Radar equipment  is used at airports, in airplanes, at  missile sites , on tanks, and on ships. The radar technology commonly used in the 20th century produced X-rays as a technically unavoidable by-product in the  high-voltage electronics  of the equipment. [ 259 ]  In the 1960s and 1970s, German soldiers and technicians were largely unaware of the dangers, as were those in the GDR's  National People's Army . [ 259 ]  The problem had been known internationally since the 1950s, and to the German Armed Forces since at least 1958. [ 260 ]  However, no radiation protection measures were taken, such as the wearing of lead aprons. Until about the mid-1980s, radiation shielding was inadequate, especially for pulse switch tubes. [ 259 ]  Particularly affected were maintenance technicians (radar mechanics) who were exposed to the X-ray generating parts for hours without any protection. The permissible annual limit value could be exceeded after just 3 minutes. It was not until 1976 that warning notices were put up and protective measures taken in the German Navy, and not until the early 1980s in general. [ 259 ]  As late as the 1990s, the German Armed Forces denied any connection between radar equipment and cancer or genetic damage. [ 261 ]  The number of victims amounted to several thousand. The connection was later acknowledged by the German Armed Forces and in many cases a supplementary pension was paid. In 2012, a foundation was set up to provide unbureaucratic compensation for the victims. [ 262 ]


The harmful effects of X-rays were recognized during the  National Socialist  era. The function of the  gonads  ( ovaries  or  testicles ) was destroyed by ionizing radiation, leading to  infertility . In July 1942,  Heinrich Himmler  (1900-1945) decided to conduct forced sterilization experiments at the  Auschwitz-Birkenau concentration camp , which were carried out by  Horst Schumann  (1906-1983), previously a doctor in  Aktion T4 . [ 263 ]  Each test victim had to stand between two X-ray machines, which were arranged in such a way that the test victim had just enough space between them. Opposite the x-ray machines was a booth with lead walls and a small window. From the booth, Schumann could direct X-rays at the test victims' sexual organs without endangering himself. [ 264 ]  Human radiation castration experiments were also conducted in  concentration camps  under the direction of  Viktor Brack  (1904-1948). As part of the "Law for the Prevention of Hereditary Diseases," people were often subjected to radiation castration during interrogations without their knowledge. [ 265 ]  Approximately 150 radiologists from hospitals throughout Germany participated in the forced castration of approximately 7,200 people using X-rays or radium. [ 266 ]


On November 23, 2006,  Alexander Alexanderovich Litvinenko  (1962-2006) was murdered under unexplained circumstances as a result of radiation sickness caused by  polonium . [ 267 ]  This was also briefly suspected in the case of  Yasser Arafat  (1929-2004), who died in 2004.


The misuse of ionizing radiation is a radiation offence under  German criminal law . The use of ionizing radiation to harm persons or property is punishable. Since 1998, the regulations can be found in


§ 309
 StGB  (in German)
(previously § 311a StGB old version); the regulations go back to § 41 AtG old version. In the Austrian Criminal Code, relevant criminal offenses are defined in the seventh section, " Criminal acts dangerous to the public " and " Criminal acts against the environment ". In Switzerland, endangerment by nuclear energy, radioactive substances or ionizing radiation is punishable under Art. 326 of the Swiss Criminal Code and disregard of safety regulations under Chapter 9 of the Nuclear Energy Act of 21 March 2003.


Originally, the term radiation protection referred only to ionizing radiation. Today, non-ionizing radiation is also included and is the responsibility of the Federal Office for Radiation Protection, the Radiation Protection Division [ 2 ]  of the  Federal Office of Public Health [ 3 ]  and the  Ministry of Climate Action and Energy (Austria) . [ 4 ]  The project collected, evaluated and compared data on the legal situation in all European countries (47 countries plus Germany) and major non-European countries (China, India, Australia, Japan, Canada, New Zealand and the USA) regarding electric, magnetic and electromagnetic fields (EMF) and optical radiation (OS). The results were very different and in some cases deviated from the recommendations of the  International Commission on Non-Ionizing Radiation Protection  (ICNIRP). [ 268 ]


For many centuries, the  Inuit  ( Eskimos ) have used  snow goggles  with narrow slits, carved from seal bones or reindeer antlers, to protect against snow blindness (photokeratitis).


In the 1960s, Australia - particularly  Queensland  - launched the first awareness campaign on the dangers of  ultraviolet (UV) radiation  in the spirit of primary prevention. In the 1980s, many countries in Europe and overseas initiated similar UV protection campaigns. UV radiation has a thermal effect on the skin and eyes and can lead to skin cancer (malignant melanoma) and eye inflammation or cataracts. [ 269 ]  To protect the skin from harmful UV radiation, such as  photodermatosis ,  acne aestivalis ,  actinic keratosis  or urticaria solaris, normal clothing, special UV protective clothing (SPF 40-50) and high SPF sunscreen can be used. The Australian-New Zealand Standard (AS/NZS 4399) of 1996 measures new textile materials in an unstretched and dry state for the manufacture of protective clothing worn while bathing, especially by children, and for the manufacture of shading textiles (sunshades, awnings). The UV Standard 801 assumes a maximum radiation intensity with the solar spectrum in Melbourne, Australia, on January 1 of a year (at the height of the Australian summer), the most sensitive skin type of the wearer, and under wearing conditions. As the  solar spectrum  in the  northern hemisphere  differs from that in Australia, the measurement method according to the European standard EN 13758-1 is based on the solar spectrum of  Albuquerque  (New Mexico, USA), which corresponds approximately to that of southern Europe. [ 270 ]


To protect your eyes, wear sunglasses with UV protection or special goggles that also shield the sides to prevent snow blindness. A defensive reaction of the skin is the formation of a light callus, the skin's own sun protection, which corresponds to a protection factor of about 5. At the same time, the production of brown skin pigments ( melanin ) in the corresponding cells ( melanocytes ) is stimulated.


A  solar control film  is usually a film made of  polyethylene terephthalate  (PET) that is applied to windows to reduce the light and heat from the sun's rays. The film filters UV-A and UV-B radiation. Polyethylene terephthalate goes back to an invention by the two Englishmen  John Rex Whinfield  (1902-1966) and  James Tennant Dickson  in 1941.


The fact that UV-B radiation (Dorno radiation, after  Carl Dorno  (1865-1942)) is a proven carcinogen, but is also required for the body's own synthesis of  vitamin-D 3  (cholecalciferol), leads to internationally conflicting recommendations regarding health-promoting UV exposure. [ 271 ]  In 2014, based on the scientific evidence of the last decades, 20 scientific authorities, professional societies and associations from the fields of radiation protection, health, risk assessment, medicine and nutrition published a recommendation on "UV exposure for the formation of the body's own vitamin D". It was the first interdisciplinary recommendation on this topic worldwide. Using a  solarium  for the first time at a young age (<35 years) almost doubles the risk of developing malignant melanoma. In Germany, the use of tanning beds by minors has been prohibited by law since March 2010. As of August 1, 2012, sunbeds must not exceed a maximum irradiance of 0.3 watts per square meter of skin. Sunbeds must be labeled accordingly. The new irradiance limit corresponds to the highest UV dose that can be measured on Earth at 12 noon under a cloudless sky at the equator. [ 272 ]


The minimum erythema dose (MED) is determined for medical applications. The MED is defined as the lowest dose of radiation that produces a barely visible erythema. It is determined 24 hours after the test irradiation. It is performed with the type of lamp intended for the therapy by applying so-called light stairs to skin that is not normally exposed to light (for example, on the buttocks). [ 273 ]


Richard Küch (1860-1915) was able to melt  quartz glass  - the basis for UV radiation sources - for the first time in 1890 and founded the  Heraeus Quarzschmelze . He developed the first quartz lamp (sun lamp) for generating UV radiation in 1904, thus laying the foundation for this form of light therapy.


Despite the dosage problems, doctors increasingly used quartz lamps in the early 20th century.  Internal medicine  specialists and  dermatologists  were among the most eager testers. After successful treatment of  skin tuberculosis , internal medicine began to treat tuberculous  pleurisy , glandular tuberculosis and intestinal tuberculosis. In addition, doctors tested the effect of quartz lamps on other infectious diseases such as  syphilis ,  metabolic diseases ,  cardiovascular diseases , nerve pain such as  sciatica , or  nervous diseases  such as  neurasthenia  and  hysteria . In dermatology,  fungal diseases ,  ulcers  and wounds,  psoriasis ,  acne , freckles and hair loss were also treated with quartz lamps, while in gynecology, abdominal diseases were treated with quartz lamps. Rejuvenation specialists used artificial high-altitude sunlight to stimulate gonadal activity and treated infertility,  impotentia generandi  (inability to conceive), and  lack of sexual desire  by irradiating the genitals. For this purpose, Philipp Keller (1891-1973) developed an erythema dosimeter with which he measured the amount of radiation not in Finsen units (UV radiation with a wavelength λ of 296.7 nm and an irradiance E of 10 −5  W/m 2 ), but in height solar units (HSE). It was the only instrument in use around 1930, but it was not widely accepted in medical circles. [ 274 ] [ 275 ]


Treatment of acne with ultraviolet radiation is still controversial. Although UV radiation can have an antibacterial effect, it can also induce  proliferative hyperkeratosis . This can lead to the formation of  comedones  ("blackheads").  Phototoxic  effects may also occur. In addition, it is carcinogenic and promotes skin aging. UV therapy is increasingly being abandoned in favor of  photodynamic therapy . [ 276 ]


The  ruby laser  was developed in 1960 by  Theodore Maiman  (1927-2007) as the first  laser  based on the  ruby maser . Soon after, the dangers of lasers were discovered, especially for the eyes and skin, due to the laser's low penetration depth. Lasers have numerous applications in technology and research as well as in everyday life, from simple  laser pointers  to  distance measuring devices , cutting and  welding tools , reproduction of  optical storage media  such as CDs, DVDs and Blu-ray discs, communication, laser scalpels and other devices using laser light in everyday medical practice. The Radiation Protection Commission requires that laser applications on human skin be performed only by a specially trained physician. Lasers are also used for show effects in discotheques and at events.


Lasers can cause biological damage due to the properties of their radiation and their sometimes extremely concentrated electromagnetic power. For this reason, lasers must be labeled with standardized warnings depending on the  laser class . The classification is based on the  DIN standard EN 60825-1 , which distinguishes between ranges of  wavelengths  and exposure times that lead to characteristic injuries and injury thresholds for power or  energy density .


