What statement best describes phase coherence achieved by the RF pulse in MRI?

• A. Spins align with the main field
• B. No net magnetization
• C. All spins are in phase briefly after excitation
• D. Signal decays immediately

Answer: (C)

Phase coherence is the moment right after the RF pulse when all the spins have been tipped into the transverse plane with the same phase in their precession. The RF pulse applies a uniform flip across the voxel, so every spin experiences the same rotation about the transverse field. As a result, their transverse components add up constructively, producing a strong net transverse magnetization and a detectable MR signal. Over time, slight field inhomogeneities and spin–spin interactions cause the spins to dephase, leading to signal decay, but immediately after excitation they are in phase, which is what the best description is.