Spin–spin relaxation

T2 relaxation curve

Spin-spin relaxation time, known as T₂, is a time constant characterizing the signal decay in nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI). It is named in contrast to T₁, the spin-lattice relaxation time.

T₂ is the rate at which M_xy, the transverse component of the magnetization vector, exponentially decays towards zero. It is the time it takes for the magnetic resonance signal to reach 37% (1/e) of its initial value after its generation by tipping the longitudinal magnetization towards the magnetic transverse plane. Hence the relation:

$M_{xy}(t)=M_{xy}(0)e^{-t/T_{2}}\,$

T₂ relaxation generally proceeds more rapidly than T₁ recovery, and different samples and different biological tissues have different T₂. For example, fluids have the longest T₂s (in the order of seconds for Protons), and water based tissues are in the 40-200 ms range, while fat based tissues are in the 10-100 ms range. Amorphous solids have T₂s in the range of milliseconds, while the transverse magnetization of crystalline samples decays in around 1/20 ms.

T₂ weighted images

In MRI, T₂ weighted images can be obtained by setting long repetition time(TR) ( >1500 ms) and echo time(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.

T₂ weighted images in MRI are often thought of as "pathology scans" because collections of abnormal fluid are bright against the darker normal tissue (reference needed). In the MRI of prostate cancer, cancerous regions of the peripheral zone of the gland often appear as hypo-intense regions of the gland in T2 weighted MR images.

See also

• Relaxation (NMR)

References

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