The dephasing of the protons is named the T2, spin-spin or transverse relaxation. The T2 time constant is the time taken for spinning protons to lose phase coherence among the nuclei spinning perpendicular to the main field. This interaction between spins results in a reduction in the transverse magnetization. The value of T2 depends on the mobility of the protons. A large mobility results in an average magnetic field variation of zero, resulting in a long T2 period of this tissue.

See also T2 Time.

(T2* or T two star) The observed time constant of the FID due to loss of phase coherence among spins oriented at an angle to the static magnetic field. Commonly due to a combination of magnetic field inhomogeneities, dB, and spin spin transverse relaxation, with the result of rapid loss in transverse magnetization and MRI signal. MRI signals can usually still be recovered as a spin echo in times less than or on the order of T2.
1/T2 * @ 1/T2 + Dw/2; Dw = gDB. The FID will generally not be exponential, so that T2* will not be unique.

The T2 relaxation time (spin spin relaxation time or transverse relaxation time), is a biological parameter that is used in MRIs to distinguish between tissue types and is termed 'Time 2' or T2. It is a tissue-specific time constant for protons and is dependent on the exchanging of energy with near by nuclei. T2 weighted images rely upon local dephasing of spins following the application of the transverse energy pulse. T2 is the decay of magnetization perpendicular to the main magnetic field (in an ideal homogeneous field).
Due to interaction between the spins, they lose their phase coherence, which results in a loss of transverse magnetization and MRI signal. After time T2 transverse magnetization has lost 63% of its original value. This tissue parameter determines the contrast.
The T2 relaxation is temperature dependent. At a lower temperature molecular motion is reduced and the decay times are reduced.
Fat has a very efficient energy exchange and therefore it has a relatively short T2.
Water is less efficient than fat in the exchange of energy, and therefore it has a long T2 time.

See also T2 Weighted Image and Magnetic Resonance Imaging MRI.

(T2 Star) The characteristic time constant that describes the decay of transverse magnetization, taking into account the inhomogeneity in static magnetic fields and the spin spin relaxation in the human body. This results in a rapid loss of phase coherence and the MRI signal. The T2* time is always less than the T2 time.

The T2 time constant, which determines the rate at which excited protons reach equilibrium, or go out of phase with each other. A measure of the time taken for spinning protons to lose phase coherence among the nuclei spinning perpendicular to the main field due to interaction between spins, resulting in a reduction in the transverse magnetization. The transverse magnetization value will drop from maximum to a value of about 37% of its original value in a time of T2.