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Thursday, May 16, 2019

Health physics

From Wikipedia, the free encyclopedia

Health Physics for your protection.
Health physics is the applied physics of radiation protection for health and health care purposes. It is the science concerned with the recognition, evaluation, and control of health hazards to permit the safe use and application of ionizing radiation. Health physics professionals promote excellence in the science and practice of radiation protection and safety. Health physicists principally work at facilities where radionuclides or other sources of ionizing radiation (such as X-ray generators) are used or produced; these include hospitals, government laboratories, academic and research institutions, nuclear power plants, regulatory agencies, and manufacturing plants.

Scope

There are many sub-specialties in the field of health physics, including

Operational health physics

The subfield of operational health physics, also called applied health physics in older sources, focuses on field work and the practical application of health physics knowledge to real-world situations, rather than basic research.

Medical physics

The field of Health Physics is related to the field of medical physics and they are similar to each other in that practitioners rely on much of the same fundamental science (i.e., radiation physics, biology, etc.) in both fields. Health physicists, however, focus on the evaluation and protection of human health from radiation, whereas medical health physicists and medical physicists support the use of radiation and other physics-based technologies by medical practitioners for the diagnosis and treatment of disease.

Radiation protection instruments

Practical ionising radiation measurement is essential for health physics. It enables the evaluation of protection measures, and the assessment of the radiation dose likely, or actually received by individuals. The provision of such instruments is normally controlled by law. In the UK it is the Ionising Radiation Regulations 1999. 

The measuring instruments for radiation protection are both "installed" (in a fixed position) and portable (hand-held or transportable).

Installed instruments

Installed instruments are fixed in positions which are known to be important in assessing the general radiation hazard in an area. Examples are installed "area" radiation monitors, Gamma interlock monitors, personnel exit monitors, and airborne contamination monitors.

The area monitor will measure the ambient radiation, usually X-Ray, Gamma or neutrons; these are radiations which can have significant radiation levels over a range in excess of tens of metres from their source, and thereby cover a wide area.

Interlock monitors are used in applications to prevent inadvertent exposure of workers to an excess dose by preventing personnel access to an area when a high radiation level is present.

Airborne contamination monitors measure the concentration of radioactive particles in the atmosphere to guard against radioactive particles being deposited in the lungs of personnel.

Personnel exit monitors are used to monitor workers who are exiting a "contamination controlled" or potentially contaminated area. These can be in the form of hand monitors, clothing frisk probes, or whole body monitors. These monitor the surface of the workers body and clothing to check if any radioactive contamination has been deposited. These generally measure alpha or beta or gamma, or combinations of these.

The UK National Physical Laboratory has published a good practice guide through its Ionising Radiation Metrology Forum concerning the provision of such equipment and the methodology of calculating the alarm levels to be used.

Portable instruments

Portable instruments are hand-held or transportable. The hand-held instrument is generally used as a survey meter to check an object or person in detail, or assess an area where no installed instrumentation exists. They can also be used for personnel exit monitoring or personnel contamination checks in the field. These generally measure alpha, beta or gamma, or combinations of these. 

Transportable instruments are generally instruments that would have been permanently installed, but are temporarily placed in an area to provide continuous monitoring where it is likely there will be a hazard. Such instruments are often installed on trolleys to allow easy deployment, and are associated with temporary operational situations.

Instrument types

A number of commonly used detection instruments are listed below.
The links should be followed for a fuller description of each.

Guidance on use

In the United Kingdom the HSE has issued a user guidance note on selecting the correct radiation measurement instrument for the application concerned. This covers all ionising radiation instrument technologies, and is a useful comparative guide.

Radiation dosimeters

Dosimeters are devices worn by the user which measure the radiation dose that the user is receiving. Common types of wearable dosimeters for ionizing radiation include:

Units of measure

External dose quantities used in radiation protection and dosimetry
 
Graphic showing relationship of SI radiation dose units

Absorbed dose

The fundamental units do not take into account the amount of damage done to matter (especially living tissue) by ionizing radiation. This is more closely related to the amount of energy deposited rather than the charge. This is called the absorbed dose.
  • The gray (Gy), with units J/kg, is the SI unit of absorbed dose, which represents the amount of radiation required to deposit 1 joule of energy in 1 kilogram of any kind of matter.
  • The rad (radiation absorbed dose), is the corresponding traditional unit, which is 0.01 J deposited per kg. 100 rad = 1 Gy.

