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In which ways is a stronger magnet better for magnetic resonance imaging? I read that:

The field strength of the magnet will influence the quality of the MR image regarding chemical shift artifacts, the signal to noise ratio (SNR), motion sensitivity and susceptibility artifacts.

but I don't understand why. I suppose the dephasing is quicker, reducing the transverse net magnetization more quickly. Does this go for the longitudinal net magnetization too? How would this reduce motion sensitivity and SNR?

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this is not a a bad question for biology - i was just answering it when you closed it. Mark the v short answer is that stronger magnetic fields allow the machine to distinguish between the biological state of the atoms they are reading. lots of biologists use MRI and other physical methods to investigate biology. where do all those protein structures come frome? –  shigeta Mar 3 '12 at 15:03
    
BTW Mark, If you want to reestablish control as the poster of the question create an account here on Physics.SE and link it with your Biology account. And welcome aboard. –  dmckee Mar 3 '12 at 17:56
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1 Answer

The magnetic moment of the nuclei being read by the MRI instrument does have a stronger coherence when the magnet is stronger - the energy difference between the excited and ground state of the atom's nuclear spin is larger.

This does lengthen the relaxation time as you say, but the really important effect is that the resolution of the MRI instrument becomes greater. When differentiating between nuclear spins of protons in cancer tissue vs healthy tissues is very small - but fortunately it is there. Most of the medical applications need as much resolution as they can get because there is a massive amount of protons in water or phosophorus the machine has to differentiate over - most if the 'signal' is miniscule compared to native state biological atoms the MRI is also seeing.

@DrSAR I'm sort of confused by your comment. Not sure what you are saying.

1) MRI is one of the more common ways to identify cancer. Maybe if you could be more explicit in why you do not believe NMR (I use this term interchangably with MRI) can differentiate physiological state of tissues? This has been pretty well discussed since the 1980s.

this quote passed wikipedia's editorial process okay:

MRI provides good contrast between the different soft tissues of the body, which makes it especially useful in imaging the brain, muscles, the heart, and cancers compared with other medical imaging techniques such as computed tomography (CT) or X-rays. Unlike CT scans or traditional X-rays, MRI does not use ionizing radiation.

2) MRI does not need protons, any nucleus with a spin =1/2 is a decent target for an experiment. again P31 is one of the more popular targets because it has a high natural abundance, but O17 and C13 are also accessible through most machines (and their high intensity magnetic fields) today. More exotic spins (3/2 5/2 etc) can also be used, but the sensitivity and analysis of the returned signal are more complicated. So many different kinds of atoms can and are used in MRI experiments.

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Sorry, but there are multiple mistakes or at least misleading statements: differentiating cancer from healthy tissues based on either relaxation times or spectral resolution (not clear which you propose) is not that easy at best, impossible most of the time. There are also no protons in phosphorus and atoms are not classified based on being biological or not. –  DrSAR Oct 24 '12 at 7:47
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