The  CO 2 -Laser  was developed in 1964 by the Indian electrical engineer and physicist  Chandra Kumar Naranbhai Patel  (*1938) [ 277 ]  at the same time as the  Nd:YAG laser  (neodymium-doped yttrium aluminum garnet laser) at  Bell Laboratories  by  LeGrand Van Uitert  (1922-1999) and Joseph E. Geusic (*1931) and the  Er:YAG laser  (erbium-doped yttrium aluminum garnet laser) and has been used in dentistry since the early 1970s. In the hard laser field, two systems in particular are emerging for use in the oral cavity: the CO2 laser for use in soft tissue and the Er:YAG laser for use in dental hard and soft tissue. The goal of  soft laser treatment  is to achieve  biostimulation  with low energy densities. [ 278 ]


The Commission on Radiological Protection strongly recommends that the possession and purchase of class 3B and 4 laser pointers be regulated by law to prevent misuse. [ 279 ]  This is due to the increase in dangerous dazzle attacks caused by high-power laser pointers. In addition to pilots, these include truck and car drivers, train operators, soccer players, referees, and even spectators at soccer games. [ 280 ]  Such glare can lead to serious accidents and, in the case of pilots and truck drivers, to occupational disability due to eye damage. The first accident prevention regulation was published on April 1, 1988 as BGV B2, followed on January 1, 1997 by DGUV Regulation 11 of the German Social Accident Insurance. [ 281 ]  Between January and mid-September 2010, the  German Federal Aviation Office  registered 229 dazzle attacks on helicopters and airplanes of German airlines nationwide. [ 282 ]  On October 18, 2017, a perpetrator of a dazzle attack on a federal police helicopter was sentenced to one year and six months in prison without parole. [ 283 ]


Electrosmog is colloquially understood as the exposure of humans and the environment to electric, magnetic and  electromagnetic fields , some of which are believed to have undesirable biological effects. [ 284 ]  Electromagnetic environmental compatibility (EMC) refers to the effects on living organisms, some of which are considered  electrosensitive . Fears of such effects have existed since the beginning of technological use in the mid-19th century. In 1890, for example, officials of the Royal General Directorate in Bavaria were forbidden to attend the opening ceremony of Germany's first alternating current power plant, the Reichenhall Electricity Works, or to enter the machine room. With the establishment of the first radio  telegraphy  and its telegraph stations, the U.S. magazine The Atlanta Constitution reported in April 1911 on the potential dangers of radio telegraph waves, which, in addition to "tooth loss," were said to cause hair loss and make people "crazy" over time. [ 285 ]  Full-body protection was recommended as a preventive measure.


During the second half of the 20th century, other sources of electromagnetic fields have become the focus of health concerns, such as power lines,  photovoltaic systems , microwave ovens, computer and television screens, security devices, radar equipment, and more recently, cordless telephones ( DECT ), cell phones, their  base stations ,  energy-saving lamps , and  Bluetooth  connections. Electrified railroad lines, tram overhead lines and subway tracks are also strong sources of electrosmog. In 1996, the  World Health Organization  (WHO) launched the EMF (ElectroMagnetic Fields) Project to bring together current knowledge and available resources from key international and national organizations and scientific institutions on electromagnetic fields. [ 286 ] [ 287 ]   The German Federal Office for Radiation Protection  ( BfS ) published the following recommendation in 2006:


"In order to avoid possible health risks, the German Federal Office for Radiation Protection recommends that you minimize your personal exposure to radiation through your own initiative."


As of 2016, the EMF Guideline 2016 of EUROPAEM ( European Academy For Environmental Medicine ) on the prevention, diagnosis and treatment of EMF-related complaints and diseases applies. [ 289 ]


A  microwave oven , invented in 1950 by U.S. researcher  Percy Spencer  (1894-1970), is used to quickly heat food using  microwave  radiation at a frequency of 2.45 gigahertz. In an intact microwave oven,  leakage radiation  is relatively low due to the  shielding  of the cooking chamber. An "emission limit of five milliwatts per square centimeter (equivalent to 50 watts per square meter) at a distance of five centimeters from the surface of the appliance" (radiation density or power flux density) is specified. Children should not stand directly in front of or next to the appliance while food is being prepared. In addition, the  Federal Office for Radiation Protection  lists pregnant women as particularly at risk. [ 290 ]


In microwave therapy, electromagnetic waves are generated for heat treatment. The penetration depth and energy distribution vary depending on the frequency of application (short waves, ultra short waves, microwaves). To achieve greater penetration, pulsed microwaves are used, each of which delivers high energy to the tissue. A pulse pause ensures that no burns occur. Metal  implants  and  pacemakers  are contraindications. [ 291 ]


The discussion about possible  health risks from mobile phone radiation  has been controversial to date, although there are currently no valid results. According to the German Federal Office for Radiation Protection


"there are still uncertainties in the risk assessment that could not be completely eliminated by the German Mobile Telecommunication Research Program, in particular possible health risks of long-term exposure to high-frequency electromagnetic fields from cell phone calls in adults (intensive cell phone use over more than 10 years) and the question of whether the use of cell phones by children could have an effect on health. For these reasons, the Federal Office for Radiation Protection still considers preventive health protection (precaution) to be necessary: exposure to electromagnetic fields should be kept as low as possible."


The German Federal Office for Radiation Protection recommends, among other things, mobile phones with a low  SAR  (Specific Absorption Rate) [ 292 ] [ 293 ]  and the use of headsets or hands-free devices to keep the mobile phone away from the head. There is some discussion that mobile phone radiation may increase the incidence of  acoustic neuroma , a benign tumor that arises from the  vestibulocochlear nerve . It should therefore be reduced. [ 294 ]  In everyday life, a mobile phone transmits at maximum power only in exceptional cases. As soon as it is near a cell where maximum power is no longer needed, it is instructed by that cell to reduce its power. Electrosmog or cell phone radiation filters built into cell phones are supposed to protect against radiation. The effect is doubtful from the point of view of electromagnetic environmental compatibility, because the radiation intensity of the cell phone is increased disproportionately in order to obtain the necessary power. The same is true for use in a car without an external antenna, as the necessary radiation can only penetrate through the windows, or in areas with poor network coverage. Since 2004, radio network  repeaters  have been developed for mobile phone networks ( GSM ,  UMTS ,  Tetrapol ) that can amplify the reception of a mobile phone  cell  in shaded buildings. This reduces the SAR value of the mobile phone when making calls.


The SAR value of a  WLAN   router  is only a tenth of that of a cell phone, although this drops by a further 80% at a distance of just one meter. The router can be set so that it switches off when not in use, for example at night. [ 295 ]


Until now, electrical energy has been transported from the power plant to the consumer almost exclusively via  high-voltage lines , in which  alternating current  flows at a frequency of 50  Hertz . As part of the  energy transition ,  high-voltage direct current  (HVDC) transmission systems are also planned in Germany. Since the amendment of the 26th Federal Immission Control Ordinance (BImSchV) in 2013, emissions from HVDC systems are also regulated by law. The limit is set to prevent interference with electronic implants caused by static  magnetic fields . No limit has been set for static electric fields.


Ground fault interrupters are available to reduce  electric fields  and (in the case of  current  flow) magnetic fields from residential electrical installations. In  plaster  installations, only a small part of the electric field can escape from the wall. However, a mains disconnect switch automatically disconnects the relevant line as long as no electrical load is switched on; as soon as a load is switched on, the  mains voltage  is also switched on. [ 296 ]  Ground fault interrupters were introduced in 1973 and have been continuously improved over the decades. [ 297 ]  In 1990, for example, it became possible to disconnect the PEN conductor (formerly known as the neutral conductor). [ 298 ]  Circuit breakers can be installed in several different circuits, preferably in those that supply bedrooms. However, they only turn off when no continuous current consumers such as air conditioners, fans, humidifiers, electric alarm clocks, night lights, standby devices, alarm systems, chargers, and similar devices are turned on. Instead of the mains voltage, a  low voltage  (2-12 volts) is applied, which can be used to detect when a consumer is switched on.


Rooms can also be shielded with copper wallpaper or special wall paints containing metal, thus applying the  Faraday cage  principle.


Since about 2005,  body scanners  have been used primarily at airports for security (passenger) screening. Passive scanners detect the natural radiation emitted by a person's body and use it to locate objects worn or concealed on the body. Active systems also use artificial radiation to improve detection by analyzing the  backscatter . A distinction is made between body scanners that use ionizing radiation (usually X-rays) and those that use non-ionizing radiation ( terahertz radiation ).


The integrated components operating in the lower terahertz range emit less than 1 mW (-3 dBm), [ 299 ]  so no health effects are expected. There are conflicting studies from 2009 on whether genetic damage can be detected as a result of terahertz radiation. [ 300 ]  In the U.S., backscatter x-ray scanners make up the majority of devices used. Scientists fear that a future increase in cancer could pose a greater threat to the life and limb of passengers than terrorism itself. [ 301 ]  It is not clear to the passenger whether the body scanners used during a particular checkpoint use only terahertz or also X-ray radiation.


According to the Federal Office for Radiation Protection, the few available results from investigations in the frequency range of active whole-body scanners that work with  millimeter wave  or terahertz radiation do not yet allow a conclusive assessment from a radiation protection perspective (as of 24 May 2017). [ 302 ]


In the vicinity of the plant, where employees or other third parties may be present, the limit value of the permissible annual dose for a single person in the population of one millisievert (1 mSv, including pregnant women and children) is not exceeded, even in the case of permanent presence.


In the case of X-ray scanners for hand luggage, it is not necessary to set up a radiation protection area by Section §19  RöV , as the radiation exposure during a hand luggage check for passengers does not exceed 0.2 microsievert (μSv), even under unfavorable assumptions. For this reason, employees involved in baggage screening are not considered to be occupationally exposed to radiation in accordance with Section §31 X-ray Ordinance and therefore do not have to wear a dosimeter. [ 303 ]


Electromagnetic alternating fields  have been used in medicine since 1764, [ 304 ]  mainly for heating and increasing  blood circulation  ( diathermy ,  short-wave therapy ) to improve wound and bone healing. [ 305 ]  The relevant radiation protection is regulated by the Medical Devices Act together with the Medical Devices Operator Ordinance. [ 306 ]  The Medical Devices Act came into force in Germany on January 14, 1985. It divided the medical devices known at that time into groups according to their degree of risk to the patient. The Medical Devices Ordinance regulated the handling of medical devices until January 1, 2002, when it was replaced by the Medical Devices Act. When ionizing radiation is used in medicine, the benefit must outweigh the potential risk of tissue damage (justifiable indication). For this reason, radiation protection is of great importance. The design should be optimized according to the ALARA (As Low As Reasonably Achievable) principle as soon as an application is described as suitable. Since 1996, the European ALARA Network (EAN), founded by the  European Commission , has been working on the further implementation of the ALARA principle in radiation protection. [ 307 ]


Discovered around 1800 by the German-British astronomer, engineer and musician  Friedrich Wilhelm Herschel  (1738-1822),  infrared radiation  primarily produces heat. If the increase in body temperature and the duration of exposure exceed critical limits, heat damage and even  heat stroke  can result. Due to the still unsatisfactory data situation and the partly contradictory results, it is not yet possible to give clear recommendations for radiation protection with regard to infrared radiation. However, the findings regarding the acceleration of skin aging by infrared radiation are sufficient to describe the use of infrared radiation against wrinkles as counterproductive. [ 308 ]


In 2011, the  Institute for Occupational Safety and Health of the German Social Accident Insurance  established exposure limit values to protect the skin from burns caused by  thermal radiation . The IFA recommends that, in addition to the limit specified in EU Directive 2006/25/EC to protect the skin from burns for exposure times up to 10 seconds, a limit for exposure times between 10 and 1000 seconds should be applied. In addition, all radiation components in the wavelength range from 380 to 20000 nm should be considered for comparison with the limit values. [ 309 ]


A leaflet published by the German Radiological Society (DRG) in 1913 was the first systematic approach to  radiation protection . [ 310 ] [ 311 ]  The physicist and co-founder of the society, Bernhard Walter (1861-1950), was one of the pioneers of radiation protection.


