Spin-spin relaxation time

Spin-spin relaxation time, known as T₂, is a time constant in Nuclear Magnetic Resonance and Magnetic Resonance Imaging. It is named in contrast to T₁, the spin-lattice relaxation time.

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T₂ characterizes the rate at which the M_xy component of the magnetization vector decays in the transverse magnetic plane. It is the time it takes for the transverse signal to reach 37% (1/e) of its initial value after flipping into the magnetic transverse plane. Hence the relation:

T₂ decay occurs 5 to 10 times more rapidly than T₁ recovery, and different tissues have different T₂. For example, fluids have the longest T₂s (700-1200 ms), and water based tissues are in the 40-200 ms range, while fat based tissues are in the 10-100 ms range.

T₂ images in MRI are often thought of as "pathology scans" because collections of abnormal fluid are bright against the darker normal tissue.

T₂ weighted images

In MRI, T₂ weighted images can be obtained by setting long TR ( >1500 ms) and TE ( > 75 ms) values in conventional spin echo sequences, while in gradient echo sequences they can be obtained by using flip angles of less than 40° while setting TE values to above 30 ms.

See also

• Relaxation (NMR)

References

• McRobbie D., et al. MRI, From picture to proton. 2003
• Hashemi Ray, et al. MRI, The Basics 2ED. 2004.