Encyclopedia
X-rays are a form of
electromagnetic radiation with a
wavelength in the range of 10 to 0.01
nanometres, corresponding to
frequencies in the range 30 to 30 000 PHz . X-rays are primarily used for diagnostic radiography and
crystallography. X-rays are a form of
ionizing radiation and as such can be dangerous. In many languages it is called
Röntgen radiation after the investigator of the radiation, Wilhelm Röntgen.
Physics
X-rays are a type of electromagnetic radiation with wavelengths of around 10
-10 meter. When medical X-rays are being produced, a thin metallic sheet is placed between the emitter and the target, effectively filtering out the lower energy X-rays. The resultant X-ray is said to be
hard. Soft X-rays overlap the range of
extreme ultraviolet. The frequency of hard X-rays is higher than that of soft X-rays, and the wavelength is shorter. Hard X-rays overlap the range of "long"-wavelength
gamma rays, however the distinction between the two terms depends on the source of the radiation, not its wavelength: X-ray
photons are generated by energetic
electron processes, gamma rays by transitions within
atomic nuclei.
X-ray K-series spectral line wavelengths for some common target materials
Target | Kß1 | Kß2 | Ka1 | Ka2 |
| Fe | 0.17566 | 0.17442 | 0.193604 | 0.193998 |
|---|
| Ni | 0.15001 | 0.14886 | 0.165791 | 0.166175 |
|---|
| Cu | 0.139222 | 0.138109 | 0.154056 | 0.154439 |
|---|
| Zr | 0.070173 | 0.068993 | 0.078593 | 0.079015 |
|---|
| Mo | 0.063229 | 0.062099 | 0.070930 | 0.071359 |
|---|
The basic production of X-rays is by accelerating electrons in order to collide with a
metal target . Here the electrons suddenly decelerate upon colliding with the metal target and if enough energy is contained within the electron it is able to knock out an electron from the inner 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 causes the spectral line part of the wavelength distribution. There is also a continuum
bremsstrahlung component given off by the electrons as they are scattered by the strong electric field near the high Z nuclei.
Nowadays, for many applications, X-ray production is achieved by
synchrotrons .
Detectors
Photographic Plate
The detection of X-rays is based on various methods. The most commonly known method are a photographic plate, X-ray
film in a cassette, and
rare earth screens.
The X-ray photographic plate or film is used in
hospitals to produce images of the internal organs and bones of a patient. 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 barium or iodine, which are radiopaque, can be injected in the artery of a particular organ, or given intravenously. The contrast compounds essentially block the X-rays and hence the circulation of the organ can be more readily seen.
For some procedures, the contrast can have a syrupy consistency, which can be thinned by warming, and is introduced with a power injector, such as the Nemoto Injector.
Photostimulable Phosphors
An increasingly common method of detecting X-rays is the use of Photostimulable Luminescence . Pioneered by Fuji in the 1980's. 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. The PSP plate can be used over and over again.
Geiger counter
Initially, most common detection methods were based on the ionisation of gases, as in the
Geiger-Müller counter: a sealed volume, usually a cylinder, with a polymer or thin metal window contains a gas, and a wire, and a high voltage is applied between the cylinder and the wire . When an X-ray photon enters the cylinder, it ionises the gas. These ions accelerate toward the anode, in the process causing further ionisation 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
diffracting crystal may be used to first separate the different photons, the method is called wavelength dispersive X-ray spectroscopy . 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 equipment allows digital spectra to be created for later analysis.
For many applications, counters are not sealed but are constantly fed with purified gas . These are called "flow counter".
Scintillators
Some materials such as sodium iodide can "convert" an X-ray photon to a visible photon; an electronic detector can be built by adding a
photomultiplier. These detectors are called "scintillators", filmscreens or "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 using
fluoroscopy acquired using an
image intensifier.
Angioplasty, medical interventions of the arterial system, rely heavily on X-ray-sensitive contrast to identify potentially treatable lesions.
Direct Semiconductor Detectors
Since the 1970s, new semiconductor detectors have been developed . X-ray photons are converted to electron-hole pairs in the semiconductor and are collected to detect the X-rays. When the temperature is low enough , it is possible to directly determine the X-ray energy spectrum; this method is called energy dispersive X-ray spectroscopy ; it is often used in small
X-ray fluorescence spectrometers. These detectors are sometimes called "solid detectors".
Cadmium telluride and its alloy with
zinc, cadmium zinc telluride detectors have an increased sensitivity, which allows lower doses of X-rays to be used.
Practical application in Medical Imaging didn't start taking place until the 1990's. Currently amorphous
selenium is used in commercial large area flat panel x-ray detectors for chest radiography and
mammography.
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.
, produced by conventional
semiconductor fabrication, now provide a cost-effective and high resolving radiation measurement. They replace conventional X-ray detectors, such as Sis, as they do not need to be cooled with liquid
nitrogen.
