T2Relaxation

T2 relaxation, or transverse relaxation, is the process by which the transverse component of magnetization decays after a radiofrequency excitation in magnetic resonance imaging. It reflects the loss of phase coherence among spins due to spin-spin interactions and molecular motion, and is described by the T2 time constant: the transverse signal decays to about 37% of its initial value after a time equal to T2 in an ideally homogeneous magnetic field.

In practice, microscopic field inhomogeneities and the chemical environment introduce additional dephasing, resulting in an observed decay described by T2*, which is shorter than T2 and is most clearly measured with gradient-echo imaging rather than spin-echo sequences.

The T2 value depends on tissue type, water content, macromolecular composition, temperature, and magnetic field strength. To measure T2, sequences such as multi-echo spin-echo are used. A series of echoes is collected at increasing echo times (TE), and the signal S(TE) is fitted to S0 exp(-TE/T2) to yield T2 maps or T2-weighted images.

Tissue-specific characteristics include cerebrospinal fluid, which has a long T2 (about 2 seconds at 1.5 Tesla); white matter, around 80 milliseconds; and gray matter, around 100 milliseconds. Values vary with field strength and pathology.

Clinical and research applications of T2 imaging include providing contrast that highlights fluid-rich and edematous regions, aiding the identification of demyelination, gliosis, and various lesions. Quantitative T2 mapping offers voxelwise T2 values for monitoring disease progression or treatment response. Limitations include sensitivity to temperature, hydration, and magnetic field nonuniformities, as well as longer acquisition times for mapping.