Chemiluminescence

Chemiluminescence (also chemoluminescence) is the emission of light (luminescence), as the result of a chemical reaction. There may also be limited emission of heat. Given reactants A and B, with an excited intermediate ,

[A] + [B] → [] → [Products] + light

For example, if [A] is luminol and [B] is hydrogen peroxide in the presence of a suitable catalyst we have:

where:

Chemoluminescent reaction
A chemoluminescent reaction in an Erlenmeyer flask

General description

The decay of this excited state[] to a lower energy level causes light emission.[1] In theory, one photon of light should be given off for each molecule of reactant. This is equivalent to Avogadro's number of photons per mole of reactant. In actual practice, non-enzymatic reactions seldom exceed 1% QC, quantum efficiency.

In a chemical reaction, reactants collide to form a transition state, the enthalpic maximum in a reaction coordinate diagram, which proceeds to the product. Normally, reactants form products of lesser chemical energy. The difference in energy between reactants and products, represented as , is turned into heat, physically realized as excitations in the vibrational state of the normal modes of the product. Since vibrational energy is generally much greater than the thermal agitation, it rapidly disperses in the solvent through molecular rotation. This is how exothermic reactions make their solutions hotter. In a chemiluminescent reaction, the direct product of the reaction is an excited electronic state. This state then decays into an electronic ground state and emits light through either an allowed transition (analogous to fluorescence) or a forbidden transition (analogous to phosphorescence), depending partly on the spin state of the electronic excited state formed.

Chemiluminescence differs from fluorescence or phosphorescence in that the electronic excited state is the product of a chemical reaction rather than of the absorption of a photon. It is the antithesis of a photochemical reaction, in which light is used to drive an endothermic chemical reaction. Here, light is generated from a chemically exothermic reaction. The chemiluminescence might be also induced by an electrochemical stimulus, in this case is called electrochemiluminescence.

Lampyris Noctiluca (firefly) mating
Bioluminescence in nature: A male firefly mating with a female of the species Lampyris noctiluca.

A standard example of chemiluminescence in the laboratory setting is the luminol test. Here, blood is indicated by luminescence upon contact with iron in hemoglobin. When chemiluminescence takes place in living organisms, the phenomenon is called bioluminescence. A light stick emits light by chemiluminescence.

Liquid-phase reactions

Chemiluminescence in aqueous system is mainly caused by redox reactions[2].

Chemiluminescence
Chemiluminescence after a reaction of hydrogen peroxide and luminol

Gas-phase reactions

Knicklichter
Green and blue glow sticks
  • One of the oldest known chemiluminescent reactions is that of elemental white phosphorus oxidizing in moist air, producing a green glow. This is a gas-phase reaction of phosphorus vapor, above the solid, with oxygen producing the excited states (PO)2 and HPO.[5]
  • Another gas phase reaction is the basis of nitric oxide detection in commercial analytic instruments applied to environmental air-quality testing. Ozone is combined with nitric oxide to form nitrogen dioxide in an activated state.
NO+O3 → NO2[]+ O2
The activated NO2[] luminesces broadband visible to infrared light as it reverts to a lower energy state. A photomultiplier and associated electronics counts the photons that are proportional to the amount of NO present. To determine the amount of nitrogen dioxide, NO2, in a sample (containing no NO) it must first be converted to nitric oxide, NO, by passing the sample through a converter before the above ozone activation reaction is applied. The ozone reaction produces a photon count proportional to NO that is proportional to NO2 before it was converted to NO. In the case of a mixed sample that contains both NO and NO2, the above reaction yields the amount of NO and NO2 combined in the air sample, assuming that the sample is passed through the converter. If the mixed sample is not passed through the converter, the ozone reaction produces activated NO2[] only in proportion to the NO in the sample. The NO2 in the sample is not activated by the ozone reaction. Though unactivated NO2 is present with the activated NO2[], photons are emitted only by the activated species that is proportional to original NO. Final step: Subtract NO from (NO + NO2) to yield NO2[6]

Infrared chemiluminescence

In chemical kinetics, infrared chemiluminiscence (IRCL) refers to the emission of infrared photons from vibrationally excited product molecules immediately after their formation. The intensities of infrared emission lines from vibrationally excited molecules are used to measure the populations of vibrational states of product molecules.[7][8]

