What are T1 and T2 in MRI?
T1, the longitudinal relaxation time, is a measure of how long it takes atomic nuclei to realign longitudinally with the main magnetic field after they have been knocked over by an RF pulse. T2, the transverse relaxation time, is a measure of how long it takes a group of atomic nuclei that have been knocked over by an RF pulse to become maximally disordered in the transverse plane. Different tissues have different T1 and T2 times (usually on the order of milliseconds [ms]), which form the basis for T1-weighted and T2-weighted MR image contrast, respectively. The T2 of a tissue is always less than its T1.