What are all the different types of noises that an MRI makes when you’re getting a scan? And why is it different noises every few minutes?
In: Technology
You might be interested [in this recent Reddit post](https://www.reddit.com/r/mechanical_gifs/comments/iy4ej9/a_ct_scanner_at_full_speed_without_the_cover/?utm_source=share&utm_medium=ios_app&utm_name=iossmf) that shows a CT scanner running at full speed without its cover on. There is lots of information about these and MRI scanners in the comments, including an explanation about the sounds.
Sounds are caused by vibrations in the air.
Different kinds of vibrations cause different sounds.
An MRI generates very intense magnetic fields using electric coils.
When the magnetic field is changed this causes physical forces to act on the actual MRI machine, making it move or vibrate.
When the machine moves out vibrates, the movement is transferred to the air and you sense it as sound.
The different sounds are caused by different kinds of vibrations, which in turn are caused by different fields being generated.
The MRI needs to constantly change the field in order to focus on different areas of the body, and different kinds of magnetic waves of pulses allow for different kinds of readings, or to detect different molecules.
Not exactly ELI5 but some of the previous explanations are scientifically inaccurate- MRIs are permanent superconducting magnets, new ones are in the Tesla range of field strength. The magnetic field lines all of the protons up in your body by making them temporary magnets. Imagine all of the protons in your water and proteins oriented north-south. Then radio waves are pulsed causing the protons to flip and when they re-orient, photons (think little atomic screams) are released. The radio pulsing is at different frequencies and timing to resolve different structures. The photons are picked up by a detector, run through an amplifier and signal processor to deliver an image.
Fun fact MRIs are actually big-ass NMRs which were originally developed to study natural gas.
Essentially an MRI machine works by creating magnetic fields at very concise moments and areas along the area it is scanning. Because you want the patient to be in the machine the least amount of time as possible you need to create those magnetic fields (or apply gradients – that’s a more technical term and I won’t explain it because it doesn’t really answer the question) extremely quickly. In order to do that you need to manipulate the magnets in the machine quickly as well. The noise you hear is largely just the mechanical mechanism within the MRI spinning the magnets back and forth extremely quickly.
An MRI machine collects data by delivering electromagnetic energy into the body to ‘flip’ the spins of nuclei in the body. When the nuclei flip back, they emit electromagnetic signals of the same frequency that are then detected by the system.
The machine is set to have as homogeneous a magnetic field as possible at the center of the tube when nothing is happening. When a scan is taking place, the system rapidly changes the magnetic field within the magnet, using gradient coils, to provide spatial information.
The sounds you hear have a variety of purposes. The slow knocking sounds that precede a scan are a pulse sequence designed to tune the cool to maximize the signal returned from the body. Then, the rapid knocking is from the rapid changes in the gradient coils during the scan itself. Different scanning sequences change the gradients at different rates, so the frequency of the sound you hear differs.
The gradient coils that sit on the outside of the magnet are long loops of wire that vibrate due to the current pulsing through them during the scan. What you hear is that vibration.
Source: spent 10 years working for MRI vendor.
The noise is caused by the vibration of metal coils inside the machine. Inside an MRI is a big magnet and large coil of wire. To take a 3D picture, the high power coils pulse on and off. Energizing a coil of wire creates a magnetic field. This field is SO powerful that it causes the actual wire to vibrate. During a scan the coils are rapidly switched on and off, creating a loud pulsing sound.
During a scan the control computers modify the signals sent to the coils. A faster pulse for example will likewise sound quicker in pace. I am not an MRI tech, but I assume the different signals modify the properties of the image.
Have you ever heard the low hum of large electrical transformers / equipment? The same principle applies.
I was told the first scans are low-res and let the shot be setup or aimed correctly. Then when all’s good, hit the higher res scans. There’s differences in power, frequencies, etc. for low- vs. high-res scanning, and the machine sounds different when these variables change.
Source: had an MRI, asked the technician.
Basically the MRI spins the protons* in all the water molecules in your body in the same direction.
Knowing this, the sounds you hear are actually pulses that only some of your tissues react to (the extra energy that courses through your body knock them out of the spin). By changing the pulses (sounds), the receiver can focus on those tissues that are spinning differently.
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