The observation of IRCL was developed as a kinetic technique by John Polanyi, who used it to study the attractive or repulsive nature of the potential energy surface for gas-phase reactions. In general the IRCL is much more intense for reactions with an attractive surface, indicating that this type of surface leads to energy deposition in vibrational excitation. In contrast reactions with a repulsive potential energy surface lead to little IRCL, indicating that the energy is primarily deposited as translational energy.[9]

Enhanced chemiluminescence

Enhanced chemiluminescence is a common technique for a variety of detection assays in biology. A horseradish peroxidase enzyme (HRP) is tethered to an antibody that specifically recognizes the molecule of interest. This enzyme complex then catalyzes the conversion of the enhanced chemiluminescent substrate into a sensitized reagent in the vicinity of the molecule of interest, which on further oxidation by hydrogen peroxide, produces a triplet (excited) carbonyl, which emits light when it decays to the singlet carbonyl. Enhanced chemiluminescence allows detection of minute quantities of a biomolecule. Proteins can be detected down to femtomole quantities,[10][11] well below the detection limit for most assay systems.

Applications

  • Gas analysis: for determining small amounts of impurities or poisons in air. Other compounds can also be determined by this method (ozone, N-oxides, S-compounds). A typical example is NO determination with detection limits down to 1 ppb. Highly specialised chemiluminescence detectors have been used recently to determine concentrations as well as fluxes of NOx with detection limits as low as 5 ppt.[12][13][14]
  • Analysis of inorganic species in liquid phase
  • Analysis of organic species: useful with enzymes, where the substrate is not directly involved in the chemiluminescence reaction, but the product is
  • Detection and assay of biomolecules in systems such as ELISA and Western blots
  • DNA sequencing using pyrosequencing
  • Lighting objects. Chemiluminescence kites,[15] emergency lighting, glow sticks[16] (party decorations).
  • Combustion analysis: Certain radical species (such as CH* and OH*) give off radiation at specific wavelengths. The heat release rate is calculated by measuring the amount of light radiated from a flame at those wavelengths.[17]
  • Children's toys.
  • Glow sticks.

Biological applications

Chemiluminescence has been applied by forensic scientists to solve crimes. In this case, they use luminol and hydrogen peroxide. The iron from the blood acts as a catalyst and reacts with the luminol and hydrogen peroxide to produce blue light for about 30 seconds. Because only a small amount of iron is required for chemiluminescence, trace amounts of blood are sufficient.

In biomedical research, the protein that gives fireflies their glow and its co-factor, luciferin, are used to produce red light through the consumption of ATP. This reaction is used in many applications, including the effectiveness of cancer drugs that choke off a tumor's blood supply. This form of bioluminescence imaging allows scientists to test drugs in the pre-clinical stages cheaply. Another protein, aequorin, found in certain jellyfish, produces blue light in the presence of calcium. It can be used in molecular biology to assess calcium levels in cells. What these biological reactions have in common is their use of adenosine triphosphate (ATP) as an energy source. Though the structure of the molecules that produce luminescence is different for each species, they are given the generic name of luciferin. Firefly luciferin can be oxidized to produce an excited complex. Once it falls back down to a ground state a photon is released. It is very similar to the reaction with luminol.

Many organisms have evolved to produce light in a range of colors. At the molecular level, the difference in color arises from the degree of conjugation of the molecule, when an electron drops down from the excited state to the ground state. Deep sea organisms have evolved to produce light to lure and catch prey, as camouflage, or to attract others. Some bacteria even use bioluminescence to communicate. The common colors for the light emitted by these animals are blue and green because they have shorter wavelength than red and can transmit more easily in water.

Chemiluminescence is different from fluorescence. Hence the application of fluorescent proteins such as Green fluorescent protein is not a biological application of chemiluminescence.

