Precession is a wobbling motion that occurs when a spinning object is the subject of an external force. Relevant to MRI, the proton of a hydrogen nucleus spins around its axis giving it an angular moment (quantum mechanics). Through the protons positive charge and its spin it generates a magnetic field and gets a magnetic dipole moment (MDM) parallel to the rotation axis. If placed in a magnetic field the magnetic dipole moment will precess about the direction of the magnetic field with an angular frequency (Larmor frequency). The Larmor equation dictates that the frequency of the precession at higher field strengths is higher.

A type of magnet that utilizes the principles of electromagnetism to generate the magnetic field. Typically large current values and significant cooling of the magnet coils is required. The resistive magnet does not require cryogens, but needs a constant power supply to maintain a homogenous magnetic field, and can be quite expensive to maintain.
Resistive magnets fall into two general categories - iron-core and air-core.
Iron-core electromagnets provide the advantages of a vertically oriented magnetic field, and a limited fringe field with little, if any, missile effects due to the closed iron-flux return path.
Air-core electromagnets exhibit horizontally oriented fields, which have large fringe fields (unless magnetically shielded) and are prone to missile effects. Resistive magnets are typically limited to maximum field strengths of approximately 0.6T.

Time-dependent electromagnetic fields are significantly attenuated by conducting media (including the human body); the skin depth gives a measure of the average depth of penetration of the RF field. A high power frequency tunable RF source can be rapidly switched on and off. This produces a large RF field perpendicular to the magnetic field. This RF field is focused by the body coil. The RF source and coils must be tunable in both frequency and impedance to 'match the impedance' of the patient's body.

The skin depth may be a limiting factor in MR imaging at very high frequencies (high magnetic fields). The skin depth also affects the Q of the coils.

A coil of wire wound in the form of a long cylinder. When a current is passed through the coil, the magnetic field within the coil is relatively uniform. Solenoid RF coils are commonly used when the static magnetic field is perpendicular to the long axis of the body.
The reason against a solenoidal coil is that the signal is missed when the axis of the solenoidal coil is coaxial with the horizontal magnetic field Bo. If the coil's axis can be put near a right angle (60° to 90°) with the principal field, so the loss in the signal is greatly dismissed, and good SNR can be achieved.
The two forms are single turn solenoid and multiple turn solenoids.

Superparamagnetism occurs when the material is composed of very small crystallites (1-10 nm). In this case, even though the temperature is below the Curie or Néel temperature and the thermal energy is not sufficient to overcome the coupling forces between neighboring atoms, the thermal energy is sufficient to change the direction of magnetization of the entire crystallite. The resulting fluctuations in the direction of magnetization cause the magnetic field to average to zero. The material behaves in a manner similar to paramagnetism, except that instead of each individual atom being independently influenced by an external magnetic field, the magnetic moment of the entire crystallite tends to align with the magnetic field.
In MRI superparamagnetic iron oxide is used as a contrast agent.

See also magnetism, ferromagnetism, paramagnetism, and diamagnetism.