Anautoradiograph is an image on anX-ray film ornuclear emulsion produced by the pattern of decay emissions (e.g.,beta particles orgamma rays) from a distribution of aradioactive substance. Alternatively, the autoradiograph is also available as a digital image (digital autoradiography), due to the recent development ofscintillation gas detectors[1] or rare-earth phosphorimaging systems.[2] The film or emulsion is apposed to the labeled tissue section to obtain the autoradiograph (also called an autoradiogram). Theauto- prefix indicates that the radioactive substance is within the sample, as distinguished from the case ofhistoradiography or microradiography, in which the sample is marked using an external source. Some autoradiographs can be examined microscopically for localization of silver grains (such as on the interiors or exteriors of cells or organelles) in which the process is termed micro-autoradiography. For example, micro-autoradiography was used to examine whetheratrazine was being metabolized by thehornwort plant or byepiphyticmicroorganisms in thebiofilm layer surrounding the plant.[3]
Inbiology, this technique may be used to determine the tissue (or cell) localization of a radioactive substance, either introduced into a metabolic pathway, bound to a receptor[4][5] or enzyme, or hybridized to a nucleic acid.[6] Applications for autoradiography are broad, ranging from biomedical to environmental sciences to industry.
The use ofradiolabeled ligands to determine the tissue distributions of receptors is termed eitherin vivo orin vitroreceptor autoradiography if the ligand is administered into the circulation (with subsequent tissue removal and sectioning) or applied to the tissue sections, respectively.[7] Once the receptor density is known,in vitro autoradiography can also be used to determine the anatomical distribution and affinity of a radiolabeled drug towards the receptor. Forin vitro autoradiography, radioligand was directly applying on frozen tissue sections without administration to the subject. Thus it cannot follow the distribution, metabolism and degradation situation completely in the living body. But because target in the cryosections is widely exposed and can direct contact with radioligand,in vitro autoradiography is still a quick and easy method to screen drug candidates,PET andSPECT ligands. The ligands are generally labeled with3H (tritium),18F (fluorine),11C (carbon) or125I (radioiodine). Compare toin vitro,ex vivo autoradiography were performed after administration of radioligand in the body, which can decrease the artifacts and are closer to the inner environment.
The distribution of RNA transcripts in tissue sections by the use of radiolabeled, complementary oligonucleotides or ribonucleic acids ("riboprobes") is calledin situ hybridization histochemistry. Radioactive precursors of DNA and RNA, [3H]-thymidine and [3H]-uridine respectively, may be introduced to living cells to determine the timing of several phases of the cell cycle. RNA or DNA viral sequences can also be located in this fashion. These probes are usually labeled with32P,33P, or35S. In the realm of behavioral endocrinology, autoradiography can be used to determine hormonal uptake and indicate receptor location; an animal can be injected with a radiolabeled hormone, or the study can be conductedin vitro.
The rate of DNA replication in a mouse cell growingin vitro was measured by autoradiography as 33 nucleotides per second.[8] The rate ofphage T4 DNA elongation in phage-infectedE. coli was also measured by autoradiography as 749 nucleotides per second during the period of exponential DNA increase at 37 °C (99 °F).[9]
Phosphorylation means the posttranslational addition of aphosphate group to specific amino acids of proteins, and such modification can lead to a drastic change in the stability or the function of a protein in the cell. Protein phosphorylation can be detected on an autoradiograph, after incubating the protein in vitro with the appropriatekinase and γ-32P-ATP. The radiolabeled phosphate of latter is incorporated into the protein which is isolated viaSDS-PAGE and visualized on an autoradiograph of the gel. (See figure 3. of a recent study showing thatCREB-binding protein is phosphorylated byHIPK2.[10])
Inplant physiology, autoradiography can be used to determine sugar accumulation in leaf tissue.[11] Sugar accumulation, as it relates to autoradiography, can described thephloem-loading strategy used in a plant.[12] For example, if sugars accumulate in theminor veins of a leaf, it is expected that the leaves have fewplasmodesmatal connections which is indicative ofapoplastic movement, or an active phloem-loading strategy. Sugars, such assucrose,fructose, ormannitol, areradiolabeled with [14-C], and then absorbed into leaf tissue by simplediffusion.[13] The leaf tissue is then exposed to autoradiographic film (or emulsion) to produce an image. Images will show distinct vein patterns if sugar accumulation is concentrated in leaf veins (apoplastic movement), or images will show a static-like pattern if sugar accumulation is uniform throughout the leaf (symplastic movement).
This autoradiographic approach contrasts to techniques such asPET andSPECT where the exact 3-dimensional localization of the radiation source is provided by careful use of coincidence counting, gamma counters and other devices.
Krypton-85 is used to inspect aircraft components for small defects. Krypton-85 is allowed to penetrate small cracks, and then its presence is detected by autoradiography. The method is called "krypton gas penetrant imaging". The gas penetrates smaller openings than the liquids used indye penetrant inspection andfluorescent penetrant inspection.[14]
The task of radioactive decontamination following theBaker nuclear test atBikini Atoll duringOperation Crossroads in 1946 was far more difficult than the U.S. Navy had prepared for. Though the task's futility became apparent and the danger to cleanup crews mounted, ColonelStafford Warren, in charge of radiation safety, had difficulty persuading Vice AdmiralWilliam H. P. Blandy to abandon the cleanup and with it the surviving target ships. On August 10, Warren showed Blandy an autoradiograph made by asurgeonfish from the lagoon that was left on a photographic plate overnight. The film was exposed by alpha radiation produced from the fish's scales, evidence that plutonium, mimicking calcium, had been distributed throughout the fish. Blandy promptly ordered that all further decontamination work be discontinued. Warren wrote home, "A self X ray of a fish ... did the trick."[15]
Original publication by sole inventorAskins, Barbara S. (1 November 1976). "Photographic image intensification by autoradiography". Applied Optics. 15 (11): 2860–2865. Bibcode:1976ApOpt..15.2860A. doi:10.1364/ao.15.002860.
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