Thepertechnetate ion (/pərˈtɛknəteɪt/)[1] is anoxyanion with the chemical formulaTcO−
4. It is often used as a convenient water-soluble source of isotopes of the radioactive elementtechnetium (Tc). In particular it is used to carry the99mTc isotope (half-life 6 hours) which is commonly used innuclear medicine in several nuclear scanning procedures.
Pertechnetate is poorly hydrated as [TcO4(H2O)n]− and [TcO4(H2O)n-m]−[H3O]+m (n = 1–50, m = 1–4) clusters that have been demonstrated by simulation with DFT. First hydration shell of TcO4− is asymmetric and contains no more than 7 water molecules. Only three of the four oxygen atoms of TcO4− form hydrogen bonds with water molecules.[2]
Atechnetate(VII) salt is acompound containing this ion. Pertechnetate compounds are salts oftechnetic(VII) acid. Pertechnetate is analogous topermanganate but it has little oxidizing power. Pertechnetate has higher oxidation power than perrhenate.[3]
Understanding pertechnetate is important in understanding technetium contamination in the environment and innuclear waste management.[3]
TcO−
4 is the starting material for most of the chemistry of technetium. Pertechnetate salts are usually colorless.[4]TcO−
4 is produced by oxidizing technetium with nitric acid or with hydrogen peroxide. The pertechnetate anion is similar to thepermanganate anion but is a weakeroxidizing agent. It is tetrahedral and diamagnetic. The standard electrode potential forTcO−
4/TcO
2 is only +0.738 V in acidic solution, as compared to +1.695 V forMnO−
4/MnO
2.[5] Because of its diminished oxidizing power,TcO−
4 is stable in alkaline solution.TcO−
4 is more similar toReO−
4. Depending on the reducing agent,TcO−
4 can be converted to derivatives containing Tc(VI), Tc(V), and Tc(IV).[6] In the absence of strong complexing ligands,TcO−
4 is reduced to a +4 oxidation state via the formation ofTcO
2 hydrate.[5]
Some metals like actinides,[7] barium, scandium, yttrium[8] or zirconium[9] may form complex salts with pertechnetate thus strongly effecting its liquid-liquid extraction behavior.[10]
99m
Tc is conveniently available in high radionuclidic purity frommolybdenum-99, which decays with 87% probability to99m
Tc. The subsequent decay of99m
Tc leads to either99
Tc or99
Ru.99
Mo can be produced in a nuclear reactor viairradiation of either molybdenum-98 or naturally occurring molybdenum with thermal neutrons, but this is not the method currently in use today. Currently,99
Mo is recovered as a product of the nuclear fission reaction of235
U,[11] separated from other fission products via a multistep process and loaded onto a column of alumina that forms the core of a99
Mo/99m
Tc radioisotope generator.
As the99
Mo continuously decays to99m
Tc, the99m
Tc can be removed periodically (usually daily) by flushing a saline solution (0.15 M NaCl in water) through the alumina column: the more highly charged99
MoO2−
4 is retained on the column, where it continues to undergo radioactive decay, while the medically useful radioisotope99m
TcO−
4 is eluted in the saline. The eluate from the column must be sterile and pyrogen free, so that the Tc drug can be used directly, usually within 12 hours of elution.[5] In a few cases, sublimation or solvent extraction may be used.
| Formula | name | crystal structure | cell dimensions (Å) | unit cell volume (Å3) | remarks | references |
|---|---|---|---|---|---|---|
| LiTcO4 | lithium pertechnetate | [3] | ||||
| LiTcO4·2H2O | lithium pertechnetate dihydrate | [3] | ||||
| LiTcO4·3H2O | lithium pertechnetate trihydrate | Pt3/mc | [3] | |||
| NaTcO4 | sodium pertechnetate | tetragonal | a = 5.342, c = 1.874 | 338.91 | absorbs water from atmosphere | [3] |
| NaTcO4·H2O | sodium pertechnetate monohydrate | [3] | ||||
| NaTcO4·2H2O | sodium pertechnetate dihydrate | [3] | ||||
| NaTcO4·4H2O | sodium pertechnetate tetrahydrate | [3] | ||||
| KTcO4 | potassium pertechnetate | tetragonal | a = 5.647, c = 12.91 | 411.73 | used to prepare radiopharmaceuticals | [3] |
| RbTcO4 | rubidium pertechnetate | tetragonal | a = 5.762, c =13.543 | 449.65 | [3] | |
| α-CsTcO4 | α-caesium pertechnetate | tetragonal | a = 5.898, c = 14.38 | volatile at temperatures >470K | [3] | |
| β-CsTcO4 | β-caesium pertechnetate | orthorhombic | a = 5.737, b = 5.92, c = 14.341 | 486.38 | [3] | |
| TlTcO4 | thallium pertechnetate | orthorhombic | [3] | |||
| TlTcO4 | thallium pertechnetate | tetragonal | [3] | |||
| NH4TcO4 | ammonium pertechnetate | tetragonal | technetium may be supplied in this form | [3] | ||
| AgTcO4 | silver pertechnetate | tetragonal | [3] |
The half-life of99m
Tc is long enough that labelling synthesis of theradiopharmaceutical and scintigraphic measurements can be performed without significant loss of radioactivity.[5] The energy emitted from99m
Tc is 140 keV, which allows for the study of deep body organs. Radiopharmaceuticals have no intended pharmacologic effect and are used in very low concentrations. Radiopharmaceuticals containing99m
Tc are currently being applied in the determining morphology of organs, testing of organ function, and scintigraphic and emission tomographic imaging. The gamma radiation emitted by the radionuclide allows organs to be imagedin vivo tomographically. Currently, over 80% of radiopharmaceuticals used clinically are labelled with99m
Tc. A majority of radiopharmaceuticals labelled with99m
Tc are synthesized by the reduction of the pertechnetate ion in the presence of ligands chosen to confer organ specificity of the drug. The resulting99m
Tc compound is then injected into the body and a "gamma camera" is focused on sections or planes in order to image the spatial distribution of the99m
Tc.
