Which relaxation process is associated with recovery of longitudinal magnetization toward equilibrium after excitation?

• A. T1-relaxation
• B. T2-relaxation
• C. Proton exchange
• D. Spin diffusion

Answer: (A)

T1-relaxation is the process that governs the recovery of longitudinal magnetization along the main magnetic field after an RF pulse tips the net magnetization into the transverse plane. It represents the spin-lattice interaction, where protons exchange energy with the surrounding lattice and gradually return to their equilibrium alignment with the field. The rate of this recovery is characterized by the time constant T1, so the longitudinal magnetization grows back toward its resting value over time as Mz(t) approaches M0.

T2-relaxation, in contrast, describes the decay of transverse magnetization in the xy-plane due to dephasing among spins, not the return of magnetization to the z-axis. Proton exchange and spin diffusion involve other physical processes that influence signal under certain conditions but do not specifically describe the recovery of longitudinal magnetization toward equilibrium after excitation.