As title. Thank you. In the context of MRI, why/how does applying 90 degree radiofrequency at Lamour frequency put proton precessions into phase? I understand 90 degree RF minimises net magnetic moment, such that there is least difference between number of protons in parallel (with external field) and number of protons in antiparallel alignment (ie. those at higher energy states, and are against external field). The 90 degree RF also puts proton spins into phase. Why / how does the 90 degree RF wave manage to put protons into phase please? Thank you.