Why doesn't SE correct T2?
If the SE is able to recover components of the FID lost by field inhomogeneities, why doesn't it correct for T2 decay as well?
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The second RF pulse in a SE sequence is only capable of recovering a portion of the dephased FID signal. Specifically, the SE technique rephases only those spins whose local magnetic field does not change in the interval between RF excitation and echo formation. This condition is met only for stationary spins that experience static local field distortions (e.g., gross inhomogeneity of Bo, a constant or symmetrically-applied imaging gradient, a chemical shift, or static susceptibility gradient).
The condition for perfect RF refocusing of the FID into a SE is not met if the local field distortion changes, if the spin moves into a different part of the field, or if the spin enters a new chemical environment during the course of the imaging experiment. For example, if a water molecule diffuses within a static susceptibility gradient during the interpulse interval, its local field changes and its signal will not refocus into a SE. On the other hand, a stationary proton in a static susceptibility gradient will experience a constant field distortion that will be corrected for by the RF spin flip and refocus into an SE.
True T2 decay processes at the atomic/molecular level occur because of unpredictable time-dependent changes in the local magnetic field sensed by each proton. Accordingly, spins experiencing true T2 interactions have a random dispersal of their phases that are unrecoverable by a simple RF flip.
The condition for perfect RF refocusing of the FID into a SE is not met if the local field distortion changes, if the spin moves into a different part of the field, or if the spin enters a new chemical environment during the course of the imaging experiment. For example, if a water molecule diffuses within a static susceptibility gradient during the interpulse interval, its local field changes and its signal will not refocus into a SE. On the other hand, a stationary proton in a static susceptibility gradient will experience a constant field distortion that will be corrected for by the RF spin flip and refocus into an SE.
True T2 decay processes at the atomic/molecular level occur because of unpredictable time-dependent changes in the local magnetic field sensed by each proton. Accordingly, spins experiencing true T2 interactions have a random dispersal of their phases that are unrecoverable by a simple RF flip.
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