**4. Longitudinal relaxation time (T1) of hyperpolarized 129Xe in the brain**

Hyperpolarized 129Xe magnetization enhanced by SEOP is non-recoverable, and the T1 of an hyperpolarized gas is the time elapsed for the signal to decay, because its thermal equilibrium polarization is almost zero relative to the hyperpolarized polarization. Usually, small flip angles has to be used to ration the hyperpolarized magnetization, and it is very important to have a T1 as long as possible to ensure that the signal lasts long enough for the acquisition. Therefore, the T1 of hyperpolarized 129Xe in the brain is a critical parameter. This subsection will discuss the aspects that affect T1 of hyperpolarized xenon, and describe the methods to accurately measure the T1 in the brain (Zhou et. al, 2008).
