Figure 4.

T1 relaxation: M₀ is the initial longitudinal magnetization (M_z=M₀) before the application of the RF pulse. M₀ is tipped into the transverse plane by the application of the 90° RF pulse (M_z=0). Immediately after removal of the 90° RF pulse (see Figure 3C) (A), magnetization will recover from the transverse plane. While this is occurring (B), the net magnetization along the z-axis (M_z, blue arrows) will increase while transverse magnetization (M_xy, purple arrows) will decrease until the net magnetization returns to its original orientation (C) aligned with the B0 field (M_z = M₀). The magnitude of the net magnetization along the Z axis follows an exponential recovery curve (D), with T1 relaxation time being the amount of time it takes for magnetization on the z-axis to return to M_z.