The  International Commission on Radiological Protection  (ICRP) and the  International Commission on Radiation Units and Measurements  (ICRU) were established at the Second International Congress of Radiology in Stockholm in 1928. In the same year, the first international radiation protection recommendations were adopted and each country represented was asked to develop a coordinated radiation control program. The United States representative,  Lauriston Taylor  of the US  Bureau of Standards  (NSB), formed the Advisory Committee on X-Ray and Radium Protection, later renamed the  National Committee on Radiation Protection and Measurements  (NCRP). The NCRP received a Congressional charter in 1964 and continues to develop guidelines to protect individuals and the public from excessive radiation. In the years that followed, numerous other organizations were established by almost every president. [ 312 ]


Individuals in professions such as pilots, nuclear physicians, and nuclear power plant workers are regularly exposed to ionizing radiation. In Germany, over 400,000 workers undergo occupational radiation monitoring to safeguard against the harmful effects of radiation. Approximately 70,000 individuals employed across various industries possess a radiation pass (distinct from an X-ray pass - see below). Individuals who may receive an annual effective dose of more than 1 millisievert during their work are required to undergo radiation protection monitoring. In Germany, the effective dose from natural radiation is 2.1 millisieverts per year. Radiation dose is measured using dosimeters, and the occupational dose limit is 20 millisieverts per year. [ 313 ]  Monitoring also applies to buildings, plant components or (radioactive) substances. These are exempted from the scope of the Radiation Protection Ordinance by a special administrative act, the exemption in radiation protection. To this end, it must be ensured that the resulting radiation exposure for an individual member of the public does not exceed 10 μSv per calendar year and that the resulting collective dose does not exceed 1 person sievert per year. [ 314 ]


According to


§ 170
 StrlSchG   [Radiation Protection Act]  (in German)
all occupationally exposed persons and holders of radiation passports require a radiation protection register number (SSR number or SSRN), a unique personal identification number, as of December 31, 2018. The SSR number facilitates and improves the allocation and balancing of individual dose values from occupational radiation exposure in the radiation protection register. It replaces the former radiation passport number. It is used to monitor dose limits. Companies are obliged to deploy their employees in such a way that the  radiation dose  to which they are exposed does not exceed the limit of 20  millisieverts  per calendar year. In Germany, about 440,000 people were classified as occupationally exposed to radiation in 2016. According to


§ 145
 StrlSchG   [German Radiation Protection Act]  (in German)
paragraph 1, Sentence 1, "in the case of remediation and other measures to prevent and reduce exposure at radioactively contaminated sites, the person who carries out the measures himself or has them carried out by workers under his supervision must carry out an assessment of the body dose of the workers before starting the measures". Applications for SSR numbers must be submitted to the Federal Office for Radiation Protection ( BfS ) by March 31, 2019 for all employees currently under surveillance. [ 315 ]


The application for the SSR number at the Federal Office and the transmission of the necessary data must be ensured following


§ 170
 StrlSchG   [German Radiation Protection Act]  (in German)
paragraph 4 sentence 4 by


§ 131
 StrlSchG  (in German) paragraph 1 or


§ 145
 StrlSchG  (in German) paragraph 1 sentence 1 or by


§ 115
 StrlSchG  (in German) paragraph 2 or


§ 153
 StrlSchG  (in German) paragraph 1.
The SSR numbers must then be available for further use as part of normal communication with monitoring stations or radiation pass authorities. [ 316 ]  The SSR number is derived from the social security number and personal data using non-traceable encryption. The transmission takes place online. Approximately 420,00 persons are monitored for radiation protection in Germany (as of 2019).


Emergency responders (including volunteers) who are not occupationally exposed persons within the meaning of the Radiation Protection Act also require an SSR number retrospectively, i.e. after an operation in which they were exposed to radiation above the limits specified in the Radiation Protection Ordinance, as all relevant exposures must be recorded in the Radiation Protection Register.


Radiation protection areas  are spatial areas in which either people can receive certain body doses during their stay or in which a certain local dose rate is exceeded. They are defined in § 36 of the Radiation Protection Ordinance and in §§ 19 and 20 of the X-Ray Ordinance. According to the Radiation Protection Ordinance, radiation protection areas are divided into restricted areas (local dose rate ≥ 3 mSv/hour), control areas (effective dose > 6 mSv/year) and monitoring areas (effective dose > 1 mSv/year), depending on the hazard.


Germany, Austria and Switzerland, among many other countries, have early warning systems in place to protect the population.


The local dose rate measurement network (ODL measurement network) is a measurement system for radioactivity operated by the German Federal Office for Radiation Protection, which determines the local dose rate at the measurement site. [ 317 ]


In Austria, the Radiation Early Warning System is a measurement and reporting system established in the late 1970s to provide early detection of elevated levels of ionizing radiation in the country and to enable the necessary measures to be taken. The readings are automatically sent to the central office at the Ministry, where they can be accessed by the relevant departments, such as the Federal Warning Center or the warning centers of the federal states. [ 318 ]


NADAM (Network for Automatic Dose Alerting and Measurement) is the gamma radiation monitoring network of the Swiss National Emergency Operations Center. The monitoring network is complemented by the MADUK stations (Monitoring Network for Automatic Dose Rate Monitoring in the Environment of Nuclear Power Plants) of the Swiss Federal Nuclear Safety Inspectorate (ENSI).


In 2011-2014, the NERIS-TP project aimed to discuss the lessons learned from the European EURANOS project on nuclear emergency response with all relevant  stakeholders . [ 319 ]


The European PREPARE project aims to fill gaps in nuclear and radiological emergency preparedness identified after the Fukushima accident. The project aims to review emergency response concepts for long-lived releases, to address issues of measurement methods and food safety in the case of transboundary contamination, and to fill gaps in decision support systems (source term reconstruction, improved dispersion modeling, consideration of aquatic dispersion pathways in European river systems). [ 320 ]


Environmental radioactivity  has been monitored in Germany since the 1950s. Until 1986, this was carried out by various authorities that did not coordinate with each other. Following the confusion during the Chernobyl reactor disaster in April 1986, measurement activities were pooled in the IMIS (Integrated Measurement and Information System) project, an environmental information system for monitoring radioactivity in Germany. [ 321 ]  Previously, the measuring equipment was affiliated to the warning offices under the name  WADIS  ("Warning service information system").


The aim of the CONCERT (European Joint Programme for the Integration of Radiation Protection Research) project is to establish a joint European program for radiation protection research in Europe in 2018, based on the current strategic research programs of the European research platforms MELODI (radiation effects and radiation risks), ALLIANCE (radioecology), NERIS (nuclear and radiological emergency response), EURADOS (radiation dosimetry) and EURAMED (medical radiation protection). [ 322 ]


The REWARD (Real time wide area radiation surveillance system) project was established to address the threats of nuclear terrorism, missing radioactive sources, radioactive contamination and nuclear accidents. The consortium developed a mobile system for real time wide area radiation monitoring based on the integration of new miniaturized solid state sensors. Two sensors are used: a  cadmium zinc telluride  (CdZnTe) detector for gamma radiation and a high efficiency neutron detector based on novel silicon technologies. The gamma and neutron detectors are integrated into a single monitoring device called a tag. The sensor unit includes a wireless communication interface to remotely transmit data to a monitoring base station, which also uses a  GPS  system to calculate the tag's position. [ 323 ]


The  Nuclear Emergency Support Team  (NEST) is a US program for all types of nuclear emergencies of the National Nuclear Security Administration (NNSA) of the United States Department of Energy and is also a counter-terrorism unit that responds to incidents involving radioactive materials or nuclear weapons in US possession abroad. [ 324 ] [ 325 ]  It was founded in 1974/75 under US President  Gerald Ford  and renamed the Nuclear Emergency Support Team in 2002. [ 326 ] [ 327 ]  In 1988, a secret agreement from 1976 between the USA and the Federal Republic of Germany became known, which stipulates the deployment of NEST in the Federal Republic. In Germany, a similar unit has existed since 2003 with the name Central Federal Support Group for Serious Cases of Nuclear-Specific Emergency Response ( ZUB ). [ 328 ]


As early as 1905, the Frenchman  Viktor Hennecart [ 329 ]  called for special legislation to regulate the use of X-rays. In England, Sidney Russ (1879-1963) suggested to the British Roentgen Society in 1915 that it should develop its own set of safety standards, which it did in July 1921 with the formation of the British X-Ray and Radium Protection Committee. [ 330 ]  In the United States, the American X-Ray Society developed its own guidelines in 1922. In the German Reich, a special committee of the German X-Ray Society under  Franz Maximilian Groedel  (1881-1951),  Hans Liniger  (1863-1933) and  Heinz Lossen  (1893-1967) formulated the first guidelines after the First World War. In 1953, the employers' liability insurance associations issued the accident prevention regulation "Use of X-rays in medical facilities" based on the legal basis in § 848a of the Reich Insurance Code (RVG). In the GDR, the Occupational Safety and Health Regulation (ASAO) 950 was in effect from 1954 to 1971. It was replaced by ASAO 980 on April 1, 1971.


