiation, unlike the [[proportional counter]]. The intensity of the radiation is measurable by the Geiger counter as the counting-rate of the system.
In order to gain energy spectrum information a [[diffraction|diffracting]] crystal may be used to first separate the different photons, the method is called [[wavelength dispersive X-ray spectroscopy]] ([[WDX]] or WDS). Position-sensitive detectors are often used in conjunction with dispersive elements. Other detection equipment may be used which are inherently energy-resolving, such as the aforementioned [[proportional counter]]s. In either case, use of suitable pulse-processing (MCA) equipment allows digital spectra to be created for later analysis.
For many applications, counters are not sealed but are constantly fed with purified gas (thus reducing problems of contamination or gas aging). These are called "flow counter".
===Scintillators===
Some materials such as [[sodium iodide]] (NaI) can "convert" an X-ray photon to a visible photon; an electronic detector can be built by adding a [[photomultiplier]]. These detectors are called "[[scintillator]]s", filmscreens or "[[scintillation counter]]s". The main advantage of using these is that an adequate image can be obtained while subjecting the patient to a much lower dose of X-rays.
===Image intensification===
[[Image:Laprascopy-Roentgen.jpg|right|thumb|250px|X-ray during [[Cholecystectomy]]]]
X-rays are also used in "real-time" procedures such as [[angiography]] or contrast studies of the hollow organs (e.g. [[barium enema]] of the small or large intestine) using [[fluoroscopy]] acquired using an [[X-ray image intensifier]]. [[Angioplasty]], medical interventions of the arterial system, rely heavily on X-ray-sensitive contrast to identify potentially treatable lesions.
===Direct semiconductor detectors===
Since the 1970s, new [[semiconductor detector]]s have been developed ([[silicon]] or [[germanium]] doped with [[lithium]], Si(Li) or Ge(Li)). X-ray photons are converted to electron-hole pairs in the semiconductor and are collected to detect the X-rays. When the temperature is low enough (the detector is cooled by [[Peltier effect]] or even cooler [[liquid nitrogen]]), it is possible to directly determine the X-ray energy spectrum; this method is called [[energy dispersive X-ray spectroscopy]] (EDX or EDS); it is often used in small [[X-ray fluorescence]] [[spectroscopy|spectrometers]]. These detectors are sometimes called "[[solid state]] detectors". [[Cadmium telluride]] ([[cadmium|Cd]]Te) and its alloy with [[zinc]], [[cadmium zinc telluride]] detectors have an increased sensitivity, which allows lower doses of X-rays to be used.
Practical application in [[medical imaging]] didn't start taking place until the 1990s. Currently amorphous [[selenium]] is used in commercial large area flat panel X-ray detectors for [[mammography]] and chest [[radiography]]. Current research and development is focused around pixel detectors, such as [[CERN]]'s energy resolving [[Medipix]] detector.
Note: A standard [[semiconductor]] [[diode]], such as a 1N4007, will produce a small amount of current when placed in an X-ray beam. A test device once used by Medical Imaging Service personnel was a small project box that contained several diodes of this type in [[series and parallel circuits#Series circuits|series]], which could be connected to an [[oscilloscope]] as a quick diagnostic.
[[Silicon drift detector]]s (SDDs), produced by conventional [[semiconductor fabrication]], now provide a cost-effective and high resolving power radiation measurement. Unlike conventional X-ray detectors, such as Si(Li)s, they do not need to be cooled with liquid nitrogen.
===Scintillator plus semiconductor detectors (indirect detection)===
With the advent of large semiconductor array detectors it has become possible to design detector systems using a scintillator screen to convert from X-rays to visible light which is then converted to electrical signals in an array detector. Indirect Flat Panel Detectors (FPDs) are in widespread use today in medical, dental, veterinary and industrial applications. A common form of these detectors is based on [[amorphous silicon]] [[Thin film transistor|TFT]]/[[photodiode]] arrays.
