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This article is about the nature, production, and uses of the radiation. For the method of imaging, seeRadiography.For the medical specialty, seeRadiology.For other uses, seeX-ray (disambiguation).Not to be confused withX-waveorX-band.

Natural color X-rayphotogramof a wine scene. Note the edges of hollow cylinders as compared to the solid candle.
William Coolidgeexplains medical imaging and X-rays.

X-rays(or rarely,X-radiation) are a form of high-energyelectromagnetic radiation.In many languages, it is referred to asRöntgen radiation,after the German scientistWilhelm Conrad Röntgen,who discovered it in1895[1]and named itX-radiationto signify an unknown type of radiation.[2]

X-raywavelengthsare shorter than those ofultravioletrays and longer than those ofgamma rays.There is no universally accepted, strict definition of the bounds of the X-ray band. Roughly, X-rays have awavelengthranging from 10nanometersto 10picometers,corresponding tofrequenciesin the range of 30petahertzto 30exahertz(3×1016Hzto3×1019Hz) and photon energies in the range of 100eVto 100keV,respectively.

X-rays can penetrate many solid substances such as construction materials and living tissue, so X-rayradiographyis widely used inmedical diagnostics(e.g., checking forbroken bones) andmaterial science(e.g., identification of somechemical elementsand detecting weak points in construction materials).[3]However X-rays areionizing radiation,and exposure to high intensities can be hazardous to health, causing damage toDNA,cancer, and at high dosages, burns andradiation sickness.Their generation and use is strictly controlled by public health authorities.

History[edit]

Pre-Röntgen observations and research[edit]

Example of aCrookes tube,a type ofdischarge tubethat emitted X-rays

Before their discovery in1895,X-rays were just a type of unidentifiedradiationemanating from experimentaldischarge tubes.They were noticed by scientists investigatingcathode raysproduced by such tubes, which are energeticelectronbeams that were first observed in 1869. Many of the earlyCrookes tubes(invented around1875) undoubtedly radiated X-rays, because early researchers noticed effects that were attributable to them, as detailed below. Crookes tubes created free electrons byionizationof the residual air in the tube by a highDCvoltageof anywhere between a fewkilovoltsand 100 kV. This voltage accelerated the electrons coming from thecathodeto a high enough velocity that they created X-rays when they struck theanodeor the glass wall of the tube.[4]

The earliest experimenter thought to have (unknowingly) produced X-rays wasWilliam Morgan.In1785,he presented apaperto theRoyal Society of Londondescribing the effects of passingelectrical currentsthrough a partially evacuated glass tube, producing a glow created by X-rays.[5][6]This work was further explored byHumphry Davyand his assistantMichael Faraday.

WhenStanford Universityphysics professorFernando Sanfordcreated his "electric photography", he also unknowingly generated and detected X-rays. From1886to1888,he studied in theHermann von Helmholtzlaboratory inBerlin,where he became familiar with the cathode rays generated in vacuum tubes when a voltage was applied across separate electrodes, as previously studied byHeinrich HertzandPhilipp Lenard.His letter of6 January 1893(describing his discovery as "electric photography" ) to thePhysical Reviewwas duly published and an article entitledWithout Lens or Light, Photographs Taken With Plate and Object in Darknessappeared in theSan Francisco Examiner.[7]

Starting in 1888, Philipp Lenard conducted experiments to see whether cathode rays could pass out of the Crookes tube into the air. He built a Crookes tube with a "window" at the end made of thin aluminium, facing the cathode so the cathode rays would strike it (later called a "Lenard tube" ). He found that something came through, that would expose photographic plates and cause fluorescence. He measured the penetrating power of these rays through various materials. It has been suggested that at least some of these "Lenard rays" were actually X-rays.[8]

In1889,Ivan Puluj,a lecturer in experimental physics at thePrague Polytechnicwho since1877had been constructing various designs ofgas-filled tubesto investigate their properties, published a paper on how sealed photographic plates became dark when exposed to the emanations from the tubes.[9]

Helmholtz formulated mathematical equations for X-rays. He postulated a dispersion theory before Röntgen made his discovery and announcement. He based it on theelectromagnetic theory of light.[10][full citation needed]However, he did not work with actual X-rays.

In1894,Nikola Teslanoticed damaged film in his lab that seemed to be associated with Crookes tube experiments and began investigating this invisible,radiant energy.[11][12]After Röntgen identified the X-ray, Tesla began making X-ray images of his own using high voltages and tubes of his own design,[13]as well as Crookes tubes.

Discovery by Röntgen[edit]

Wilhelm Röntgen

On8 November 1895,German physics professorWilhelm Röntgenstumbled on X-rays while experimenting with Lenard tubes andCrookes tubesand began studying them. He wrote an initial report "On a new kind of ray: A preliminary communication" and on 28 December 1895, submitted it toWürzburg's Physical-Medical Society journal.[14]This was the first paper written on X-rays. Röntgen referred to the radiation as "X", to indicate that it was an unknown type of radiation. Some early texts refer to them as Chi-rays, having interpreted "X" as the uppercaseGreek letter Chi,Χ.[15][16][17]The name X-rays stuck, although (over Röntgen's great objections) many of his colleagues suggested calling themRöntgen rays.They are still referred to as such in many languages, including German,Hungarian,Ukrainian,Danish,Polish,Czech,Bulgarian,Swedish,Finnish,Portuguese,Estonian,Slovak,Slovenian,Turkish,Russian,Latvian,Lithuanian,Albanian,Japanese,Dutch,Georgian,Hebrew,Icelandic,andNorwegian.Röntgen received the firstNobel Prize in Physicsfor his discovery.[18]

