Positron emission

Positron emission or beta plus decay+ decay) is a subtype of radioactive decay called beta decay, in which a proton inside a radionuclide nucleus is converted into a neutron while releasing a positron and an electron neutrino (νe).[1] Positron emission is mediated by the weak force. The positron is a type of beta particle+), the other beta particle being the electron (β) emitted from the β decay of a nucleus.

An example of positron emission (β+ decay) is shown with magnesium-23 decaying into sodium-23:

23
12
Mg
23
11
Na
+
e+
+
ν
e

Because positron emission decreases proton number relative to neutron number, positron decay happens typically in large "proton-rich" radionuclides. Positron decay results in nuclear transmutation, changing an atom of one chemical element into an atom of an element with an atomic number that is less by one unit.

Positron emission should not be confused with electron emission or beta minus decay (β decay), which occurs when a neutron turns into a proton and the nucleus emits an electron and an antineutrino.

Positron emission is different from proton decay, the hypothetical decay of protons, not necessarily those bound with neutrons, not necessarily through the emission of a positron and not as part of nuclear physics, but rather of particle physics.

Discovery of positron emission

In 1934 Frédéric and Irène Joliot-Curie bombarded aluminium with alpha particles to effect the nuclear reaction 4
2
He
 + 27
13
Al
 → 30
15
P
 + 1
0
n
, and observed that the product isotope 30
15
P
emits a positron identical to those found in cosmic rays by Carl David Anderson in 1932.[2] This was the first example of
β+
 decay (positron emission). The Curies termed the phenomenon "artificial radioactivity," since 30
15
P
is a short-lived nuclide which does not exist in nature. The discovery of artificial radioactivity would be cited when the husband and wife team won the Nobel Prize.

Positron-emitting isotopes

Isotopes which undergo this decay and thereby emit positrons include carbon-11, potassium-40, nitrogen-13, oxygen-15, aluminium-26, sodium-22, fluorine-18, and iodine-124. As an example, the following equation describes the beta plus decay of carbon-11 to boron-11, emitting a positron and a neutrino:

11
6
C
 
→  11
5
B
 

e+
 

ν
e
 
0.96 MeV

Emission mechanism

Inside protons and neutrons, there are fundamental particles called quarks. The two most common types of quarks are up quarks, which have a charge of +2/3, and down quarks, with a −1/3 charge. Quarks arrange themselves in sets of three such that they make protons and neutrons. In a proton, whose charge is +1, there are two up quarks and one down quark (2/3 + 2/31/3 = 1). Neutrons, with no charge, have one up quark and two down quarks (2/31/31/3 = 0). Via the weak interaction, quarks can change flavor from down to up, resulting in electron emission. Positron emission happens when an up quark changes into a down quark.[3] (2/3 − 1 = −1/3).

Nuclei which decay by positron emission may also decay by electron capture. For low-energy decays, electron capture is energetically favored by 2mec2 = 1.022 MeV, since the final state has an electron removed rather than a positron added. As the energy of the decay goes up, so does the branching ratio towards positron emission. However, if the energy difference is less than 2mec2, then positron emission cannot occur and electron capture is the sole decay mode. Certain otherwise electron-capturing isotopes (for instance, 7
Be
) are stable in galactic cosmic rays, because the electrons are stripped away and the decay energy is too small for positron emission.

Energy conservation

A positron is ejected from the parent nucleus, and the daughter (Z−1) atom must shed an orbital electron to balance charge. The overall results is that the mass of two electrons are ejected from the atom (one for the positron and one for the electron), and the β+ decay is energetically possible only if the mass of the parent atom exceeds the mass of the daughter atom by at least two electron masses (1.02 MeV).

Isotopes which increase in mass under the conversion of a proton to a neutron, or which decrease in mass by less than 2me, cannot spontaneously decay by positron emission.