See also

References

  1. ^ Vacher, Morgane; Fdez. Galván, Ignacio; Ding, Bo-Wen; Schramm, Stefan; Berraud-Pache, Romain; Naumov, Panče; Ferré, Nicolas; Liu, Ya-Jun; Navizet, Isabelle; Roca-Sanjuán, Daniel; Baader, Wilhelm J.; Lindh, Roland (March 2018). "Chemi- and Bioluminescence of Cyclic Peroxides". Chemical Reviews. 118 (15): 6927–6974. doi:10.1021/acs.chemrev.7b00649. PMID 29493234.
  2. ^ Shah, Syed Niaz Ali; Lin, Jin-Ming (2017). "Recent advances in chemiluminescence based on carbonaceous dots". Advances in Colloid and Interface Science. 241: 24–36. doi:10.1016/j.cis.2017.01.003. PMID 28139217.
  3. ^ "Luminol chemistry laboratory demonstration". Retrieved 2006-03-29.
  4. ^ "Investigating luminol" (PDF). Salters Advanced Chemistry. Archived from the original (PDF) on September 20, 2004. Retrieved 2006-03-29.
  5. ^ Rauhut, Michael M. (1985), Chemiluminescence. In Grayson, Martin (Ed) (1985). Kirk-Othmer Concise Encyclopedia of Chemical Technology (3rd ed), pp 247 John Wiley and Sons. ISBN 0-471-51700-3
  6. ^ Air Zoom | Glowing with Pride. Fannation.com. Retrieved on 2011-11-22.
  7. ^ Atkins P. and de Paula J. Physical Chemistry (8th ed., W.H.Freeman 2006) p.886 ISBN 0-7167-8759-8
  8. ^ Steinfeld J.I., Francisco J.S. and Hase W.L. Chemical Kinetics and Dynamics (2nd ed., Prentice-Hall 1998) p.263 ISBN 0-13-737123-3
  9. ^ Atkins P. and de Paula J. p.889-890
  10. ^ Enhanced CL review. Biocompare.com (2007-06-04). Retrieved on 2011-11-22.
  11. ^ High Intensity HRP-Chemiluminescence ELISA Substrate. Haemoscan.com (2016-02-11). Retrieved on 2016-03-29.
  12. ^ ECOPHYSICS CLD790SR2 NO/NO2 analyser
  13. ^ Stella, P., Kortner, M., Ammann, C., Foken, T., Meixner, F. X., and Trebs, I.: Measurements of nitrogen oxides and ozone fluxes by eddy covariance at a meadow: evidence for an internal leaf resistance to NO2, Biogeosciences, 10, 5997-6017, doi:10.5194/bg-10-5997-2013, 2013.
  14. ^ Tsokankunku, Anywhere: Fluxes of the NO-O3-NO2 triad above a spruce forest canopy in south-eastern Germany. Bayreuth, 2014 . - XII, 184 P. ( Doctoral thesis, 2014, University of Bayreuth, Faculty of Biology, Chemistry and Earth Sciences) [1]
  15. ^ Kinn, John J "Chemiluminescent kite" U.S. Patent 4,715,564issued 12/29/1987
  16. ^ Kuntzleman, Thomas Scott; Rohrer, Kristen; Schultz, Emeric (2012-06-12). "The Chemistry of Lightsticks: Demonstrations To Illustrate Chemical Processes". Journal of Chemical Education. 89 (7): 910–916. Bibcode:2012JChEd..89..910K. doi:10.1021/ed200328d. ISSN 0021-9584.
  17. ^ Chemiluminescence as a Combustion Diagnostic Venkata Nori and Jerry Seitzman - AIAA - 2008
1,2-Dioxetanedione

The chemical compound 1,2-dioxetanedione, or 1,2-dioxacyclobutane-3,4-dione, often called peroxyacid ester, is an unstable oxide of carbon (an oxocarbon) with formula C2O4. It can be viewed as a double ketone of 1,2-dioxetane (1,2-dioxacyclobutane), or a cyclic dimer of carbon dioxide.In ordinary conditions, it quickly decomposes to carbon dioxide (CO2) even at 180 K (−93.1 °C), but can be detected by mass spectrometry and other techniques.1,2-Dioxetanedione is an intermediate in the chemoluminescent reactions used in glowsticks. The decomposition proceeds via a paramagnetic oxalate biradical intermediate.Recently it has been found that a high-energy intermediate in one of these reactions (between oxalyl chloride and hydrogen peroxide in ethyl acetate), which is presumed to be 1,2-dioxetanedione, can accumulate in solution at room temperature (up to a few micromoles at least), provided that the activating dye and all traces of metals and other reducing agents are removed from the system, and the reactions are carried out in an inert atmosphere.

9,10-Diphenylanthracene

9,10-Diphenylanthracene is a polycyclic aromatic hydrocarbon. It has the appearance of a slightly yellow powder. 9,10-Diphenylanthracene is used as a sensitiser in chemiluminescence. In lightsticks it is used to produce blue light. It is a molecular organic semiconductor, used in blue OLEDs and OLED-based displays.

