MRI Physics And Clinical Applications Practice Exam

The key idea is that rephasing transverse magnetization recovers signal that would otherwise be lost to dephasing. After the initial 90-degree flip, spins in slightly different local magnetic fields begin to accumulate phase at different rates, causing the transverse signal to decay faster than true T2 due to field inhomogeneities (T2* effects). Introducing a 180-degree RF pulse flips the spins in the transverse plane, so their subsequent phase evolution proceeds in the opposite direction. At a later time, the spins come back into phase, producing a spin echo. This refocusing cancels the dephasing caused by static field inhomogeneities, so the remaining signal decay reflects true T2 relaxation rather than T2*. The frequency-encoding and slice-select gradients SAMPLEmainly govern spatial encoding and selection and do not cause this rephasing.