Application

These isotopes are used in positron emission tomography, a technique used for medical imaging. Note that the energy emitted depends on the isotope that is decaying; the figure of 0.96 MeV applies only to the decay of carbon-11.

The short-lived positron emitting isotopes 11C, 13N, 15O and 18F used for positron emission tomography are typically produced by proton irradiation of natural or enriched targets.[4][5]

References

  1. ^ The University of North Carolina at Chapel Hill. "Nuclear Chemistry". Retrieved 2012-06-14.
  2. ^ I. Curie and F. Joliot, C. R. Acad. Sci. 198, 254 (1934)
  3. ^ How it works:Positron emission
  4. ^ Positron Emission Tomography Imaging at the University of British Columbia (accessed 11 May 2012)
  5. ^ Ledingham, K W D; McKenna, P; McCanny, T; Shimizu, S; Yang, J M; Robson, L; Zweit, J; Gillies, J M; Bailey, J; Chimon, G N; Clarke, R J; Neely, D; Norreys, P A; Collier, J L; Singhal, R P; Wei, M S; Mangles, S P D; Nilson, P; Krushelnick, K; Zepf, M (2004). "High power laser production of short-lived isotopes for positron emission tomography". Journal of Physics D: Applied Physics. 37 (16): 2341. Bibcode:2004JPhD...37.2341L. doi:10.1088/0022-3727/37/16/019.

External links

Brain positron emission tomography

Brain positron emission tomography is a form of positron emission tomography (PET) that is used to measure brain metabolism and the distribution of exogenous radiolabeled chemical agents throughout the brain. PET measures emissions from radioactively labeled metabolically active chemicals that have been injected into the bloodstream. The emission data from brain PET are computer-processed to produce multi-dimensional images of the distribution of the chemicals throughout the brain.

Cardiac PET

Cardiac PET (or cardiac positron emission tomography) is a form of diagnostic imaging in which the presence of heart disease is evaluated using a PET scanner. Intravenous injection of a radiotracer is performed as part of the scan. Commonly used radiotracers are Rubidium-82, Nitrogen-13 ammonia and Oxygen-15 water.The requirements to perform Cardiac PET imaging include:

Facility: taking into consideration clinical workflow, as well as regulatory requirements such as requisite shielding from radiation exposure

Capital equipment: PET or PET/CT scanner

Radiopharmaceutical: Rubidium-82 generator system or close access to cyclotron produced isotopes such as Nitrogen-13 ammonia

Personnel: including specially trained physician, radiographers, radiation safety supervisors and optional nursing support

Operations: stress test monitoring, as well as emergency response equipment, processing and review workstations, administrative and support personnel are additional considerationsThis form of diagnostic imaging has traditionally been perceived as cost-prohibitive in comparison to general nuclear medicine cardiac stress testing using single photon emission computed tomography (SPECT). However, due to significant gains in access to scanners, related to the widely accepted role of PET/CT in clinical oncology, cardiac PET is likely to become more widely available, particularly given various clinical and technical advantages that might make this a potential test of choice in the diagnosis of coronary artery/heart disease.Cardiac PET imaging has now been expanded to mobile services to facilitate all healthcare providers by a company called Cardiac Imaging, Inc. located in Wheaton, Illinois.

They now have the only Medicare approved mobile Cardiac PET scanner available for patient use.

Cardiac imaging

Cardiac imaging techniques include coronary catheterization, echocardiogram, Intravascular ultrasound, Cardiac PET scan, Cardiac CT scan and Cardiac MRI.

Deauville Criteria

The Deauville 5-point scoring system is an internationally Accepted and utilized five-point scoring system for the Fluorodeoxyglucose (FDG) avidity of a Hodgkin's lymphoma or Non-Hodgkin's lymphoma tumor mass as seen on FDG Positron emission tomography:

Score 1: No uptake above the background

Score 2: Uptake ≤ mediastinum

Score 3: Uptake > mediastinum but ≤ liver

Score 4: Uptake moderately increased compared to the liver at any site

Score 5: Uptake markedly increased compared to the liver at any site

Score X: New areas of uptake unlikely to be related to lymphoma

Scores of 1 and 2 are considered to be negative and 4 and 5 are considered to be positive. "Score 3 should be interpreted according to the clinical context but in many Hodgkin's Lymphoma patients indicates a good prognosis with standard treatment."

