Figure 3.

During RF excitation, if there is no R₁ relaxation or off-resonance, the ratio of R₂ to $R_2^{\text{crit}}$ dictates the excitation dynamic. With a constant applied RF field B₁ and no off-resonance, the excitation is specified by its pulse duration p or nominal excitation angle α = γB₁p. Its effect upon magnetization varies over the range of relative R₂ (thick lines). As the excitation proceeds to longer durations for higher tip angles (blue → yellow gradient), short-T₂ magnetization ceases to increase its transverse component. For all tip angles, the final orientations of magnetization across the range of R₂ (dashed lines tracing the range over R₂) can differ greatly from the nominal excitation angle (dotted lines).