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.

Contrast is the relative difference of signal intensities in two adjacent regions of an image.
Due to the T1 and T2 relaxation properties in magnetic resonance imaging, differentiation between various tissues in the body is possible. Tissue contrast is affected by not only the T1 and T2 values of specific tissues, but also the differences in the magnetic field strength, temperature changes, and many other factors. Good tissue contrast relies on optimal selection of appropriate pulse sequences (spin echo, inversion recovery, gradient echo, turbo sequences and slice profile).
Important pulse sequence parameters are TR (repetition time), TE (time to echo or echo time), TI (time for inversion or inversion time) and flip angle. They are associated with such parameters as proton density and T1 or T2 relaxation times. The values of these parameters are influenced differently by different tissues and by healthy and diseased sections of the same tissue.
For the T1 weighting it is important to select a correct TR or TI. T2 weighted images depend on a correct choice of the TE. Tissues vary in their T1 and T2 times, which are manipulated in MRI by selection of TR, TI, and TE, respectively. Flip angles mainly affect the strength of the signal measured, but also affect the TR/TI/TE parameters.
Conditions necessary to produce different weighted images:
T1 Weighted Image: TR value equal or less than the tissue specific T1 time - TE value less than the tissue specific T2 time.
T2 Weighted Image: TR value much greater than the tissue specific T1 time - TE value greater or equal than the tissue specific T2 time.
Proton Density Weighted Image: TR value much greater than the tissue specific T1 time - TE value less than the tissue specific T2 time.

See also Image Contrast Characteristics, Contrast Reversal, Contrast Resolution, and Contrast to Noise Ratio.

Quick Overview

The magic angle is a precisely defined angle, the value is approximately 54.7°. Hence, two nuclei with a dipolar coupling vector at an angle of approximately 54.7° to a strong external magnetic field have zero dipolar coupling.
Magic angle spinning is a technique in solid-state NMR spectroscopy, which employs this principle to remove or reduce dipolar couplings, thereby increasing spectral resolution. In MRI, the magic angle effect visualizes as bright spots through an increased T2 time on short echo time (TE) images, for e.g. collagen fibers of tendons and ligaments, which are oriented at the magic angle of approximately 54.7° to the magnetic field.

Take care that tendons and ligaments are not oriented at about a 54.7° angle to the main magnetic field.

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.

(FRFSE, FR-FSE) The fast relaxation fast spin echo sequence provides high signal intensity of fluids even with short repetition times, and can be used with parallel imaging techniques for short breath hold imaging or respiratory gating for free-breathing, high isotropic resolution MR imaging. After signal decay at the end of the echo train, a negative 90° pulse align spins with long T2 from the transverse plane to the longitudinal plane, leading to a much faster recovery of tissues with long T2 time to the equilibrium and thus better contrast between tissues with long and short T2.
Fast relaxation FSE has advantages also for volumetric imaging as the TR can be substantially reduced and thus the scan time. The sequence can be post processed with maximum intensity projection, surface or volume rendering algorithms to visualize anatomical details in brain or spine MRI. Cerebro spinal fluid pulsation artifacts, often problematic in the cervical or thoracic spine may be reduced by radial sampling, in particular when combined with acquisitions of the PROPELLER type.

See also Fast spin echo, Driven Equilibrium.