Hi there, I am trying to get piece this together. Wondering if you could explain why the 180 rephasing pulse doesnt nullify the phase shift embedded by the phase encoding gradient?
This is a great question! Part of the conceptual challenge is that this question is trying to reconcile both the quantum mechanical (spins) and classical (magnetization) descriptions of MRI, and quantum mechanics (QM) doesn't always behave like a classical system (magnetization experiencing forces from the magnetic field, causing it to rotate). It's also not completely correct that the 90 degree "rephases the spins". Rather, a better way to think of it is that the initial 90 degree pulse changes the net magnetization along z into a phase coherence. If there were no net magnetization along z, then from the classical viewpoint there would be no magnetization tipped into the transverse plane, and thus from the QM viewpoint the 90 degree pulse would not rephase the spins. Similarly, the 180 degree pulse changes the phase coherence along one direction (in the transverse plane) into the same coherence on the opposite side. So if you applied the 180 degree pulse immediately after the 90, you would have a lot of phase coherence. If you waited longer, there would be less phase coherence to flip to the other side. However, since the spins are flipped to the opposite side, those that were ahead in terms of phase are now behind, and the spins eventually refocus into the spin echo.
@@neuroimagingresearchmethods yeah I think the problem is understanding how the pulse changes spin direction coherence along Z into phaze coherense of particular protons, there is some heavy duty magic here
Hi there, I am trying to get piece this together. Wondering if you could explain why the 180 rephasing pulse doesnt nullify the phase shift embedded by the phase encoding gradient?
About 5:19
Doesn't the T2* have the shorter TE compared to T2?
Why does 180, degree pulse not rephase the spins completely like 90 degree one?
This is a great question! Part of the conceptual challenge is that this question is trying to reconcile both the quantum mechanical (spins) and classical (magnetization) descriptions of MRI, and quantum mechanics (QM) doesn't always behave like a classical system (magnetization experiencing forces from the magnetic field, causing it to rotate). It's also not completely correct that the 90 degree "rephases the spins". Rather, a better way to think of it is that the initial 90 degree pulse changes the net magnetization along z into a phase coherence. If there were no net magnetization along z, then from the classical viewpoint there would be no magnetization tipped into the transverse plane, and thus from the QM viewpoint the 90 degree pulse would not rephase the spins. Similarly, the 180 degree pulse changes the phase coherence along one direction (in the transverse plane) into the same coherence on the opposite side. So if you applied the 180 degree pulse immediately after the 90, you would have a lot of phase coherence. If you waited longer, there would be less phase coherence to flip to the other side. However, since the spins are flipped to the opposite side, those that were ahead in terms of phase are now behind, and the spins eventually refocus into the spin echo.
@@neuroimagingresearchmethods yeah I think the problem is understanding how the pulse changes spin direction coherence along Z into phaze coherense of particular protons, there is some heavy duty magic here