Terms Starting with Letter: R radioisotope An isotope is a chemical element having the same atomic number as another but a different atomic weight. Isotopes that spontaneously decay are called radioisotopes. Radioisotopes are used in medicine for research, diagnosis, and the treatment of disease. radiotherapy The treatment of disease using radiation. random coincidences Random or accidental coincidence refers to the chance overlap of two unrelated events within the coincidence resolving time of the detection system, giving an apparent coincidence event. The accidental coincidence is a form of background, which will be distributed randomly about the image. These events are usually measured separately and subtracted from the data. recombinant antibody Genetically engineered Antibodies. See Diabody, Fab fragment, Single-chain antibody. recombinant DNA Any DNA molecule that has been formed by the joining of DNA fragments originating from different sources. reconstruction The mathematical process whereby raw data from scanners (coincidence lines in PET) is used to produce an image of radioactivity distribution. The process usually involves mathematical techniques such as the Fast Fourier Transform or Maximum Likelihood. recovery coefficient The ratio of the measured isotope concentration of a structure in an image to the true isotope concentration (<=1). The recovery coefficient is a measure of the ability of the system to make a quantitative measurement of specific structures. The true activity for a structure is obtained by taking the measured activity and dividing this value by the recovery coefficient. region of interest Region of interest. A ROI defines that region which is of interest to an investigator. The ROI will have a finite number of pixels contained within it, and these will have a mean, a variance, etc. regulatory region The region on a DNA base sequence controling gene expression. reporter enzyme Gene product of a reporter gene. reporter gene Gene that encodes for an easily detectable protein (eg, lacZ for histology or an imaging marker gene such as tyrosinase for in vivo imaging). reporter probe Enzyme substrate that is enzymatically modified by a reporter enzyme to give a readily detectable product (eg, fluorophore). restriction enzyme Any enzyme that recognizes and cleaves a specific short sequence of DNA. These enzymes are used in recombinant DNA technology. retention fraction Fraction of the total tracer delivered to an organ (or region of an organ) that is extracted into and retained by the tissue. This fraction is the residual after clearances of the vascular component and the portion of tracer that rapidly back diffuses from tissue to blood. Usually this fraction of tracer is sequestered in more slowly turning over or membrane binding processes. retrovirus A group of eukaryotic RNA viruses that contain reverse transcriptase allowing for the replication in cells by first making cDNA of the RNA. The cDNA is then integrated into a host chromosome. Genetically modified derivatives of retroviruses can be used in gene therapy. reverse transcription Process by which DNA is synthesized from an RNA template. ribonucleic acid (RNA) Long, linear, single-stranded molecule composed of ribose nucleotides formed by transcription of DNA within the nucleus of the cell. Different types include: messenger RNA, transfer RNA, ribosomal RNA, and other small RNAs. ribozyme An RNA molecule with catalytic activity. RNA polymerase Enzyme that synthesizes RNA according to the DNA template. See Transcription. RNAi (RNA interference) The biological phenomenon whereby double stranded fragments of RNA, once transported inside the cell, efficiently knock down expression of a particular gene by causing degradation of complimentary mRNA and inhibiting its translation. ROI See region of interest.