X-ray
X-radiation (composed of X-rays) is a
form of electromagnetic radiation. Most X-rays have a wavelength in the range
of 0.01 to 10 nanometers, corresponding to frequencies in the range 30
petahertz to 30 exahertz (3×1016 Hz to 3×1019 Hz) and energies in the range 100
eV to 100 keV. However, much higher-energy X-rays can be generated for medical
and industrial uses, for example radiotherapy, which utilizes linear
accelerators to generate X-rays in the ranges of 6–20 MeV. X-ray wavelengths
are shorter than those of UV rays and typically longer than those of gamma
rays. In many languages, X-radiation is referred to with terms meaning Röntgen
radiation, after Wilhelm Röntgen, who is usually credited as its discoverer, and
who had named it X-radiation to signify an unknown type of radiation. Spelling
of X-ray(s) in the English language includes the variants x-ray(s) and X
ray(s).
X-rays with
photon energies above 5–10 keV (below 0.2–0.1 nm wavelength) are called hard
X-rays, while those with lower energy are called soft X-rays. Due to their
penetrating ability hard X-rays are widely used to image the inside of objects,
e.g. in medical radiography and airport security. As a result, the term X-ray
is metonymically used to refer to a radiographic image 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 by X-ray crystallography. By contrast, soft X-rays are easily
absorbed in air and the attenuation length of 600 eV (~2 nm) X-rays in water is
less than 1 micrometer.
There is no
universal consensus for a definition distinguishing between X-rays and gamma
rays. One common practice is to distinguish between the two types of radiation
based on their source: X-rays are emitted by electrons, while gamma rays are
emitted by the atomic nucleus.This definition has several problems; other
processes also can generate these high energy photons, 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
10−11 m (0.1 Å), defined as gamma radiation. 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 by X-ray tubes generally has a longer wavelength and lower
photon energy than the radiation emitted by radioactive nuclei. 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.
Properties
X-ray
photons carry enough energy to ionize atoms and disrupt molecular bonds. This
makes it a type of ionizing radiation and thereby harmful to living tissue. A
very high radiation dose over a short amount of time causes radiation sickness,
while lower doses can give an increased risk of radiation-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
utilized in cancer treatment to kill malignant cells using radiation therapy.
It is also used for material characterization using X-ray spectroscopy.
Hard X-rays
can traverse relatively thick objects without being much absorbed or scattered.
For this reason X-rays are widely used to image the inside of visually opaque
objects. The most often seen applications are in medical radiography and
airport security scanners, but similar techniques are also important in
industry (e.g. industrial radiography and industrial CT scanning) and research
(e.g. small animal CT). The penetration depth varies with several orders of
magnitude over the X-ray spectrum. This allows the photon energy to be adjusted
for the application so as to give sufficient transmission through the object
and at the same time good contrast in the image.
X-rays have
much shorter wavelength than visible light, which makes it possible to probe
structures much smaller than what can be seen using a normal microscope. This
can be used in X-ray microscopy to acquire high resolution images, but also in
X-ray crystallography to determine the positions of atoms in crystals.
Interaction
with matter
X-rays
interact with matter in three main ways, through photoabsorption, Compton
scattering, and Rayleigh scattering. The strength of these interactions depend
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 the compton effect dominates.
Photoelectric
absorption
The
probability of a photoelectric absorption per unit mass is approximately
proportional to Z3/E3, where Z is the atomic number and E is the energy of the
incident photon. This rule is not valid close to inner shell electron binding
energies where there are abrupt changes in interaction probability, so called
absorption edges. However, the general trend of high absorption coefficients
and thus short penetration depths for 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 of calcium (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 the produce either a
characteristic photon or an Auger electron. These effects can be used for
elemental detection through X-ray spectroscopy or Auger electron spectroscopy.
Compton
scattering
Compton
scattering is the predominant interaction between X-rays and soft tissue in medical
imaging. Compton scattering is an inelastic scattering of 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 a bit more likely, especially for high-energy
X-rays. The probability for different scattering angles are described by the
Klein–Nishina formula. The transferred energy can be directly obtained from the
scattering angle from the conservation of energy and momentum.
Rayleigh
scattering
Rayleigh
scattering is the dominant elastic scattering mechanism in the X-ray regime.
The inelastic forward scattering is what gives rise to the refractive index,
which for X-rays is only slightly below 1.
Sources
Since X-rays
are emitted by electrons, they can be generated by an X-ray tube, a vacuum tube
that uses a high voltage to accelerate the electrons released by a hot cathode
to a high velocity. The high velocity electrons collide with a metal target,
the anode, creating the X-rays. In medical X-ray tubes the target 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 softer
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.