There are conflicting accounts of his discovery because Röntgen had hislab notesburned after his death, but this is a likely reconstruction by his biographers:[19][20]Röntgen was investigating cathode rays from a Crookes tube which he had wrapped in black cardboard so that the visible light from the tube would not interfere, using afluorescentscreen painted with bariumplatinocyanide.He noticed a faint green glow from the screen, about 1 meter (3.3 ft) away. Röntgen realized some invisible rays coming from the tube were passing through the cardboard to make the screen glow. 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.[21]

Hand mit Ringen(Hand with Rings): print of Wilhelm Röntgen's first "medical" X-ray, of his wife's hand, taken on 22 December 1895 and presented toLudwig Zehnderof the Physik Institut,University of Freiburg,on 1 January 1896[22][23]

Röntgen discovered their medical use when he made a picture of his wife's hand on a photographic plate formed due to X-rays. The photograph of his wife's hand was the first photograph of a human body part using X-rays. When she saw the picture, she said "I have seen my death."[24]

The discovery of X-rays generated significant interest. Röntgen's biographer Otto Glasser estimated that, in1896alone, as many as 49 essays and 1044 articles about the new rays were published.[25]This was probably a conservative estimate, if one considers that nearly every paper around the world extensively reported about the new discovery, with a magazine such asSciencededicating as many as 23 articles to it in that year alone.[26]Sensationalist reactions to the new discovery included publications linking the new kind of rays to occult and paranormal theories, such as telepathy.[27][28]

Advances in radiology[edit]

Taking an X-ray image with earlyCrookes tubeapparatus, late 1800s. The Crookes tube is visible in center. The standing man is viewing his hand with afluoroscopescreen. The seated man is taking aradiographof his hand by placing it on aphotographic plate.No precautions against radiation exposure are taken; its hazards were not known at the time.
Surgical removal of a bullet whose location was diagnosed with X-rays (see inset) in 1897

Röntgen immediately noticed X-rays could have medical applications. Along with his 28 December Physical-Medical Society submission, he sent a letter to physicians he knew around Europe (1 January 1896).[29]News (and the creation of "shadowgrams" ) spread rapidly with Scottish electrical engineerAlan Archibald Campbell-Swintonbeing the first after Röntgen to create an X-ray (of a hand). Through February, there were 46 experimenters taking up the technique in North America alone.[29]

The first use of X-rays under clinical conditions was byJohn Hall-Edwardsin Birmingham, England on 11 January 1896, when he radiographed a needle stuck in the hand of an associate. On 14 February 1896, Hall-Edwards was also the first to use X-rays in a surgical operation.[30]

Images by James Green, from "Sciagraphs of British Batrachians and Reptiles" (1897), featuring (from left)Rana esculenta(nowPelophylax lessonae),Lacerta vivipara(nowZootoca vivipara), andLacerta agilis

In early 1896, several weeks after Röntgen's discovery,Ivan Romanovich Tarkhanovirradiated frogs and insects with X-rays, concluding that the rays "not only photograph, but also affect the living function".[31]At around the same time, the zoological illustrator James Green began to use X-rays to examine fragile specimens.George Albert Boulengerfirst mentioned this work in a paper he delivered before theZoological Society of Londonin May 1896. The bookSciagraphs of British Batrachians and Reptiles(sciagraph is an obsolete name for an X-ray photograph), by Green and James H. Gardiner, with a foreword by Boulenger, was published in 1897.[32][33]

The first medical X-ray made in the United States was obtained using a discharge tube of Puluj's design. In January 1896, on reading of Röntgen's discovery, Frank Austin ofDartmouth Collegetested all of the discharge tubes in the physics laboratory and found that only the Puluj tube produced X-rays. This was a result of Puluj's inclusion of an oblique "target" ofmica,used for holding samples offluorescentmaterial, within the tube. On 3 February 1896, Gilman Frost, professor of medicine at the college, and his brother Edwin Frost, professor of physics, exposed the wrist of Eddie McCarthy, whom Gilman had treated some weeks earlier for a fracture, to the X-rays and collected the resulting image of the broken bone ongelatin photographic platesobtained from Howard Langill, a local photographer also interested in Röntgen's work.[34]

1896 plaque published in"Nouvelle Iconographie de la Salpetrière",a medical journal. In the left a hand deformity, in the right same hand seen usingradiography.The authors named the technique Röntgen photography.

Many experimenters, including Röntgen himself in his original experiments, came up with methods to view X-ray images "live" using some form of luminescent screen.[29]Röntgen used a screen coated with bariumplatinocyanide.On 5 February 1896, live imaging devices were developed by both Italian scientist Enrico Salvioni (his "cryptoscope" ) andWilliam Francis MagieofPrinceton University(his "Skiascope" ), both using barium platinocyanide. American inventorThomas Edisonstarted research soon after Röntgen's discovery and investigated materials' ability to fluoresce when exposed to X-rays, finding thatcalcium tungstatewas the most effective substance. In May 1896, he developed the first mass-produced live imaging device, his "Vitascope", later called thefluoroscope,which became the standard for medical X-ray examinations.[29]Edison dropped X-ray research around 1903, before the death ofClarence Madison Dally,one of his glassblowers. Dally had a habit of testing X-ray tubes on his own hands, developing a cancer in them so tenacious that both arms wereamputatedin a futile attempt to save his life; in 1904, he became the first known death attributed to X-ray exposure.[29]During the time the fluoroscope was being developed, Serbian American physicistMihajlo Pupin,using a calcium tungstate screen developed by Edison, found that using a fluorescent screen decreased the exposure time it took to create an X-ray for medical imaging from an hour to a few minutes.[35][29]