Equivalent dose

Equal doses of different types or energies of radiation cause different amounts of damage to living tissue. For example, 1 Gy of alpha radiation causes about 20 times as much damage as 1 Gy of X-rays. Therefore, the equivalent dose was defined to give an approximate measure of the biological effect of radiation. It is calculated by multiplying the absorbed dose by a weighting factor WR, which is different for each type of radiation. This weighting factor is also called the Q (quality factor), or RBE (relative biological effectiveness of the radiation).
  • The sievert (Sv) is the SI unit of equivalent dose. Although it has the same units as the gray, J/kg, it measures something different. For a given type and dose of radiation(s) applied to a certain body part(s) of a certain organism, it measures the magnitude of an X-rays or gamma radiation dose applied to the whole body of the organism, such that the probabilities of the two scenarios to induce cancer is the same according to current statistics.
  • The rem (Roentgen equivalent man) is the traditional unit of equivalent dose. 1 sievert = 100 rem. Because the rem is a relatively large unit, typical equivalent dose is measured in millirem (mrem), 10−3 rem, or in microsievert (μSv), 10−6 Sv. 1 mrem = 10 μSv.
  • A unit sometimes used for low-level doses of radiation is the BRET (Background Radiation Equivalent Time). This is the number of days of an average person's background radiation exposure the dose is equivalent to. This unit is not standardized, and depends on the value used for the average background radiation dose. Using the 2000 UNSCEAR value (below), one BRET unit is equal to about 6.6 μSv.
For comparison, the average 'background' dose of natural radiation received by a person per day, based on 2000 UNSCEAR estimate, makes BRET 6.6 μSv (660 μrem). However local exposures vary, with the yearly average in the US being around 3.6 mSv (360 mrem), and in a small area in India as high as 30 mSv (3 rem). The lethal full-body dose of radiation for a human is around 4–5 Sv (400–500 rem).

History

In 1898, The Röntgen Society (Currently the British Institute of Radiology) established a committee on X-ray injuries, thus initiating the discipline of radiation protection.

The term "health physics"

According to Paul Frame:
"The term Health Physics is believed to have originated in the Metallurgical Laboratory at the University of Chicago in 1942, but the exact origin is unknown. The term was possibly coined by Robert Stone or Arthur Compton, since Stone was the head of the Health Division and Arthur Compton was the head of the Metallurgical Laboratory. The first task of the Health Physics Section was to design shielding for reactor CP-1 that Enrico Fermi was constructing, so the original HPs were mostly physicists trying to solve health-related problems. The explanation given by Robert Stone was that '...the term Health Physics has been used on the Plutonium Project to define that field in which physical methods are used to determine the existence of hazards to the health of personnel.'


A variation was given by Raymond Finkle, a Health Division employee during this time frame. 'The coinage at first merely denoted the physics section of the Health Division... the name also served security: 'radiation protection' might arouse unwelcome interest; 'health physics' conveyed nothing.'"

Radiation-related quantities

The following table shows radiation quantities in SI and non-SI units.

Ionising radiation related quantities
Quantity Unit Symbol Derivation Year SI equivalence
Activity (A) curie Ci 3.7 × 1010 s−1 1953 3.7×1010 Bq
becquerel Bq s−1 1974 SI unit
rutherford Rd 106 s−1 1946 1,000,000 Bq
Exposure (X) röntgen R esu / 0.001293 g of air 1928 2.58 × 10−4 C/kg
Absorbed dose (D) erg
erg⋅g−1 1950 1.0 × 10−4 Gy
rad rad 100 erg⋅g−1 1953 0.010 Gy
gray Gy J⋅kg−1 1974 SI unit
Dose equivalent (H) röntgen equivalent man rem 100 erg⋅g−1 1971 0.010 Sv
sievert Sv J⋅kg−1 × WR 1977 SI unit

Although the United States Nuclear Regulatory Commission permits the use of the units curie, rad, and rem alongside SI units, the European Union European units of measurement directives required that their use for "public health ... purposes" be phased out by 31 December 1985.

