Transverse nuclear magnetization decay ($T_2$)

Transverse relaxation describes loss of ensemble transverse magnetization. It must not automatically be identified with pure dephasing. In a simple completely positive two-level relaxation model,where $T_{\varphi }$ describes additional pure dephasing. Thus population recovery itself contributes to transverse decay. A spread of static magnetic fields can further cancel ensemble phases without the same microscopic relaxation; an observed free signal may have an effective $T_2^{\ast }$ rather than homogeneous $T_2$. The simple exponential examples below assume a uniform field and a single homogeneous $T_2$.