In 1901,U.S. President William McKinley was shot twicein an assassination attempt while attending thePan American ExpositioninBuffalo, New York.While one bullet only grazed hissternum,another had lodged somewhere deep inside hisabdomenand could not be found. A worried McKinley aide sent word to inventor Thomas Edison to rush anX-ray machineto Buffalo to find the stray bullet. It arrived but was not used. While the shooting itself had not been lethal,gangrenehad developed along the path of the bullet, and McKinley died ofseptic shockdue to bacterial infection six days later.[36]

Hazards discovered[edit]

With the widespread experimentation with X‑rays after their discovery in1895by scientists, physicians, and inventors came many stories of burns, hair loss, and worse in technical journals of the time. In February 1896, Professor John Daniel andWilliam Lofland DudleyofVanderbilt Universityreported hair loss after Dudley was X-rayed. A child who had been shot in the head was brought to the Vanderbilt laboratory in 1896. Before trying to find the bullet, an experiment was attempted, for which Dudley "with his characteristic devotion to science"[37][38][39]volunteered. Daniel reported that 21 days after taking a picture of Dudley'sskull(with an exposure time of one hour), he noticed a bald spot 5 centimeters (2 in) in diameter on the part of his head nearest the X-ray tube: "A plate holder with the plates towards the side of the skull was fastened and acoinplaced between the skull and the head. The tube was fastened at the other side at a distance of one-half-inch [1.3 cm] from the hair. "[40]Beyond burns, hair loss, and cancer, X-rays can be linked to infertility in males based on the amount of radiation used.

In August 1896, H. D. Hawks, a graduate of Columbia College, suffered severe hand and chest burns from an X-ray demonstration. It was reported inElectrical Reviewand led to many other reports of problems associated with X-rays being sent in to the publication.[41]Many experimenters includingElihu Thomsonat Edison's lab,William J. Morton,andNikola Teslaalso reported burns. Elihu Thomson deliberately exposed a finger to an X-ray tube over a period of time and suffered pain, swelling, and blistering.[42]Other effects were sometimes blamed for the damage including ultraviolet rays and (according to Tesla) ozone.[11]Many physicians claimed there were no effects from X-ray exposure at all.[42]On 3 August 1905, in San Francisco, California,Elizabeth Fleischman,an American X-ray pioneer, died from complications as a result of her work with X-rays.[43][44][45]

Hall-Edwards developed a cancer (then called X-ray dermatitis) sufficiently advanced by 1904 to cause him to write papers and give public addresses on the dangers of X-rays. His left arm had to be amputated at the elbow in 1908,[46][47]and four fingers on his right arm soon thereafter, leaving only a thumb. He died of cancer in 1926. His left hand is kept atBirmingham University.

20th century and beyond[edit]

A patient being examined with a thoracicfluoroscopein1940,which displayed continuous moving images. This image was used to argue thatradiation exposureduring the X-ray procedure would be negligible.

The many applications of X-rays immediately generated enormous interest. Workshops began making specialized versions of Crookes tubes for generating X-rays and these first-generationcold cathodeor Crookes X-ray tubes were used until about 1920.

A typical early 20th-century medical X-ray system consisted of aRuhmkorff coilconnected to acold cathode Crookes X-ray tube.A spark gap was typically connected to the high voltage side in parallel to the tube and used for diagnostic purposes.[48]The spark gap allowed detecting the polarity of the sparks, measuring voltage by the length of the sparks thus determining the "hardness" of the vacuum of the tube, and it provided a load in the event the X-ray tube was disconnected. To detect the hardness of the tube, the spark gap was initially opened to the widest setting. While the coil was operating, the operator reduced the gap until sparks began to appear. A tube in which the spark gap began to spark at around 6.4 centimeters (2.5 in) was considered soft (low vacuum) and suitable for thin body parts such as hands and arms. A 13-centimeter (5 in) spark indicated the tube was suitable for shoulders and knees. An 18-to-23-centimeter (7 to 9 in) spark would indicate a higher vacuum suitable for imaging the abdomen of larger individuals. Since the spark gap was connected in parallel to the tube, the spark gap had to be opened until the sparking ceased to operate the tube for imaging. Exposure time for photographic plates was around half a minute for a hand to a couple of minutes for a thorax. The plates may have a small addition of fluorescent salt to reduce exposure times.[48]

Crookes tubes were unreliable. They 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. However, as time passed, the X-rays caused the glass to absorb the gas, causing the tube to generate "harder" X-rays until it soon stopped operating. Larger and more frequently used tubes were provided with devices for restoring the air, known as "softeners". These often took the form of a small side tube that contained a small piece ofmica,a mineral that traps relatively large quantities of air within its structure. A small electrical heater heated the mica, causing it to release a small amount of air, thus restoring the tube's efficiency. However, the mica had a limited life, and the restoration process was difficult to control.

In1904,John Ambrose Fleminginvented thethermionic diode,the first kind ofvacuum tube.This used ahot cathodethat caused anelectric currentto flow in avacuum.This idea was quickly applied to X-ray tubes, and hence heated-cathode X-ray tubes, called "Coolidge tubes", completely replaced the troublesome cold cathode tubes by about 1920.

In about 1906, the physicistCharles Barkladiscovered that X-rays could be scattered by gases, and that each element had a characteristicX-ray spectrum.He won the1917Nobel Prize in Physicsfor this discovery.

In1912,Max von Laue,Paul Knipping, and Walter Friedrich first observed thediffractionof X-rays by crystals. This discovery, along with the early work ofPaul Peter Ewald,William Henry Bragg,andWilliam Lawrence Bragg,gave birth to the field ofX-ray crystallography.[49]

In1913,Henry Moseleyperformed crystallography experiments with X-rays emanating from various metals and formulatedMoseley's lawwhich relates the frequency of the X-rays to the atomic number of the metal.

