Terms Starting with Letter: S sarcoma Cancer arising in the mesenchyme-derived tissues, that is in connective tissues and muscles. scatter coincidence A coincident event in which one or both photons originating from the same annihilation event, and, therefore, can be in true coincidence, are scattered (i.e., deflected) before reaching the detectors. The result of a scatter coincidence is the mispositioning of the event in the image. sequencing Determination of the order of nucleotides in a DNA or RNA. Sequencing also refers to extracting the order of amino acids in proteins. signal transduction The sequence of intermolecular reactions involved in forwarding and amplificating extracellular signals within the cell. single-chain antibody (scAb) Genetically engineered antibody derivatives with pharmacokinetcs for imaging more ideal than natural antibodies. small animal imaging Any imaging technology (nuclear medicine, MR imaging, CT, US, optical imaging) that is optimized for the geometry and size of a mouse or similar sized small animal. This allows for higher spatial resolution, improved signal-to-noise ratio, and lower system costs compared with systems designed for human imaging. smart probe Probe that, after target interaction, actually changes its physicochemical properties. smoothing The most commonly used noise reduction method is image smoothing, which reduces statistical noise at the cost of a reduction in spatial resolution. specific activity The amount of radioactivity of a specific radionuclide divided by the mass of the radionuclide or labeled compound into which it has been incorporated (i.e., mCi/g or mCi/mmole). specific binding The difference between total binding to the tissue and binding that occurs in the presence of an excess concentration of unlabeled ligand (i.e., 100 times the Kd concentration). Binding of a compound to a specific receptor as opposed to general solubility or nonspecific binding of a compound in tissue. steady state Rate of transport or reaction sequence that is not changing with time. stem cell Earliest progenitor cell that divides to give rise to a cell with an identical developmental potential and/or one that is more restricted. suicide gene therapy Gene therapy scheme based on the enzymatic conversion of a nontoxic pro-drug into a lethal drug by a transgene.