Which of the following describes T1 relaxation in MRI?

T1 relaxation in MRI specifically refers to the process of regrowth of the longitudinal magnetization of protons after they have been disturbed by a radiofrequency pulse. When protons are excited, they are flipped out of alignment with the main magnetic field. T1 relaxation, also known as spin-lattice relaxation, describes how these protons return to their equilibrium state, aligning back with the magnetic field while also releasing energy to their surrounding lattice (the molecular environment). This process is characterized by a time constant known as T1, which varies depending on the tissue type and its environment.

The other options describe different phenomena related to magnetic resonance but do not accurately define T1 relaxation. Transverse magnetization decay refers to T2 relaxation, where the coherence of spins in the transverse plane diminishes over time due to interactions among them. The exchange of energy among protons is a broader concept that may occur in various contexts within magnetic resonance but is not specifically indicative of T1 relaxation. Dispersion of magnetic fields is not relevant to the definition of T1 relaxation and does not pertain to how protons regain their equilibrium state after excitation. Thus, the correct choice captures the essence of T1 relaxation by focusing on the regrowth of longitudinal magnetization.