The reconstruction matrix is the array of rows and columns of pixels in the reconstructed image.

An anaphylactic reaction is a generalized allergic effect (also called anaphylactic shock). Allergic or anaphylactoid reactions range from sneezing, urticaria and itching, bronchospasm, facial and laryngeal edema to life-threatening symptoms including cardiovascular collapse, shock and respiratory distress.
Iodinated contrast materials are safe and widely used. However, anaphylactoid reactions occur rarely after administration of x-ray contrast agents. Most hypersensitivity symptoms appear short time after the intravenous, oral, rectal or other application (e.g., retrograde pyelography), only few are delayed by hours.
Patients with a history of allergic, asthmatic or reactions to contrast agents are at increased risk of anaphylaxis. Pre-treatment with corticosteroids and antihistamines decreases the incidence of an adverse reaction.

(AC) The attenuation correction is usually applied to achieve count rate values independent from tissue densities.

The bioaccumulation factor is the ratio of the concentration of a radionuclide in a biological organism or tissue to the concentration in water or soil.

When a group of spins is placed in a magnetic field, each spin aligns in one of the two possible orientations. The relative numbers of spins with different alignments will be given by the Boltzmann distribution.
Definition: if a system of particles, which are able to exchange energy in collisions is in thermal equilibrium, then the relative number (population) of particles, N1 and N2, in two particular energy levels with corresponding energies, E1 and E2, is given by N1/N2 = exp [-(E1 - E2)/kT ] where k is the Boltzmann constant and T is the absolute temperature. For example, in NMR of protons at room temperature in a magnetic field of 0.25 tesla, the difference in relative numbers of spins aligned with the magnetic field and against the field is about one part in a million; the small excess of nuclei in the lower energy state is the basis of the net magnetization and the resonance phenomenon.