New technologies evolving especially in the last century have solved many problems in human life, paved the way for diverse application in different aspects that makes our life easier, comfortable and effortless. On the other hand, this great impact at human life over the years shown that this new technologies have serious consequences with respect to toxicological, environmental and social issues. One example is the X-radiation (X-rays) which was discovered by Wilhelm Conrad Röntgen in 1895; and it is also called Röntgen radiation as an honored for the discoverer.
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 (figure 1) was the first ever photograph of a human body part using X-rays [1]. The photograph electrified the general public and aroused great scientific interest in the new form of radiation.
Figure 1: (Hand with Rings): print of Wilhelm Röntgen's first "medical" X-ray, of his wife's hand, taken on 22 December 1895
X-radiation features and properties:
It is part of the electromagnetic spectrum figure 2, which lies between ultraviolet rays and Gamma rays; ranging in frequencies from 30 petahertz to 30 exahertz, which give a wavelength from 0.01 to 10 nanometers. In general the properties of electromagnetic radiation which have the wave and particle behavior apply to x-radiation as it is considered a form of electromagnetic radiation.
Figure 2: Electromagnetic Spectrum
Exposure is the measuring unit of X-rays ionizing ability; the coulomb per kilogram (C/kg) is the SI unit of ionizing radiation exposure, whereas The roentgen (R) is an obsolete traditional unit of exposure where 1.00 roentgen = 2.58×10-4 C/kg.
Another feature of ionized radiation is related to the amount of energy deposited on the matter; this measure of effected energy absorbed is called the absorbed dose: The gray (Gy), which has units of (joules/kilogram), is the SI unit of absorbed dose, and it is the amount of radiation required to deposit one joule of energy in one kilogram of any kind of matter, whereas the rad is the (obsolete) corresponding traditional unit where 100 rad = 1.00 gray.
There are a number of sources of X-ray radiation; basically X-rays 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 [3].
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, so an 80 kV tube cannot create X-rays with 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.
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, and thus the highest energies of the X-rays, range from roughly 20 to 150 kV [4].
X-rays from about 0.12 to 12 keV (10 to 0.10 nm wavelength) are classified as "soft" X-rays, and from about 12 to 120 keV (0.10 to 0.01 nm wavelength) as "hard" X-rays, due to their penetrating abilities [5].
History of X-ray:
Although Röntgen discovered X-radiation, but other scientist have observed their effect and studied ways of improving the generating of these radiation in parallel to their potential applications over the years. Scientist began making specialized versions of tubes for generating X-rays and these first generation cold cathode X-ray tubes were used until about 1920. William Coolidge invented the X-ray tube popularly called the Coolidge tube. His invention revolutionized the generation of X-rays and is the model upon which all X-ray tubes for medical applications are based.
Growing control over technology and increasingly regulated competence paved the way for development in generating X-radiation and expansion their applications; a breakthrough came from physicist Charles Barkla discovered that X-rays could be scattered by gases and that each element had a characteristic X-ray, This discovery, along with the early works of other scientist gave birth to the field of X-ray crystallography. It is hard to summaries this history is such a short research.
Uses and implementation:
The breakthrough discovery in using X-ray in diagnostics radiography was not the only field of x-ray potential applications; on the contrary the many applications of X-rays immediately generated enormous interests over the years which exploited different fields of applications. Industrial radiography uses X-rays for inspection of industrial parts, Airport security luggage scanners use X-rays for inspecting the interior of luggage for security threats also in borders security it uses the same principle to look over trucks. Moreover X-ray is used in microscopic analysis, spectroscopy and crystallography in which the pattern produced by the diffraction of X-rays is analyzed to reveal the nature of that lattice of atoms, A related technique, fiber diffraction, was used to discover the double helical structure of DNA [2].
However, among all different applications of X-radiation; medical X-ray is one of the most interesting applications because of their direct touch and impact on human lives. X-rays are capable of penetrating some thickness of matter. Medical x-rays are produced by letting a stream of fast electrons come to a sudden stop at a metal plate; the images produced by X-rays are due to the different absorption rates of different tissues. Calcium in bones absorbs X-rays the most, so bones look white on a film recording of the X-ray image, called a radiograph. Fat and other soft tissues absorb less, and look gray. Air absorbs the least, so lungs look black on a radiograph. X-rays are the oldest and most frequently used form of medical imaging.
Figure 3: examples of medical x-ray images.
The radiation issues of using x-ray in medical imaging:
The relationship between radiation dose and cancer risk is controversial, as radiation is considered one of the most extensively researched carcinogens. Diagnostic X-rays are the largest man-made source of radiation exposure to the general population, contributing about 14% of total worldwide exposure from man-made and natural sources [6]. However, although diagnostic X-rays provide great benefits, that their use involves some risk of developing cancer is generally accepted. The risk to an individual is probably small because radiation doses are usually low, but the large number of people exposed annually means that even small individual risks could translate into a considerable number of cancer cases [7].
In 2006, Americans were exposed to more than seven times as much ionizing radiation from medical procedures as was the case in the early 1980s, according to a new report on population exposure released by the National Council on Radiation Protection and Measurements (NCRP) at its annual meeting. In 2006, medical exposure (figure 4) constituted nearly half of the total radiation exposure of the U.S. population from all sources [8].
Figure 4: All exposure categories, collective dose (percent) 2006. (Credit: Image courtesy of National Council on Radiation Protection & Measurements)
These facts can reflect the potential risk of using x-radiation in medical imaging; especially with the increase of using it for imaging diagnostics. de González at el [8] reported the risk of cancer attributed to diagnostic X-ray exposures (figure 5) for 15 countries studied, the UK had the lowest annual frequency of diagnostic X-rays and Japan the highest (table 6 and figure 3).1 Japan also had the highest attributable risks, with 3·2% of the cumulative risk of cancer attributable to diagnostic X-rays, equivalent to 7587 cases of cancer per year. In all other populations less than 2% of the cumulative cancer risk was attributable to diagnostic X-rays; Croatia and Germany had the highest proportions at 1·8% and 1·5%, respectively, whereas Poland and the UK had the lowest (both 0·6%). A survey of UK practice has suggested that the comparatively low frequency of diagnostic X-ray use is due in part to the detailed guidance for doctors on the indicators for X-ray examinations issued by the Royal College of Radiologists.
Figure 5: Risk of cancer attributable to diagnostic X-ray exposures versus annual X-ray frequency (*Taken from worldwide survey)
Although there are clear benefits from the use of diagnostic X-rays that their use involves some risk of cancer is generally acknowledged. That requires a special care to be taken during x-ray examinations to use the lowest radiation dose possible while producing the best images for evaluation. National and international radiology protection councils continually review and update the technique standards used by radiology professionals.
State-of-the-art x-ray systems have tightly controlled x-ray beams with significant filtration and dose control methods to minimize stray or scatter radiation. This ensures that those parts of a patient's body not being imaged receive minimal radiation exposure.
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