TheCoolidge X-ray tubewas invented the same year byWilliam D. Coolidge.It made possible the continuous emissions of X-rays. Modern X-ray tubes are based on this design, often employing the use of rotating targets which allow for significantly higher heat dissipation than static targets, further allowing higher quantity X-ray output for use in high-powered applications such as rotational CT scanners.

Chandra's image of the galaxy cluster Abell 2125 reveals a complex of several massive multimillion-degree-Celsius gas clouds in the process of merging.

The use of X-rays for medical purposes (which developed into the field ofradiation therapy) was pioneered by MajorJohn Hall-EdwardsinBirmingham,England. Then in 1908, he had to have his left arm amputated because of the spread ofX-ray dermatitison his arm.[50]

Medical science also used the motion picture to study human physiology. In 1913, a motion picture was made in Detroit showing a hard-boiled egg inside a human stomach. This early X-ray movie was recorded at a rate of one still image every four seconds.[51]Dr Lewis Gregory Cole of New York was a pioneer of the technique, which he called "serial radiography".[52][53]In 1918, X-rays were used in association withmotion picture camerasto capture the human skeleton in motion.[54][55][56]In 1920, it was used to record the movements of tongue and teeth in the study of languages by the Institute of Phonetics in England.[57]

In1914,Marie Curiedeveloped radiological cars to support soldiers injured inWorld War I.The cars would allow for rapid X-ray imaging of wounded soldiers so battlefield surgeons could quickly and more accurately operate.[58]

From the early 1920s through to the 1950s, X-ray machines were developed to assist in the fitting of shoes[59]and were sold to commercial shoe stores.[60][61][62]Concerns regarding the impact of frequent or poorly controlled use were expressed in the 1950s,[63][64]leading to the practice's eventual end that decade.[65]

TheX-ray microscopewas developed during the 1950s.

TheChandra X-ray Observatory,launched on23 July 1999,has been allowing the exploration of the very violent processes in theuniversethat produce X-rays. Unlikevisible light,which gives a relatively stable view of the universe, the X-ray universe is unstable. It featuresstarsbeing torn apart byblack holes,galactic collisions,andnovae,andneutron starsthat build up layers ofplasmathat thenexplodeintospace.

Phase-contrast X-ray image of a spider

AnX-ray laserdevice was proposed as part of theReagan Administration'sStrategic Defense Initiativein the 1980s, but the only test of the device (a sort of laser "blaster" ordeath ray,powered by a thermonuclear explosion) gave inconclusive results. For technical and political reasons, the overall project (including the X-ray laser) was defunded (though was later revived by the secondBush AdministrationasNational Missile Defenseusing different technologies).

Phase-contrast X-ray imagingrefers to a variety of techniques that use phase information of an X-ray beam to form the image. Due to its good sensitivity to density differences, it is especially useful for imaging soft tissues. It has become an important method for visualizing cellular and histological structures in a wide range of biological and medical studies. There are several technologies being used for X-ray phase-contrast imaging, all using different principles to convert phase variations in the X-rays emerging from an object into intensity variations.[66][67]These include propagation-based phase contrast,[68]Talbotinterferometry,[67]refraction-enhanced imaging,[69]and X-ray interferometry.[70]These methods provide higher contrast compared to normal absorption-based X-ray imaging, making it possible to distinguish from each other details that have almost similar density. A disadvantage is that these methods require more sophisticated equipment, such assynchrotronormicrofocusX-ray sources,X-ray optics,and high resolution X-ray detectors.

Energy ranges[edit]

X-rays are part of theelectromagnetic spectrum,with wavelengths shorter thanUV light.Different applications use different parts of the X-ray spectrum.

Soft and hard X-rays[edit]

X-rays with highphoton energiesabove 5–10 keV (below 0.2–0.1 nm wavelength) are calledhard X-rays,while those with lower energy (and longer wavelength) are calledsoft X-rays.[71]The intermediate range with photon energies of several keV is often referred to astender X-rays.Due to their penetrating ability, hard X-rays are widely used to image the inside of objects (e.g. inmedical radiographyandairport security). The termX-rayismetonymicallyused to refer to aradiographicimage produced using this method, in addition to the method itself. Since the wavelengths of hard X-rays are similar to the size of atoms, they are also useful for determining crystal structures byX-ray crystallography.By contrast, soft X-rays are easily absorbed in air; theattenuation lengthof 600 eV (~2 nm) X-rays in water is less than 1 micrometer.[72]

Gamma rays[edit]

There is no consensus for a definition distinguishing between X-rays andgamma rays.One common practice is to distinguish between the two types of radiation based on their source: X-rays are emitted byelectrons,while gamma rays are emitted by theatomic nucleus.[73][74][75][76]This definition has several problems: other processes can also generate these high-energyphotons,or sometimes the method of generation is not known. One common alternative is to distinguish X- and gamma radiation on the basis of wavelength (or, equivalently, frequency or photon energy), with radiation shorter than some arbitrary wavelength, such as 10−11m (0.1Å), defined as gamma radiation.[77]This criterion assigns a photon to an unambiguous category, but is only possible if wavelength is known. (Some measurement techniques do not distinguish between detected wavelengths.) However, these two definitions often coincide since the electromagnetic radiation emitted byX-ray tubesgenerally has a longer wavelength and lower photon energy than the radiation emitted byradioactivenuclei.[73]Occasionally, one term or the other is used in specific contexts due to historical precedent, based on measurement (detection) technique, or based on their intended use rather than their wavelength or source. Thus, gamma-rays generated for medical and industrial uses, for exampleradiotherapy,in the ranges of 6–20MeV,can in this context also be referred to as X-rays.[78]

Properties[edit]

Ionizing radiation hazard symbol

X-rayphotonscarry enough energy toionizeatoms and disruptmolecular bonds.This makes it a type ofionizing radiation,and therefore harmful to livingtissue.A very highradiation doseover a short period of time causes burns andradiation sickness,while lower doses can give an increased risk ofradiation-induced cancer.In medical imaging, this increased cancer risk is generally greatly outweighed by the benefits of the examination. The ionizing capability of X-rays can be used incancer treatmentto killmalignantcellsusingradiation therapy.It is also used for material characterization usingX-ray spectroscopy.

