Terms Starting with Letter: P p53 Discovery in 1979, p53 is a tumor suppressor gene, i.e., its activity stops the formation of tumors, located on chromosome 17. Those with only one funcitonal copy of p53 are predisposed to cancer and usually develop several independent tumors in a variety of tissues in early adulthood. partial-volume effect The partial-volume effect describes the relationship between object size and physical image resolution. Tissue concentrations will be underestimated if the object size, e.g., the thickness of the adult left ventricular myocardium (usually 8-12 mm), is less than two times the spatial resolution of the imaging device . The spatial resolution of first-generation tomographs ranged from 15-20 mm, which caused underestimation of myocardial tissue concentrations by about 50%. The magnitude of this underestimation declines with increased spatial resolution. Corrections for underestimation are possible from the relationship between count recovery and object size and the thickness of, for example, the left ventricular myocardium. The relationship between object size and count recovery or the recovery coefficient (as the ratio of observed to true counts) can be established with bar phantoms. "Object size" or wall thickness is measured by echocardiography or, with newer high-resolution tomographs, directly from ECG-gated cross-sectional PET images. Correcting for the partial volume effect is critical for absolute quantitation using PET. partition coefficient The equilibrium ratio of concentrations of a solute in two immiscible solvents. For example, the ratio of the concentration of a tracer in tissue to that in blood, assuming tracer distributes uniformly in blood and tissue. The partition coefficient is unitless since it is a ratio of concentrations. The unit of ml/g frequently used originates from tracer measurements of tissue/blood partition coefficient determined by amount/g in tissue divided by amount/ml in blood. peptide Any of various amides that are derived from two or more amino acids linked by combination of the carboxyl group of one amino acid to the amino group of another. peptide nucleic acid (PNA) An analog of DNA where a peptide replaces a ribose sugar as the backbone. PNA behaves like DNA but is more stable due to its stronger bonding. PNA is an example of a biomimetic entity. peptidomimetics Compounds that have been biologically engineered to mimic the behavior of naturally occurring peptides but are more stable and remain in the blood stream longer. Such qualities make them potentially useful imaging agents. perfusion Perfusion refers to the flow of blood through a vascular bed. Its units are often expressed as the volume transferred per unit time per mass of tissue (e.g., ml/min/g). perfusion pressure Driving pressure for flow of blood through coronary circulation, usually determined as pressure in aorta (inflow pressure) and right atrium (outflow pressure) of coronary circulation. PET See: Positron Emission Tomography. phage display A combinatorial method for identifying specifically binding peptide sequences using a large pool of genetically engineered viruses. pharmacodynamics The study of the time-dependent interactions involved in the process of the distribution, absoption, localization within tissues, biotransformation and excretion of radiopharmaceutical in the body. pharmacogenomics The development of genome-based drugs or contrast agents for individualized medicine. pharmacokinetics The use of compartmental and noncompartmental mathematical models to measure the time-dependent course of a pharmaceutical as it passes through the fluids, tissues, excreta of the body. phenotype The observable properties of a cell or organism as they have developed under genetic and environmental factors. In molecular imaging, probes can be developed to target these phenotypic characteristics. plasma The fluid (non-cellular) component of the blood. plasmid Small, circular extrachromosomal DNA that may replicate independently of the chromosome. Plasmids are commonly genetically engineered and used as cloning vectors. polyclonal antibodies Miscellaneous antibodies produced during an immune reaction in response to antigen immunization. polymerase chain reaction (PCR) Technique for amplifying specific oligonucleotide primers in a mixture of DNA by successive cycles of DNA synthesis followed by brief heat treatment to separate the complementary strands. Real-time quantitative PCR utilizes fluorophore-labeled labeled oligonuleotides that provide a fluorescence signal signal that correlates with the amount of the desired product that is being amplified. positron Positively charged electron (anti-electron) that is emitted from the nucleus of an unstable isotope that has an excessive number or protons. The positron will combine with an electron and convert the mass of the electron and positron into electromagnetic radiation of two equal energy 511-keV annihilation photons. positron 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. It is this annihilation radiation that can be detected externally and is used to measure both the quantity and the location of the positron emitter. Positron Emission Tomography A method of scanning the human body that uses radioactive tracers injected into the patient to view organ systems and functions. As a diagnostic tool, PET is particularly effective in viewing cancer in organs, and monitoring for spread of cancer into other organ systems that other methods simply would not detect. This ability to detect and view cancer cells throughout the body allows doctors to plan the best possible course of treatment for the patient. postgenome era Period following the completion of the Human Genome Project focusing on the actual function of the genes that have been sequenced. Also called functional genomics. pretargeting A two-step procedure for imaging agent or drug targeted delivery. printing The deposit of DNA or proteins onto a DNA chip. pro-drug Physiologically inactive drug precursor that can be made active enzymatically in vivo. progenitor cell Immature cells that have the ability of self renewal as well as differentiate. promotor DNA sequence that determines the initial binding site for RNA polymerase to initiate transcription. protease A proteolytic enzyme. protein One of the most fundamental building substances of living organisms. These macromolecules are made up of amino acids linked together in a specific sequence. They function as enzymes, antibodies, hormones, etc. and are involved in nearly all activities in the cell. proteomics System-wide gene expression analysis to identify, quantitate and characterize proteins. proto-oncogene Proto-oncogenes encode receptor proteins and growth factors. They can be mutated into a cancer-promoting oncogene by either altering the regulation of the protein or changing the protein-coding segment. proximal nearest the point of origin PS product PS (permeability-surface) product is the product of the permeability (ml/min/cm-2) of a substance or tracer across a capillary wall and the capillary surface area per unit weight of tissue (cm2/g). It determines the rate at which a substance or tracer is transported from the vascular to tissue space. Note that PS has the same units as blood flow (i.e., ml/min/g).