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.

Magnetic permeability is the degree of magnetization of a material that responds linearly to an applied magnetic field. The SI units of permeability are henry per meter (H/m) or newton per square ampere (N/A²).

A vector is a quantity characterized by a magnitude (in mathematics a number, in physics a number times a unit) and a direction (and a point of application), often represented graphically by an arrow. The length of the line segment represents the magnitude, and its orientation in space represents its direction. Vector quantities can be added to or subtracted from one another. Used in diagnostic imaging to describe forces, e.g. magnetic moment, spin, magnetization etc.