Terms Starting with Letter: T targeting molecule Molecule that is used to attract radioactive isotopes, enzymes, fluorophores or other labels to it (eg, antibody). thymidine kinase Enzyme that phosphorylates thymidine and related pro-drugs, such as ganciclovir, and has been used as a reporter enzyme when combined with radiolabeled ganciclovir. thymine (T) One of the bases found in nucleic acids. See Base pair. time activity curve A curve representing time vs. radioactivity concentration in some region of interest. tissue curve In PET, "tissue curve" refers to the dynamic data obtained from analyzing a tissue region of interest over a sequence of PET scans. Depending on such factors as the tissue, the region, and the tomograph resolution, the tissue curve may include a vascular or plasma component, due to such factors as the degree of vascularization, the nearness of the region to a major blood supply (or a heart ventricle), and the inherent properties of the tomographic reconstruction algorithm. tracer A measurable substance used to mimic, follow, or trace a chemical compound or process without disturbing the process under study. transcription Process of RNA synthesis whereby one strand of DNA is used as the template to build the complementary RNA. transcytosis Transport of a macromolecule across an epithelial cell layer into the interstitial fluid. Contrast imaging agents can be trasported into the interstitial fluid via transcytosis. transduction Successful uptake, incorporation and expression of foreign DNA into bacterial cells by a viral system. transfection Successful uptake, incorporation and expression of foreign DNA into cells in a culture. transfer RNA (tRNA) A group of small RNA molecules that function as amino acid donors in protein syntheses. transformation Permanent and heritable change to a cell resulting from the uptake and incorporation of foreign DNA. The immortalization of normal mammal cells by treatment with a virus or other cancer-causing agent. transgene A cloned gene that has been introduced and successfully incorporated into a plant or animal and can then be passed on to successive generations. transgenic organisms Plants, animals, or bacteria that have been altered by genetic engineering. translation Synthesis of protein in the cell according to the mRNA amino acid sequence. transmission scan A scan usually obtained prior to the emission scan in order to determine a map of attenuation through a given cross-section. This scan is obtained by using radioactivity placed around the object of interest, and then detecting the amount that actually passes through the object. This scan can be directly used to form attenuation coefficients in order to correct the emission scan. tumor markers Any molecule whose abnormal levels in the body fluids or tissues can be measured indicaticating malignancy. tumor suppressor genes Genes that encode proteins which inhibit the progression of tumor growth through the cell cycle. tumor-associated antigen (TAA) Any antigen that is predominantly released by or displayed on the surface of tumor cells. tumorigenesis Development of a normal cell into a tumor cell by way of multiple mutations. Signs associated with tumor growth are mechanisms to escape apoptosis, invade tissues, form metastases and sustained angiogenesis. two-photon microscopy Microscopic method in which a chromophore is excited by two lower-energy (infrared) photons that are absorbed simultaneously. As only dye molecules within the focus of the beam are excited, the tissue outside the plane of focus is not photobleached or exposed to phototoxicity. tyrosinase Reporter enzyme that catalyzes two fundamental reactions during melanin synthesis helping intensify the MR signals of tyrosinase reporter gene expressing cells.