Hard X-rays can traverse relatively thick objects without being muchabsorbedorscattered.For this reason, X-rays are widely used toimagethe inside of visually opaque objects. The most often seen applications are in medicalradiographyandairport securityscanners, but similar techniques are also important in industry (e.g.industrial radiographyandindustrial CT scanning) and research (e.g.small animal CT). Thepenetration depthvaries with severalorders of magnitudeover the X-ray spectrum. This allows the photon energy to be adjusted for the application so as to give sufficienttransmissionthrough the object and at the same time provide goodcontrastin the image.

X-rays have much shorter wavelengths than visible light, which makes it possible to probe structures much smaller than can be seen using a normalmicroscope.This property is used inX-ray microscopyto acquire high-resolution images, and also inX-ray crystallographyto determine the positions ofatomsincrystals.

Interaction with matter[edit]

Attenuation length of X-rays in water showing the oxygenabsorption edgeat 540 eV, the energy−3dependence ofphotoabsorption,as well as a leveling off at higher photon energies due toCompton scattering.The attenuation length is about four orders of magnitude longer for hard X-rays (right half) compared to soft X-rays (left half).

X-rays interact with matter in three main ways, throughphotoabsorption,Compton scattering,andRayleigh scattering.The strength of these interactions depends on the energy of the X-rays and the elemental composition of the material, but not much on chemical properties, since the X-ray photon energy is much higher than chemical binding energies. Photoabsorption or photoelectric absorption is the dominant interaction mechanism in the soft X-ray regime and for the lower hard X-ray energies. At higher energies, Compton scattering dominates.

Photoelectric absorption[edit]

The probability of a photoelectric absorption per unit mass is approximately proportional toZ3/E3,whereZis theatomic numberandEis the energy of the incident photon.[79]This rule is not valid close to inner shell electron binding energies where there are abrupt changes in interaction probability, so calledabsorption edges.However, the general trend of highabsorption coefficientsand thus shortpenetration depthsfor low photon energies and high atomic numbers is very strong. For soft tissue, photoabsorption dominates up to about 26 keV photon energy where Compton scattering takes over. For higher atomic number substances, this limit is higher. The high amount ofcalcium(Z= 20) in bones, together with their high density, is what makes them show up so clearly on medical radiographs.

A photoabsorbed photon transfers all its energy to the electron with which it interacts, thus ionizing the atom to which the electron was bound and producing a photoelectron that is likely to ionize more atoms in its path. An outer electron will fill the vacant electron position and produce either a characteristic X-ray or anAuger electron.These effects can be used for elemental detection throughX-ray spectroscopyorAuger electron spectroscopy.

Compton scattering[edit]

Compton scattering is the predominant interaction between X-rays and soft tissue in medical imaging.[80]Compton scattering is aninelastic scatteringof the X-ray photon by an outer shell electron. Part of the energy of the photon is transferred to the scattering electron, thereby ionizing the atom and increasing the wavelength of the X-ray. The scattered photon can go in any direction, but a direction similar to the original direction is more likely, especially for high-energy X-rays. The probability for different scattering angles is described by theKlein–Nishina formula.The transferred energy can be directly obtained from the scattering angle from theconservation of energyandmomentum.

Rayleigh scattering[edit]

Rayleigh scattering is the dominantelastic scatteringmechanism in the X-ray regime.[81]Inelastic forward scattering gives rise to the refractive index, which for X-rays is only slightly below 1.[82]

Production[edit]

Whenever charged particles (electrons or ions) of sufficient energy hit a material, X-rays are produced.

Production by electrons[edit]

Characteristic X-ray emission lines for some common anode materials.[83][84]
Anode
material		 Atomic
number		 Photon energy [keV] Wavelength [nm]
Kα1 Kβ1 Kα1 Kβ1
W 74 59.3 67.2 0.0209 0.0184
Mo 42 17.5 19.6 0.0709 0.0632
Cu 29 8.05 8.91 0.154 0.139
Ag 47 22.2 24.9 0.0559 0.0497
Ga 31 9.25 10.26 0.134 0.121
In 49 24.2 27.3 0.0512 0.0455
Spectrum of the X-rays emitted by an X-ray tube with arhodiumtarget, operated at 60kV.The smooth, continuous curve is due tobremsstrahlung,and the spikes arecharacteristic K linesfor rhodium atoms.

X-rays can be generated by anX-ray tube,avacuum tubethat uses a high voltage to accelerate theelectronsreleased by ahot cathodeto a high velocity. The high velocity electrons collide with a metal target, theanode,creating the X-rays.[85]In medical X-ray tubes the target is usuallytungstenor a more crack-resistant alloy ofrhenium(5%) and tungsten (95%), but sometimesmolybdenumfor more specialized applications, such as when softer X-rays are needed as in mammography. In crystallography, a copper target is most common, withcobaltoften being used when fluorescence from iron content in the sample might otherwise present a problem.

