In MRI physics, what term describes the alignment of randomly precessing protons into a common phase after an RF pulse?

• A. phase coherence
• B. relaxation
• C. spin density
• D. magnetization flip

Answer: (A)

Phase coherence is the alignment of randomly precessing protons into a common phase in the transverse plane after the RF pulse. When the pulse tips the net magnetization away from the longitudinal axis, the spins momentarily precess together with the same phase, creating a coherent transverse magnetization that generates the MR signal detected by the coil. Over time, dephasing from field inhomogeneities and spin-spin interactions causes the signal to decay, which is transverse relaxation. The other terms don’t describe this synchronized phase alignment: relaxation refers to recovery of longitudinal magnetization and loss of transverse signal over time; spin density is about how many spins contribute to the signal; magnetization flip refers to the angle of rotation caused by the RF pulse, not the subsequent phase alignment.