The European Atomic Energy Community (EURATOM) was founded on March 25, 1957, by the Treaty of Rome between France, Italy, the Benelux countries and the Federal Republic of Germany, and remains almost unchanged to this day. Chapter 3 of the Euratom Treaty regulates measures to protect the health of the population. Article 35 requires facilities for the continuous monitoring of soil, air and water for radioactivity. As a result, monitoring networks have been set up in all Member States and the data collected is sent to the EU's central database (EURDEP, European Radiological Data Exchange Platform). [ 331 ]  The platform is part of the EU's ECURIE system for the exchange of information in the event of radiological emergencies and became operational in 1995. [ 332 ]  Switzerland also participates in this information system. [ 333 ] [ 334 ]


In Germany, the first X-ray regulation ( RGBl . I p. 88) was issued in 1941 and originally applied to non-medical companies. The first medical regulations were issued in October 1953 by the Main Association of Industrial Employer's Liability Insurance Associations as accident prevention regulations for the Reich Insurance Code. Basic standards for radiation protection were introduced by directives of the European Atomic Energy Community ( EURATOM ) on February 2, 1959. The Atomic Energy Act of December 23, 1959 is the national legal basis for all radiation protection legislation in the Federal Republic of Germany (West) with the Radiation Protection Ordinance of June 24, 1960 (only for radioactive substances), the Radiation Protection Ordinance of July 18, 1964 (for the medical sector) and the X-ray Ordinance of March 1, 1973. [ 335 ]  Radiation protection was formulated in § 1, according to which  life, health and property are to be protected from the dangers of nuclear energy and the harmful effects of ionizing radiation and damage caused by nuclear energy or ionizing radiation is to be compensated.  The Radiation Protection Ordinance sets dose limits for the general population and for occupationally exposed persons. In general, any use of ionizing radiation must be justified and radiation exposure must be kept as low as possible even below the limit values. To this end, physicians, dentists and veterinarians, for example, must provide proof every five years - by Section 18a (2) X-ray Ordinance .  in the version dated April 30, 2003 - that their specialist knowledge in radiation protection has been updated and must complete a full-day course with a final examination. Specialist knowledge in radiation protection is required by the Technical Knowledge Guideline according to X-ray Ordinance .  - R3 for persons who work with baggage screening equipment, industrial measuring equipment and interfering emitters. Since 2019, the regulatory areas of the previous X-ray and radiation protection ordinances have been merged in the amended Radiation Protection Ordinance.


The  Radiation Protection Commission  ( SSK ) was founded in 1974 as an  advisory body  to the  Federal Ministry of the Interior. [ 336 ]  It emerged from Commission IV "Radiation Protection" of the German Atomic Energy Commission, which was founded on January 26, 1956. After the Chernobyl nuclear disaster in 1986, the  Federal Ministry for the Environment, Nature Conservation, Nuclear Safety and Consumer Protection  was established in the Federal Republic of Germany. The creation of this ministry was primarily a response to the perceived lack of coordination in the political response to the Chernobyl disaster and its aftermath. On December 11, 1986, the German Bundestag passed the Precautionary Radiation Protection Act ( StrVG ) to protect the population, to monitor radioactivity in the environment, and to minimize human exposure to radiation and  radioactive contamination  of the environment in the event of radioactive accidents or incidents. The last revision of the X-Ray Ordinance was issued on January 8, 1987. As part of a comprehensive modernization of German radiation protection law, [ 337 ]  which is largely based on Directive 2013/59/Euratom, [ 338 ]  the provisions of the X-Ray Ordinance have been incorporated into the revised Radiation Protection Ordinance.


Among many other measures, contaminated food was withdrawn from the market on a large scale. Parents were strongly advised not to let their children play in sandboxes. Some of the contaminated sand was replaced. In 1989, the Federal Office for Radiation Protection was incorporated into the Ministry of the Environment. On April 30, 2003, a new precautionary radiation protection law was promulgated to implement two EU directives on the health protection of persons against the dangers of ionizing radiation during medical exposure. [ 339 ] [ 340 ]  The protection of workers from optical radiation (infrared radiation (IR), visible light (VIS) and ultraviolet radiation (UV)), which falls under the category of  non-ionizing radiation, is regulated by the Ordinance on the Protection of Workers from Artificial Optical Radiation of 19 July 2010 . [ 341 ]  It is based on the  EU Directive  2006/25/EC of April 27, 2006. [ 342 ]  On March 1, 2010, the "Act on the Protection of Humans from Non-Ionizing Radiation" ( NiSG ), [ 343 ]   BGBl . I p. 2433, came into force, according to which the use of sunbeds by minors has been prohibited since August 4, 2009, in accordance with


§ 4
 NiSG   [Network and Information Systems Security Ordinance – NIS Ordinance]  (in German)
A new Radiation Protection Act came into force in Germany on October 1, 2017. [ 344 ]


In Germany, a radiation protection officer directs and supervises activities to ensure radiation protection when handling radioactive materials or ionizing radiation. Their duties are described in


§ 31-33
 StrlSchV  (in German)
of the Radiation Protection Ordinance and


§ 13-15
 RöV  (in German)
of the X-Ray Ordinance. They are appointed by the radiation protection officer, who is responsible for ensuring that all radiation protection regulations are observed.


Since 2002, an x-ray pass is a document in which the examining physician or dentist must enter information about the x-ray examinations performed on the patient. The main aim was to avoid unnecessary repeat examinations. According to the new Radiation Protection Ordinance ( StrlSchV ), [ 345 ]  practices and clinics are no longer obliged to offer their patients X-ray passports and to enter examinations in them. The Radiation Protection Ordinance came into force on December 31, 2018, together with the Radiation Protection Act (StrlSchG) passed in 2017, replacing the previous Radiation Protection Ordinance and the X-ray Ordinance. The  Federal Office for Radiation Protection  ( BfS ) continues to advise patients to keep records of their own radiation diagnostic examinations. On its website, the BfS provides a downloadable document that can be used for personal documentation. [ 346 ]


In Switzerland, institutionalized radiation protection began in 1955 with the issuance of  guidelines for protection against ionizing radiation in medicine, laboratories, industry and manufacturing plants , although these were only recommendations. The legal basis was created by a new constitutional article (Art. 24), according to which the federal government issues regulations on protection against the dangers of ionizing radiation. On this basis, a corresponding federal law entered into force on July 1, 1960. The first Swiss ordinance on radiation protection entered into force on May 1, 1963. On October 7, 1963, the  Federal Department of Home Affairs  (EDI) issued the following decrees to supplement the ordinance:


Another 40 regulations followed. The monitoring of such facilities took many years due to a lack of personnel. From 1963, dosimeters were to be used for personal protection, but this met with great resistance. It was not until 1989 that an updated radiation protection law was passed, accompanied by radiation protection training for the people concerned. [ 347 ]


The legal basis for radiation protection in Austria is the Radiation Protection Act (BGBl. 277/69 as amended) of June 11, 1969. [ 348 ]  The tasks of radiation protection extend to the fields of medicine, commerce and industry, research, schools, worker protection and food. The General Radiation Protection Ordinance, Federal Law Gazette II No. 191/2006, has been in force since June 1, 2006. [ 349 ]  Based on the Radiation Protection Act, it regulates the handling of radiation sources and measures for protection against ionizing radiation. The Optical Radiation Ordinance ( VOPST ) is a detailed ordinance to the Occupational Safety and Health Act ( ASchG ).


On August 1, 2020, a new radiation protection law came into force, which largely harmonized the radiation protection regulations for artificial radioactive substances and terrestrial natural radioactive substances. They are now enshrined in the General Radiation Protection Ordinance 2020. Companies that carry out activities with naturally occurring radioactive substances are now subject to the licensing or notification requirements pursuant to Sections 15 to 17 of the Radiation Protection Act 2020, unless an exemption provision pursuant to Sections 7 or 8 of the General Radiation Protection Ordinance 2020 applies. Cement production including maintenance of  clinker  kilns, production of primary iron and tin, lead and copper smelting are included in the scope. If a company falls within the scope of the General Radiation Protection Ordinance 2020, its owner must commission an officially authorized monitoring body. The mandate includes dose assessment for workers who may be exposed to increased radiation exposure and, if necessary, determination of the activity concentration of residues and radioactive substances discharged with the air or waste water. [ 350 ]


The  Hounsfield scale  ( / ˈ h aʊ n z f iː l d /   HOWNZ -feeld ), named after Sir  Godfrey Hounsfield , is a quantitative scale for describing  radiodensity . It is frequently used in  CT scans , where its value is also termed  CT number .


The Hounsfield unit (HU) scale is a linear transformation of the original  linear attenuation coefficient  measurement into one in which the  radiodensity  of  distilled water  at standard  pressure  and  temperature  ( STP ) is defined as 0 Hounsfield units (HU), while the radiodensity of  air  at STP is defined as −1000 HU. In a  voxel  with average linear attenuation coefficient

 μ

 {\displaystyle \mu }

 , the corresponding HU value is therefore given by: [ 1 ] [ 2 ]


H
 U
 =
 1000
 ×

 μ
 −

 μ

 water

 μ

 water

 −

 μ

 air

 {\displaystyle HU=1000\times {\frac {\mu -\mu _{\textrm {water}}}{\mu _{\textrm {water}}-\mu _{\textrm {air}}}}}


where

 μ

 water

 {\displaystyle \mu _{\textrm {water}}}

  and

 μ

 air

 {\displaystyle \mu _{\textrm {air}}}

  are respectively the linear attenuation coefficients of water and air.


Thus, a change of one Hounsfield unit (HU) represents a change of 0.1% of the attenuation coefficient of water since the attenuation coefficient of air is nearly zero. [ 3 ] : 259


Calibration tests of HU with reference to water and other materials may be done to ensure standardised response. This is particularly important for CT scans used in  radiotherapy   treatment planning , where HU is converted to  electron density . [ 4 ]  Variation in the measured values of reference materials with known composition, and variation between and within slices may be used as part of test procedures. [ 3 ] : 283  [ 5 ]


The above standards were chosen as they are universally available references and suited to the key application for which computed axial tomography was developed: imaging the internal anatomy of living creatures based on organized water structures and mostly living in air,  e.g.   humans . [ citation needed ]


HU-based differentiation of material applies to  medical-grade   dual-energy CT scans  but not to  cone beam computed tomography  (CBCT) scans, as CBCT scans provide  unreliable HU readings . [ 6 ]


Values reported here are  approximations . Different dynamics are reported from one study to another. [ citation needed ]


Exact HU dynamics can vary from one CT acquisition to another due to CT acquisition and reconstruction parameters (kV, filters, reconstruction algorithms, etc.). The use of  contrast agents  modifies HU as well in some body parts (mainly blood).


