INTRODUCTION
Autoradiography is a photographic technique by which the location of radioactive isotopes may be detected. Radiography is the visualization of the pattern of distribution of radiation. In general, the radiation consists of X-rays, alpha, gamma (g ) or beta (b ) rays, and the recording medium is a photographic film. For classical X-rays, the specimen to be examined is placed between the source of radiation and the film, and the absorption and scattering of radiation by the specimen produces its image on the film. In contrast, in Autoradiography the specimen itself is the source of the radiation, which originates from radioactive material incorporated into it. The recording medium which makes visible the resultant image is usually, though not always, photographic emulsion.
The first autoradiography was obtained accidently around 1867 when a blackening was produced on emulsions of silver chloride and iodide by uranium salts. Such studies and the work of the Curies in 1898 demonstrated autoradiography before, and contributed directly to, the discovery of radioactivity. The development of autoradiography as a biological technique really started to happen after World War II with the development of photographic emulsions and then stripping film (see Rogers, 1979) made of silver halide. Radioactivity is now no longer the property of a few rare elements of minor biological interest (such as radium, thorium or uranium) as now any biological compound can be labeled with radioactive isotopes opening up many possibilities in the study of living systems.
As we all know that Autoradiography has some major advantages and disadvantages. But due to the increase in the demand of radioautographic photographs in the Medical field, the major disadvantages-viz-its sluggishness to yielding results especially during quantitative studies, lack of specificity, have been ruled out to almost nothing due to the recent technical and technological advances in Modern Autoradiography, of which we will broadly elucidate.
REVIEW ON RECENT ADVANCES IN AUTORADIOGRAPHY
In recent autoradiography, the major concern of the scientist is not only to view the quantitative details of the specimen but also its qualitative clarity, amidst other advances, two major innovations has brought autoradiography from darkness to light. These are:
v Gas-Counter Technology in Autoradiography
v Electronic Autoradiography
GAS-COUNTER TECHNOLOGY IN AUTORADIOGRAPHY
Recently, the two major gas counters—Geiger-Muller and Scintillating Counter—have been found more efficient than the Conventional Film Emulsion in the capture of the Radioactive rays(alpha, beta, Gamma) on the photographic plate.
A Geiger counter is a sealed metal tube filled with an inert gas, such as argon. One fine wire, called an electrode, runs down the center of the tube. When the Geiger counter is on, the difference in electric voltage between the positively charged electrode and the negatively charged walls of the tube is about 1000 volts. Although the tube carries such high voltage, no electric current passes between the electrode and the walls because the inert gas does not normally conduct electricity.
When a charged particle, such as an electron, enters the tube, it interacts with the gas to create ions, atoms or molecules with an electric charge. The positive ions move toward the negatively charged walls of the tube and the negative ions move toward the positively charged electrode, creating a momentary electric current between the electrode and the tube wall. The Geiger counter relays the current to the user with an audible click or a visual signal to indicate the presence of a radioactive particle. Each signal represents that one particle has entered the detector.
On the other hand, a scintillation counter functions by the ionization produced by charged particles moving at high speed within certain transparent solids and liquids, known as scintillating materials, causing flashes of visible light. The gases argon, krypton, and xenon produce ultraviolet light, and hence are used in scintillation counters. A primitive scintillation device, known as the spinthariscope, was invented in the early 1900s and was of considerable importance in the development of nuclear physics. The spinthariscope required, however, the counting of the scintillations by eye. Because of the uncertainties of this method, physicists turned to other detectors, including the Geiger-Müller counter. The scintillation method was revived in 1947 by placing the scintillating material in front of a photomultiplier tube, a type of photoelectric cell. The light flashes are converted into electrical pulses that can be amplified and recorded electronically.
Various organic and inorganic substances such as plastic, zinc sulfide, sodium iodide, and anthracene are used as scintillating materials. Certain substances react more favorably to specific types of radiation than others, making possible highly diversified instruments. The scintillation counter is superior to all other radiation-detecting devices in a number of fields of current research. It has replaced the Geiger-Müller counter in the detection of biological tracers and as a surveying instrument in prospecting for radioactive ores.
The Use of a Multistep Avalanche Counter Detector
By using a multistep avalanche/multiwire proportional counter (MSA/MWPC) detector system fitted with delay line readout, high speed digital beta Autoradiographic imaging is demonstrated with submillimeter spatial resolution. In the case of autoradiography with a tritium label, image acquisition requires about one hour compared with several weeks for conventional film techniques. Good proportionality of observed counting rate relative to the known tritium activity is demonstrated. This gas counter system in autoradiography can be used in immunoelectrophoresis, histopathology and DNA sequencing (using 125I, 14C and 35S labels in addition to 3H).
ELECTRONIC AUTORADIOGRAPHY
The Use of CCD and CMOS
CCD (charged coupled device) and CMOS (Complimentary Metal-Oxide Semiconductor) imaging technologies can be applied to thin tissue autoradiography as potential imaging alternatives to using Conventional Film Emulsion. Both imaging sensors (CCD and CMOS) have been operated at room temperature using direct irradiation with images produced from calibrated microscales and radiolabelled tissue samples. We then compare these digital image sensor technologies with the use of conventional film. We show comparative results obtained with 14C calibrated microscales and 35S radiolabelled tissue sections. We also present the first results of 3H images produced under direct irradiation of a CCD sensor operating at room temperature. Compared to film, silicon based imaging technologies exhibit enhanced sensitivity, dynamic range and clarity.