The maximum energy of the produced X-rayphotonis limited by the energy of the incident electron, which is equal to the voltage on the tube times the electron charge, so an 80 kV tube cannot create X-rays with an energy greater than 80 keV. When the electrons hit the target, X-rays are created by two different atomic processes:

  1. Characteristic X-rayemission(X-ray electroluminescence): If the electron has enough energy, it can knock an orbital electron out of the innerelectron shellof the target atom. After that, electrons from higher energy levels fill the vacancies, and X-ray photons are emitted. This process produces anemission spectrumof X-rays at a few discrete frequencies, sometimes referred to as spectral lines. Usually, these are transitions from the upper shells to the K shell (called K lines), to the L shell (called L lines) and so on. If the transition is from 2p to 1s, it is called Kα, while if it is from 3p to 1s it is Kβ. The frequencies of these lines depend on the material of the target and are therefore called characteristic lines. The Kα line usually has greater intensity than the Kβ one and is more desirable in diffraction experiments. Thus the Kβ line is filtered out by a filter. The filter is usually made of a metal having one proton less than the anode material (e.g. Ni filter for Cu anode or Nb filter for Mo anode).
  2. Bremsstrahlung:This is radiation given off by the electrons as they are scattered by the strong electric field near the nuclei. These X-rays have acontinuous spectrum.The frequency ofBremsstrahlungis limited by the energy of incident electrons.

So, the resulting output of a tube consists of a continuousBremsstrahlungspectrum falling off to zero at the tube voltage, plus several spikes at the characteristic lines. The voltages used in diagnostic X-ray tubes range from roughly 20 kV to 150 kV and thus the highest energies of the X-ray photons range from roughly 20 keV to 150 keV.[86]

Both of these X-ray production processes are inefficient, with only about one percent of the electrical energy used by the tube converted into X-rays, and thus most of theelectric powerconsumed by the tube is released as waste heat. When producing a usable flux of X-rays, the X-ray tube must be designed to dissipate the excess heat.

A specialized source of X-rays which is becoming widely used in research issynchrotron radiation,which is generated byparticle accelerators.Its unique features are X-ray outputs many orders of magnitude greater than those of X-ray tubes, wide X-ray spectra, excellentcollimation,andlinear polarization.[87]

Short nanosecond bursts of X-rays peaking at 15 keV in energy may be reliably produced by peeling pressure-sensitive adhesive tape from its backing in a moderate vacuum. This is likely to be the result of recombination of electrical charges produced bytriboelectric charging.The intensity of X-raytriboluminescenceis sufficient for it to be used as a source for X-ray imaging.[88]

Production by fast positive ions[edit]

X-rays can also be produced by fast protons or other positive ions. The proton-induced X-ray emission orparticle-induced X-ray emissionis widely used as an analytical procedure. For high energies, the productioncross sectionis proportional toZ12Z2−4,whereZ1refers to theatomic numberof the ion,Z2refers to that of the target atom.[89]An overview of these cross sections is given in the same reference.

Production in lightning and laboratory discharges[edit]

X-rays are also produced in lightning accompanyingterrestrial gamma-ray flashes.The underlying mechanism is the acceleration of electrons in lightning related electric fields and the subsequent production of photons throughBremsstrahlung.[90]This produces photons with energies of some fewkeVand several tens of MeV.[91]In laboratory discharges with a gap size of approximately 1 meter length and a peak voltage of 1 MV, X-rays with a characteristic energy of 160 keV are observed.[92]A possible explanation is the encounter of twostreamersand the production of high-energyrun-away electrons;[93]however, microscopic simulations have shown that the duration of electric field enhancement between two streamers is too short to produce a significant number of run-away electrons.[94]Recently, it has been proposed that air perturbations in the vicinity of streamers can facilitate the production of run-away electrons and hence of X-rays from discharges.[95][96]

Detectors[edit]

Main article:X-ray detector

X-ray detectors vary in shape and function depending on their purpose. Imaging detectors such as those used forradiographywere originally based onphotographic platesand laterphotographic film,but are now mostly replaced by variousdigitaldetector types such asimage platesandflat panel detectors.Forradiation protectiondirect exposure hazard is often evaluated usingionization chambers,whiledosimetersare used to measure theradiation dosethe person has been exposed to. X-rayspectracan be measured either by energy dispersive or wavelength dispersivespectrometers.ForX-ray diffractionapplications, such asX-ray crystallography,hybrid photon counting detectorsare widely used.[97]

Medical uses[edit]

Patient undergoing an X-ray exam in a hospital radiology room
Achest radiographof a female patient, demonstrating ahiatal hernia

Since Röntgen's discovery that X-rays can identify bone structures, X-rays have been used formedical imaging.[98]The first medical use was less than a month after his paper on the subject.[34]Up to 2010, five billion medical imaging examinations had been conducted worldwide.[99]Radiation exposure from medical imaging in 2006 made up about 50% of total ionizing radiation exposure in the United States.[100]

Projectional radiographs[edit]

Main article:Projectional radiography
Plain radiograph of the right knee

Projectional radiographyis the practice of producing two-dimensional images using X-ray radiation. Bones contain a high concentration ofcalcium,which, due to its relatively highatomic number,absorbs X-rays efficiently. This reduces the amount of X-rays reaching the detector in the shadow of the bones, making them clearly visible on the radiograph. The lungs and trapped gas also show up clearly because of lower absorption compared to tissue, while differences between tissue types are harder to see.[101]

Projectional radiographs are useful in the detection ofpathologyof theskeletal systemas well as for detecting some disease processes insoft tissue.Some notable examples are the very commonchest X-ray,which can be used to identify lung diseases such aspneumonia,lung cancer, orpulmonary edema,and theabdominal x-ray,which can detectbowel (or intestinal) obstruction,free air (from visceral perforations), and free fluid (inascites). X-rays may also be used to detect pathology such asgallstones(which are rarelyradiopaque) orkidney stoneswhich are often (but not always) visible. Traditional plain X-rays are less useful in the imaging of soft tissues such as the brain ormuscle.One area where projectional radiographs are used extensively is in evaluating how an orthopedicimplant,such as a knee, hip or shoulder replacement, is situated in the body with respect to the surrounding bone. This can be assessed in two dimensions from plain radiographs, or it can be assessed in three dimensions if a technique called '2D to 3D registration' is used. This technique purportedly negates projection errors associated with evaluating implant position from plain radiographs.[102]

Dental radiographyis commonly used in the diagnoses of common oral problems, such ascavities.