A practical application of this is in evaluation of tumors, where, for example, an  adrenal tumor  with a radiodensity of less than 10 HU is rather fatty in composition and almost certainly a benign  adrenal adenoma . [ 30 ]


In  medical or research imaging , an  incidental imaging finding  (also called an  incidentaloma ) is an unanticipated finding which is not related to the original diagnostic inquiry. As with other types of  incidental medical findings , they may represent a diagnostic, ethical, and philosophical dilemma because their significance is unclear. While some coincidental findings may lead to beneficial diagnoses, others may lead to  overdiagnosis  that results in unnecessary testing and treatment, sometimes called the "cascade effect". [ 1 ]


Incidental findings are common in imaging. For instance, around 1 in every 3 cardiac  MRIs  result in an incidental finding. [ 2 ]  Incidence is similar for chest  CT scans  (~30%). [ 2 ]


As the use of medical imaging increases, the number of incidental findings also increases. [ citation needed ]


Incidental adrenal masses on imaging are common (0.6 to 1.3% of all abdominal CT). Differential diagnosis include  adenoma ,  myelolipoma , cyst,  lipoma ,  pheochromocytoma ,  adrenal cancer ,  metastatic cancer ,  hyperplasia , and  tuberculosis . [ 3 ]   Some of these lesions are easily identified by radiographic appearance; however, it is often adenoma vs. cancer/metastasis that is most difficult to distinguish.  Thus, clinical guidelines have been developed to aid in diagnosis and decision-making. [ 4 ]  Although adrenal incidentalomas are common, they are not commonly cancerous - less than 1% of all adrenal incidentalomas are malignant. [ 2 ]


The first considerations are size and radiographic appearance of the mass.  Suspicious adrenal masses or those ≥4 cm are recommended for complete removal by adrenalectomy.  Masses <4 cm may also be recommended for removal if they are found to be hormonally active, but are otherwise recommended for observation. [ 5 ]   All adrenal masses should receive hormonal evaluation. Hormonal evaluation includes: [ 6 ]


On CT scan, benign  adenomas  typically are of low  radiodensity  (due to fat content). A radiodensity equal to or below 10  Hounsfield units  (HU) is considered diagnostic of an adenoma. [ 7 ]  An adenoma also shows rapid  radiocontrast washout  (50% or more of the contrast medium washes out at 10 minutes). If the hormonal evaluation is negative and imaging suggests benign lesion, follow up may be considered. Imaging at 6, 12, and 24 months and repeat hormonal evaluation yearly for 4 years is often recommended, [ 6 ]  but there exists controversy about harm/benefit of such screening as there is a high subsequent false-positive rate (about 50:1) and overall low incidence of adrenal carcinoma. [ 8 ]


Autopsy  series have suggested that  pituitary  incidentalomas may be quite common. It has been estimated that perhaps 10% of the adult population may harbor such  endocrinologically  inert lesions. [ 9 ]  Most of these lesions, especially those which are small, will not grow. However, some form of long-term surveillance has been recommended based on the size and presentation of the lesion. [ 10 ]  With pituitary adenomas larger than 1 cm, a baseline pituitary hormonal function test should be done, including measurements of serum levels of  TSH ,  prolactin ,  IGF-1  (as a test of  growth hormone  activity), adrenal function (i.e. 24 hour urine cortisol, dexamethasone suppression test),  testosterone  in men, and  estradiol  in  amenorrheic  women. [ 11 ]


Incidental  thyroid  masses may be found in 9% of patients undergoing bilateral carotid duplex ultrasonography. [ 12 ]


Some experts [ 13 ]  recommend that nodules > 1 cm (unless the  TSH  is suppressed) or those with ultrasonographic features of malignancy should be biopsied by  fine needle aspiration .  Computed tomography   is inferior to  ultrasound  for evaluating thyroid nodules. [ 14 ]  Ultrasonographic markers of malignancy are: [ 15 ]


Incidental  parathyroid  masses may be found in 0.1% of patients undergoing bilateral carotid duplex ultrasonography. [ 12 ]


The American College of Radiology  recommends the following workup for thyroid nodules as incidental imaging findings on  CT ,  MRI  or  PET-CT : [ 16 ]


Studies of whole body screening  computed tomography  find abnormalities in the lungs of 14% of patients. [ 17 ]   Clinical practice guidelines  by the  American College of Chest Physicians  advise on the evaluation of the  solitary pulmonary nodule . [ 18 ]


Most  renal cell carcinomas  are now found incidentally. [ 19 ]  Tumors less than 3 cm in diameter less frequently have aggressive  histology . [ 20 ]


A  CT scan  is the first choice modality for workup of solid masses in the kidneys. Nevertheless, hemorrhagic cysts can resemble renal cell carcinomas on CT, but they are easily distinguished with  Doppler ultrasonography  (Doppler US). In renal cell carcinomas, Doppler US often shows vessels with high velocities caused by neovascularization and arteriovenous shunting. Some renal cell carcinomas are hypovascular and not distinguishable with Doppler US. Therefore, renal tumors without a Doppler signal, which are not obvious simple cysts on US and CT, should be further investigated with  contrast-enhanced ultrasound , as this is more sensitive than both Doppler US and CT for the detection of hypovascular tumors. [ 21 ]


The increasing use of MRI, often during diagnostic work-up for back or lower extremity pain, has led to a significant increase in the number of incidental findings that are most often clinically inconsequential.  The most common include: [ 22 ]


Sometimes normally asymptomatic findings can present with symptoms and these cases when identified cannot then be considered as incidentalomas. [ citation needed ]


The concept of the "incidentaloma" has been criticized, as such lesions do not have much in common other than the history of an incidental identification and the assumption that they are clinically inert. It has been proposed just to say that such lesions have been "incidentally found." [ 23 ]  The underlying pathology shows no unifying histological concept. [ citation needed ]


The  International Day of Radiology  (IDoR) is an  annual event  promoting the role of medical imaging in modern  healthcare . It is celebrated on  November 8  each year and coincides with the anniversary of the discovery of  x-rays . It was first introduced in 2012, as a joint initiative of the  European Society of Radiology  (ESR), the  Radiological Society of North America  (RSNA), and the  American College of Radiology  (ACR). [ 1 ]  The International Day of Radiology is acknowledged and celebrated by nearly 200 national, sub-speciality, and related societies around the world.


The International Day of Radiology is a successor to the  European Day of Radiology  which was launched in 2011. The first and only European Day of Radiology (EDoR) was held on February 10, 2011, to commemorate the anniversary of Röntgen's death and was organised by the  European Society of Radiology (ESR). Due to the success of the EDoR, the ESR entered into cooperation with the RSNA and the ACR to establish the International Day of Radiology. It was also decided that the date of the celebration should be moved from the anniversary of Röntgen's death to that of his discovery of the  x-ray . The day was officially confirmed by the three founding societies during the annual RSNA meeting in Chicago on November 28, 2011.


On November 8, 1895  Wilhelm Conrad Röntgen  discovered  x-rays  by chance while investigating cathode rays, effectively laying the foundation for the medical discipline of  radiology . This discovery would grow to include various methods of imaging and establish itself as a crucial element of modern medicine. November 8 was eventually chosen as the appropriate day to mark the celebrations which are observed by radiological societies the world over. [ 2 ]


In addition to the general recognition of radiology, a theme is chosen every year, focusing on various specialities and sub-specialities of radiology. [ 3 ]  These themes have included:


In and around November 8 of every year, international radiological societies all over the world celebrate the day with their own organised events. These celebrations come in the form of exhibitions, workshops, lectures, and social media campaigns which invite radiologists as well as the general public to participate and learn more about radiology.


In 2018, various radiological and partner societies organised events to draw attention to radiology and that year's theme of cardiac imaging. Examples include:


In further support of the day, the  European Society of Radiology  publishes a book every year on the selected theme. In 2018, the book,  The HEART revealed,  was published as a free pdf download on the IDoR website. The book, authored by professional radiologists, contains descriptions of various cardiac diseases where imaging is helpful in diagnosis, treatment, and follow-up. [ 14 ]


Other published texts addressing the yearly theme include:


In addition to the themed publications, the  European Society of Radiology  in collaboration with the International Society for the History of Radiology published a three-volume series on the history of radiology,  The Story of Radiology .


The  International Subarachnoid Aneurysm Trial  ( ISAT ) was a large  multicenter  prospective  randomized   clinical medical trial  comparing the safety and efficacy of  endovascular coil  treatment and  surgical clipping  for the treatment of  brain aneurysms . The study began in 1994 with the first results being published in  The Lancet  in 2002, and the 10-year data were published again in  The Lancet  in early September 2005. A total of 2,143 study participants were mostly drawn from  U.K.  hospitals with the rest drawn from North American and European hospitals.


The study found superior results with  endovascular  coil treatment compared to surgical clipping. However, subsequent studies have questioned this conclusion. [ 1 ]  The study was criticized by many clinicians and not well accepted by surgeons. [ 2 ]  Primary criticisms were related to the study's generalizability and the long-term prognosis of coil embolization. [ 3 ]


ISAT sought to measure outcomes of  cerebral aneurysm  patients at 2 and 12 months using a type of a  Rankin scale . [ 4 ] : 114   The study was prematurely terminated in 2002 after the oversight committee determined there was increased morbidity with surgical clipping compared to  endovascular  coiling. [ 4 ] : 114


ISAT was criticized on several factors related to the randomization of the patient population. The randomized patient population in the ISAT was younger on average, the majority had aneurysms under 10mm and in anterior circulation compared to the population of  subarachnoid hemorrhage  patients in the U.S. and Japan. [ 1 ] [ 5 ] : 210   In response to these criticisms, a facility that participated in ISAT compared the clinical outcomes of their patients who were not selected for the study to those who were. They reported similar outcomes to the ISAT. [ 1 ]


Although the initial ISAT analysis appeared to favor endovascular coiling over microsurgical clipping, subsequent meta-analysis have questioned that conclusion finding higher incidences [ spelling? ]  of recurrence. [ 6 ]  A large meta-analysis from  Johns Hopkins University  published in  Neurosurgery  concluded that "there is no clear consensus in these two studies or in the 45 observational studies included." [ 6 ]


Updated data from the ISAT group in March 2008 shows that the higher aneurysm rate of recurrence is also associated with a higher re-bleeding rate, given that the re-bleed rate of coiled aneurysms appears to be 8 times higher than that of aneurysms treated with surgical clipping in this study. [ 7 ]  The ISAT authors conclude that "when treating ruptured cerebral aneurysms, the advantage of coil embolization over clip ligation cannot be assumed for patients younger than 40 years old." [ 7 ]  Other studies have directly questioned the ISAT's conclusions. [ 8 ]  This conclusion is based on a number of methodological assumptions itself and other authors have cautioned about extending it to other patient populations. [ 9 ]


It appears that although endovascular coiling is associated with a shorter recovery period as compared to surgical clipping, it is also associated with a significantly higher  recurrence  rate after treatment. The long-term data for unruptured  aneurysms  are still being gathered.


