A magnet is a magnetic dipole and it can be represented by a magnetic vector. A moving magnetic field induces a current in a loop of wire. For example, the rotating magnet below induces a sinusoidal current that can be recorded. MRI coils can be used for transmitting and/or receiving. As it is not possible to receive RF signal in the same axis as B0, coils are only sensitive to variations of transverse magnetization vector. Quadrature RF coils (circularly polarized coils) consist of at least two coils that are oriented orthogonal to each over (and both are othogonal to B0 axis). They have a better signal to noise ratio than linear RF coils.