Part 1: The Science Behind TMS and MRI Machines
A couple weeks ago, my FemSTEM group had the opportunity of visiting the UCSB Medical Neuroscience Labs. The researchers were working on a study regarding women’s hormones and their relation to the brain. We also got demonstrations of TMS and fMRI machines, both of which have cool physics and mathematics principles behind them.
Both types of machinery have the same kind of physics behind them. In physics, electric currents generate magnetic fields and magnetic fields generate electric currents. Starting with the first principle (electric currents generate magnetic fields), honestly, scientists fall short at fully answering this. What we do know, however, is that the velocity and direction of moving, charged participles affect the magnetic field generated. With a larger velocity, a larger magnetic field is generated. The direction also matters since magnetic fields have directions as well. If you were to look at a basic bar magnet, the magnetic field goes from the north pole to the south pole. One of the “right-had-rules“ demonstrates which way a field would be generated with a positive moving particle. To try it yourself, imagine a NEGATIVELY charged particle moving towards you. Point your RIGHT thumb towards you as well. Now, with the rest of your fingers, pretend they are spinning around your thumb and determining that direction. They should be “moving“ in an anti-clockwise direction. Because the charged particle is negative, however, the actual magnetic field generated with the opposite of this and is instead in the clockwise direction. In theory, electric currents and magnetic fields are the same, so the converse of the described relationship is also true.
But hoe does a TMS machine use this? Below is what TMS machine looks like:
Notice the figure-8-shaped paddle in the upper left corner. Inside of this paddle, there are two separate coils of electromagnetic material that send currents in opposite directions. Using the right-hand rule, two currents traveling in opposite circular directions generate two magnetic fields in the same direction. This strengthens the magnetic field and generates roughly 2 Teslas, units of measurements for magnetic field strength. For comparison, the fridge magnet that holds your fridge together is roughly 0.001 teslas, so this TMS magnetic field is huge.
After this magnetic field is generated, it generates an electric action in the human brain and neurons. Using the principle magnetic fields generate currents, the TMS machine can activate the electric currents that cause movement. Neurons in the human body communicate via “action potentials“ and neurotransmitters. Action potentials are created with fluctuating ion concentration levels that rise and fall on various sides of the neuron membrane. Because these ions have a charge and are reliant on that charge difference (voltage,) The TMS inducing a current mimics the events caused by a natural action potential. As the neurotransmitters communicate from neuron to neuron, eventually a motor neuron releases a chemical that induces muscle fibers to contract causing movement. When the TMS paddle is near either arm tendons or near the motor strip in the human brain, the magnetic field from the TMS machine and its relationship to electric currents causes muscle movement. A TMS machine can also be used to stimulate brain activity used to treat mental disorders such as depression, all because of the relationships between physics properties.
After we got demonstrations of the TMS machine, the UCSB Researchers started the MRI demonstration. The MRI machine consists of strong magnets and radio wave generators all connected to a computer to generate imagines of soft tissue. The large magnet is shaped into a tube (the tube you do into) which creates a unified magnetic field around certain parts of the body. This magnet affects the water molecules in the human body making them all move at the same frequency.
There is a little more detail in how the generated magnetic field affects atoms in tissue. Unlike the TMS machine, this magnetic field is not looking to induce current or movement. Instead, it utilizes a property of elementary particles: their spin. In a magnetic field, a particle with spin has to either have “spin up“ (aligned with the field) or “spin down“ (aligned opposite the field). Each position is then associated with different energy levels. During the MRI process, the magnetic field relaxes back to 2 equilibrium states (T1 and T2) each with times specific to certain tissues such as healthy tissue versus cancerous tissue. Some scans rely on detection gradients to then construct images of tissue. This detection gradient is subjected to various magnetic fields to control its angle. Each T1 and T2 value on a “plane“ or slice of the magnetic field through tissue is then reconstructed into a computer image, a process called tomography.
Even without a contrast agent, the various particles and water molecules in tissue still have different T1 and T2 values. Places with less tissue concentration and more water molecules will appear darker than places of high tissue concentration. Thsi generates a picture of what the inside of a brain looks like without harmful radiation, allowing multiple scans of the same person to be done safely and consistently. See this in the image of the brain below:
Because the MRI machine only uses the properties of magnetism, researchers at the UCSB lab can scan patients daily and monthly without any negative side effects. This enables consistent research without risks for the subjects or researchers. In the case of the UCSB Neuroscience study on women’s hormones, this also protects the women’s reproductive systems for safe future pregnancies.
Any form of x-ray machine uses tomography to produce the virtual images through series of scans. In the next post, I will try to go a little more in depth into the actual math behind medical imagining through tomography.
Sources:
“Transcranial Magnetic Stimulation - Mayo Clinic.” 27 Nov. 2018, www.mayoclinic.org/tests-procedures/transcranial-magnetic-stimulation/about/pac-20384625.
https://www.nibib.nih.gov/science-education/science-topics/magnetic-resonance-imaging-mri#:~:text=How%20does%20MRI%20work%3F,-MRI%20of%20a&text=MRIs%20employ%20powerful%20magnets%20which,pull%20of%20the%20magnetic%20field.
https://www.nde-ed.org/Physics/Magnetism/fieldcreation.xhtml#:~:text=As%20Ampere%20suggested%2C%20a%20magnetic,direction%20of%20the%20magnetic%20field.
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