Isomeric transition (IT).Decay of the excited state of an isomer of a nuclide
to a lower excited state or the ground state.
Isomers.Nuclides having the same atomic and mass numbers but differing
in energy and spin of the nuclei. For example,^99 Tc and 99mTc are isomers.
Isotones.Nuclides have the same number of neutrons in the nucleus. For
example,^13153 I and^13254 Xe are isotones.
Isotopes.Nuclides having the same atomic number, that is, the same number
of protons in the nucleus. Examples are^146 C and^126 C.
LD50/60. A quantity of a substance that, when administered or applied to a
group of any living species, kills 50% of the group in 60 days.
Linear energy transfer (LET).Energy deposited by radiation per unit length
of the matter through which the radiation passes. Its usual unit is keV/mm.
Mass defect.The difference between the mass of the nucleus and the com-
bined masses of individual nucleons of the nucleus of a nuclide.
Mass number (A).The total number of protons and neutrons in a nucleus
of a nuclide.
Mean life (t).The average expected lifetime of a group of radionuclides
before disintegration. It is related to the half-life and decay constant by
t=1/l=1.44.
Metastable state (m).An excited state of a nuclide that decays to a lower
excited or the ground state by isomeric transition with a measurable
half-life.
Modulation transfer function.A quantitative value of the spatial resolution
of an imaging system.
Neutrino ().A particle of no charge and mass emitted with variable energy
during b+, and electron capture decays of radionuclides. An antineutrino
( ) is emitted in b−decay.
No carrier added (NCA).A term used to characterize the state of a radio-
active material to which no stable isotope of the compound has been
added purposely.
Nucleon.A common term for neutrons or protons in the nucleus of a
nuclide.
Organ, critical.The organ that is functionally essential for the body and
receives the highest radiation dose after administration of radioactivity.
Organ, target.The organ intended to be imaged and expected to receive the
greatest concentration of administered radioactivity.
Pair production.g-Rays with energy greater than 1.02 MeV interact with
the nucleus of an absorber atom, and a positron and an electron are pro-
duced at the expense of the photon.
Photoelectric effect.A process in which a g-ray, while passing through an
absorber, transfers all its energy to an orbital electron, primarily
the K-shell electron of an absorber, and the photoelectron is ejected from
the shell.
Photofraction. The fraction of all detected g-rays that contributes to
the photopeak.
Physical half-life (Tp). SeeHalf-life.
t^12Appendix B. Terms Used in Text 305