99m
Tc is used primarily in the study of the thyroid gland - its morphology, vascularity, and function.TcO−
4 andiodide, due to their comparable charge/radius ratio, are similarly incorporated into the thyroid gland. The pertechnetate ion is not incorporated into thethyroglobulin. It is also used in the study of blood perfusion, regional accumulation, and cerebral lesions in the brain, as it accumulates primarily in thechoroid plexus.
Pertechnetate salts, such as sodium pertechnetate, cannot pass through theblood–brain barrier. In addition to the salivary and thyroid glands,99m
TcO−
4 localizes in the stomach.99m
TcO−
4 is renally eliminated for the first three days after being injected. After a scanning is performed, it is recommended that a patient drink large amounts of water in order to expedite elimination of the radionuclide.[15] Other methods of99m
TcO−
4 administration include intraperitoneal, intramuscular, subcutaneous, as well as orally. The behavior of the99m
TcO−
4 ion is essentially the same, with small differences due to the difference in rate of absorption, regardless of the method of administration.[16]
99m
TcO−
4 is advantageous for the synthesis of a variety of radiopharmaceuticals because Tc can adopt a number of oxidation states.[5] The oxidation state and coligands dictate the specificity of the radiopharmaceutical. The starting materialNa[99m
TcO
4], made available after elution from the generator column, as mentioned above, can be reduced in the presence of complexing ligands. Many different reducing agents can be used, but transition metal reductants are avoided because they compete with99m
Tc for ligands.Oxalates,formates,hydroxylamine, andhydrazine are also avoided because they form complexes with the technetium. Electrochemical reduction is impractical.
Ideally, the synthesis of the desired radiopharmaceutical from99m
TcO−
4, a reducing agent, and desired ligands should occur in one container after elution, and the reaction must be performed in a solvent that can be injected intravenously, such as a saline solution. Kits are available that contain the reducing agent, usually tin(II) and ligands. These kits are sterile, pyrogen-free, easily purchased, and can be stored for long periods of time. The reaction with99m
TcO−
4 takes place directly after elution from the generator column and shortly before its intended use. A high organ specificity is important because the injected activity should accumulate in the organ under investigation, as there should be a high activity ratio of the target organ to nontarget organs. If there is a high activity in organs adjacent to the one under investigation, the image of the target organ can be obscured. Also, high organ specificity allows for the reduction of the injected activity, and thus the exposure to radiation, in the patient. The radiopharmaceutical must be kinetically inert, in that it must not change chemicallyin vivo en route to the target organ.
Atechnetium-99m generator provides the pertechnetate containing the short-lived isotope99mTc for medical uses. This compound is generated directly frommolybdate held on alumina within the generator (see this topic for detail).
Pertechnetate has a wide variety of uses in diagnosticnuclear medicine. Since technetate(VII) can substitute foriodine in the Na/I symporter (NIS) channel in follicular cells of thethyroid gland, inhibiting uptake of iodine into the follicular cells,99mTc-pertechnetate can be used as an alternative to123I in imaging of the thyroid, although it specifically measures uptake and not organification.[17] It has also been used historically to evaluate fortesticular torsion, althoughultrasound is more commonly used in current practice, as it does not deliver a radiation dose to thetestes. It is also used in labeling of autologousred blood cells forMUGA scans to evaluate left ventricular cardiac function, localization of gastrointestinal bleeding prior to embolization or surgical management, and in damaged red blood cells to detect ectopicsplenic tissue.
It is actively accumulated and secreted by the mucoid cells of the gastric mucosa,[18] and therefore, technetate(VII) radiolabeled with technetium-99m is injected into the body when looking for ectopic gastric tissue as is found in aMeckel's diverticulum with Meckel's scans.[19]
All technetium salts are mildly radioactive, but some of them have explored use of the element for its chemical properties. In these uses, its radioactivity is incidental, and generally the least radioactive (longest-lived) isotopes of Tc are used. In particular,99Tc (half-life 211,000 years) is used in corrosion research, because it is the decay product of the easily obtained commercial99mTc isotope.[3] Solutions of technetate(VII) react with the surface ofiron to form technetium dioxide, in this way it is able to act as an anodiccorrosion inhibitor.[20]
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