Technetium-99m

From Wikipedia, the free encyclopedia

Technetium-99m,  99mTc
First technetium-99m generator - 1958.jpg
The first technetium-99m generator, 1958.
A Tc-99m pertechnetate solution is
being eluted from Mo-99 molybdate
bound to a chromatographic substrate
General
Name, symbolTechnetium-99m,99mTc
Neutrons56
Protons43
Nuclide data
Half-life6.0067 hours
Parent isotopes99Mo (65.976 h)
Decay products99Tc
Isotope mass98.9063 u
Spin1/2-
Excess energy-87327.195 keV
Binding energy8613.603 keV
Decay modes
Decay modeDecay energy (MeV)
Isomeric transition
γ emission 87.87%
98.6%: 0.1405 MeV
1.4%: 0.1426

A technetium injection contained in a shielded syringe
 
Technetium-99m is a metastable nuclear isomer of technetium-99 (itself an isotope of technetium), symbolized as 99mTc, that is used in tens of millions of medical diagnostic procedures annually, making it the most commonly used medical radioisotope.

Technetium-99m is used as a radioactive tracer and can be detected in the body by medical equipment (gamma cameras). It is well suited to the role, because it emits readily detectable gamma rays with a photon energy of 140 keV (these 8.8 pm photons are about the same wavelength as emitted by conventional X-ray diagnostic equipment) and its half-life for gamma emission is 6.0058 hours (meaning 93.7% of it decays to 99Tc in 24 hours). The relatively "short" physical half-life of the isotope and its biological half-life of 1 day (in terms of human activity and metabolism) allows for scanning procedures which collect data rapidly but keep total patient radiation exposure low. The same characteristics make the isotope suitable only for diagnostic but never therapeutic use.

Technetium-99m was discovered as a product of cyclotron bombardment of molybdenum. This procedure produced molybdenum-99, a radionuclide with a longer half-life (2.75 days), which decays to Tc-99m. At present, molybdenum-99 (Mo-99) is used commercially as the easily transportable source of medically used Tc-99m. In turn, this Mo-99 is usually created commercially by fission of highly enriched uranium in aging research and material testing nuclear reactors in several countries.

History

Discovery

In 1938, Emilio Segrè and Glenn T. Seaborg isolated for the first time the metastable isotope technetium-99m, after bombarding natural molybdenum with 8 MeV deuterons in the 37-inch (940 mm) cyclotron of Ernest Orlando Lawrence's Radiation laboratory. In 1970 Seaborg explained that
we discovered an isotope of great scientific interest, because it decayed by means of an isomeric transition with emission of a line spectrum of electrons coming from an almost completely internally converted gamma ray transition. [actually, only 12% of the decays are by internal conversion] (...) This was a form of radioactive decay which had never been observed before this time. Segrè and I were able to show that this radioactive isotope of the element with the atomic number 43 decayed with a half-life of 6.6 h [later updated to 6.0 h] and that it was the daughter of a 67-h [later updated to 66 h] molybdenum parent radioactivity. This chain of decay was later shown to have the mass number 99, and (...) the 6.6-h activity acquired the designation ‘technetium-99m.
Later in 1940, Emilio Segrè and Chien-Shiung Wu published the experimental results of the analysis of fission products of uranium-235, among which was present molybdenum-99, and detected the 6-h activity of element 43, later labelled as technetium-99m.

Early medical applications in the United States

Tc-99m remained a scientific curiosity until the 1950s when Powell Richards realized the potential of technetium-99m as a medical radiotracer and promoted its use among the medical community. While Richards was in charge of the radioisotope production at the Hot Lab Division of the Brookhaven National Laboratory, Walter Tucker and Margaret Greene were working on how to improve the separation process purity of the short-lived eluted daughter product iodine-132 from tellurium-132, its 3.2-days parent, produced in the Brookhaven Graphite Research Reactor. They detected a trace contaminant which proved to be Tc-99m, which was coming from Mo-99 and was following tellurium in the chemistry of the separation process for other fission products. Based on the similarities between the chemistry of the tellurium-iodine parent-daughter pair, Tucker and Greene developed the first technetium-99m generator in 1958. It was not until 1960 that Richards became the first to suggest the idea of using technetium as a medical tracer.

The first US publication to report on medical scanning of Tc-99m appeared in August 1963. Sorensen and Archambault demonstrated that intravenously injected carrier-free Mo-99 selectively and efficiently concentrated in the liver, becoming an internal generator of Tc-99m. After build-up of Tc-99m, they could visualize the liver using the 140 keV gamma ray emission.

