In MRI, what term represents the return of protons to magnetic equilibrium after excitation?

In MRI, what term represents the return of protons to magnetic equilibrium after excitation?

• A. Echo time
• B. Repetition time
• C. Transverse relaxation time
• D. Relaxation time

Answer: (D)

After excitation, protons are perturbed from alignment with the main magnetic field and gradually return to equilibrium through two processes: longitudinal relaxation (recovery of magnetization along the field, described by T1) and transverse relaxation (decay of magnetization in the transverse plane, described by T2). The general term that encompasses this whole return to equilibrium is relaxation time. Echo time and repetition time are imaging timings, not the fundamental relaxation processes themselves—TE is when signal is measured after the pulse, and TR is the interval between successive pulses, influencing how much relaxation occurs between excitations. Transverse relaxation time refers specifically to T2, the decay of transverse magnetization, not the full return to equilibrium. Since the question describes the overall return to equilibrium after excitation, relaxation time is the best fit.

After excitation, protons are perturbed from alignment with the main magnetic field and gradually return to equilibrium through two processes: longitudinal relaxation (recovery of magnetization along the field, described by T1) and transverse relaxation (decay of magnetization in the transverse plane, described by T2). The general term that encompasses this whole return to equilibrium is relaxation time.

Echo time and repetition time are imaging timings, not the fundamental relaxation processes themselves—TE is when signal is measured after the pulse, and TR is the interval between successive pulses, influencing how much relaxation occurs between excitations. Transverse relaxation time refers specifically to T2, the decay of transverse magnetization, not the full return to equilibrium. Since the question describes the overall return to equilibrium after excitation, relaxation time is the best fit.