Dihydrotetrabenazine

Dihydrotetrabenazine or DTBZ is an organic compound with the chemical formula C19H29NO3. It is a close analog of tetrabenazine. DTBZ and its derivatives, when labeled with positron emitting isotopes such as carbon-11 and fluorine-18, are used as PET radioligands for examining VMAT2.

Electron capture

Electron capture (K-electron capture, also K-capture, or L-electron capture, L-capture) is a process in which the proton-rich nucleus of an electrically neutral atom absorbs an inner atomic electron, usually from the K or L electron shell. This process thereby changes a nuclear proton to a neutron and simultaneously causes the emission of an electron neutrino.

p + e− → n + νeSince this single emitted neutrino carries the entire decay energy, it has this single characteristic energy. Similarly, the momentum of the neutrino emission causes the daughter atom to recoil with a single characteristic momentum.

The resulting daughter nuclide, if it is in an excited state, then transitions to its ground state. Usually, a gamma ray is emitted during this transition, but nuclear de-excitation may also take place by internal conversion.

Following capture of an inner electron from the atom, an outer electron replaces the electron that was captured and one or more characteristic X-ray photons is emitted in this process. Electron capture sometimes also results in the Auger effect, where an electron is ejected from the atom's electron shell due to interactions between the atom's electrons in the process of seeking a lower energy electron state.

Following electron capture, the atomic number is reduced by one, the neutron number is increased by one, and there is no change in mass number. Simple electron capture by itself results in a neutral atom, since the loss of the electron in the electron shell is balanced by a loss of positive nuclear charge. However, a positive atomic ion may result from further Auger electron emission.

Electron capture is an example of weak interaction, one of the four fundamental forces.

Electron capture is the primary decay mode for isotopes with a relative superabundance of protons in the nucleus, but with insufficient energy difference between the isotope and its prospective daughter (the isobar with one less positive charge) for the nuclide to decay by emitting a positron. Electron capture is always an alternative decay mode for radioactive isotopes that do not have sufficient energy to decay by positron emission.

Electron capture is sometimes included as a type of beta decay, because the basic nuclear process, mediated by the weak force, is the same. In nuclear physics, beta decay is a type of radioactive decay in which a beta ray (fast energetic electron or positron) and a neutrino are emitted from an atomic nucleus.

Electron capture is sometimes called inverse beta decay, though this term usually refers to the interaction of an electron antineutrino with a proton.If the energy difference between the parent atom and the daughter atom is less than 1.022 MeV, positron emission is forbidden as not enough decay energy is available to allow it, and thus electron capture is the sole decay mode. For example, rubidium-83 (37 protons, 46 neutrons) will decay to krypton-83 (36 protons, 47 neutrons) solely by electron capture (the energy difference, or decay energy, is about 0.9 MeV).

Emission computed tomography

Emission computed tomography (ECT) is a type of tomography involving radioactive emissions.

Types include positron emission tomography (PET) and Single-photon emission computed tomography (SPECT).

The imaging agent used in SPECT emits gamma rays, as opposed to the positron emitters (such as 18F) used in PET. There are a range of radiotracers (such as 99mTc, 111In, 123I, 201Tl) that can be used, depending on the specific application.

Fluorine-18

Fluorine-18 (18F) is a fluorine radioisotope which is an important source of positrons. It has a mass of 18.0009380(6) u and its half-life is 109.771(20) minutes. It decays by positron emission 97% of the time and electron capture 3% of the time. Both modes of decay yield stable oxygen-18.

