The principles and differences of T1, T2 and t2* in magnetic resonance

From the point of view of physics, to understand the differences between these concepts, we need to understand the magnetization of the nucleus, this article through some illustrations of these concepts concise introduction.

First, the most basic principle of magnetic resonance is the quantum mechanical properties of the hydrogen nucleus in the magnetic field of spin motion. In a uniform magnetic field B0, the rotation of the hydrogen nucleus (spin) occurs in two spin states, one in the direction of the magnetic field (up), and the other in the opposite direction (down state) along the magnetic field. The frequency of rotation is related to the strength of the magnetic field, called the Rameau frequency. On average, most nuclei are rotated in the direction of the magnetic field, so when the balance is reached, a magnetic M0 (magnetization) with the same direction as the B0 is produced, and the M0 is the source of the MRI signal.

The direction of the B0 is defined as the z-axis direction, this time adding a direction perpendicular to the z-axis of the magnetic field B1, so that the B1 also along the B0 direction with Rameau frequency rotation:

For the sake of simplification, it is envisaged that there is a rotating reference system in which the rotational frequency of the reference system is also the Rameau frequency, and the B1 is stationary relative to the reference system. In the role of the B1, M0 will be rotated B1 for the rotation axis, after a very short time, M0 rotated 90 degrees, fell on the X-y plane.

This B1 is called a 90-degree pulse, at which point the b1,x-y plane is removed from the magnetization of MXY, its size is the same as M0, the z-axis magnetization is MZ, its size is 0. By the way, the signal acquisition coil in MRI is measured mxy, if the size of the MXY is 0, there is no signal output.

When the B1 is removed, the magnetization state is gradually restored to its original equilibrium state, which is called relaxation (relaxation) and is manifested in two ways: Mxy gradually reverts to 0,mz and gradually restores to M0.

In the relaxation process, MZ exponential growth, the time constant is t1,mxy in the relaxation process exponential attenuation, the time constant is T2.

T1 relaxation occurs because of the energy exchange between the rotating nucleus and the surrounding environment (that is, the lattice, the lattice), the number of nuclei that cause the up state and the down state to change, and the number distribution to revert back to the non-B1 equilibrium, so MZ reverts to M0, and T1 is also called spin-lattice relaxation time.

The occurrence of T2 relaxation also has a certain degree of these factors, but in addition, because the rotating nuclei have energy exchange between each other, the rotation of each nucleus of the phase becomes random, its magnetization vector of the net (MXY) gradually decay. So T2 is also called spin-spin relaxation time.

So here's the question, t2*, what's going on?

In fact, the main magnetic field B0 can not achieve absolute uniformity, because the hydrogen atom rotation frequency and the strength of the B0, uneven B0 will lead to different positions of the hydrogen atom rotation frequency is different, so the hydrogen atom rotation will be out of sync, thus accelerating the mxy attenuation, the attenuation is exponential decay, the time constant is t2*. T2* is smaller than T2.