Worldwide expansion

The production and medical use of Tc-99m rapidly expanded across the world in the 1960s, benefiting from the development and continuous improvements of the gamma cameras.
Americas
Between 1963 and 1966, numerous scientific studies demonstrated the use of Tc-99m as radiotracer or diagnostic tool. As a consequence the demand for Tc-99m grew exponentially and by 1966, Brookhaven National Laboratory was unable to cope with the demand. Production and distribution of Tc-99m generators were transferred to private companies. "TechneKow-CS generator", the first commercial Tc-99m generator, was produced by Nuclear Consultants, Inc. (St. Louis, Missouri) and Union Carbide Nuclear Corporation (Tuxedo, New York). From 1967 to 1984, Mo-99 was produced for Mallinckrodt Nuclear Company at the Missouri University Research Reactor (MURR).

Union Carbide actively developed a process to produce and separate useful isotopes like Mo-99 from mixed fission products that resulted from the irradiation of highly enriched uranium (HEU) targets in nuclear reactors developed from 1968 to 1972 at the Cintichem facility (formerly the Union Carbide Research Center built in the Sterling forest in Tuxedo, New York (41°14′6.88″N 74°12′50.78″W)). The Cintichem process originally used 93% highly enriched U-235 deposited as UO2 on the inside of a cylindrical target.

At the end of the 1970s, 200,000 Ci (7.4×1015 Bq) of total fission product radiation were extracted weekly from 20-30 reactor bombarded HEU capsules, using the so-called "Cintichem [chemical isolation] process." The research facility with its 1961 5-MW pool-type research reactor was later sold to Hoffman-LaRoche and became Cintichem Inc. In 1980, Cintichem, Inc. began the production/isolation of Mo-99 in its reactor, and became the single U.S. producer of Mo-99 during the 1980s. However, in 1989, Cintichem detected an underground leak of radioactive products that led to the reactor shutdown and decommissioning, putting an end to the commercial production of Mo-99 in the USA.

The production of Mo-99 started in Canada in the early 1970s and was shifted to the NRU reactor in the mid 1970s. By 1978 the reactor provided technetium-99m in large enough quantities that were processed by AECL's radiochemical division, which was privatized in 1988 as Nordion, now MDS Nordion. In the 1990s a substitution for the aging NRU reactor for production of radioisotopes was planned. The Multipurpose Applied Physics Lattice Experiment (MAPLE) was designed as a dedicated isotope-production facility. Initially, two identical MAPLE reactors were to be built at Chalk River Laboratories, each capable of supplying 100% of the world's medical isotope demand. However, problems with the MAPLE 1 reactor, most notably a positive power co-efficient of reactivity, led to the cancellation of the project in 2008.

The first commercial Tc-99m generators were produced in Argentina in 1967, with Mo-99 produced in the CNEA's RA-1 Enrico Fermi reactor. Besides its domestic market CNEA supplies Mo-99 to some South American countries.
Asia
In 1967, the first Tc-99m procedures were carried out in Auckland, New Zealand. Mo-99 was initially supplied by Amersham, UK, then by the Australian Nuclear Science and Technology Organisation (ANSTO) in Lucas Heights, Australia.
Europe
In May 1963, Scheer and Maier-Borst were the first to introduce the use of Tc-99m for medical applications. In 1968, Philips-Duphar (later Mallinckrodt, today Covidien) marketed the first technetium-99m generator produced in Europe and distributed from Petten, the Netherlands.

Shortage

Global shortages of technetium-99m emerged in the late 2000s because two aging nuclear reactors (NRU and HFR) that provided about two-thirds of the world’s supply of molybdenum-99, which itself has a half-life of only 66 hours, were shut down repeatedly for extended maintenance periods. In May 2009 the Atomic Energy of Canada Limited announced the detection of a small leak of heavy water in the NRU reactor that remained out of service until completion of the repairs in August 2010. After the observation of gas bubble jets released from one of the deformations of primary cooling water circuits in August 2008, the HFR reactor was stopped for a thorough safety investigation. NRG received in February 2009 a temporary license to operate HFR only when necessary for medical radioisotope production. HFR stopped for repairs at the beginning of 2010 and was restarted in September 2010.

Two replacement Canadian reactors constructed in the 1990s were closed before beginning operation, for safety reasons. A construction permit for a new production facility to be built in Columbia, MO was issued in May 2018.