In medical diagnostic applications, the low energy (soft) X-rays are unwanted, since they are totally absorbed by the body, increasing the radiation dose without contributing to the image. Hence, a thin metal sheet, often of aluminium, called anX-ray filter,is usually placed over the window of the X-ray tube, absorbing the low energy part in the spectrum. This is calledhardeningthe beam since it shifts the center of the spectrum towards higher energy (or harder) X-rays.

To generate an image of thecardiovascular 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 an iodinatedcontrast agenthas been injected into the blood vessels within this area. These two images are then digitally subtracted, leaving an image of only the iodinated contrast outlining the blood vessels. Theradiologistor surgeon then compares the image obtained to normal anatomical images to determine whether there is any damage or blockage of the vessel.

Computed tomography[edit]

Main article:CT scan
HeadCT scan(transverse plane) slice – a modern application ofmedical radiography

Computed tomography(CT scanning) is a medical imaging modality wheretomographic imagesor slices of specific areas of the body are obtained from a large series of two-dimensional X-ray images taken in different directions.[103]These cross-sectional images can be combined into athree-dimensionalimage of the inside of the body.[104]CT scans are a quicker and more cost effective imaging modality that can be used for diagnostic and therapeutic purposes in various medical disciplines.[104]

Fluoroscopy[edit]

Main article:Fluoroscopy

Fluoroscopyis an imaging technique commonly used by physicians orradiation therapiststo obtain real-time moving images of the internal structures of a patient through the use of a fluoroscope.[105]In its simplest form, a fluoroscope consists of an X-ray source and a fluorescent screen, between which a patient is placed. However, modern fluoroscopes couple the screen to anX-ray image intensifierandCCDvideo cameraallowing the images to be recorded and played on a monitor. This method may use a contrast material. Examples include cardiac catheterization (to examine forcoronary artery blockages) and barium swallow (to examine foresophageal disordersand swallowing disorders). As of recent, modern fluoroscopy utilizes short bursts of x-rays, rather than a continuous beam, to effectively lower radiation exposure for both the patient and operator.[105]

Radiotherapy[edit]

The use of X-rays as a treatment is known asradiation therapyand is largely used for the management (includingpalliation) of cancer; it requires higher radiation doses than those received for imaging alone. X-rays beams are used for treating skin cancers using lower energy X-ray beams while higher energy beams are used for treating cancers within the body such as brain, lung, prostate, and breast.[106][107]

Adverse effects[edit]

Abdominal radiographof a pregnant woman

X-rays are a form ofionizing radiation,and are classified as acarcinogenby both the World Health Organization'sInternational Agency for Research on Cancerand the U.S. government.[99][108]Diagnostic X-rays (primarily from CT scans due to the large dose used) increase the risk of developmental problems and cancer in those exposed.[109][110][111]It is estimated that 0.4% of current cancers in the United States are due tocomputed tomography(CT scans) performed in the past and that this may increase to as high as 1.5–2% with 2007 rates of CT usage.[112]

Experimental and epidemiological data currently do not support the proposition that there is athreshold dose of radiationbelow which there is no increased risk of cancer.[113]However, this is under increasing doubt.[114]Cancer risk can start at the exposure of 1100 mGy.[115]It is estimated that the additional radiation from diagnostic X-rays will increase the average person's cumulative risk of getting cancer by age 75 by 0.6–3.0%.[116]The amount of absorbed radiation depends upon the type of X-ray test and the body part involved.[112]CT and fluoroscopy entail higher doses of radiation than do plain X-rays.

To place the increased risk in perspective, a plain chest X-ray will expose a person to the same amount frombackground radiationthat people are exposed to (depending upon location) every day over 10 days, while exposure from a dental X-ray is approximately equivalent to 1 day of environmental background radiation.[117]Each such X-ray would add less than 1 per 1,000,000 to the lifetime cancer risk. An abdominal or chest CT would be the equivalent to 2–3 years of background radiation to the whole body, or 4–5 years to the abdomen or chest, increasing the lifetime cancer risk between 1 per 1,000 to 1 per 10,000.[117]This is compared to the roughly 40% chance of a US citizen developing cancer during their lifetime.[118]For instance, the effective dose to the torso from a CT scan of the chest is about 5 mSv, and the absorbed dose is about 14 mGy.[119]A head CT scan (1.5 mSv, 64 mGy)[120]that is performed once with and once without contrast agent, would be equivalent to 40 years of background radiation to the head. Accurate estimation of effective doses due to CT is difficult with the estimation uncertainty range of about ±19% to ±32% for adult head scans depending upon the method used.[121]

The risk of radiation is greater to a fetus, so in pregnant patients, the benefits of the investigation (X-ray) should be balanced with the potential hazards to the fetus.[122][123]If there is 1 scan in 9 months, it can be harmful to the fetus.[124]Therefore, women who are pregnant get ultrasounds as their diagnostic imaging because this does not use radiation.[124]If there is too much radiation exposure there could be harmful effects on the fetus or the reproductive organs of the mother.[124]In the US, there are an estimated 62 million CT scans performed annually, including more than 4 million on children.[112]Avoiding unnecessary X-rays (especially CT scans) reduces radiation dose and any associated cancer risk.[125]