The maximum
energy of the produced X-ray photon is 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:
X-ray
fluorescence: If the electron has enough energy it can knock an orbital
electron out of the inner electron shell of a metal atom, and as a result
electrons from higher energy levels then fill up the vacancy and X-ray photons
are emitted. This process produces an emission spectrum of X-rays at a few discrete
frequencies, sometimes referred to as the spectral lines. The spectral lines
generated depend on the target (anode) element used and thus are called
characteristic lines. Usually these are transitions from upper shells into K
shell (called K lines), into L shell (called L lines) and so on.
Bremsstrahlung: This is radiation given off by the
electrons as they are scattered by the strong electric field near the high-Z
(proton number) nuclei. These X-rays have a continuous spectrum. The intensity
of the X-rays increases linearly with decreasing frequency, from zero at the
energy of the incident electrons, the voltage on the X-ray tube.
So the
resulting output of a tube consists of a continuous bremsstrahlung spectrum
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 to 150 kV and thus the highest energies of the X-ray photons range
from roughly 20 to 150 keV.
Both of
these X-ray production processes are inefficient, with a production efficiency
of only about one percent, and hence, to produce a usable flux of X-rays, most
of the electric power consumed by the tube is released as waste heat. The X-ray
tube must be designed to dissipate this excess heat.
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 by triboelectric charging. The intensity of X-ray
triboluminescence is sufficient for it to be used as a source for X-ray
imaging. Using sources considerably more advanced than sticky tape, at least
one startup firm is exploiting tribocharging in the development of highly
portable, ultra-miniaturized X-ray devices.
A
specialized source of X-rays which is becoming widely used in research is
synchrotron radiation, which is generated by particle accelerators. Its unique
features are X-ray outputs many orders of magnitude greater than those of X-ray
tubes, wide X-ray spectra, excellent collimation, and linear polarization.
Detectors
X-ray
detectors vary in shape and function depending on their purpose. Imaging
detectors such as those used for radiography were originally based on
photographic plates and later photographic film but are now mostly replaced by
various digital detector types such as image plates or flat panel detectors.
For radiation protection direct exposure hazard is often evaluated using
ionization chambers, while dosimeters are used to measure the radiation dose a
person has been exposed to. X-ray spectra can be measured either by energy
dispersive or wavelength dispersive spectrometers.
Computed
tomography (CT scanning) is a medical imaging modality where tomographic images
or slices of specific areas of the body are obtained from a large series of
two-dimensional X-ray images taken in different directions. These
cross-sectional images can be combined into a three-dimensional image of the
inside of the body and used for diagnostic and therapeutic purposes in various
medical disciplines.
Fluoroscopy
Fluoroscopy
is an imaging technique commonly used by physicians or radiation therapists to
obtain real-time moving images of the internal structures of a patient through
the use of a fluoroscope. In its simplest form, a fluoroscope consists of an
X-ray source and fluorescent screen between which a patient is placed. However,
modern fluoroscopes couple the screen to an X-ray image intensifier and CCD
video camera allowing the images to be recorded and played on a monitor. This
method may use a contrast material. Examples include cardiac catheterization
(to examine for coronary artery blockages) and barium swallow (to examine for
esophageal disorders).
Radiotherapy
The use of
X-rays as a treatment is known as radiation therapy and is largely used for the
management (including palliation) of cancer; it requires higher radiation
energies than for imaging alone.
Adverse effects
Diagnostic
X-rays (primarily from CT scans due to the large dose used) increase the risk
of developmental problems and cancer in those exposed. X rays are classified as
a carcinogen by both the World Health Organization's International Agency for Research
on Cancer and the U.S. government. It is estimated that 0.4% of current cancers
in the United States are due to computed 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.
Experimental
and epidemiological data currently do not support the proposition that there is
a threshold dose of radiation below which there is no increased risk of
cancer.[31] However, this is under increasing doubt. It is estimated that the
additional radiation will increase a person's cumulative risk of getting cancer
by age 75 by 0.6–1.8%. The amount of absorbed radiation depends upon the type
of X-ray test and the body part involved. 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 from background radiation that we 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. 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. This is compared
to the roughly 40% chance of a US citizen developing cancer during their
lifetime. 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. A head CT scan
(1.5mSv, 64mGy) 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.
The risk of
radiation is greater to unborn babies, so in pregnant patients, the benefits of
the investigation (X-ray) should be balanced with the potential hazards to the
unborn fetus. In the US, there are an estimated 62 million CT scans performed
annually, including more than 4 million on children. Avoiding unnecessary
X-rays (especially CT scans) will reduce radiation dose and any associated
cancer risk.
Medical
X-rays are a significant source of man-made radiation exposure. In 1987, they
accounted for 58% of exposure from man-made sources in the United States. Since
man-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%
of total American radiation exposure; medical procedures as a whole (including
nuclear 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 particular computed tomography (CT), and to
the growth in the use of nuclear medicine.
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 0.5 to 4 mrem. A full
mouth series may therefore result in an exposure of up to 6 (digital) to 18
(film) mrem, for a yearly average of up to 40 mrem.