Nuclear properties

Technetium-99m is a metastable nuclear isomer, as indicated by the "m" after its mass number 99. This means it is a decay product whose nucleus remains in an excited state that lasts much longer than is typical. The nucleus will eventually relax (i.e., de-excite) to its ground state through the emission of gamma rays or internal conversion electrons. Both of these decay modes rearrange the nucleons without transmuting the technetium into another element. 

Tc-99m decays mainly by gamma emission, slightly less than 88% of the time. (99mTc → 99Tc + γ) About 98.6% of these gamma decays result in 140.5 keV gamma rays and the remaining 1.4% are to gammas of a slightly higher energy at 142.6 keV. These are the radiations that are picked up by a gamma camera when 99mTc is used as a radioactive tracer for medical imaging. The remaining approximately 12% of 99mTc decays are by means of internal conversion, resulting in ejection of high speed internal conversion electrons in several sharp peaks (as is typical of electrons from this type of decay) also at about 140 keV (99mTc → 99Tc+ + e). These conversion electrons will ionize the surrounding matter like beta radiation electrons would do, contributing along with the 140.5 keV and 142.6 keV gammas to the total deposited dose.

Pure gamma emission is the desirable decay mode for medical imaging because other particles deposit more energy in the patient body (radiation dose) than in the camera. Metastable isomeric transition is the only nuclear decay mode that approaches pure gamma emission. 

Tc-99m's half-life of 6.0058 hours is considerably longer (by 14 orders of magnitude, at least) than most nuclear isomers, though not unique. This is still a short half-life relative to many other known modes of radioactive decay and it is in the middle of the range of half lives for radiopharmaceuticals used for medical imaging

After gamma emission or internal conversion, the resulting ground-state technetium-99 then decays with a half-life of 211,000 years to stable ruthenium-99. This process emits soft beta radiation without a gamma. Such low radioactivity from the daughter product(s) is a desirable feature for radiopharmaceuticals.

Production

Production of Mo-99 in nuclear reactors

Neutron irradiation of U-235 targets
The parent nuclide of Tc-99m, Mo-99, is mainly extracted for medical purposes from the fission products created in neutron-irradiated U-235 targets, the majority of which is produced in five nuclear research reactors around the world using highly enriched uranium (HEU) targets. Smaller amounts of 99Mo are produced from low-enriched uranium in at least three reactors.
Nuclear reactors producing 99Mo from U-235 targets. The year indicates the date of the first criticality of the reactor.
Type Reactor Location Target/Fuel Year
Large-scale producers NRU Canada HEU/LEU 1957
BR2 Belgium HEU/HEU 1961
SAFARI-1 South Africa LEU/LEU 1965
HFR the Netherlands HEU/LEU 1961
Osiris reactor France LEU/HEU 1966
Regional producers OPAL Australia LEU/LEU 2006
MPR RSG-GAS Indonesia LEU/LEU 1987
RA-3 Argentina LEU/LEU 1961
MARIA Poland HEU/HEU 1974
LVR-15 Czech Republic HEU/HEU 1957
Neutron activation of Mo-98
Production of 99Mo by neutron activation of natural molybdenum, or molybdenum enriched in Mo-98, is another, currently smaller, route of production.

Production of Tc-99m/Mo-99 in particle accelerators

Production of "Instant" Tc-99m
The feasibility of Tc-99m production with the 22-MeV-proton bombardment of a Mo-100 target in medical cyclotrons was demonstrated in 1971. The recent shortages of Tc-99m reignited the interest in the production of "instant" 99mTc by proton bombardment of isotopically enriched Mo-100 targets (>99.5%) following the reaction 100Mo(p,2n)99mTc. Canada is commissioning such cyclotrons, designed by Advanced Cyclotron Systems, for Tc-99m production at the University of Alberta and the Université de Sherbrooke, and is planning others at the University of British Columbia, TRIUMF, University of Saskatchewan and Lakehead University.
Indirect routes of production of Mo-99
Other particle accelerator-based isotope production techniques have been investigated. The supply disruptions of Mo-99 in the late 2000s and the aging of the producing nuclear reactors forced the industry to look into alternative methods of production. The use of cyclotrons to produce Mo-99 from Mo-100 via (n,2n) or (γ,n) reactions has been further investigated.

