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.

T2 characterizes the rate at which the Mxy 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:

Mxy(t)= M0e-t/T2

T2 decay occurs 5 to 10 times more rapidly than T1 recovery, and different tissues have different T2s. For example, fluids have the longest T2s (700-1200 mSec), and water based tissues are in the 40-200 mSec range, while fat based tissues are in the 10-100 mSec 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 mSec ) and TE ( > 75mSec ) values in conventional Spin Echo sequences, while in Gradient Echo Sequences they can be obtained by using flip angles of less than 40o while setting TE values to above 30 mSec.