In MRI, what term describes the return of nuclei to thermal equilibrium after the RF pulse is turned off?

• A. phase coherence
• B. relaxation
• C. signal decay
• D. spin density

Answer: (B)

Relaxation is the process by which excited nuclei return to their resting, thermally equilibrated state after the RF pulse ends. In MRI, the pulse tips the net magnetization away from alignment with the main magnetic field. After the pulse, spins exchange energy with their surroundings (spin-lattice) and with each other, gradually restoring the original longitudinal magnetization along the main field and, at the same time, losing transverse coherence. This overall return to equilibrium comprises two related processes: T1 relaxation, the recovery of longitudinal magnetization, and T2 relaxation, the decay of transverse magnetization. The term relaxation thus best describes the system returning to thermal equilibrium. Phase coherence refers to the uniformity of spin phases in the transverse plane, which is a feature within the relaxation process but not the overall return to equilibrium. Signal decay specifically denotes the loss of transverse signal over time, a component of relaxation, while spin density is about how many spins are present, not their return to equilibrium.