Technetium-99m generators

Technetium-99m's short half-life of 6 hours makes storage impossible and would make transport very expensive. It is instead its parent nuclide 99Mo is supplied to hospitals after its extraction from the neutron-irradiated uranium targets and its purification in dedicated processing facilities. It is shipped by specialised radiopharmaceutical companies in the form of technetium-99m generators worldwide or directly distributed to the local market. The generators, colloquially known as a moly cows, are devices designed to provide radiation shielding for transport and to minimize the extraction work done at the medical facility. A typical dose rate at 1 metre from Tc-99m generator is 20-50 μSv/h during transport. These generators' output declines with time and must be replaced weekly, since the half-life of 99Mo is still only 66 hours.

Molybdenum-99 spontaneously decays to excited states of 99Tc through beta decay. Over 87% of the decays lead to the 142 keV excited state of Tc-99m. A
β
electron and a
ν
e
electron antineutrino are emitted in the process (99Mo → 99mTc +
β
+
ν
e
). The
β
electrons are easily shielded for transport, and 99mTc generators are only minor radiation hazards, mostly due to secondary X-rays produced by the electrons (also known as Bremsstrahlung). 

At the hospital, the 99mTc that forms through 99Mo decay is chemically extracted from the technetium-99m generator. Most commercial 99Mo/99mTc generators use column chromatography, in which 99Mo in the form of water-soluble molybdate, MoO42− is adsorbed onto acid alumina (Al2O3). When the 99Mo decays, it forms pertechnetate TcO4, which, because of its single charge, is less tightly bound to the alumina. Pulling normal saline solution through the column of immobilized 99MoO42− elutes the soluble 99mTcO4, resulting in a saline solution containing the 99mTc as the dissolved sodium salt of the pertechnetate. One technetium-99m generator, holding only a few micrograms of 99Mo, can potentially diagnose 10,000 patients because it will be producing 99mTc strongly for over a week.

Technetium scintigraphy of a neck of a Graves' disease patient

Preparation

Technetium exits the generator in the form of the pertechnetate ion, TcO4. The oxidation state of Tc in this compound is +7. This is directly suitable for medical applications only in bone scans (it is taken up by osteoblasts) and some thyroid scans (it is taken up in place of iodine by normal thyroid tissues). In other types of scans relying on Tc-99m, a reducing agent is added to the pertechnetate solution to bring the oxidation state of the Tc down to +3 or +4. Secondly, a ligand is added to form a coordination complex. The ligand is chosen to have an affinity for the specific organ to be targeted. For example, the exametazime complex of Tc in oxidation state +3 is able to cross the blood–brain barrier and flow through the vessels in the brain for cerebral blood flow imaging. Other ligands include sestamibi for myocardial perfusion imaging and mercapto acetyl triglycine for MAG3 scan to measure renal function.

Medical uses

In 1970, Eckelman and Richards presented the first "kit" containing all the ingredients required to release the Tc-99m, "milked" from the generator, in the chemical form to be administered to the patient.

Technetium-99m is used in 20 million diagnostic nuclear medical procedures every year. Approximately 85% of diagnostic imaging procedures in nuclear medicine use this isotope as radioactive tracer. Klaus Schwochau's book Technetium lists 31 radiopharmaceuticals based on 99mTc for imaging and functional studies of the brain, myocardium, thyroid, lungs, liver, gallbladder, kidneys, skeleton, blood, and tumors. Depending on the procedure, the 99mTc is tagged (or bound to) a pharmaceutical that transports it to its required location. For example, when 99mTc is chemically bound to exametazime (HMPAO), the drug is able to cross the blood–brain barrier and flow through the vessels in the brain for cerebral blood-flow imaging. This combination is also used for labeling white blood cells (99mTc labeled WBC) to visualize sites of infection. 99mTc sestamibi is used for myocardial perfusion imaging, which shows how well the blood flows through the heart. Imaging to measure renal function is done by attaching 99mTc to mercaptoacetyl triglycine (MAG3); this procedure is known as a MAG3 scan.

Technetium-99m can be readily detected in the body by medical equipment because it emits 140.5 keV gamma rays (these are about the same wavelength as emitted by conventional X-ray diagnostic equipment), and its half-life for gamma emission is six hours (meaning 94% of it decays to 99Tc in 24 hours). The "short" physical half-life of the isotope and its biological half-life of 1 day (in terms of human activity and metabolism) allows for scanning procedures which collect data rapidly, but keep total patient radiation exposure low.