The array technology is a variant on the amorphous silicon TFT arrays used in many [[flat panel display]]s, like the ones in computer laptops. The array consists of a sheet of glass covered with a thin layer of silicon that is in an amorphous or disordered state. At a microscopic scale, the silicon has been imprinted with millions of transistors arranged in a highly ordered array, like the grid on a sheet of graph paper. Each of these [[thin film transistor]]s (TFTs) are attached to a light-absorbing photodiode making up an individual [[pixel]] (picture element). Photons striking the photodiode are converted into two [[charge carrier|carriers of electrical charge]], called electron-hole pairs. Since the number of charge carriers produced will vary with the intensity of incoming light photons, an electrical pattern is created that can be swiftly converted to a voltage and then a digital signal, which is interpreted by a computer to produce a digital image. Although silicon has outstanding electronic properties, it is not a particularly good absorber of X-ray photons. For this reason, X-rays first impinge upon [[scintillator]]s made from eg. [[gadolinium oxysulfide]] or [[caesium iodide]]. The scintillator absorbs the X-rays and converts them into visible light photons that then pass onto the photodiode array.
===Visibility to the human eye===
While generally considered invisible to the human eye, in special circumstances X-rays can be visible.<ref>{{cite web
| last = Martin
| first = Dylan
| authorlink =
| coauthors =
| title = X-Ray Detection
| work =
| publisher = University of Arizona Optical Sciences Center
| date = 2005
| url = http://www.u.arizona.edu/~dwmartin/
| format =
| doi =
| accessdate = 2008-05-19}}</ref> Brandes, in an experiment a short time after [[Wilhelm Röntgen|Röntgen's]] landmark 1895 paper, reported after dark adaptation and placing his eye close to an X-ray tube, seeing a faint "blue-gray" glow which seemed to originate within the eye itself.<ref>{{cite web
| accessdate = 2008-05-19}}</ref> Upon hearing this, Röntgen reviewed his record books and found he too had seen the effect. When placing an X-ray tube on the opposite side of a wooden door Röntgen had noted the same blue glow, seeming to emanate from the eye itself, but thought his observations to be spurious because he only saw the effect when he used one type of tube. Later he realized that the tube which had created the effect was the only one powerful enough to make the glow plainly visible and the [[experiment]] was thereafter readily repeatable. The knowledge that X-rays are actually faintly visible to the dark-adapted naked eye has largely been forgotten today; this is probably due to the desire not to repeat what would now be seen as a recklessly dangerous and potentially harmful experiment with [[ionizing radiation]]. It is not known what exact mechanism in the eye produces the visibility: it could be due to conventional detection (excitation of [[rhodopsin]] molecules in the retina), direct excitation of retinal nerve cells, or secondary detection via, for instance, X-ray induction of [[phosphorescence]] in the eyeball with conventional retinal detection of the secondarily produced visible light.
==Medical uses==
==Medical uses==
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==Other uses==
==Other uses==
[[Image:X-ray diffraction pattern 3clpro.jpg|thumb|Each dot, called a reflection, in this diffraction pattern forms from the constructive interference of scattered X-rays passing through a crystal. The data can be used to determine the crystalline structure.]]
Other notable uses of X-rays include
Other notable uses of X-rays include
*[[X-ray crystallography]] in which the pattern produced by the [[diffraction]] of X-rays through the closely spaced lattice of atoms in a crystal is recorded and then analyzed to reveal the nature of that lattice. A related technique, [[fiber diffraction]], was used by [[Rosalind Franklin]] to discover the [[Double helix|double helical]] structure of [[DNA]]).<ref>{{cite book
*[[X-ray crystallography]] in which the pattern produced by the [[diffraction]] of X-rays through the closely spaced lattice of atoms in a crystal is recorded and then analyzed to reveal the nature of that lattice. A related technique, [[fiber diffraction]], was used by [[Rosalind Franklin]] to discover the [[Double helix|double helical]] structure of [[DNA]]).<ref>{{cite book
X-Ray Image of the Paranasal Sinuses, Lateral Projection
Since Röntgen's discovery that X-rays can identify bony structures, X-rays have been developed for their use in medical imaging. Radiology is a specialized field of medicine. Radiographers employ radiography and other techniques for diagnostic imaging. Indeed, this is probably the most common use of X-ray technology.