The Use of Microchannel Plate Analyzers
The use of Microchannel Plate Analyzers, the so called Electronic Autoradiography, in Radiopharmacy is also described. The system can be used for quality control of radiopharmaceuticals as well as for scientific research purposes. Quantitative analysis of 2-dimensional radioactive samples of all radionuclides used in Nuclear Medicine (especially 99mTc) can be performed in a very short time with little effort.
RECENT APPLICATIONS OF AUTORADIOGRAPHY
As said earlier, the recent advances in autoradiography have brought about its importance in the world. It is broadly used in the following Medical field:
v Histopathology
v Immunoelectrophoresis
v Pharmacology
v Radiopharmaceuticals
v Biochemistry
v DNA Sequencing
HISTOPATHOLOGY
Histopathology is a branch of pathology concerned with the study of the microscopic changes in diseased tissues. On the other hand, pathology is the branch of medicine concerned with determining the nature and course of diseases by analyzing body tissues and fluids. Pathology is divided into anatomic and clinical pathology. Anatomic pathologists perform autopsies and analyze tissues taken from patients during surgery or by biopsy. Clinical pathologists contribute to the diagnosis of disease by measuring chemicals and cells in blood, sputum, bone marrow, and urine.
The use of the normal film emulsion has a major disadvantage which is time. As we all know that time in the Medical field is very essential to humanity. Normally results from the film could take up to weeks to show full details and this could lead to more serious problems, especially in carcinomas. So, the application of the gas counter technology in this field is a major breakthrough. This technology can be used in;
i. Quick detection of pancreatic Carcinoma by imaging lactose
ii. Assessment of treatment response
IMMUNOELECTROPHORESIS
Immunoelectrophoresis is a method of separating and identifying a mixture of antigens using electrophoresis to separate them and an antigen-antibody reaction to identify them. While electrophoresis on its own is of the movement of electrically charged particles through a gas or liquid as a result of an electric field formed between electrodes immersed in the medium.
Recent developments In autoradiography has helped in producing a high quality imaging of antigen-antibody reaction in Immunoelectrophoresis—a field in Immunology.
PHARMACOLOGY
Yet again, Autoradiography has become a very important and highly specific tool to pharmacologically characterize receptors in tissue (unlike tissue bath preparations). It provides the location of receptors (etc) in tissue. Very importantly, it enables characterization of receptors in different tissues between different animals or brain regions. And lastly, it is technically easy to handle.
An important aspect of its breakthrough in pharmacology is its use to check the increased Nicotinic Receptors in Brains of Smokers. Nicotinic cholinergic receptors are widely distributed in the vertebrate central nervous system (CNS), where they appear to be associated with the axons and cell bodies of neurons that comprise several major neurotransmitter systems in the brain, including neurons that use dopamine, norepinephrine, acetylcholine, g-aminobutyric acid, and glutamate. Because of the strategic locations of these receptors, nicotine may exert widespread influences on the function of several important neural pathways in the CNS. Nicotinic receptors in the cerebral cortex and hippocampus are of particular interest because of their possible roles in cognition, memory, arousal, attention, and anxiety, all of which are reported to be affected by nicotine and/or withdrawal from nicotine (Levin, 1992). In the present study, we compared neuronal nicotinic receptor binding sites in the prefrontal cerebral cortex (area 10 and 11), temporal cerebral cortex (area 38), and the hippocampus from autopsied brains from smokers and age-matched nonsmokers.
CONCLUSION
In summary, modern advancement in science and technology has been able to carry Autoradiography along. That is, its problems which are;
Ø Speed
Ø Non-specificity
Ø Image quality
have been almost completely taken care of. So, in a nutshell tissue preparations which could take longer time can now be done in couple of minutes. This has made things easy for scientists, especially the Autoradiographers.
REFERENCES
1. Barthe N, Coulon P, Hennion C, Ducassou D, Basse-Cathalinat B, Charpak G (May 1999). "Optimization of a new scintillation gas detector used to localize electrons emitted by 99mTc". J Nucl Med. 40 (5): 868–75. PMID 10319763.
2. Encyclopedia of Life Sciences: Phosphorimager
3. Kuhar M, Yamamura HI (Jul 1976). "Localization of cholinergic muscarinic receptors in rat brain by light microscopic radioautography". Brain Res. 110 (2): 229–43.doi:10.1016/0006-8993(76)90399-1. PMID 938940.
4. Young WS, Kuhar MJ (Dec 1979). "A new method for receptor autoradiography: [3H]opioid receptors in rat brain". Brain Res. 179 (2): 255–70. doi:10.1016/0006-8993(79)90442-6. PMID 228806.
5. Jin L, Lloyd RV (1997). "In situ hybridization: methods and applications". J Clin Lab Anal. 11 (1): 2–9. doi:10.1002/(SICI)1098-2825(1997)11:1<2::aid-jcla2>3.0.CO;2-F.PMID 9021518.