Radiation side-effects

Diagnostic treatment involving technetium-99m will result in radiation exposure to technicians, patients, and passers-by. Typical quantities of technetium administered for immunoscintigraphy tests, such as SPECT tests, range from 400 to 1,100 MBq (11 to 30 mCi) (millicurie or mCi; and Mega-Becquerel or MBq) for adults. These doses result in radiation exposures to the patient around 10 mSv (1000 mrem), the equivalent of about 500 chest X-ray exposures. This level of radiation exposure carries a 1 in 1000 lifetime risk of developing a solid cancer or leukemia in the patient. The risk is higher in younger patients, and lower in older ones. Unlike a chest x-ray, the radiation source is inside the patient and will be carried around for a few days, exposing others to second-hand radiation. A spouse who stays constantly by the side of the patient through this time might receive one thousandth of patient's radiation dose this way. 

The short half-life of the isotope allows for scanning procedures that collect data rapidly. The isotope is also of a very low energy level for a gamma emitter. Its ~140 keV of energy make it safer for use because of the substantially reduced ionization compared with other gamma emitters. The energy of gammas from 99mTc is about the same as the radiation from a commercial diagnostic X-ray machine, although the number of gammas emitted results in radiation doses more comparable to X-ray studies like computed tomography

Technetium-99m has several features that make it safer than other possible isotopes. Its gamma decay mode can be easily detected by a camera, allowing the use of smaller quantities. And because technetium-99m has a short half-life, its quick decay into the far less radioactive technetium-99 results in relatively low total radiation dose to the patient per unit of initial activity after administration, as compared to other radioisotopes. In the form administered in these medical tests (usually pertechnetate), technetium-99m and technetium-99 are eliminated from the body within a few days.

3-D scanning technique: SPECT

Single photon emission computed tomography (SPECT) is a nuclear medicine imaging technique using gamma rays. It may be used with any gamma-emitting isotope, including Tc-99m. In the use of technetium-99m, the radioisotope is administered to the patient and the escaping gamma rays are incident upon a moving gamma camera which computes and processes the image. To acquire SPECT images, the gamma camera is rotated around the patient. Projections are acquired at defined points during the rotation, typically every three to six degrees. In most cases, a full 360° rotation is used to obtain an optimal reconstruction. The time taken to obtain each projection is also variable, but 15–20 seconds are typical. This gives a total scan time of 15–20 minutes.

The technetium-99m radioisotope is used predominantly in bone and brain scans. For bone scans, the pertechnetate ion is used directly, as it is taken up by osteoblasts attempting to heal a skeletal injury, or (in some cases) as a reaction of these cells to a tumor (either primary or metastatic) in the bone. In brain scanning, Tc-99m is attached to the chelating agent HMPAO to create technetium (99mTc) exametazime, an agent which localizes in the brain according to region blood flow, making it useful for the detection of stroke and dementing illnesses that decrease regional brain flow and metabolism.
Most recently, technetium-99m scintigraphy has been combined with CT coregistration technology to produce SPECT/CT scans. These employ the same radioligands and have the same uses as SPECT scanning, but are able to provide even finer 3-D localization of high-uptake tissues, in cases where finer resolution is needed. An example is the sestamibi parathyroid scan which is performed using the Tc-99m radioligand sestamibi, and can be done in either SPECT or SPECT/CT machines.

Bone scan

The nuclear medicine technique commonly called the bone scan usually uses Tc-99m. It is not to be confused with the "bone density scan", DEXA, which is a low-exposure X-ray test measuring bone density to look for osteoporosis and other diseases where bones lose mass without rebuilding activity. The nuclear medicine technique is sensitive to areas of unusual bone rebuilding activity, since the radiopharmaceutical is taken up by osteoblast cells which build bone. The technique therefore is sensitive to fractures and bone reaction to bone tumors, including metastases. For a bone scan, the patient is injected with a small amount of radioactive material, such as 700–1,100 MBq (19–30 mCi) of 99mTc-medronic acid and then scanned with a gamma camera. Medronic acid is a phosphate derivative which can exchange places with bone phosphate in regions of active bone growth, so anchoring the radioisotope to that specific region. To view small lesions (less than 1 centimetre (0.39 in)) especially in the spine, the SPECT imaging technique may be required, but currently in the United States, most insurance companies require separate authorization for SPECT imaging.