X-rays are especially useful in the detection of pathology of the skeletal system, but are also useful for detecting some disease processes in soft tissue. Some notable examples are the very common chest X-ray, which can be used to identify lung diseases such as pneumonia, lung cancer or pulmonary edema, and the abdominal X-ray, which can detect ileus (blockage of the intestine), free air (from visceral perforations) and free fluid (in ascites). In some cases, the use of X-rays is debatable, such as gallstones (which are rarely radiopaque) or kidney stones (which are often visible, but not always). Also, traditional plain X-rays pose very little use in the imaging of soft tissues such as the brain or muscle. Imaging alternatives for soft tissues are computed axial tomography (CAT or CT scanning), magnetic resonance imaging (MRI) or ultrasound. Since 2005, X-rays are listed as a carcinogen by the U.S. government.[1]
Radiotherapy, a curative medical intervention, now used almost exclusively for cancer, employs higher energies of radiation.
The efficiency of X-ray tubes is less than 2%. Most of the energy is used to heat up the anode.
X-ray fluorescence, a technique in which X-rays are generated within a specimen and detected. The outgoing energy of the X-ray can be used to identify the composition of the sample.
Paintings are often X-rayed to reveal the underdrawing and pentimenti or alterations in the course of painting, or by later restorers. Many pigments such as lead white show well in X-ray photographs.
Physicist Johann Hittorf (1824 – 1914) observed tubes with energy rays extending from a negative electrode. These rays produced a fluorescence when they hit the glass walls of the tubes. In 1876 the effect was named "cathode rays" by Eugen Goldstein, and today are known to be streams of electrons. Later, English physicist William Crookes investigated the effects of electric currents in gases at low pressure, and constructed what is called the Crookes tube. It is a glass cylinder mostly (but not completely) evacuated, containing electrodes for discharges of a high voltage electric current. He found, when he placed unexposed photographic plates near the tube, that some of them were flawed by shadows, though he did not investigate this effect. Crookes also noted that his cathode rays caused the glass walls of his tube to glow a dull blue colour. Crookes failed to realise that it wasn't actually the cathode rays that caused the blue glow, but the low-level X-rays produced when the cathode rays struck the glass.
Ivan Pulyui
In 1877 Ukranian-born Pulyui, a lecturer in experimental physics at the University of Vienna, constructed various designs of vacuum discharge tube to investigate their properties.[3] He continued his investigations when appointed professor at the Prague Polytechnic and in 1886 he found that that sealed photographic plates became dark when exposed to the emanations from the tubes. Early in 1896, just a few weeks after Röntgen published his first X-ray photograph, Pulyui published high-quality x-ray images in journals in Paris and London.[3] Although Pulyui had studied with Röntgen at the University of Strasbourg in the years 1873-75, his biographer Gaida (1997) asserts that his subsequent research was conducted independently.[3]
The first medical X-ray made in the United States was obtained using a discharge tube of Pulyui's design. In January 1896, on reading of Röntgen's discovery, Frank Austin of Dartmouth College tested all of the discharge tubes in the physics laboratory and found that only the Pulyui tube produced X-rays. This was a result of Pulyui's inclusion of an oblique "target" of mica, used for holding samples of fluorescent material, within the tube. On 3 February1896 Gilman Frost, professor of medicine at the college, and his brother Edwin Frost, professor of physics, exposed the wrist of Eddie McCarthy, whom Edwin had treated some weeks earlier for a fracture, to the x-rays and collected the resulting image of the broken bone on gelatin photographic plates obtained from Howard Langill, a local photographer also interested in Röntgen's work.[4]
Nikola Tesla