The  John Thomas sign , [ 1 ]  also known as the  Throckmorton sign , [ 2 ]  is a slang or joke term used in the field of  radiology . It refers to the position of a  penis  as it relates to  pathology  on an  X-ray  of a  pelvis . When the penis (visible on the X-ray as a shadow) points towards the same side as a unilateral medical condition such as a broken bone, this is considered a "positive John Thomas sign," and if the shadow points to the other side, it is a "negative John Thomas sign." [ 3 ]


Studies have shown that the "sign" is no better than chance at identifying the location of a  hip fracture . [ 4 ] [ 5 ]  In those cases where the John Thomas sign is positive, it has been proposed that a person with a displaced hip fracture may try to lie on the injured side to immobilize the fracture and reduce pain; the penis then inclines toward the downward (injured) side. [ 6 ]


Andy Murray , British professional  tennis  player, released a picture of his pelvic  X-ray  following his hip resurfacing surgery on January 29, 2019 [ 7 ]  clearly demonstrating an example of a negative John Thomas or Throckmorton sign where his penis pointed away from the site of injury. The release of the X-ray image with visible genitalia was discussed by  Piers Morgan  on  Good Morning Britain , prompting Murray, who was watching at the time, to message the show, stating, "Please can you stop discussing my genitals on national TV, I was heavily medicated at the time of posting." [ 8 ]


Laboratory Unit for Computer Assisted Surgery   is a system used for virtual  surgical planning . Starting with 1998, LUCAS was developed at the  University of Regensburg, Germany , with the support of the  Carl Zeiss Company . The resulting surgical planning is then  reproduced  onto the patient by using a  navigation system . In fact, LUCAS is integrated into the same  platform  together with the  Surgical Segment Navigator (SSN) , the Surgical Tool Navigator (STN), the Surgical Microscope Navigator (SMN) and the 6 DOF  Manipulator (or, in German, "Mehrkoordinatenmanipulator" - MKM), also from the  Carl Zeiss Company .


Data from separate  bidimensional  slices generated by a  CT  or  MRI  scan are uploaded into the LUCAS system. The resulting  dataset  is then processed, in order to eliminate  image noise , and to enhance the anatomical contours and also the general contrast of the images. The next step is to create a virtual  3D  model from the gathered collection of  2D  images. The bone segment that is to be repositioned is marked, on the 3D grid reconstructed model; then, the actual repositioning of that bone segment is done on the virtual model, until the optimal anatomical position is obtained. The criteria for the optimal position of the bone segment are: symmetry with the opposite side, the continuity of the normal bone contours, or the normal volume of an anatomical region (such as the  Orbit . Afterwards, a  textured  final image is rendered. The calculated vectors for the bone segment repositioning, together with the whole virtual model are finally transferred to the  Surgical Segment Navigator .


A  limited radiology technician  perform  x-rays  of patients and deliver the images to requester. They make no  diagnosis  but still work closely with patients, explaining procedures, operating the X-ray and other associated equipment. Technical aspects include positioning patients for X-rays, determining appropriate angle and height of X-ray equipment, and calculating  radiation  dosages needed to create X-rays of the appropriate density, detail, and contrast, enabling the  physician  to make an accurate diagnosis.


The  Lubberts effect  is the non-uniform response of an  imaging system  to  X-rays  that are absorbed at different depths within the input  phosphor .  It indicates an input phosphor depth-dependent response of the imaging system. It is named [ 1 ]  for G. Lubberts, who published a report of it in 1968 while working at  Kodak . [ 2 ]  The Lubberts effect is related to the  Swank effect , which relates the  signal-to-noise ratio  of a scintillator-based imaging system to the amount of random variation in the strength of the emitted photons. [ 3 ]


Magnetic resonance imaging  ( MRI ) is a  medical imaging  technique used in  radiology  to generate  pictures of the  anatomy  and the  physiological  processes inside the body.  MRI scanners  use strong  magnetic fields , magnetic field gradients, and  radio waves  to form images of the organs in the body. MRI does not involve  X-rays  or the use of  ionizing radiation , which distinguishes it from  computed tomography  (CT) and  positron emission tomography  (PET) scans. MRI is a  medical application  of  nuclear magnetic resonance  (NMR) which can also be used for imaging in other  NMR applications , such as  NMR spectroscopy . [ 1 ]


MRI is widely used in hospitals and clinics for  medical diagnosis ,  staging  and follow-up of disease. Compared to CT, MRI provides better  contrast  in images of soft tissues, e.g. in the  brain  or abdomen. However, it may be perceived as less comfortable by patients, due to the usually longer and louder measurements with the subject in a long, confining tube, although "open" MRI designs mostly relieve this. Additionally,  implants  and other non-removable metal in the body can pose a risk and may exclude some patients from undergoing an MRI examination safely.


MRI was originally called NMRI (nuclear magnetic resonance imaging), but "nuclear" was dropped to avoid  negative associations . [ 2 ]  Certain  atomic nuclei  are able to absorb  radio frequency  (RF) energy when placed in an external  magnetic field ; the resultant evolving  spin polarization  can induce an RF signal in a radio frequency coil and thereby be detected. [ 3 ]  In other words, the nuclear magnetic spin of protons in the hydrogen nuclei resonates with the RF incident waves and emit coherent radiation with compact direction, energy (frequency) and phase. This coherent amplified radiation is easily detected by RF antennas close to the subject being examined. It is a process similar to  masers . In clinical and research MRI,  hydrogen atoms  are most often used to generate a macroscopic polarized radiation that is detected by the antennas. [ 3 ]  Hydrogen atoms are  naturally abundant in humans  and other biological organisms, particularly in  water  and  fat . For this reason, most MRI scans essentially map the location of water and fat in the body. Pulses of radio waves excite the  nuclear spin  energy transition, and magnetic field gradients localize the polarization in space. By varying the parameters of the  pulse sequence , different contrasts may be generated between tissues based on the  relaxation  properties of the hydrogen atoms therein.


Since its development in the 1970s and 1980s, MRI has proven to be a versatile imaging technique. While MRI is most prominently used in  diagnostic medicine  and biomedical research, it also may be used to form images of non-living objects, such as  mummies .  Diffusion MRI  and  functional MRI  extend the utility of MRI to capture neuronal tracts and blood flow respectively in the nervous system, in addition to detailed spatial images. The sustained increase in demand for MRI within  health systems  has led to concerns about  cost effectiveness  and  overdiagnosis . [ 4 ] [ 5 ] [ dubious  –  discuss ]


In most medical applications,  hydrogen  nuclei, which consist solely of a  proton , that are in tissues create a signal that is processed to form an image of the body in terms of the density of those nuclei in a specific region. Given that the protons are affected by fields from other atoms to which they are bonded, it is possible to separate responses from hydrogen in specific compounds. To perform a study, the person is positioned within an  MRI scanner  that forms a strong  magnetic field  around the area to be imaged. First, energy from an  oscillating  magnetic field is temporarily applied to the patient at the appropriate  resonance  frequency. Scanning with X and Y gradient coils causes a selected region of the patient to experience the exact magnetic field required for the energy to be absorbed. The atoms are  excited  by a  RF  pulse and the resultant signal is measured by a  receiving coil . The RF signal may be processed to deduce position information by looking at the changes in RF level and phase caused by varying the local magnetic field using  gradient coils . As these coils are rapidly switched during the excitation and response to perform a moving line scan, they create the characteristic repetitive noise of an MRI scan as the windings move slightly due to  magnetostriction . The contrast between different tissues is determined by the rate at which excited atoms return to the  equilibrium state .  Exogenous   contrast agents  may be given to the person to make the image clearer. [ 6 ]


The major components of an MRI scanner are the main  magnet , which polarizes the sample, the  shim coils  for correcting shifts in the  homogeneity  of the main magnetic field, the gradient system which is used to localize the region to be scanned and the RF system, which excites the sample and detects the resulting NMR signal. The whole system is controlled by one or more computers.


MRI requires a magnetic field that is both strong and  uniform  to a few parts per million across the scan volume. The field strength of the magnet is measured in  teslas  – and while the majority of systems operate at 1.5 T, commercial systems are available between 0.2 and 7 T.  3T MRI  systems, also called 3 Tesla MRIs, have stronger magnets than 1.5 systems and are considered better for images of organs and soft tissue. [ 7 ]  Whole-body MRI systems for research applications operate in e.g. 9.4T, [ 8 ] [ 9 ]  10.5T, [ 10 ]  11.7T. [ 11 ]  Even higher field whole-body MRI systems e.g. 14 T and beyond are in conceptual proposal [ 12 ]  or in engineering design. [ 13 ]  Most clinical magnets are  superconducting  magnets, which require  liquid helium  to keep them at low temperatures. Lower field strengths can be achieved with permanent magnets, which are often used in "open" MRI scanners for  claustrophobic  patients. [ 14 ]  Lower field strengths are also used in a  portable MRI  scanner approved by the FDA in 2020. [ 15 ]  Recently, MRI has been demonstrated also at ultra-low fields, i.e., in the microtesla-to-millitesla range, where sufficient signal quality is made possible by prepolarization (on the order of 10–100 mT) and by measuring the  Larmor precession  fields at about 100 microtesla with highly sensitive superconducting quantum interference devices ( SQUIDs ). [ 16 ] [ 17 ] [ 18 ]


Each tissue returns to its equilibrium state after excitation by the independent relaxation processes of T 1  ( spin-lattice ; that is, magnetization in the same direction as the static magnetic field) and T 2  ( spin-spin ; transverse to the static magnetic field).
To create a T 1 -weighted image, magnetization is allowed to recover before measuring the MR signal by changing the  repetition time  (TR). This image weighting is useful for assessing the cerebral cortex, identifying fatty tissue, characterizing focal liver lesions, and in general, obtaining morphological information, as well as for  post-contrast  imaging.

To create a T 2 -weighted image, magnetization is allowed to decay before measuring the MR signal by changing the  echo time  (TE). This image weighting is useful for detecting  edema  and inflammation, revealing  white matter lesions , and assessing zonal anatomy in the  prostate  and  uterus .


The information from MRI scans comes in the form of  image contrasts  based on differences in the rate of relaxation of  nuclear spins  following their perturbation by an oscillating magnetic field (in the form of radiofrequency pulses through the sample). [ 19 ]  The relaxation rates are a measure of the time it takes for a signal to decay back to an equilibrium state from either the longitudinal or transverse plane.