CSPD (molecule)

CSPD ([3-(1-chloro-3'-methoxyspiro[adamantane-4,4'-dioxetane]-3'-yl)phenyl] dihydrogen phosphate) is a chemical substance with formula C18H22ClO7P. It is a component of enhanced chemiluminescence enzyme-linked immunosorbent assay (ELISA) kits, used for the detection of minute amounts of various substances such as proteins.

Chromatography detector

A chromatography detector is a device used in gas chromatography (GC) or liquid chromatography (LC) to detect components of the mixture being eluted off the chromatography column. There are two general types of detectors: destructive and non-destructive. The destructive detectors perform continuous transformation of the column effluent (burning, evaporation or mixing with reagents) with subsequent measurement of some physical property of the resulting material (plasma, aerosol or reaction mixture). The non-destructive detectors are directly measuring some property of the column eluent (for example UV absorption) and thus affords greater analyte recovery.

Electrochemiluminescence

Electrochemiluminescence or electrogenerated chemiluminescence (ECL) is a kind of luminescence produced during electrochemical reactions in solutions. In electrogenerated chemiluminescence, electrochemically generated intermediates undergo a highly exergonic reaction to produce an electronically excited state that then emits light upon relaxation to a lower-level state. This wavelength of the emitted photon of light corresponds to the energy gap between these two states. ECL excitation can be caused by energetic electron transfer (redox) reactions of electrogenerated species. Such luminescence excitation is a form of chemiluminescence where one/all reactants are produced electrochemically on the electrodes.ECL is usually observed during application of potential (several volts) to electrodes of electrochemical cell that contains solution of luminescent species (polycyclic aromatic hydrocarbons, metal complexes, Quantum Dots or Nanoparticles ) in aprotic organic solvent (ECL composition).

In organic solvents both oxidized and reduced forms of luminescent species can be produced at different electrodes simultaneously or at a single one by sweeping its potential between oxidation and reduction. The excitation energy is obtained from recombination of oxidized and reduced species.

In aqueous medium, which is mostly used for analytical applications, simultaneous oxidation and reduction of luminescent species is difficult to achieve due to electrochemical splitting of water itself so the ECL reaction with the coreactants is used. In the later case luminescent species are oxidized at the electrode together with the coreactant which gives a strong reducing agent after some chemical transformations (the oxidative reduction mechanism).

Electroluminescence

Electroluminescence (EL) is an optical phenomenon and electrical phenomenon in which a material emits light in response to the passage of an electric current or to a strong electric field. This is distinct from black body light emission resulting from heat (incandescence), from a chemical reaction (chemiluminescence), sound (sonoluminescence), or other mechanical action (mechanoluminescence).

Glow stick

A glow stick is a self-contained, short-term light-source. It consists of a translucent plastic tube containing isolated substances that, when combined, make light through chemiluminescence, so it does not require an external energy source. The light cannot be turned off and can only be used once. Glow sticks are often used for recreation, but may also be relied upon for light during military, police, fire, or EMS operations. They are also used by military and police to mark ‘clear’ areas.

Horseradish peroxidase

The enzyme horseradish peroxidase (HRP), found in the roots of horseradish, is used extensively in biochemistry applications. It is a metalloenzyme with many isoforms, of which the most studied type is C. It catalyzes the oxidation of various organic substrates by hydrogen peroxide.

List of light sources

This is a list of sources of light, including both natural and artificial processes that emit light. This article focuses on sources that produce wavelengths from about 390 to 700 nanometers, called visible light.

Luminescence (journal)

Luminescence: The Journal of Biological and Chemical Luminescence is a bimonthly peer-reviewed scientific journal publishing original scientific papers, short communications, technical notes, and reviews on fundamental and applied aspects of all forms of luminescence, including bioluminescence, chemiluminescence, electrochemiluminescence, sonoluminescence, triboluminescence, fluorescence, time-resolved fluorescence, and phosphorescence. The current editor-in-chief is L.J. Kricka (University of Pennsylvania). It was established in 1986 by John Wiley & Sons as the Journal of Bioluminescence and Chemiluminescence and obtained its current title in 1999.

Luminol

Luminol (C8H7N3O2) is a chemical that exhibits chemiluminescence, with a blue glow, when mixed with an appropriate oxidizing agent. Luminol is a white-to-pale-yellow crystalline solid that is soluble in most polar organic solvents, but insoluble in water.

