Why doesn't the MRI magnet interfere with neural electrical signals?

[eok20]

I only have a vague idea of how the brain and MRI work but from my understanding the brain uses electric impusles and an MRI uses a huge magnet. Since a magnetic force redirects a moving charge, why doesn't someone go crazy when they are in an MRI because the neurons don't go where they are supposed to?

Thanks.

 

[selfAdjoint]

I only have a vague idea of how the brain and MRI work but from my understanding the brain uses electric impusles and an MRI uses a huge magnet. Since a magnetic force redirects a moving charge, why doesn't someone go crazy when they are in an MRI because the neurons don't go where they are supposed to?

Thanks.

Neurons don't travel in the brain, they are cells that have long "arms" called axons down which electrochemical signals can pass, and the axons end in bulbs which almost touch the bulbs of other neurons across gaps called synapses, and some bulb emit molecules of various kinds and others absorb certain kinds, and the rates at which these synaptic processes happen have a lot to do with our thinking.

Now way down below this chemical, molecular level is the atomic level, and the nuclei of the atoms have spin, and what the magnet does is get the axes of spins of those nuclei lined up. Then when a chemical event happens in a molecule the resulting electrical force will make the spinning nucleus precess like a gyroscope, and according to the laws of electromagnetism it will emit a photon of EM radiation, whose frequency will depend on the rate of precession and hence on the force and hence will encode the strength of that chemical event. And because the axes were all lined up, the signals from different atoms will coordinate. This is called a Nuclear Magnetic Resonance (NMR) and they have software that can read all these resonances from different nuclei and build pictures, which is called Magnetic Resonance Imaging (MRI). And all this jiggling of molecules is otherwise invisible at the molecular level where thinking is generated.

 

[eeka chu]

selfAdjoint pretty much has it.

Although, there have been some studies into the effects of magnetic fields on the brain. Obviously, the MRI NMR guys have done testing to make sure it's safe to go into the scanners.

But I've also heard of at least one group who've tried stimulating bulk areas of brain tissue using a magnetic field projected from what's basically a coil in a plastic paddle thingy.

They noticed some interesting effects when it was positioned over certain areas of the brain; the person's ability to draw might improve for example. Although, these were highly qualitative experiments as you can imagine.

The total field emitted from an MRI is very high, but the actual density of the field is comparable to a normal permanent magnet. And the ionic currents that make neurons work are tiny and quite rough in operation (they work in a digital, all or nothing manner), so they'd seem to need quite a strong field to make a difference (if you think about them in terms of a hall sensor, were you have a current flowing in one direction, a field penetrating that current region like it would in a bubble chamber and then a voltage difference produced on either side of the device as a result of the eletrons' paths bending left or right slightly, the bigger the quiescent current you have the greater the voltage difference you'll see).

 

[imabug]

But I've also heard of at least one group who've tried stimulating bulk areas of brain tissue using a magnetic field projected from what's basically a coil in a plastic paddle thingy.

They noticed some interesting effects when it was positioned over certain areas of the brain; the person's ability to draw might improve for example. Although, these were highly qualitative experiments as you can imagine.

It's called Transcranial Magnetic Stimulation (TMS), and is being used tested at a lot of different places for various things. A Pubmed search on Transcranial Magnetic Stimulation will yield plenty of papers on the topic.

Depending on what area it's being used on, it can be called TMS, VNS (Vagal Nerve Stimulation) or DBS (Deep Brain Stimulation), but all are variants of the TMS technique. The most common application being examined is as a potential treatment for depression. Some people at the hospital I work at got some research money from DoD a few years ago to see if it can be used to temporarily ward off the effects of sleep deprivation. The TMS technique can pretty much be used to stimulate any area of the brain more efficiently than electrical methods (i.e. electroconvulsive shock), including the motor cortex.

 

[eeka chu]

Some people at the hospital I work at got some research money from DoD a few years ago to see if it can be used to temporarily ward off the effects of sleep deprivation.

Very, very lame, "can we make it into a gun?"

The only problem with magnetic stimulation is the resolution of the field, which can easily by surpassed by a physical probe touching the brain. But if you wanted to stimulate bulk tissue, no problem.