Magnetization  builds up along the z-axis in the presence of a magnetic field, B 0 , such that the  magnetic dipoles  in the sample will, on average, align with the z-axis summing to a total magnetization M z . This magnetization along z is defined as the equilibrium magnetization; magnetization is defined as the sum of all magnetic dipoles in a sample. Following the equilibrium magnetization, a 90° radiofrequency (RF) pulse flips the direction of the magnetization vector in the xy-plane, and is then switched off. The initial magnetic field B 0 , however, is still applied. Thus, the spin magnetization vector will slowly return from the xy-plane back to the equilibrium state. The time it takes for the magnetization vector to return to its equilibrium value, M z , is referred to as the longitudinal relaxation time, T 1 . [ 20 ]  Subsequently, the rate at which this happens is simply the reciprocal of the relaxation time:

 1

 T
 1

 =
 R
 1

 {\displaystyle {\frac {1}{T1}}=R1}

 . Similarly, the time in which it takes for M xy  to return to zero is T 2 , with the rate

 1

 T
 2

 =
 R
 2

 {\displaystyle {\frac {1}{T2}}=R2}

 . [ 21 ]  Magnetization as a function of time is defined by the  Bloch equations .


T 1  and T 2  values are dependent on the chemical environment of the sample; hence their utility in MRI. Soft tissue and muscle tissue relax at different rates, yielding the image contrast in a typical scan.


The standard display of MR images is to represent fluid characteristics in  black-and-white  images, where different tissues turn out as follows:


MRI has a wide range of applications in  medical diagnosis  and around 50,000 scanners are estimated to be in use worldwide. [ 25 ]  MRI affects diagnosis and treatment in many specialties although the effect on improved health outcomes is disputed in certain cases. [ 26 ] [ 27 ]


MRI is the investigation of choice in the preoperative  staging  of  rectal  and  prostate cancer  and has a role in the diagnosis, staging, and follow-up of other tumors, [ 28 ]  as well as for determining areas of tissue for sampling in biobanking. [ 29 ] [ 30 ]


MRI is the investigative tool of choice for neurological cancers over CT, as it offers better visualization of the  posterior cranial fossa , containing the  brainstem  and the  cerebellum . The contrast provided between  grey  and  white matter  makes MRI the best choice for many conditions of the  central nervous system , including  demyelinating diseases ,  dementia ,  cerebrovascular disease ,  infectious diseases ,  Alzheimer's disease  and  epilepsy . [ 31 ] [ 32 ] [ 33 ]  Since many images are taken milliseconds apart, it shows how the brain responds to different stimuli, enabling researchers to study both the functional and structural brain abnormalities in psychological disorders. [ 34 ]  MRI also is used in  guided   stereotactic surgery  and  radiosurgery  for treatment of intracranial tumors, arteriovenous malformations, and other surgically treatable conditions using a device known as the  N-localizer . [ 35 ] [ 36 ] [ 37 ]  New tools that implement  artificial intelligence in healthcare  have demonstrated higher image quality and morphometric analysis in  neuroimaging  with the application of a denoising system. [ 38 ]


The record for the highest spatial resolution of a whole intact brain (postmortem) is 100 microns, from Massachusetts General Hospital. The data was published in NATURE on 30 October 2019. [ 39 ] [ 40 ]


Though MRI is used widely in research on mental disabilities, based on a 2024 systematic literature review and meta analysis commissioned by the Patient-Centered Outcomes Research Institute (PCORI), available research using MRI scans to diagnose ADHD showed great variability. [ 41 ]  The authors conclude that MRI cannot be reliably used to assist in making a clinical diagnosis of ADHD. [ 41 ]


Cardiac MRI is complementary to other imaging techniques, such as  echocardiography ,  cardiac CT , and  nuclear medicine . It can be used to assess the structure and the function of the heart. [ 42 ]  Its applications include assessment of  myocardial ischemia and viability ,  cardiomyopathies ,  myocarditis ,  iron overload , vascular diseases, and  congenital heart disease . [ 43 ]


Applications in the musculoskeletal system include  spinal imaging , assessment of  joint  disease, and  soft tissue tumors . [ 44 ]  Also, MRI techniques can be used for diagnostic imaging of
 systemic muscle diseases  including genetic muscle diseases. [ 45 ] [ 46 ]


Swallowing movement of throat and oesophagus can cause motion artifact over the imaged spine. Therefore, a saturation pulse [ clarification needed ]  applied over this region the throat and oesophagus can help to avoid this artifact. Motion artifact arising due to pumping of the heart can be reduced by timing the MRI pulse according to heart cycles. [ 47 ]  Blood vessels flow artifacts can be reduced by applying saturation pulses above and below the region of interest. [ 48 ]


Hepatobiliary  MR is used to detect and characterize lesions of the  liver ,  pancreas , and  bile ducts . Focal or diffuse disorders of the liver may be evaluated using  diffusion-weighted , opposed-phase imaging and  dynamic contrast enhancement  sequences. Extracellular contrast agents are used widely in liver MRI, and newer hepatobiliary contrast agents also provide the opportunity to perform functional biliary imaging. Anatomical imaging of the bile ducts is achieved by using a heavily T2-weighted sequence in magnetic resonance cholangiopancreatography (MRCP). Functional imaging of the pancreas is performed following administration of  secretin . MR enterography provides non-invasive assessment of inflammatory bowel disease and small bowel tumors. MR-colonography may play a role in the detection of large polyps in patients at increased risk of colorectal cancer. [ 49 ] [ 50 ] [ 51 ] [ 52 ]


Magnetic resonance  angiography  (MRA) generates pictures of the arteries to evaluate them for  stenosis  (abnormal narrowing) or  aneurysms  (vessel wall dilatations, at risk of rupture). MRA is often used to evaluate the arteries of the neck and brain, the thoracic and abdominal aorta, the renal arteries, and the legs (called a "run-off"). A variety of techniques can be used to generate the pictures, such as administration of a  paramagnetic  contrast agent ( gadolinium ) or using a technique known as "flow-related enhancement" (e.g., 2D and 3D time-of-flight sequences), where most of the signal on an image is due to blood that recently moved into that plane (see also  FLASH MRI ). [ 53 ]


Techniques involving phase accumulation (known as phase contrast angiography) can also be used to generate flow velocity maps easily and accurately. Magnetic resonance venography (MRV) is a similar procedure that is used to image veins. In this method, the tissue is now excited inferiorly, while the signal is gathered in the plane immediately superior to the excitation plane—thus imaging the venous blood that recently moved from the excited plane. [ 54 ]


MRI for imaging anatomical structures or blood flow do not require contrast agents since the varying properties of the tissues or blood provide natural contrasts. However, for more specific types of imaging,  exogenous  contrast agents may be given  intravenously ,  orally , or  intra-articularly . [ 6 ]  Most contrast agents are either paramagnetic (e.g.: gadolinium, manganese, europium), and are used to shorten T1 in the tissue they accumulate in, or super-paramagnetic (SPIONs), and are used to shorten T2 and T2* in healthy tissue reducing its signal intensity (negative contrast agents). The most commonly used intravenous contrast agents are based on  chelates  of  gadolinium , which is highly paramagnetic. [ 55 ]  In general, these agents have proved safer than the iodinated contrast agents used in X-ray radiography or CT.  Anaphylactoid reactions  are rare, occurring in approx. 0.03–0.1%. [ 56 ]  Of particular interest is the lower incidence of nephrotoxicity, compared with iodinated agents, when given at usual doses—this has made contrast-enhanced MRI scanning an option for patients with renal impairment, who would otherwise not be able to undergo  contrast-enhanced CT . [ 57 ]


Gadolinium-based contrast reagents are typically  octadentate  complexes of  gadolinium(III) . The complex is  very stable  (log K > 20) so that, in use, the concentration of the un-complexed Gd 3+  ions should be below the toxicity limit. The 9th place in the metal ion's  coordination sphere  is occupied by a water molecule which exchanges rapidly with water molecules in the reagent molecule's immediate environment, affecting the magnetic resonance  relaxation time . [ 58 ]


In December 2017, the  Food and Drug Administration  (FDA) in the  United States  announced in a drug safety communication that new warnings were to be included on all gadolinium-based contrast agents (GBCAs). The FDA also called for increased patient education and requiring gadolinium contrast vendors to conduct additional animal and clinical studies to assess the safety of these agents. [ 59 ]
Although gadolinium agents have proved useful for patients with kidney impairment, in patients with severe  kidney failure  requiring dialysis there is a risk of a rare but serious illness,  nephrogenic systemic fibrosis , which may be linked to the use of certain gadolinium-containing agents. The most frequently linked is  gadodiamide , but other agents have been linked too. [ 60 ]  Although a causal link has not been definitively established, current guidelines in the  United States  are that dialysis patients should only receive gadolinium agents where essential and that  dialysis  should be performed as soon as possible after the scan to remove the agent from the body promptly. [ 61 ] [ 62 ]


In Europe, where more gadolinium-containing agents are available, a classification of agents according to potential risks has been released. [ 63 ] [ 64 ]  In 2008, a new contrast agent named  gadoxetate , brand name Eovist (US) or Primovist (EU), was approved for diagnostic use: This has the theoretical benefit of a dual excretion path. [ 65 ]


An  MRI sequence  is a particular setting of radiofrequency pulses and gradients, resulting in a particular image appearance. [ 66 ]  The  T1 and T2  weighting can also be described as MRI sequences.


edit This table does not include  uncommon and experimental sequences .


Standard foundation and comparison for other sequences


Magnetic resonance spectroscopy  (MRS) is used to measure the levels of different  metabolites  in body tissues, which can be achieved through a variety of single voxel or imaging-based techniques. [ 96 ]  The MR signal produces a spectrum of resonances that corresponds to different molecular arrangements of the isotope being "excited". This signature is used to diagnose certain metabolic disorders, especially those affecting the brain, [ 97 ]  and to provide information on tumor  metabolism . [ 98 ]


Magnetic resonance spectroscopic imaging (MRSI) combines both spectroscopic and imaging methods to produce spatially localized spectra from within the sample or patient. The spatial resolution is much lower (limited by the available  SNR ), but the spectra in each voxel contains information about many metabolites. Because the available signal is used to encode spatial and spectral information, MRSI requires high SNR achievable only at higher field strengths (3 T and above). [ 99 ]  The high procurement and maintenance costs of MRI with extremely high field strengths [ 100 ]  inhibit their popularity. However, recent  compressed sensing -based software algorithms ( e.g. ,  SAMV [ 101 ] ) have been proposed to achieve  super-resolution  without requiring such high field strengths.