Forensic investigators use luminol to detect trace amounts of blood at crime scenes, as it reacts with the iron in hemoglobin. Biologists use it in cellular assays to detect copper, iron, cyanides, as well as specific proteins via western blotting.When luminol is sprayed evenly across an area, trace amounts of an activating oxidant make the luminol emit a blue glow that can be seen in a darkened room. The glow only lasts about 30 seconds, but investigators can document the effect with a long-exposure photograph. Crime scene investigators must apply it evenly to avoid misleading results, as blood traces appear more concentrated in areas that receive more spray. The intensity of the glow does not indicate the amount of blood or other activator present, but only shows the distribution of trace amounts in the area.

Peroxyoxalate

Peroxyoxalates are esters initially formed by the reaction of hydrogen peroxide with oxalate diesters or oxalyl chloride, with or without base, although the reaction is much faster with base:

Peroxyoxalates are intermediates that will rapidly transform into 1,2-dioxetanedione, another high-energy intermediate. The likely mechanism of 1,2-dioxetanedione formation from peroxyoxalate in base is illustrated below:

1,2-Dioxetanedione will rapidly decompose into carbon dioxide (CO2). If there is no fluorescer present, only heat will be released. However, in the presence of a fluorescer, light can be generated (chemiluminescence).

Peroxyoxalate chemiluminescence (CL) was first reported by Rauhut in 1967 [1] in the reaction of diphenyl oxalate. The emission is generated by the reaction of an oxalate ester with hydrogen peroxide in the presence of a suitably fluorescent energy acceptor. This reaction is used in glow sticks.

The three most widely used oxalates are bis(2,4,6-trichlorophenyl)oxalate (TCPO), Bis(2,4,5-trichlorophenyl-6-carbopentoxyphenyl)oxalate (CPPO) and bis(2,4-dinitrophenyl) oxalate (DNPO). Other aryl oxalates have been synthesized and evaluated with respect to their possible analytical applications [2]. Divanillyl oxalate, a more eco-friendly or "green" oxalate for chemiluminescence, decomposes into the nontoxic, biodegradable compound vanillin and works in nontoxic, biodegradable triacetin [16] . Peroxyoxalate CL is an example of indirect or sensitized chemiluminescence in which the energy from an excited intermediate is transferred to a suitable fluorescent molecule, which relaxes to the ground state by emitting a photon. Rauhut and co-workers have reported that the intermediate responsible for providing the energy of excitation is 1,2-dioxetanedione [1,3]. The peroxyoxalate reaction is able to excite many different compounds, having emissions spanning the visible and infrared regions of the spectrum [3,4], and the reaction can supply up to 440 kJ mol-1, corresponding to excitation at 272 nm [5]. It has been found, however, that the chemiluminescence intensity corrected for quantum yield decreases as the singlet excitation energy of the fluorescent molecule increases [6]. There is also a linear relationship between the corrected chemiluminescence intensity and the oxidation potential of the molecule [6]. This suggests the possibility of an electron transfer step in the mechanism, as demonstrated in several other chemiluminescence systems [7-10]. It has been postulated that a transient charge transfer complex is formed between the intermediate 1,2-dioxetanedione and the fluorescer [11], and a modified mechanism was proposed involving the transfer of an electron from the fluorescer to the reactive intermediate [12]. The emission of light is thought to result from the annihilation of the fluorescer radical cation with the carbon dioxide radical anion formed when the 1,2-dioxetanedione decomposes [13]. This process is called chemically induced electron exchange luminescence (CIEEL).

Chemiluminescent reactions are widely used in analytical chemistry [14, 15]

Phosphorescence

Phosphorescence is a type of photoluminescence related to fluorescence. Unlike fluorescence, a phosphorescent material does not immediately re-emit the radiation it absorbs. The slower time scales of the re-emission are associated with "forbidden" energy state transitions in quantum mechanics. As these transitions occur very slowly in certain materials, absorbed radiation is re-emitted at a lower intensity for up to several hours after the original excitation.

Everyday examples of phosphorescent materials are the glow-in-the-dark toys, stickers, paint, and clock dials that glow after being charged with a bright light such as in any normal reading or room light. Typically, the glow slowly fades out, sometimes within a few minutes or up to a few hours in a dark room.The study of phosphorescent materials led to the discovery of radioactivity in 1896.

