X-ray

Unit of measure and exposure

The rem is the traditional unit of dose equivalent. This describes the Energy delivered by γ 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), or one thousandth of a rem.

The average person living in the United States is exposed to approximately 150 mrem annually from 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^[3], 40^[4], 300^[5], or as many as 900^[6] mrems.

Physics

X-rays are a type of atomic nuclei.

X-ray K-series spectral line wavelengths (nm) for some common target materials.^[7]

Target	Kβ₁	Kβ₂	Kα₁	Kα₂
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
Me	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 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 proton number) nuclei.

X-rays can detect cancer, cysts, and tumors. 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 crystallagraphy, where their wave-like nature is most important.

Nowadays, for many (non-medical) applications, X-ray production is achieved by synchrotrons (see synchrotron light).

To create a blood or artery X-ray, also called digital angiography, iodine is injected into the veins and a digitized image is created. Then, a second image is established of only the parts of the X-rayed section without iodine. The first image is subtracted then a final image is produced containing both the first and second images together. Lastly, the results are printed. The doctor or surgeon then compares the results of the angiography to a perfect angiography structure to see if there are any malfunctions.

To take an X-ray of the bones, no iodization is required. Short X-ray pulses are shot through a body at first. Next, the bones absorb the most waves because they are more dense and contain calcium which absorbs more rays than the carbon, oxygen, and nitrogen atoms of the soft tissue because there are more electrons in a calcium atom.

Detectors

Photographic plate

The detection of X-rays is based on various methods. The most commonly known method are a rare earth screens.

A photographic plate or film is used in hospitals to produce images of the internal organs and bones of a patient. They are also used in industrial radiography processes. Since photographic plates are sensitive to X-rays, they provide a convenient and easy means of recording the image. X-ray film is usually provided as pre-loaded paper cartridges with the film inside a light proof paper envelope. An additional paper coated in a thin layer of lead is often included in contact with the photographic film. The lead reflects the x-rays back through the photo film thus more or less doubling the sensitivity of the assembly. Thus the photographic film has to be used the right way round, and is marked as such. The emulsion is frequently coated on both sides of the film or plate in order to increase the sensitivity further.

The part of the patient to be X-rayed is placed between the X-ray source and the photographic receptor to produce what is a shadow of all the internal structure of that particular part of the body being X-rayed. The X-rays are blocked by dense tissues such as bone and pass through soft tissues. Those areas where the X-rays strike the photographic receptor turn black when it is 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 thorium was used as a contrast medium (Thorotrast) — this caused many people to be injured or even die from the effects of the radiation from the thorium.

Photographic plates are losing favour in many X-ray facilities because of the necessity to have processing facilities readily to hand, and because the photographic plates themselves, plus the processing chemicals are relatively expensive consumables.

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 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 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 counters. 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 scintillation counters". 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

  X-rays are also used in "real-time" procedures such as angiography or contrast studies of the hollow organs (e.g. 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 detectors have been developed (zinc, cadmium zinc telluride detectors have an increased sensitivity, which allows lower doses of X-rays to be used.

Practical application in 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, which could be connected to an oscilloscope as a quick diagnostic.

Silicon drift detectors (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 photodiode arrays.

The array technology is a variant on the amorphous silicon TFT arrays used in many flat panel displays, 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 transistors (TFTs) are attached to a light-absorbing photodiode making up an individual pixel (picture element). Photons striking the photodiode are converted into two scintillators made from eg. gadolinium oxysulfide or cesium 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. Brandes, in an experiment a short time after phosphorescence in the eyeball with conventional retinal detection of the secondarily produced visible light.

If the intensity of an X-ray beam is high enough, the ionization of the air will make the beam visible with a white glow.

Medical uses

  Since Röntgen's discovery that X-rays can identify bony structures, X-rays have been developed for their use in 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 carcinogen by the U.S. government.

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.

Other uses

  Other notable uses of X-rays include

• DNA).
• X-ray astronomy, which is an observational branch of astronomy, which deals with the study of X-ray emission from celestial objects.
• electromagnetic radiation in the soft X-ray band to produce images of very small objects.
• 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.

History

Among the important early researchers in X-rays were Professor Ivan Pulyui, Sir Wilhelm Conrad Röntgen.

William Morgan

Welsh Actuary, Physicist and fellow of the Royal Society, William Morgan^[8] (1750 - 1833) was the first to record an experiment which produced X-rays.^[9] In 1785 he presented the paper "Electrical Experiments Made in Order to Ascertain the Non-Conducting Power of a Perfect Vacuum"^[10]. The experiment involved creating a potential difference in a vacuum and slowly reducing the completeness of the vacuum by introducing mercury vapour into it. From the paper: "according to the length of time during which the mercury was boiled, the 'electric' light turned violet, then purple, then a beautiful green...and then the light became invisible". This progression was the result of the wavelength of the radiation caused by the electric current decreasing beyond the visible range and into X-ray wavelengths.