Other
uses
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
analysed to reveal the nature of that lattice. A related technique, fiber
diffraction, was used by Rosalind Franklin to discover the double helical
structure of DNA.
X-ray
astronomy, which is an observational branch of astronomy, which deals with the
study of X-ray emission from celestial objects.
X-ray
microscopic analysis, which uses 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.
Industrial
radiography uses X-rays for inspection of industrial parts, particularly welds.
Industrial
CT (computed tomography) is a process which 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.
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.
X-ray
spectromicroscopy has been used to analyse the reactions of pigments in
paintings. For example, in analysing colour degradation in the paintings of van
Gogh
Airport
security luggage scanners use X-rays for inspecting the interior of luggage for
security threats before loading on aircraft.
Border
control truck scanners use X-rays for inspecting the interior of trucks.
X-ray art
and fine art photography, artistic use of X-rays, for example the works by
Stane Jagodič
X-ray hair
removal, a method popular in the 1920s but now banned by the FDA.
Shoe-fitting
fluoroscopes were popularized in the 1920s, banned in the US in the 1960s,
banned in the UK in the 1970s, and even later in continental Europe.
Roentgen
Stereophotogrammetry is used to track movement of bones based on the
implantation of markers
X-ray
photoelectron spectroscopy is a chemical analysis technique relying on the
photoelectric effect, usually employed in surface science.
History
Discovery
German
physicist Wilhelm Röntgen is usually credited as the discoverer of X-rays in
1895, because he was the first to systematically study them, though he is not the
first to have observed their effects. He is also the one who gave them the name
"X-rays", though many referred to these as "Röntgen rays"
(and the associated X-ray radiograms as, "Röntgenograms") for several
decades after their discovery and even to this day in some languages, including
Röntgen's native German.
X-rays were
found emanating from Crookes tubes, experimental discharge tubes invented
around 1875, by scientists investigating the cathode rays, that is energetic
electron beams, that were first created in the tubes. Crookes tubes created
free electrons by ionization of the residual air in the tube by a high DC
voltage of anywhere between a few kilovolts and 100 kV. This voltage
accelerated the electrons coming from the cathode to a high enough velocity
that they created X-rays when they struck the anode or the glass wall of the
tube. Many of the early Crookes tubes undoubtedly radiated X-rays, because
early researchers noticed effects that were attributable to them, as detailed
below. Wilhelm Röntgen was the first to systematically study them, in 1895.
Early
research
The
important early researchers in X-rays were Ivan Pulyui, William Crookes, Johann
Wilhelm Hittorf, Eugen Goldstein, Heinrich Hertz, Philipp Lenard, Hermann von
Helmholtz, Thomas Edison, Charles Glover Barkla, Nikola Tesla, Max von Laue,
and Wilhelm Conrad Röntgen.
German
physicist Johann Hittorf (1824–1914), a co-inventor and early researcher of the
Crookes tube, 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.[citation needed]
In 1877
Ukrainian-born Pulyui, a lecturer in experimental physics at the University of
Vienna, constructed various designs of vacuum discharge tube to investigate
their properties. He continued his investigations when appointed professor at
the Prague Polytechnic and in 1886 he found 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. 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.
Wilhelm
Röntgen
On November
8, 1895, German physics professor Wilhelm Röntgen stumbled on X-rays while
experimenting with Lenard and Crookes tubes and began studying them. He wrote
an initial report "On a new kind of ray: A preliminary communication"
and on December 28, 1895 submitted it to the Würzburg's Physical-Medical
Society journal. 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. 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, Danish, Polish, Swedish, Finnish,
Estonian, Russian, Japanese, Dutch, and Norwegian. 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:
Röntgen was investigating cathode rays using 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 would not interfere. He noticed a
faint green glow from the screen, about 1 meter 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.
Röntgen
discovered its 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 ever photograph of a human body part using X-rays. When she saw the
picture, she said "I have seen my death."
Advances
in radiology
n 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,
even before 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.
In 1901,
U.S. President William McKinley was shot twice in an assassination attempt.
While one bullet only grazed his sternum, another had lodged somewhere deep
inside his abdomen and could not be found. "A worried McKinley aide sent
word to inventor Thomas Edison to rush an X-ray machine to Buffalo to find the
stray bullet. It arrived but wasn't used." While the shooting itself had
not been lethal, "gangrene had developed along the path of the bullet, and
McKinley died of septic shock due to bacterial infection" six days
later.[70]
The first
use of X-rays under clinical conditions was by John Hall-Edwards in Birmingham,
England on 11 January 1896, when he radiographed a needle stuck in the hand of
an associate.[71] On 14 February 1896 Hall-Edwards was also the first to use
X-rays in a surgical operation.[72]
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 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 on gelatin photographic plates obtained from
Howard Langill, a local photographer also interested in Röntgen's work
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