this technique purportedly negates projection errors associated with evaluating implant position from plain radiographs. dental radiography is commonly used in the diagnoses of common oral problems, such as cavities. 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 an x-ray filter, is usually placed over the window of the x-ray tube, absorbing the low energy part in the spectrum. this is called hardening the beam since it shifts the center of the spectrum towards higher energy (or harder) x-rays. to generate an image of the cardiovascular 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 iodinated contrast agent has 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. the radiologist or surgeon then compares the image obtained to normal anatomical images to determine whether there is any damage or blockage of the vessel. computed tomography main article: ct scan head ct scan (transverse plane) slice – a modern application of medical radiography 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 main article: 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 a 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 and swallowing 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 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. adverse effects abdominal radiograph of a pregnant woman, a procedure that should be performed only after proper assessment of benefit versus risk 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 a form of ionizing radiation, and 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. however, this is under increasing doubt. cancer risk can start at the exposure of 1100 mgy. 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%. 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 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. 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 (1.5 msv, 64 mgy) 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 a fetus, so in pregnant patients, the benefits of the investigation (x-ray) should be balanced with the potential hazards to the fetus. if there is 1 scan in 9 months, it can be harmful to the fetus. therefore, women who are pregnant get ultrasounds as their diagnostic imaging because this does not use radiation. if there is too much radiation exposure there could be harmful effects on the fetus or the reproductive organs of the mother. 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) reduces radiation dose and any associated cancer risk. 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% 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 of x-rays may result in an exposure of up to 6 (digital) to 18 (film) mrem, for a yearly average of up to 40 mrem. 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. early photon tomography or ept (as of 2015) along with other techniques are being researched as potential alternatives to x-rays for imaging applications. other uses 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. 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. radiography of cultural objects, most often x-rays of paintings to reveal underdrawing, pentimenti alterations in the course of painting or by later restorers, and sometimes previous paintings on the support. many pigments such as lead white show well in radiographs. 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. 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. 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 security luggage scanners use x-rays for inspecting the interior of luggage for security threats before loading on aircraft. border control truck scanners and domestic police departments use x-rays for inspecting the interior of trucks. x-ray fine art photography of needlefish by peter dazeley 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, in the uk in the 1970s, and 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. radiation implosion is the use of high energy x-rays generated from a fission explosion (an a-bomb) to compress nuclear fuel to the point of fusion ignition (an h-bomb). visibility 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. 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 with ionizing radiation. it is not known what exact mechanism in the eye produces the visibility: it could be due to conventional detection (excitation of rhodopsin molecules in the retina), direct excitation of retinal nerve cells, or secondary detection via, for instance, x-ray induction of phosphorescence in the eyeball with conventional retinal detection of the secondarily produced visible light. though x-rays are otherwise invisible, it is possible to see the ionization of the air molecules if the intensity of the x-ray beam is high enough. the beamline from the wiggler at the european synchrotron radiation facility is one example of such high intensity. units of measure and exposure the measure of x-rays ionizing ability is called the exposure: the coulomb per kilogram (c/kg) is the si unit of ionizing radiation exposure, and it is the amount of radiation required to