Gallium scan

A gallium scan (also called "gallium imaging") is a type of nuclear medicine test that uses either a gallium-67 (67Ga) or gallium-68 (68Ga) radiopharmaceutical to obtain images of a specific type of tissue, or disease state of tissue. Gallium salts like gallium citrate and gallium nitrate may be used. The form of salt is not important, since it is the freely dissolved gallium ion Ga3+ which is active. Both 67Ga and 68Ga salts have similar uptake mechanisms. Gallium can also be used in other forms, for example 68Ga-PSMA is used for cancer imaging. The gamma emission of gallium 67 is imaged by a gamma camera, while the positron emission of gallium 68 is imaged by positron emission tomography (PET).

Gallium salts are taken up by tumors, inflammation, and both acute and chronic infection, allowing these pathological processes to be imaged. Gallium is particularly useful in imaging osteomyelitis that involves the spine, and in imaging older and chronic infections that may be the cause of a fever of unknown origin.

Lortalamine

Lortalamine (LM-1404) is an antidepressant which was synthesized in the early 1980s. It acts as a potent and highly selective norepinephrine reuptake inhibitor. Lortalamine was under development for clinical use but was shelved, likely due to the finding that it produced ocular toxicity in animals. It has been used to label the norepinephrine transporter in positron emission tomography studies.

Neuroimaging

Neuroimaging or brain imaging is the use of various techniques to either directly or indirectly image the structure, function, or pharmacology of the nervous system. It is a relatively new discipline within medicine, neuroscience, and psychology. Physicians who specialize in the performance and interpretation of neuroimaging in the clinical setting are neuroradiologists.

Neuroimaging falls into two broad categories:

Structural imaging, which deals with the structure of the nervous system and the diagnosis of gross (large scale) intracranial disease (such as a tumor) and injury.

Functional imaging, which is used to diagnose metabolic diseases and lesions on a finer scale (such as Alzheimer's disease) and also for neurological and cognitive psychology research and building brain-computer interfaces.Functional imaging enables, for example, the processing of information by centers in the brain to be visualized directly. Such processing causes the involved area of the brain to increase metabolism and "light up" on the scan. One of the more controversial uses of neuroimaging has been researching "thought identification" or mind-reading.

Nitrogen-13

Nitrogen-13 is a radioisotope of nitrogen used in positron emission tomography (PET). It has a half-life of a little under ten minutes, so it must be made at the PET site. A cyclotron may be used for this purpose.

Nitrogen-13 is used to tag ammonia molecules for PET myocardial perfusion imaging.

PET-CT

Positron emission tomography–computed tomography (better known as PET-CT or PET/CT) is a nuclear medicine technique which combines, in a single gantry, a positron emission tomography (PET) scanner and an x-ray computed tomography (CT) scanner, to acquire sequential images from both devices in the same session, which are combined into a single superposed (co-registered) image. Thus, functional imaging obtained by PET, which depicts the spatial distribution of metabolic or biochemical activity in the body can be more precisely aligned or correlated with anatomic imaging obtained by CT scanning. Two- and three-dimensional image reconstruction may be rendered as a function of a common software and control system.

PET-CT has revolutionized medical diagnosis in many fields, by adding precision of anatomic localization to functional imaging, which was previously lacking from pure PET imaging. For example, many diagnostic imaging procedures in oncology, surgical planning, radiation therapy and cancer staging have been changing rapidly under the influence of PET-CT availability, and centers have been gradually abandoning conventional PET devices and substituting them by PET-CTs. Although the combined/hybrid device is considerably more expensive, it has the advantage of providing both functions as stand-alone examinations, being, in fact, two devices in one.

The only other obstacle to the wider use of PET-CT is the difficulty and cost of producing and transporting the radiopharmaceuticals used for PET imaging, which are usually extremely short-lived (for instance, the half life of radioactive Fluorine-18 (18F) used to trace glucose metabolism (using fluorodeoxyglucose, FDG) is two hours only. Its production requires a very expensive cyclotron as well as a production line for the radiopharmaceuticals.