Medical X-rays are a significant source of human-made radiation exposure. In 1987, they accounted for 58% of exposure from human-made sources in the United States. Since human-made sources accounted for only 18% of the total radiation exposure, most of which came from natural sources (82%), medical X-rays only accounted for 10% oftotalAmerican radiation exposure; medical procedures as a whole (includingnuclear medicine) accounted for 14% of total radiation exposure. By 2006, however, medical procedures in the United States were contributing much more ionizing radiation than was the case in the early 1980s. In 2006, medical exposure constituted nearly half of the total radiation exposure of the U.S. population from all sources. The increase is traceable to the growth in the use of medical imaging procedures, in particularcomputed tomography(CT), and to the growth in the use of nuclear medicine.[100][126]

An X-ray protective window atBirmingham Dental Hospital,England. The maker's sticker states that it is equivalent to 2.24mm of lead at 150Kv.

Dosage due to dental X-rays varies significantly depending on the procedure and the technology (film or digital). Depending on the procedure and the technology, a single dental X-ray of a human results in an exposure of 5 to 40 μSv. A full mouth series of X-rays may result in an exposure of up to 60 (digital) to 180 (film) μSv, for a yearly average of up to 400 μSv.[127][128][129][130][131][132][133]

Financial incentives have been shown to have a significant impact on X-ray use with doctors who are paid a separate fee for each X-ray providing more X-rays.[134]

Early photon tomographyor EPT[135](as of 2015) along with other techniques[136]are being researched as potential alternatives to X-rays for imaging applications.

Other uses[edit]

Other notable uses of X-rays include:

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.
Using X-ray for inspection and quality control: the differences in the structures of the die and bond wires reveal the left chip to be counterfeit.[139]
  • Authentication and quality control of packaged items.
  • Industrial CT(computed tomography), a process that uses X-ray equipment to produce three-dimensional representations of components both externally and internally. This is accomplished through computer processing of projection images of the scanned object in many directions.
  • Airport securityluggage scanners use X-rays for inspecting the interior of luggage for security threats before loading on aircraft.
  • Border controltruck scanners anddomestic police departmentsuse X-rays for inspecting the interior of trucks.
X-ray fine art photography ofneedlefishbyPeter Dazeley

Visibility[edit]

While generally considered invisible to the human eye, in special circumstances X-rays can be visible. Brandes, in an experiment a short time after 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.[141]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 withionizing radiation.It is not known what exact mechanism in the eye produces the visibility: it could be due to conventional detection (excitation ofrhodopsinmolecules in the retina), direct excitation of retinal nerve cells, or secondary detection via, for instance, X-ray induction ofphosphorescencein the eyeball with conventional retinal detection of the secondarily produced visible light.

Though X-rays are otherwise invisible, it is possible to see theionizationof the air molecules if the intensity of the X-ray beam is high enough. The beamline from thewigglerat theEuropean Synchrotron Radiation Facility[142]is one example of such high intensity.[143]

Units of measure and exposure[edit]

The measure of X-raysionizingability is called the exposure:

  • Thecoulombper kilogram (C/kg) is theSIunit ofionizing radiationexposure, and it is the amount of radiation required to create one coulomb of charge of each polarity in one kilogram of matter.
  • Theroentgen(R) is an obsolete traditional unit of exposure, which represented the amount of radiation required to create oneelectrostatic unitof charge of each polarity in one cubic centimeter of dry air. 1 roentgen =2.58×10−4C/kg.

However, the effect of ionizing radiation on matter (especially living tissue) is more closely related to the amount of energy deposited into them rather than thechargegenerated. This measure of energy absorbed is called theabsorbed dose:

  • Thegray(Gy), which has units of (joules/kilogram), is the SI unit ofabsorbed dose,and it is the amount of radiation required to deposit onejouleof energy in one kilogram of any kind of matter.
  • Theradis the (obsolete) corresponding traditional unit, equal to 10 millijoules of energy deposited per kilogram. 100 rad = 1 gray.

Theequivalent doseis the measure of the biological effect of radiation on human tissue. For X-rays it is equal to theabsorbed dose.

  • TheRoentgen equivalent man(rem) is the traditional unit of equivalent dose. For X-rays it is equal to therad,or, in other words, 10 millijoules of energy deposited per kilogram. 100 rem = 1 Sv.
  • Thesievert(Sv) is the SI unit ofequivalent dose,and also ofeffective dose.For X-rays the "equivalent dose" is numerically equal to aGray(Gy). 1 Sv = 1 Gy. For the "effective dose" of X-rays, it is usually not equal to the Gray (Gy).
Ionizing radiation related quantities
Quantity Unit Symbol Derivation Year SIequivalent
Activity(A) becquerel Bq s−1 1974 SI unit
curie Ci 3.7×1010s−1 1953 3.7×1010Bq
rutherford Rd 106s−1 1946 1000000Bq
Exposure(X) coulombperkilogram C/kg C⋅kg−1of air 1974 SI unit
röntgen R esu/0.001293gof air 1928 2.58×10−4C/kg
Absorbed dose(D) gray Gy J⋅kg−1 1974 SI unit
ergper gram erg/g erg⋅g−1 1950 1.0×10−4Gy
rad rad 100 erg⋅g−1 1953 0.010 Gy
Equivalent dose(H) sievert Sv J⋅kg−1×WR 1977 SI unit
röntgen equivalent man rem 100 erg⋅g−1×WR 1971 0.010 Sv
Effective dose(E) sievert Sv J⋅kg−1×WR×WT 1977 SI unit
röntgen equivalent man rem 100 erg⋅g−1×WR×WT 1971 0.010 Sv

See also[edit]

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External links[edit]

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