Terms Starting with Letter: D dead time The time during which information is being processed and a system cannot respond to new information. This is an important issue for PET scanners with high count rates where corrections for dead time are necessary in order to insure that the true counts are not underestimated. decay Proton-rich radioisotopes have two means of decay that will reduce excess positive charge on the nucleus (a) the nucleus can capture an orbital electron and neutralize positive charge with the negative charge of the electron, or (b) a positive electron (a positron) can be emitted from the nucleus. The positron is an antielectron that, after traveling a short distance, will combine with an electron from the surroundings and annihilate. On annihilation the masses of both the electron and the positron are converted into electromagnetic radiation. In order to conserve energy and linear momentum, the electromagnetic radiation is in the form of two gamma rays of equal energy (511 keV), which are emitted 180 degrees to each other. deoxyribonucleic acid (DNA) The fundamental hereditarty material of all living organisms composed of four kinds of deoxyribose nucleotides (adenine, thymine, cytosine, guanine) creating a long double-helical molecule. diabody A recombinant antibody made up of two single chain antibodies linked together. Diabodies that have been radiolabeled can be used as imaging agents. diffuse optical tomography (DOT) Optical imaging method that uses photon density waves to probe the object (tissue). Photon sources and detectors are used in multiple geometric configurations around the object. Images of deep structures are formed by employing a mathematical reconstruction algorithm based on the solution of diffusion equations, under the assumption that photons have been scattered multiple times. distal Farther from the origin of a body part or the point of attachment of a limb to the body trunk. distribution volume The equivalent volume that a tracer can distribute in tissue with the same concentration as in blood. It has the unit of ml/g. In a closed system, the distribution volume of a compound (e.g., a PET tracer) is the "hypothetical volume" in which the compound would be distributed if its concentration, independent of where it was distributed, were the same as at the sampling site (e.g., the blood plasma). In an open system, if the concentration of the compound at the sampling site remains constant, then the distribution volume of the compound is the amount in the system at the measurement time (total injected – total excreted) divided by the concentration of the compound in the sample. DNA chip A solid carrier (e.g., glass slide, silicon chip) that arranges DNAs, cDNAs, or oligonucleotides into a specific two-dimensional pattern to perform thousands of simultaneous hybridization experiments. See Microarray technology. DNA sequence The order of base pairs comprising a DNA fragment, a chromosome, a gene or an entire genome. drug efficacy The ability of a therapeutic compound to act against a known pathogen, cancer, or other disease-causing agent in a living organism. dual-modality imaging The use of two imaging modalities together (i.e.: CT and PET) which will provide the clinician with more information than could be available than using one by itself. Usually, one modality will provide better anatomic detail while the other will provide physiological or functional information.