PET-MRI, like PET-CT, combines modalities to produce co-registered images.

PET-MRI

Positron emission tomography–magnetic resonance imaging (PET–MRI) is a hybrid imaging technology that incorporates magnetic resonance imaging (MRI) soft tissue morphological imaging and positron emission tomography (PET) functional imaging.Simultaneous PET/MR detection was first demonstrated in 1997, however it took another 13 years, and new detector technologies, for clinical systems to become commercially available.

PET radiotracer

PET radiotracer is a type of radioligand that is used for the diagnostic purposes via positron emission tomography imaging technique.

Positron emission mammography

Positron emission mammography (PEM) is a nuclear medicine imaging modality used to detect or characterise breast cancer. Mammography typically refers to x-ray imaging of the breast, while PEM uses an injected positron emitting isotope and a dedicated scanner to locate breast tumors. Scintimammography is another nuclear medicine breast imaging technique, however it is performed using a gamma camera. Breasts can be imaged on standard whole-body PET scanners, however dedicated PEM scanners offer advantages including improved resolution.PEM is not recommended for routine use or for breast cancer screening, in part due to higher radiation dose compared to other modalities. Compared to breast MRI, PEM offers higher specificity. Specific indications can include "high-risk patients with masses > 2 cm or aggressive malignancy and serum tumor marker elevation". 18F-FDG is the most common radiopharmaceutical used for PEM.

Positron emission tomography

Positron-emission tomography (PET) is a nuclear medicine functional imaging technique that is used to observe metabolic processes in the body as an aid to the diagnosis of disease. The system detects pairs of gamma rays emitted indirectly by a positron-emitting radionuclide, most commonly fluorine-18, which is introduced into the body on a biologically active molecule called a radioactive tracer. Three-dimensional images of tracer concentration within the body are then constructed by computer analysis. In modern PET-CT scanners, three-dimensional imaging is often accomplished with the aid of a CT X-ray scan performed on the patient during the same session, in the same machine.

If the biologically active tracer molecule chosen for PET is fludeoxyglucose (FDG), an analogue of glucose, the concentrations of tracer imaged will indicate tissue metabolic activity as it corresponds to the regional glucose uptake. Use of this tracer to explore the possibility of cancer metastasis (i.e., spreading to other sites) is the most common type of PET scan in standard medical care (representing 90% of current scans). Metabolic trapping of the radioactive glucose molecule allows the PET scan to be utilized. The same tracer may also be used for PET investigation and diagnosis of types of dementia. Less often, other radioactive tracers, usually but not always labeled with fluorine-18, are used to image the tissue concentration of other types of molecules of interest.

One of the disadvantages of PET scanners is their operating cost.

Scintigraphy

Scintigraphy ("scint", Latin scintilla, spark), also known as a Gamma scan, is a diagnostic test in nuclear medicine, where radioisotopes attached to drugs that travel to a specific organ or tissue (radiopharmaceuticals) are taken internally and the emitted gamma radiation is captured by external detectors (gamma cameras) to form two-dimensional images in a similar process to the capture of x-ray images. In contrast, SPECT and positron emission tomography (PET) form 3-dimensional images, and are therefore classified as separate techniques to scintigraphy, although they also use gamma cameras to detect internal radiation. Scintigraphy is unlike a diagnostic X-ray where external radiation is passed through the body to form an image.

Setoperone

Setoperone is a compound that is a ligand to the 5-HT2A receptor.

It can be radiolabeled with the radioisotope fluorine-18 and used as a radioligand with positron emission tomography (PET).

Several research studies have used the radiolabeled setoperone in neuroimaging for the studying neuropsychiatric disorders, such as depression

or schizophrenia.

Radioactive
decay
Stellar
nucleosynthesis
Other
processes

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