In April 1887, Nikola Tesla began to investigate X-rays using high voltages and tubes of his own design, as well as Crookes tubes. From his technical publications, it is indicated that he invented and developed a special single-electrode X-ray tube [5][6], which differed from other X-ray tubes in having no target electrode. The principle behind Tesla's device is called the Bremsstrahlung process, in which a high-energy secondary X-ray emission is produced when charged particles (such as electrons) pass through matter. By 1892, Tesla performed several such experiments, but he did not categorize the emissions as what were later called X-rays. Tesla generalized the phenomenon as radiant energy of "invisible" kinds.[7][8] Tesla stated the facts of his methods concerning various experiments in his 1897 X-ray lecture [9] before the New York Academy of Sciences. Also in this lecture, Tesla stated the method of construction and safe operation of X-ray equipment. His X-ray experimentation by vacuum high field emissions also led him to alert the scientific community to the biological hazards associated with X-ray exposure.[10]
Fernando Sanford
X-rays were first generated and detected by Fernando Sanford (1854-1948), the foundation Professor of Physics at Stanford University, in 1891. From 1886 to 1888 he had studied in the Hermann Helmholtz laboratory in Berlin, where he became familiar with the cathode rays generated in vacuum tubes when a voltage was applied across separate electrodes, as previously studied by Heinrich Hertz and Philipp Lenard. His letter of January 6, 1893 (describing his discovery as "electric photography") to The Physical Review was duly published and an article entitled Without Lens or Light, Photographs Taken With Plate and Object in Darkness appeared in the San Francisco Examiner.[11]
Heinrich Hertz
In 1892, Heinrich Hertz began experimenting and demonstrated that cathode rays could penetrate very thin metal foil (such as aluminium). Philipp Lenard, a student of Heinrich Hertz, further researched this effect. He developed a version of the Crookes tube and studied the penetration by X-rays of various materials. Philipp Lenard, though, did not realize that he was producing X-rays. Hermann von Helmholtz formulated mathematical equations for X-rays. He postulated a dispersion theory before Röntgen made his discovery and announcement. It was formed on the basis of the electromagnetic theory of light (Wiedmann's Annalen, Vol. XLVIII). However, he did not work with actual X-rays.
Wilhelm Röntgen
On November 81895, Wilhelm Conrad Röntgen, a German physics professor, began observing and further documenting X-rays while experimenting with Lenard and Crookes tubes. Röntgen, on December 28, 1895, wrote a preliminary report "On a new kind of ray: A preliminary communication". He submitted it to the Würzburg's Physical-Medical Society journal.[12] This was the first formal and public recognition of the categorization of X-rays. Röntgen referred to the radiation as "X", to indicate that it was an unknown type of radiation. The name stuck, although (over Röntgen's great objections), many of his colleagues suggested calling them Röntgen rays. They are still referred to as such in many languages, including German. Röntgen received the first Nobel Prize in Physics for his discovery.
There are conflicting accounts of his discovery because Röntgen had his lab notes burned after his death, but this is a likely reconstruction by his biographers.[13] Röntgen was investigating cathode rays with a fluorescent screen painted with barium platinocyanide and a Crookes tube which he had wrapped in black cardboard so the visible light from the tube wouldn't interfere. He noticed a faint green glow from the screen, about 1 meter away. The invisible rays coming from the tube to make the screen glow were passing through the cardboard. He found they could also pass through books and papers on his desk. Röntgen threw himself into investigating these unknown rays systematically. Two months after his initial discovery, he published his paper translated "On a New Kind of Radiation" and gave a demonstration in 1896.
Röntgen discovered its medical use when he saw a picture of his wife's hand on a photographic plate formed due to X-rays. His wife's hand's photograph was the first ever photograph of a human body part using X-rays.
