(N) The SI units is moles/m³.
Definition: The concentration of nuclei in tissue processing at the Larmor frequency in a given region; one of the principal determinants of the strength of the NMR signal from the region.
For water, there are about 1.1 x 105 moles of hydrogen per m³, or 0.11 moles of hydrogen/cm³.
The signal intensity measured is related to the square of the xy-magnetization, which in a SE pulse sequence is given by
Mxy = Mxy0(1-exp(-TR/T1)) exp(-TE/T2)
where Mxy0 = Mz0 is proportional to the proton or spin density, and corresponds to the z-magnetization present at zero time of the experiment when it is tilted into the xy-plane.
True spin density is not imaged directly, but must be calculated from signals received with different interpulse times. The spin density contrast can be generated by using a long TR and sampling the data immediately after the RF pulse (with a TE as short as possible).

(I) Property of all nuclei related to the largest measurable component of the nuclear angular momentum. Non-zero values of nuclear angular momentum are quantized (fixed) as integral or half-integral multiples of (h/2p), where h is Planck's constant. The number of possible energy levels for a given nucleus in a fixed magnetic field is equal to 2I + 1. Similarly, an unpaired electron has a spin of 1/2 and two possible energy levels.

(T) The SI unit of magnetic flux density.
Definition: 1 T is the field intensity generating 1 newton of force per ampere of current per meter of conductor. The tesla unit value is defined as a field strength of 1 weber per square meter of area, where 1 weber represents 1 x 108 (100 000 000) flux lines.
One T is equal to 10 000 gauss, the older (CGS) unit. A field of 1 tesla is quite strong, the Earth's magnetic flux density, at its surface, is about 50 microteslas (μT). The slew rate of MRI devices is measured in mT/m/msec or T/m/sec.

(V) The SI unit of electric potential.
Definition: The amount of potential energy present per unit of charge. 1 volt representing a potential of 1 joule per coulomb of charge.
The unit is named for the Italian scientist Count Alessandro Volta.

(W) The SI unit of power.
Definition: 1 watt is equal to a power rate of one joule of work per second of time or in electrical terms it is the power produced by a current of one ampere flowing through an electric potential of one volt.
Power is the rate at which work is done, or the rate at which energy is expended and is used both in mechanics and in electricity.
The unit is named for James Watt, a British engineer.

(Wb) The SI unit of magnetic flux.
Definition: The rate (per unit of time) in which something crosses a surface perpendicular to the flow. If the something is a magnetic field, then the magnetic flux across a perpendicular surface is the product of the magnetic flux density, in teslas, and the surface area, in square meters.