Autoradiography involves localization, recording of a radiolabel within a solid specimen, and production of image in a photographic emulsion. These photographic emulsions consist of silver halide crystals suspended in a clear phase composed mainly of gelatin. In 1867, first autoradiography was obtained accidentally, when a blackening was produced on emulsions of silver chloride and iodide by the uranium salts.
When a β particle or γ-ray from a radionuclide passes through the emulsion, the silver ions are converted to silver atoms. This results in the production of latent image, which is later converted to a visible image after development. During this development (amplification), silver atoms cause entire silver halide crystal to be reduced to metallic silver. Unexposed crystals are removed by dissolution in fixer gives an autoradiographic image, which represents distribution of radiolabel in original sample. Autoradiography can be broadly classified as direct and indirect autoradiography. An autoradiograph is an image recorded on a photographic film, plate or nuclear emulsion produced by the radiation produced by the specimen.
Basic autoradiography procedure (protocol) involves following steps as shown below.
Resolution of activation autoradiography is done by digital computers and several technical means of signals treatments. Analysis of autoradiographic features relates to the elucidation of optical distribution of exposed and treated photo-emulsion used as detector of secondary beta irradiation. It has been used in heart imaging, receptor studies in brain, and for the bone histology.
Labeling detected by the imagers in digital autoradiography is similar to SPECT and PET, but autoradiograph requires a preparation of tissue sections. Digital autoradiography provides information of accurate localization of signal with a better resolution, precise quantification, and possibility to quantify regions of interest. It has better repeatability of the ex vivo acquisitions in comparision to the variability of in vivo acquisitions.
Applications of Autoradiography
The autoradiography is used for the detection of the radiolabeled products since the 1950s.
- Autoradiography is used to study bacterial conjugation, and DNA sequencing (Southern blotting). A comparative study of DNA repair in fibroblast of normal person and patient suffering from xenoderma pigmentosum was done by semi-automated autoradiography.
- It has also been used to determine localization of a radioactive substance in tissues either by introducing it via metabolic pathway, binding it to a receptor or enzyme, or hybridizing it to a nucleic acid.
- Track autoradiography is applied in diagnostics to study sealed radiation sources with low activities and emission of α particles.
- It is used for identification, localization and measurement of the receptors important for studying the mechanism of action and efficacy of drugs, neurotransmitters, neuropeptides, and hormones.
- Receptor autoradiography is used to construct high-resolution maps of the receptor distributions and to examine relationships between specific cellular pathologies and changes in the densities of the receptor. It offers increased anatomical resolution and sensitivity compared with biochemical methods and can provide information without destruction of the tissue sample.
- 3H and 125I are used as the ligands. The autoradiography is carried out at electron microscope level to obtain ultrastructural details in histology and molecular biology.
- A high-speed, alpha autoradiograph has been developed using silver-activated zinc sulfide as an intensifier in conjunction with a high-speed film to provide greater contrast in autoradiograph.
Digital autoradiography is used for the analysis of biological samples and for studying the process of drug metabolism. In recent years, there has been an increasing interest in the use of whole body autoradiography (WBA), to study absorption, distribution, metabolism, and elimination (ADME) of drugs, which is done by image analysis.