Why Don't Electrons Replace Protons in MRI Imaging?

[K^2]

I think his confusion stems from the fact that precession direction doesn't change, unlike in the car analogy. But the thing is, if you flip the track, along with the cars, and have cars keep driving in the same direction, it's the same thing as if the cars turned around. That's a bit closer to what the 180° pulse does.

 

[marcusl]

Hmm, even that's not quite the best picture. Remember that the slowest car (spin) must end up closest to the finish line after the flip...
I think the wiki cartoon shows it clearly.

 

[dangus]

I think his confusion stems from the fact that precession direction doesn't change, unlike in the car analogy. But the thing is, if you flip the track, along with the cars, and have cars keep driving in the same direction, it's the same thing as if the cars turned around. That's a bit closer to what the 180° pulse does.

Correct. That was my confusion. After drawing some pictures and staring at it for a bit I see it now. The analogy of flipping the track around was actually quite helpful.

Thank you

 

[K^2]

I talked to an actual NMR theorist about all this. He set me straight on some off the issues. While my understanding of T2 contrast wasn't far off, the thing I overlooked with EPR is the frequencies. Basically, molecular movement that's relevant to T2 in NMR is irrelevant in EPR. So EPR T2 contrast is going to be effectively useless.

T1 contrast in EPR will give you some information about the tissues, and it might or might not be useful, but it would be different from information you'd get from T1 using NMR. Again, different frequencies mean that lattice relaxation happens due to completely different vibrational modes.

So marcusl was absolutely correct. As far as medical imaging goes, EPR wouldn't be able to replace NMR.

 

[marcusl]

Thanks for the new information, K^2. It's nice to know the reasons behind the differences.