More tesla = more resolution, is there limit of human body of how much strong magnetic fields it can handle safely which means there is limit to how strong MRI magnets we can use?
Engineering problems aside, could we scan live humans if we had 20 or 40 tesla MRI machine?
Technically, higher main magnetic field strength doesn't mean more resolution. There are three parts to the scanner: 1) main magnetic field, 2) gradient coils and 3) RF coils.
The main magnetic field strength (1.5T, 3T, 7T, 9T, the last two typically more for research) determine more of the signal to noise (in a very simple sense). This is static - it doesn't ever change.
The gradient coils are what determines the resolution and the rate of switching is the biggest determining factor. There are limits on what they can switch at when used on humans as they can cause peripheral nerve stimulation, vertigo, nausea, metallic tastes etc.
I don't believe static field of 20T etc would be a problem, though there is not much research to prove or disprove that. The problem would be moving into that field as that will cause the same types of effects as the gradient coils switching. Even for 7T, one must not run into the scan room as it can cause peripheral nerve stimulation (and some other effects). Currently, I believe the strongest full body human MRI scanner is 10.5T. Another article talks about 11.7T human scanners (one of them is a "head only" scanner. It also has some nice pictures).
is there limit of human body of how much strong magnetic fields it can handle safely which means there is limit to how strong MRI magnets we can use?
The 2018 {1} study concluded:
Where feasible, effects are quantified for magnetic fields beyond 7T with the conclusion that there are no foreseen barriers either in the technical or human safety aspects of brain MRI and MRS at fields up to 20T. This conclusion is conditioned on results of recommended experiments to verify the predicted level of physiological effects beyond 9.4T. This technology is predicted to enable quantification of biochemical components of the functioning brain not detectable heretofore.
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