Reaction dynamics

Reaction dynamics is a field within physical chemistry, studying why chemical reactions occur, how to predict their behavior, and how to control them. It is closely related to chemical kinetics, but is concerned with individual chemical events on atomic length scales and over very brief time periods. It considers state-to-state kinetics between reactant and product molecules in specific quantum states, and how energy is distributed between translational, vibrational, rotational, and electronic modes.Experimental methods of reaction dynamics probe the chemical physics associated with molecular collisions. They include crossed molecular beam and infrared chemiluminescence experiments, both recognized by the 1986 Nobel Prize in Chemistry awarded to Dudley Herschbach, Yuan T. Lee, and John C. Polanyi "for their contributions concerning the dynamics of chemical elementary processes", In the crossed beam method used by Herschbach and Lee, narrow beams of reactant molecules in selected quantum states are allowed to react in order to determine the reaction probability as a function of such variables as the translational, vibrational and rotational energy of the reactant molecules and their angle of approach. In contrast the method of Polanyi measures vibrational energy of the products by detecting the infrared chemiluminescence emitted by vibrationally excited molecules, in some cases for reactants in defined energy states.Spectroscopic observation of reaction dynamics on the shortest time scales is known as femtochemistry, since the typical times studied are of the order of 1 femtosecond = 10−15 s. This subject has been recognized by the award of the 1999 Nobel Prize in Chemistry to Ahmed Zewail.

In addition, theoretical studies of reaction dynamics involve calculating the potential energy surface for a reaction as a function of nuclear positions, and then calculating the trajectory of a point on this surface representing the state of the system. A correction can be applied to include the effect of quantum tunnelling through the activation energy barrier, especially for the movement of hydrogen atoms.

Serology

Serology is the scientific study of serum and other bodily fluids. In practice, the term usually refers to the diagnostic identification of antibodies in the serum. Such antibodies are typically formed in response to an infection (against a given microorganism), against other foreign proteins (in response, for example, to a mismatched blood transfusion), or to one's own proteins (in instances of autoimmune disease).

Serological tests may be performed for diagnostic purposes when an infection is suspected, in rheumatic illnesses, and in many other situations, such as checking an individual's blood type. Serology blood tests help to diagnose patients with certain immune deficiencies associated with the lack of antibodies, such as X-linked agammaglobulinemia. In such cases, tests for antibodies will be consistently negative.

Serological methods are diagnostic methods that are used to identify antibodies and antigens in patients sample which is serum and plasma. There are some classical serological methods like Agglutination and Precipitation that are used to identify infectious diseases and human blood grouping typing.There are several serology techniques that can be used depending on the antibodies being studied. These include: ELISA, agglutination, precipitation, complement-fixation, and fluorescent antibodies and more recently chemiluminescence.

Some serological tests are not limited to blood serum, but can also be performed on other bodily fluids such as semen and saliva, and cerebrospinal fluid (CSF) which may contain antibodies.

Serological tests may also be used in forensic serology, specifically for a piece of evidence (e.g., linking a rapist to a semen sample).

TCPO

TCPO, or bis(2,4,6-trichlorophenyl) oxalate, is a chemical used in some types of glow sticks.

Tomography

Tomography is imaging by sections or sectioning, through the use of any kind of penetrating wave. The method is used in radiology, archaeology, biology, atmospheric science, geophysics, oceanography, plasma physics, materials science, astrophysics, quantum information, and other areas of science. The word tomography is derived from Ancient Greek τόμος tomos, "slice, section" and γράφω graphō, "to write" (see also Etymology). A device used in tomography is called a tomograph, while the image produced is a tomogram.

In many cases, the production of these images is based on the mathematical procedure tomographic reconstruction, such as X-ray computed tomography technically being produced from multiple projectional radiographs. Many different reconstruction algorithms exist. Most algorithms fall into one of two categories: filtered back projection (FBP) and iterative reconstruction (IR). These procedures give inexact results: they represent a compromise between accuracy and computation time required. FBP demands fewer computational resources, while IR generally produces fewer artifacts (errors in the reconstruction) at a higher computing cost.Although MRI and ultrasound are transmission methods, they typically do not require movement of the transmitter to acquire data from different directions. In MRI, both projections and higher spatial harmonics are sampled by applying spatially-varying magnetic fields; no moving parts are necessary to generate an image. On the other hand, since ultrasound uses time-of-flight to spatially encode the received signal, it is not strictly a tomographic method and does not require multiple acquisitions at all.

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