T2 relaxation

one of two principle contrast determining processes of the NMR phenomenon (the other being spin-lattice, longitudinal or T1 relaxation (I), also known under the names of transverse relaxation and spin - spin relaxation. After a radiofrequency RF pulse has been applied to generate the precessing coherent transverse magnetization Mxy , the energy of the spin system will be redistributed and the transverse magnetization (and thus the measurable signal) will decay away as the spins 'dephase' (dephasing). Long T2 relaxation times only occur in molecules which are tumbling rapidly in solution (see T1 relaxation (I), Fig. 1). More immobile species and solids have rapid spin - spin relaxation and often produce MR signals which decay away before they can even be detected in, for example, a MR imaging pulse sequence. There are several distinct mechanisms occurring at the molecular level which can contribute to relaxation, but dipole-dipole interactions are the major cause of T2 relaxation in spin nuclei within small molecules.

Effective T2 relaxation T2* is much more rapid than T2 relaxation because spin coherence is lost quickly due to the movement of spins in static gradient fields. The decay is also exponential and also called free induction decay FID . This more rapid decay is due to:

1. local macrosopic and microscopic susceptibility changes in the imaged object,

2. the presence of immobile paramagnetic contrast medium (see MR contrast medium)

3. the imperfections of the MR systems, or

4. the intermittent use of linear gradient fields for spatial locailization of MR signals in MR imaging and chemical shift imaging CSI .

While all effects can be reversed with the use of spin echo pulse sequences, gradient echo pulse sequences can only revert the effects of the external localizing gradients. Hence, the relevant T2 relaxation parameter in spin echo imaging is T2, it is T2* in gradient echo imaging.

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