Scintillator + semiconductor detectors
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.
Visibility to the human eye
While generally considered invisible to the human eye, in special circumstances, X-rays can be visible. An effect first discovered by Brandes in experimentation a short time after Röntgen's landmark 1895 paper; he 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. Upon hearing this, Röntgen reviewed his record books and found he in fact, also saw the effect. When placing an X-ray tube on the opposite side of a wooden door Röntgen saw the same blue glow seeming to emanate from the eye itself, but thought his observations were spurious due to the fact that he only saw the effect when he used one type of tube. Later he realized that the tube which
created the effect was the only one which produced X-rays powerful enough to make the glow plainly visible and the
experiment was thereafter repeated readily. The fact that X-rays are actually faintly visible to the dark-adapted naked eye has largely been forgotten today is probably due to the lack of desire to repeat what we would now see as a recklessly dangerous and harmful experiment with
ionizing radiation. It is not known what the exact mechanism in the eye is which produces the visibility and it could be due to either conventional detection , direct excitation of retinal nerve cells, or secondary detection via, for instance, X-ray induction of
phosphorescence in the eyeball and then conventional retinal detection of the secondarily produced visible light.
Medical uses
Since Röntgen's discovery that X-rays can identify bony structures, X-rays have been developed for their use in medical imaging.
Radiology is a specialized field of
medicine. Radiographers employ radiography and other techniques for diagnostic imaging. Indeed, this is probably the most common use of X-ray technology.
X-rays are especially useful in the detection of pathology of the
skeletal system, but are also useful for detecting some disease processes in soft tissue. Some notable examples are the very common chest X-ray, which can be used to identify lung diseases such as
pneumonia,
lung cancer or pulmonary edema, and the abdominal X-ray, which can detect ileus , free air and free fluid . In some cases, the use of X-rays is debatable, such as gallstones or
kidney stones . Also, traditional plain X-rays pose very little use in the imaging of soft tissues such as the
brain or
muscle. Imaging alternatives for soft tissues are
computed axial tomography ,
magnetic resonance imaging or
ultrasound. Since 2005, X-rays are listed as a 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.
History
Among the important early researchers in X-rays were Professor
Ivan Pului, Sir
William Crookes, Johann Wilhelm Hittorf, Eugen Goldstein,
Heinrich Hertz,
Philipp Lenard,
Hermann von Helmholtz,
Nikola Tesla,
Thomas Edison, Charles Glover Barkla,
Max von Laue, and
Wilhelm Conrad Röntgen.
Physicist Johann Hittorf 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. Later, English physicist
William Crookes investigated the effects of energy discharges on rare gases, and constructed what is called the Crookes tube. It is a glass vacuum cylinder, 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.
Tesla
In April 1887,
Nikola Tesla began to investigate X-rays using high voltages and vacuum 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, which differed from other X-ray tubes in having no target electrode. He stated these facts in his 1897 X-ray lecture before the New York Academy of Sciences.
The principle behind Tesla's device is nowadays called the
Bremsstrahlung process, in which a high-energy secondary X-ray emission is produced when charged particles pass through matter. By 1892, Tesla performed several such experiments, but he did not categorize the emissions as what were later called X-rays, instead generalizing the phenomenon as radiant energy. He did not publicly declare his findings nor did he make them widely known. His subsequent X-ray experimentation by vacuum high field emissions led him to alert the scientific community to the biological hazards associated with X-ray exposure.
Hertz
In 1892,
Heinrich Hertz began experimenting and demonstrated that cathode rays could penetrate very thin metal foil .
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 . However, he did not work with actual X-rays.
Röntgen
On November 8 1895,
Wilhelm Conrad Röntgen, a
German scientist, 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 , 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 vacuum 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 . 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.
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
In 1906, 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.
In 1912,
Max von Laue, Paul Knipping and Walter Friedrich observed for the first time the
diffraction of X-rays by crystals. This discovery, along with the early works of Paul Peter Ewald,
William Henry Bragg and
William Lawrence Bragg gave birth to the field of X-ray crystallography.
The use of X-rays for medical purposes 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. In the 1950s X-rays were used to produce an X-ray microscope.
In the 1980s an X-ray laser device was proposed as part of the
Reagan administration's
Strategic Defense Initiative, but the first and only test of the device gave inconclusive results. For technical and political reasons, the overall project was de-funded .
In the 1990s the
Chandra X-Ray Observatory was launched, allowing the exploration of the very violent processes in the universe which produce X-Rays. Unlike visible light, which is a relatively stable view of the universe, the X-ray universe is unstable, it features stars being torn apart by
black holes, galactic collisions, and novas,
neutron stars that build up layers of plasma that then explode into space.
References
- 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 . ISBN 0-7432-0023-3
See also
External links