Johann Hittorf

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 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

As a result of experiments into what he called cold light Ivan Pulyui is reputed to have developed an X-ray emitting device as early as 1881. He reputedly first demonstrated an X-ray photograph of a 13-year-old boy's broken arm and an X-ray photograph of his daughter's hand with a pin lying under it. The device became known as the Pulyui lamp and was mass-produced for a period. Reputedly, Pulyui personally presented one to Wilhelm Conrad Röntgen who went on to be credited as the major developer of the technology. Pulyui published his results in a scientific paper, Luminous Electrical Matter and the Fourth State of Matter in the Notes of the Austrian Imperial Academy of Sciences (1880-1883), but expressed his ideas in an obscure manner using obsolete terminology. Pulyui did gain some recognition when the work was translated and published as a book by the Royal Society in the UK.

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 ^{[11] [12]}, which differed from other X-ray tubes in having no target electrode. The principle behind Tesla's device is nowadays called the radiant energy of "invisible" kinds.^{[13] [14]} Tesla stated the facts of his methods concerning various experiments in his 1897 X-ray lecture ^[15] 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.^[16]

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. ^[17]

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 cathode 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 8 1895, Wilhelm Conrad Röntgen, a German physics professor, began observing and further documenting X-rays while experimenting with vacuum 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. 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. Röntgen received the first Nobel Prize in Physics for his discovery.

Röntgen was working on a primitive cathode ray generator that was projected through a glass partially evacuated tube. Suddenly he noticed a faint green light against the wall. The odd thing he had noticed, was that the light from the cathode ray generator was traveling through a bunch of the materials in its way (paper, wood, and books). He then started to put various objects in front of the generator, and as he was doing this, he noticed that the outline of the bones from his hand were displayed on the wall. Röntgen said he did not know what to think and kept experimenting. Two months after his initial discovery, he published his paper translated "On a New Kind of Radiation" and gave a demonstration in 1896.

Rontgen 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

 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.

The 20th century and beyond

Prior to 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, Flemming 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.

Two years later, physicist Charles Barkla discovered that X-rays could be scattered by gases, and that each element had a characteristic X-ray. He won the 1917 Nobel Prize in Physics for this discovery. Max von Laue, Paul Knipping and Walter Friedrich observed for the first time the Coolidge tube was invented the following year by William D. Coolidge which permitted continuous production of X-rays; this type of tube is still in use today.

 

The use of X-rays for medical purposes (to develop into the field of radiation therapy) was pioneered by Major John Hall-Edwards in Birmingham, England. In 1908, he had to have his left arm amputated owing to the spread of X-ray dermatitis[2].

The 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).

See also

References

1. ^ Kevles, Bettyann Holtzmann (1996). Naked to the Bone Medical Imaging in the Twentieth Century. Camden, NJ: Rutgers University Press, pp19-22. ISBN 0813523583. 
2. ^ Sample, Sharron (2007-03-27). X-rays. The electromagnetic spectrum. NASA. Retrieved on 2007-12-03.
3. ^ http://www.doctorspiller.com/Dental%20_X-Rays.htm and http://www.dentalgentlecare.com/x-ray_safety.htm
4. ^ http://hss.energy.gov/NuclearSafety/NSEA/fire/trainingdocs/radem3.pdf
5. ^ http://www.hawkhill.com/114s.html
6. ^ http://www.solarstorms.org/SWChapter8.html and http://www.powerattunements.com/x-ray.html
7. ^ in David R. Lide: CRC Handbook of Chemistry and Physics 75th edition. CRC Press, 10-227. ISBN 0-8493-0475-X. 
8. ^ William Morgan: Bridgend Hall of Fame (HTML). Bridgend County Borough Council.
9. ^ Anderson, J.G., " ", Transactions of the Faculty of Actuaries 17: pp219-221,
10. ^ Electrical Experiments Made in Order to Ascertain the Non-Conducting Power of a Perfect Vacuum (PDF). Philosophical Transactions of the Royal Society of London, Vol. 75, 1785. JSTOR.
11. ^ Morton, William James, and Edwin W. Hammer, American Technical Book Co., 1896. Page 68.
12. ^ U.S. Patent 514,170 , Incandescent Electric Light, and U.S. Patent 454,622 , System of Electric Lighting.
13. ^ Cheney, Margaret, "Tesla: Man Out of Time ". Simon and Schuster, 2001. Page 77.
14. ^ 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.)
15. ^ Nikola Tesla, "The stream of Lenard and Roentgen and novel apparatus for their production", Apr. 6, 1897.
16. ^ Cheney, Margaret, Robert Uth, and Jim Glenn, "Tesla, master of lightning". Barnes & Noble Publishing, 1999. Page 76. ISBN 0760710058
17. ^ Wyman, Thomas (Spring 2005). "Fernando Sanford and the Discovery of X-rays". "Imprint", from the Associates of the Stanford University Libraries: pp. 5-15.

• NASA Goddard Space Flight centre introduction to X-rays.
• Way Out There in the Blue: Reagan, Star Wars and the End of the Cold War, Frances Fitzgerald, Simon & Schuster (2001). ISBN 0-7432-0023-3

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