When TR is shorter than a tissue's T2 decay time, the remaining transverse magnetization is known as:

When the repetition time (TR) is shorter than a tissue’s T2 decay time, the relaxation of transverse magnetization follows a specific behavior. In this context, the remaining transverse magnetization after one TR period is referred to as residual transverse magnetization.

This phenomenon occurs because, during TR, not all of the transverse magnetization has decayed back to zero before the next excitation pulse is applied. The T2 decay time represents how quickly the transverse magnetization diminishes due to interactions at the microscopic level within the tissue. If TR is shorter than T2, it essentially means that there is still some transverse magnetization remaining when the next pulse is applied, which is effectively what we measure as residual transverse magnetization.

The other terms do not accurately describe this situation: residual longitudinal magnetization pertains to the z-axis and reflects recovery in the longitudinal direction, while persistent transverse signal and available magnetization do not specifically define the situation of remaining transverse magnetization after incomplete decay. Thus, the term residual transverse magnetization correctly identifies the leftover signal in the transverse plane at the time of the following excitation.