Figure 4.

Graphical illustration of the rationale behind Eq. [2]. Immediately after the tip-down pulse α, the transverse signal component has magnitude M_⊥ and is aligned with the axis of the tip-down pulse (x). During the free precession interval, the transverse component is reduced to M_⊥ e^−T_free/T2 (not indicated in the figure). We then assume that there is a mismatch Δθ between the transverse spin phase and the phase of the tip-up pulse. As a result, only the component of M_⊥ e^−T_free/T2 along the axis of the tip-up pulse, i.e. M_⊥ e^−T_free/T2 cos(Δθ), is tipped back up. Equation [2] is obtained by defining an effective transverse relaxation rate T2_eff defined by the relation M_⊥ e^−T_free/T2 cos(Δθ) ≡ M_⊥ e^{−T_free/T2_eff}.