Re:Ask HN: What is one tech product you still can’t comprehe...
by mikewarot on Monday June 06, 2022 @07:23PM
(#31647789)
MRI is easy, conceptually, at least. The nucleus of the hydrogen atom (or other target of choice) wobbles at a frequency directly proportional to the ambient magnetic field. MRI machines have a VERY uniform but strong magnetic field. If you left all the other scanning coils off... it would detect ALL of the hydrogen in your body at the same time, at one very narrow frequency band.
By adding a gradient to the field along your spine, for example... they can cause the magnitude of the field to ramp as you go from head to toe. By sampling at frequencies above and below the resonant field, you can now detect that hydrogen in a plane slicing at right angles to your spine, it's still a single value, but now you've narrowed the selectivity to a small slice of a single axis. By sweeping the frequency, you can get a set of values that vary with the hydrogen content of your body along that axis.
You repeat across all 3 axis, and you can effectively sample very small areas of the body sequentially. The modern MRI machines are a highly optimized version of this process. (And I think they sound like dancing chipmunks when they start)
It is possible to use the earth's magnetic field to do MRI scanning. The rate at which you can discriminate signals is proportional to the frequency, and this has to work at audio frequencies (about 2 khz), so images take hours or days.
By adding a gradient to the field along your spine, for example... they can cause the magnitude of the field to ramp as you go from head to toe. By sampling at frequencies above and below the resonant field, you can now detect that hydrogen in a plane slicing at right angles to your spine, it's still a single value, but now you've narrowed the selectivity to a small slice of a single axis. By sweeping the frequency, you can get a set of values that vary with the hydrogen content of your body along that axis.
You repeat across all 3 axis, and you can effectively sample very small areas of the body sequentially. The modern MRI machines are a highly optimized version of this process. (And I think they sound like dancing chipmunks when they start)
It is possible to use the earth's magnetic field to do MRI scanning. The rate at which you can discriminate signals is proportional to the frequency, and this has to work at audio frequencies (about 2 khz), so images take hours or days.