Real-time magnetic resonance imaging  (RT-MRI) refers to the continuous monitoring of moving objects in real time. Traditionally, real-time MRI was possible only with low image quality or low temporal resolution. An  iterative reconstruction  algorithm removed limitations. Radial  FLASH MRI  (real-time) yields a temporal resolution of 20 to 30 milliseconds for images with an in-plane resolution of 1.5 to 2.0 mm. [ 103 ]  Real-time MRI adds information about diseases of the  joints  and the  heart . In many cases MRI examinations become easier and more comfortable for patients, especially for the patients who cannot calm their breathing [ 104 ]  or who have  arrhythmia .


The lack of harmful effects on the patient and the operator make MRI well-suited for  interventional radiology , where the images produced by an MRI scanner guide minimally invasive procedures. Such procedures use no  ferromagnetic  instruments. [ 105 ]


A specialized growing subset of  interventional MRI  is  intraoperative MRI , in which an MRI is used in surgery. Some specialized MRI systems allow imaging concurrent with the surgical procedure. More typically, the surgical procedure is temporarily interrupted so that MRI can assess the success of the procedure or guide subsequent surgical work. [ 106 ]


In guided therapy,  high-intensity focused ultrasound  (HIFU) beams are focused on a tissue, that are controlled using MR thermal imaging. Due to the high energy at the focus, the temperature rises to above 65  °C  (150 °F) which completely destroys the tissue. This technology can achieve precise  ablation  of diseased tissue. MR imaging provides a three-dimensional view of the target tissue, allowing for the precise focusing of ultrasound energy. The MR imaging provides quantitative, real-time, thermal images of the treated area. This allows the physician to ensure that the temperature generated during each cycle of ultrasound energy is sufficient to cause thermal ablation within the desired tissue and if not, to adapt the parameters to ensure effective treatment. [ 107 ]


Hydrogen has the most frequently imaged  nucleus  in MRI because it is present in biological tissues in great abundance, and because its high  gyromagnetic ratio  gives a strong signal. However, any nucleus with a net  nuclear spin  could potentially be imaged with MRI. Such nuclei include  helium-3 ,  lithium-7 ,  carbon-13 ,  fluorine -19,  oxygen-17 ,  sodium -23,  phosphorus -31 and  xenon-129 .  23 Na and  31 P are naturally abundant in the body, so they can be imaged directly. Gaseous isotopes such as  3 He or  129 Xe must be  hyperpolarized  and then inhaled as their nuclear density is too low to yield a useful signal under normal conditions.  17 O  and  19 F can be administered in sufficient quantities in liquid form (e.g.  17 O -water) that hyperpolarization is not a necessity. [ 108 ]  Using helium or xenon has the advantage of reduced background noise, and therefore increased contrast for the image itself, because these elements are not normally present in biological tissues. [ 109 ]


Moreover, the nucleus of any atom that has a net nuclear spin and that is bonded to a hydrogen atom could potentially be imaged via heteronuclear magnetization transfer MRI that would image the high-gyromagnetic-ratio hydrogen nucleus instead of the low-gyromagnetic-ratio nucleus that is bonded to the hydrogen atom. [ 110 ]  In principle, heteronuclear magnetization transfer MRI could be used to detect the presence or absence of specific chemical bonds. [ 111 ] [ 112 ]


Multinuclear imaging is primarily a research technique at present. However, potential applications include functional imaging and imaging of organs poorly seen on  1 H MRI (e.g., lungs and bones) or as alternative contrast agents. Inhaled hyperpolarized  3 He can be used to image the distribution of air spaces within the lungs. Injectable solutions containing  13 C or stabilized bubbles of hyperpolarized  129 Xe have been studied as contrast agents for angiography and perfusion imaging.  31 P can potentially provide information on bone density and structure, as well as functional imaging of the brain. Multinuclear imaging holds the potential to chart the distribution of lithium in the human brain, this element finding use as an important drug for those with conditions such as bipolar disorder. [ 113 ]


MRI has the advantages of having very high spatial resolution and is very adept at morphological imaging and functional imaging. MRI does have several disadvantages though. First, MRI has a sensitivity of around 10 −3   mol/L   to 10 −5  mol/L, which, compared to other types of imaging, can be very limiting. This problem stems from the fact that the population difference between the nuclear spin states is very small at room temperature. For example, at 1.5  teslas , a typical field strength for clinical MRI, the difference between high and low energy states is approximately 9 molecules per 2 million. Improvements to increase MR sensitivity include increasing magnetic field strength and  hyperpolarization  via optical pumping or dynamic nuclear polarization. There are also a variety of signal amplification schemes based on chemical exchange that increase sensitivity. [ 114 ]


To achieve molecular imaging of disease biomarkers using MRI, targeted MRI  contrast agents  with high specificity and high relaxivity (sensitivity) are required. To date, many studies have been devoted to developing targeted-MRI contrast agents to achieve molecular imaging by MRI. Commonly, peptides, antibodies, or small ligands, and small protein domains, such as HER-2 affibodies, have been applied to achieve targeting. To enhance the sensitivity of the contrast agents, these targeting moieties are usually linked to high payload MRI contrast agents or MRI contrast agents with high relaxivities. [ 115 ]  A new class of gene targeting MR contrast agents has been introduced to show gene action of unique mRNA and gene transcription factor proteins. [ 116 ] [ 117 ]  These new contrast agents can trace cells with unique mRNA, microRNA and virus; tissue response to inflammation in living brains. [ 118 ]  The MR reports change in gene expression with positive correlation to TaqMan analysis, optical and electron microscopy. [ 119 ]


It takes time to gather MRI data using sequential applications of magnetic field gradients. Even for the most streamlined of  MRI sequences , there are physical and physiologic limits to the rate of gradient switching. Parallel MRI circumvents these limits by gathering some portion of the data simultaneously, rather than in a traditional sequential fashion. This is accomplished using arrays of radiofrequency (RF) detector coils, each with a different 'view' of the body. A reduced set of gradient steps is applied, and the remaining spatial information is filled in by combining signals from various coils, based on their known spatial sensitivity patterns. The resulting acceleration is limited by the number of coils and by the signal to noise ratio (which decreases with increasing acceleration), but two- to four-fold accelerations may commonly be achieved with suitable coil array configurations, and substantially higher accelerations have been demonstrated with specialized coil arrays. Parallel MRI may be used with most  MRI sequences .


After a number of early suggestions for using arrays of detectors to accelerate imaging went largely unremarked in the MRI field, parallel imaging saw widespread development and application following the introduction of the SiMultaneous Acquisition of Spatial Harmonics (SMASH) technique in 1996–7. [ 120 ]  The SENSitivity Encoding (SENSE) [ 121 ]  and Generalized Autocalibrating Partially Parallel Acquisitions (GRAPPA) [ 122 ]  techniques are the parallel imaging methods in most common use today. The advent of parallel MRI resulted in extensive research and development in image reconstruction and RF coil design, as well as in a rapid expansion of the number of receiver channels available on commercial MR systems. Parallel MRI is now used routinely for MRI examinations in a wide range of body areas and clinical or research applications.


Most MRI focuses on qualitative interpretation of MR data by acquiring spatial maps of relative variations in signal strength which are "weighted" by certain parameters. [ 123 ]  Quantitative methods instead attempt to determine spatial maps of accurate tissue relaxometry parameter values or magnetic field, or to measure the size of certain spatial features.


Examples of quantitative MRI methods are:


Quantitative MRI aims to increase the  reproducibility  of MR images and interpretations, but has historically require longer scan times. [ 123 ]


Quantitative MRI (or qMRI) sometimes more specifically refers to multi-parametric quantitative MRI, the mapping of multiple tissue relaxometry parameters in a single imaging session. [ 128 ]
Efforts to make multi-parametric quantitative MRI faster have produced sequences which map multiple parameters simultaneously, either by building separate encoding methods for each parameter into the sequence, [ 129 ]
or by fitting MR signal evolution to a multi-parameter model. [ 130 ] [ 131 ]


Traditional MRI generates poor images of lung tissue because there are fewer water molecules with protons that can be excited by the magnetic field. Using hyperpolarized gas an MRI scan can identify ventilation defects in the lungs. Before the scan, a patient is asked to inhale hyperpolarized  xenon  mixed with a buffer gas of helium or nitrogen. The resulting lung images are much higher quality than with traditional MRI.


MRI is, in general, a safe technique, although injuries may occur as a result of failed safety procedures or human error. [ 132 ]   Contraindications  to MRI include most  cochlear implants  and  cardiac pacemakers ,  shrapnel , and metallic  foreign bodies  in the  eyes .  Magnetic resonance imaging in pregnancy  appears to be safe, at least during the second and third  trimesters  if done without contrast agents. [ 133 ]  Since MRI does not use any ionizing radiation, its use is generally favored in preference to  CT  when either modality could yield the same information. [ 134 ]  Some patients experience claustrophobia and may require sedation or shorter MRI protocols. [ 135 ] [ 136 ]  Amplitude and rapid switching of gradient coils during image acquisition may cause peripheral nerve stimulation. [ 137 ]


MRI uses powerful magnets and can therefore cause  magnetic materials  to move at great speeds, posing a projectile risk, and may cause fatal accidents. [ 138 ]  However, as millions of MRIs are performed globally each year, [ 139 ]  fatalities are extremely rare. [ 140 ]


MRI machines can produce loud noise, up to 120  dB(A) . [ 141 ]  This can cause  hearing loss ,  tinnitus  and  hyperacusis , so appropriate  hearing protection  is essential for anyone inside the MRI scanner room during the examination.


Medical societies issue guidelines for when physicians should use MRI on patients and recommend against overuse. MRI can detect health problems or confirm a diagnosis, but medical societies often recommend that MRI not be the first procedure for creating a plan to diagnose or manage a patient's complaint. A common case is to use MRI to seek a cause of  low back pain ; the  American College of Physicians , for example, recommends against imaging (including MRI) as unlikely to result in a positive outcome for the patient. [ 26 ] [ 27 ]


An  MRI artifact  is a  visual artifact , that is, an anomaly during visual representation. Many different artifacts can occur during magnetic resonance imaging (MRI), some affecting the diagnostic quality, while others may be confused with pathology. Artifacts can be classified as patient-related, signal processing-dependent and hardware (machine)-related. [ 142 ]


MRI is used industrially mainly for routine analysis of chemicals. The  nuclear magnetic resonance  technique is also used, for example, to measure the ratio between water and fat in foods, monitoring of flow of corrosive fluids in pipes, or to study molecular structures such as catalysts. [ 1 ]


Being non-invasive and non-damaging, MRI can be used to study the anatomy of plants, their water transportation processes and water balance. [ 143 ]  It is also applied to veterinary radiology for diagnostic purposes. Outside this, its use in zoology is limited due to the high cost; but it can be used on many species. [ 144 ]


In palaeontology it is used to examine the structure of fossils. [ 145 ]