Thomas Edison
Diagram of a water cooled X-ray tube. (simplified/outdated)
In 1895, Thomas Edison investigated materials' ability to fluoresce when exposed to X-rays, and found that calcium tungstate was the most effective substance. Around March 1896, the fluoroscope he developed became the standard for medical X-ray examinations. Nevertheless, Edison dropped X-ray research around 1903 after the death of Clarence Madison Dally, one of his glassblowers. Dally had a habit of testing X-ray tubes on his hands, and acquired a cancer in them so tenacious that both arms were amputated in a futile attempt to save his life. "At the 1901 Pan-American Exposition in Buffalo, New York, an assassin shot President William McKinley twice at close range with a .32 caliber revolver." The first bullet was removed but the second remained lodged somewhere in his stomach. McKinley survived for some time and requested that Thomas Edison "rush an X-ray machine to Buffalo to find the stray bullet. It arrived but wasn't used . . . McKinley died of septic shock due to bacterial infection."[14]
The 20th century and beyond
Before the 20th century and for a short while after, X-rays were generated in cold cathode tubes. These tubes had to contain a small quantity of gas (invariably air) as a current will not flow in such a tube if they are fully evacuated. One of the problems with early X-ray tubes is that the generated X-rays caused the glass to absorb the gas and consequently the efficiency quickly falls off. Larger and more frequently used tubes were provided with a means of restoring the air. This often took the form of small side tube which contained a small piece of mica – a substance that traps comparatively large quantities of air within its structure. A small electrical heater heats the mica and causes it to release a small amount of air restoring the tube's efficiency. However the mica itself has a limited life and the restore process was consequently difficult to control.
In 1904, Sir John Ambrose Fleming invented the thermionic diode valve (tube). This used a heated cathode which permitted current to flow in a vacuum. The principle was quickly applied to X-ray tubes, and hard vacuum heated cathode X-ray tubes completely solved the problem of efficiency reduction.
The X-ray microscope was invented in the 1950s. The Chandra X-ray Observatory launched on July 23, 1999, has been allowing the exploration of the very violent processes in the universe which produce X-rays. Unlike visible light, which is a relatively stable view of the universe, the X-ray universe is unstable, it features stars being torn apart by black holes, galactic collisions, and novas, neutron stars that build up layers of plasma that then explode into space.
An X-ray laser device was proposed as part of the Reagan administration's Strategic Defense Initiative in the 1980s, but the first and only test of the device (a sort of laser "blaster", or death ray, powered by a thermonuclear explosion) gave inconclusive results. For technical and political reasons, the overall project (including the X-ray laser) was de-funded (though was later revived by the second Bush administration as National Missile Defense using different technologies).
↑Spiegel, Peter K (1995). "The first clinical X-ray made in America—100 years"(PDF). American Journal of Roentgenology. Leesburg, VA: American Roentgen Ray Society. 164 (1): pp241–243. ISSN: 1546-3141.CS1 maint: Extra text (link)
↑Morton, William James, and Edwin W. Hammer, American Technical Book Co., 1896. Page 68.
↑Thomas Commerford Martin (ed.), "The Inventions, Researches and Writings of Nikola Tesla". Page 252 "When it forms a drop, it will emit visible and invisible waves. [...]". (ed., this material originally appeared in an article by Nikola Tesla in The Electrical Engineer of 1894.)
↑Nikola Tesla, "The stream of Lenard and Roentgen and novel apparatus for their production", Apr. 6, 1897.
↑Cheney, Margaret, Robert Uth, and Jim Glenn, "Tesla, master of lightning". Barnes & Noble Publishing, 1999. Page 76. ISBN 0760710058
↑Wyman, Thomas (2005). "Fernando Sanford and the Discovery of X-rays". "Imprint", from the Associates of the Stanford University Libraries: pp. 5–15. Unknown parameter |month= ignored (help)CS1 maint: Extra text (link)