Myocardial perfusion imaging

Myocardial perfusion imaging (MPI) is a form of functional cardiac imaging, used for the diagnosis of ischemic heart disease. The underlying principle is, under conditions of stress, diseased myocardium receives less blood flow than normal myocardium. MPI is one of several types of cardiac stress test. As a nuclear stress test the average radiation exposure is 9.4 mSV which compared to a typical 2 view Chest X-Ray (.1 mSV) is equivalent to 94 Chest X-Rays.

Several radiopharmaceuticals and radionuclides may be used for this, each giving different information. In the myocardial perfusion scans using Tc-99m, the radiopharmaceuticals 99mTc-tetrofosmin (Myoview, GE Healthcare) or 99mTc-sestamibi (Cardiolite, Bristol-Myers Squibb) are used. Following this, myocardial stress is induced, either by exercise or pharmacologically with adenosine, dobutamine or dipyridamole(Persantine), which increase the heart rate or by regadenoson(Lexiscan), a vasodilator. (Aminophylline can be used to reverse the effects of dipyridamole and regadenoson). Scanning may then be performed with a conventional gamma camera, or with SPECT/CT.

Cardiac ventriculography

In cardiac ventriculography, a radionuclide, usually 99mTc, is injected, and the heart is imaged to evaluate the flow through it, to evaluate coronary artery disease, valvular heart disease, congenital heart diseases, cardiomyopathy, and other cardiac disorders. As a nuclear stress test the average radiation exposure is 9.4 mSV which compared to a typical 2 view Chest X-Ray (.1 mSV) is equivalent to 94 Chest X-Rays. It exposes patients to less radiation than to comparable chest X-ray studies.

Functional brain imaging

Usually the gamma-emitting tracer used in functional brain imaging is 99mTc-HMPAO (hexamethylpropylene amine oxime, exametazime). The similar 99mTc-EC tracer may also be used. These molecules are preferentially distributed to regions of high brain blood flow, and act to assess brain metabolism regionally, in an attempt to diagnose and differentiate the different causal pathologies of dementia. When used with the 3-D SPECT technique, they compete with brain FDG-PET scans and fMRI brain scans as techniques to map the regional metabolic rate of brain tissue.

Sentinel-node identification

The radioactive properties of 99mTc can be used to identify the predominant lymph nodes draining a cancer, such as breast cancer or malignant melanoma. This is usually performed at the time of biopsy or resection.99mTc-labelled isosulfan blue dye is injected intradermally around the intended biopsy site. The general location of the sentinel node is determined with the use of a handheld scanner with a gamma-sensor probe that detects the technetium-99m–labeled sulfur colloid that was previously injected around the biopsy site. An incision is then made over the area of highest radionuclide accumulation, and the sentinel node is identified within the incision by inspection; the isosulfan blue dye will usually stain any draining nodes blue.

Immunoscintigraphy

Immunoscintigraphy incorporates 99mTc into a monoclonal antibody, an immune system protein, capable of binding to cancer cells. A few hours after injection, medical equipment is used to detect the gamma rays emitted by the 99mTc; higher concentrations indicate where the tumor is. This technique is particularly useful for detecting hard-to-find cancers, such as those affecting the intestines. These modified antibodies are sold by the German company Hoechst (now part of Sanofi-Aventis) under the name "Scintium".

Blood pool labeling

When 99mTc is combined with a tin compound, it binds to red blood cells and can therefore be used to map circulatory system disorders. It is commonly used to detect gastrointestinal bleeding sites as well as ejection fraction, heart wall motion abnormalities, abnormal shunting, and to perform ventriculography.

Pyrophosphate for heart damage

A pyrophosphate ion with 99mTc adheres to calcium deposits in damaged heart muscle, making it useful to gauge damage after a heart attack.

Sulfur colloid for spleen scan

The sulfur colloid of 99mTc is scavenged by the spleen, making it possible to image the structure of the spleen.

Meckel's diverticulum

Pertechnetate is actively accumulated and secreted by the mucoid cells of the gastric mucosa, and therefore, technetate(VII) radiolabeled with Tc99m is injected into the body when looking for ectopic gastric tissue as is found in a Meckel's diverticulum with Meckel's Scans.

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