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Computed tomography

Head CT scan (transverse plane) slice - a modern application ofmedical 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.[26] 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 a fluorescent screen, between which a patient is placed. However, modern fluoroscopes couple the screen to anX-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 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.[27][28]

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.[29][30][31] X-rays are classified as acarcinogen by both the World Health Organization's International Agency for Research on Cancer and the U.S. government.[24][32] 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.[33]

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.[34] However, this is under increasing doubt.[35] It is estimated that the additional radiation will increase a person's cumulative risk of getting cancer by age 75 by 0.61.8%.[36] The amount of absorbed radiation depends upon the type of X-ray test and the body part involved.[37] 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 radiationthat 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.[38] 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 23 years of background radiation to the whole body, or 45 years to the abdomen or chest, increasing the lifetime cancer risk between 1 per 1,000 to 1 per 10,000.[38] This is compared to the roughly 40% chance of a US citizen developing cancer during their lifetime.[39] 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.[40] A head CT scan (1.5mSv, 64mGy)[41] 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.[42]



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.[43][44] In the US, there are an estimated 62 million CT scans performed annually, including more than 4 million on children.[37] Avoiding unnecessary X-rays (especially CT scans) reduces radiation dose and any associated cancer risk.[45]

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.[25][46]

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.[47][48][49][50][51][52][53]

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 byRosalind Franklin to discover the double helical structure of DNA.[54]

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 underdrawings and pentimenti, alterations in the course of painting or by later restorers. 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[55]

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 fine art photography ofneedlefish 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.[56]

Shoe-fitting fluoroscopes were popularized in the 1920s, banned in the US in the 1960s, ba

 

History

Discovery

Wilhelm Röntgen

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" (signifying an unknown quantity[57]) though many others 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 nativeGerman.

Hand mit Ringen (Hand with Rings): print of Wilhelm Röntgen's first "medical" X-ray, of his wife's hand, taken on 22 December 1895 and presented to Ludwig Zehnderof the Physik Institut, University of Freiburg, on 1 January 1896[58][59]

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.[60]

Early research

Both William Crookes (in the 1880s)[61] and German physicist Johann Hittorf[citation needed], a co-inventor and early researcher of the Crookes tube, found that photographic plates placed near the tube became unaccountably fogged or flawed by shadows. Neither found the cause nor investigated this effect.

In 1877 Ukrainian-born Ivan Pulyui, a lecturer in experimental physics at the University of Vienna, constructed various designs ofvacuum discharge tube to investigate their properties.[62] He continued his investigations when appointed professor at thePrague 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.[62] Although Pulyui had studied with Röntgen at the University of Strasbourg in the years 187375, his biographer Gaida (1997) asserts that his subsequent research was conducted independently.[62]

Taking an X-ray image with early Crookes tube apparatus, late 1800s. The Crookes tube is visible in center. The standing man is viewing his hand with a fluoroscope screen. No precautions against radiation exposure are taken; its hazards were not known at the time.

X-rays were generated and detected by Fernando Sanford (18541948), the foundation Professor of Physics at Stanford University, in 1891. From 1886 to 1888 he had studied in theHermann 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 Darknessappeared in the San Francisco Examiner.[63]

Starting in 1888, Philipp Lenard, a student of Heinrich Hertz, conducted experiments to see whether cathode rays could pass out of the Crookes tube into the air. He built a Crookes tube (later called a "Lenard tube") with a "window" in the end made of thin aluminum, facing the cathode so the cathode rays would strike it. He found that something came through, that would expose photographic plates and cause fluorescence. He measured the penetrating power of these rays through various materials. It has been suggested that at least some of these "Lenard rays" were actually X-rays.[64]

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.[65] However, he did not work with actual X-rays.

In 1894 Nikola Tesla noticed damaged film in his lab that seemed to be associated with Crookes tube experiments and began investigating this radiant energy of "invisible" kinds.[66][67] After Röntgen identified the x-ray Tesla began making X-ray images of his own using high voltages and tubes of his own design,[68] as well as Crookes tubes.


Date: 2015-12-24; view: 788


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