Noises from MRI Imaging: Sources Explained

[Lisa!]

What is the source of noises we hear during MRI imaging?

Thanks!

 

[Lok]

Big electromagnets create powerful fields, and not very stable ones on a small scale, so might be that oscillations can occur as the conductor has it's own elasticity.

Air might get ionized.

I've never had an MRI but there might be a small 50 or 60 Hz humm from the power source which should be powerful at least.

Electrics have a few specific noises.

 

[f95toli]

I've never had an MRI, but I suspect most of the noise comes from the compressor for the cryocooler (which is part of the cooling system for the magnet); they tend to be extremely noisy.

 

[russ_watters]

No, it wouldn't be the utilities - they are housed in a separate room.

 

[marcusl]

What is the source of noises we hear during MRI imaging?

Thanks!

To the other respondents: if you have no clue, please don't offer your incorrect guesses. Lok, your response is ludicrous. f95toli, systems with cryocoolers do often have background noise when you walk into imaging suite, but it is generally inaudible inside the bore and in any case is unrelated to the intense noise that every MRI patient is familiar with. To answer your question, Lisa, you need a little background on how the instrument works:

MRI systems require a very strong magnetic field to align the magnetic moments ("spins") of the hydrogen nuclei in your body. In a typical system this field is produced by a superconducting solenoid (you slide into the cylindrical cavity at the center of the magnet). The frequency of magnetic resonance is directly proportional to the field strength. Big GE systems, for instance, use a 1.5T field and excite/detect spins at 64 MHz.

To image, the locations of spins in the body must be differentiated. This is done by adding weaker position-dependent magnetic fields on top of the static strong one, so a nucleus's magnetic resonance frequency depends on where it is. Field gradients are applied in three orthogonal directions (x, y and z) in a certain time sequence, and, to create an image rapidly, they are switched on and off over and over very quickly. This is how the intensity in each pixel of the image is built up.

Ok, now to the noise. The gradients are produced by big electrical coils that also sit in the main bore. When current (typically around 150A) flows, the wires feel a large sideways Lorentz force because they are in a strong static field. Consequently, these coils are wound on strong formers and are potted to minimize wire movement. Because the current is turned on and off very rapidly, however, the wires exert a strong impulsive force that is heard as a bang even though gross wire motion is controlled. The repetitive banging you hear is the three coils being pulsed over and over with various strengths, timings and sequences to build up the image.

Here is a two-page summary of MRI. The second page shows a sketch of the main magnet and the gradient coils.
http://people.ee.duke.edu/~jshorey/MRIHomepage/work.html"

Here's a much more detailed tutorial if you want to learn more about spins, nuclear magnetic resonance, and how it is used to produce images:
http://orthopedics.about.com/gi/dynamic/offsite.htm?zi=1/XJ&sdn=orthopedics&cdn=health&tm=12&f=10&su=p284.9.336.ip_p736.3.336.ip_&tt=2&bt=1&bts=1&zu=http%3A//electronics.howstuffworks.com/mri.htm"

 

[TurtleMeister]

I had an MRI many years ago and I wondered about that myself. It sounded like a mechanical thump or knock with a frequency of about four thumps per second. But it was a long time ago so I'm not sure about the frequency.

I thought it may be the current in the magnets but I wasn't sure. I once worked in a factory that used large electric spot welders. They made a similar thumping sound.

 

[Dale]

Ok, now to the noise. The gradients are produced by big electrical coils that also sit in the main bore. When current (typically around 150A) flows, the wires feel a large sideways Lorentz force because they are in a strong static field. Consequently, these coils are wound on strong formers and are potted to minimize wire movement. Because the current is turned on and off very rapidly, however, the wires exert a strong impulsive force that is heard as a bang even though gross wire motion is controlled. The repetitive banging you hear is the three coils being pulsed over and over with various strengths, timings and sequences to build up the image.

To emphasize what marcusl said, this works exactly the same way as a speaker does. In a speaker you have a strong permanent speaker magnet and an electromagnet that is switched on and off rapidly to move the speaker cone. In the same way the gradient coils marcusl mentioned are switched rapidly and the 1.5T main magnetic field he mentioned serve the role of the permanent magnet.

Considering the physics it is actually amazing that they are not louder. The manufacturers go to great lengths to reduce acoustic noise by making it stiff and even encasing it in vacuum, but the power amplifiers that drive the gradient coils are basically beefed up audio amplifiers. Many modern systems have over 1 MW worth of amplifiers! I have always wanted to hook them up to a real huge sound system

 

[marcusl]

DaleSpam, I wouldn't say it "works exactly the same way as a speaker." The Lorentz force powers them both, but a speaker is a linear device--the motion of the diaphragm is proportional to the voice coil current--while the MRI gradient coil produces something closer to an acoustic shock wave. Also, the gradient coils are not encased in a vacuum.

 

[Dale]

As you say, the physical principle behind both is the same. Speakers aren't really linear either, as any audiophile knows. It is just different degrees of non-linearity.

Also, the gradient coils are not encased in a vacuum.

http://www.toshiba-medical.eu/en/Our-Product-Range/MRI/Technologies/Pianissimo/

 

[marcusl]

http://www.toshiba-medical.eu/en/Our-Product-Range/MRI/Technologies/Pianissimo/

Well I'll be! I hadn't heard of the vacuum-encasement technology before. Thanks!

 

[tiny-tim]

audio segements

Here is a two-page summary of MRI. The second page shows a sketch of the main magnet and the gradient coils.
http://people.ee.duke.edu/~jshorey/MRIHomepage/work.html"

Fascinating!

And that summary gives a link to several audio segments of the noise, near the bottom of "Imaging Hardware: Safety", at http://www.cis.rit.edu/htbooks/mri/chap-9/chap-9.htm#9.6 in the very detailed "The Basics of MRI" by Joseph P Hornak, and also near the bottom of "Your MRI Exam" at the same site, at http://www.cis.rit.edu/htbooks/mri/chap-13/chap-13.htm

The former link also contains the following cautionary tales:

Despite numerous safety warnings issued by the manufacturers, professional societies, and the government, I have heard numerous stories of ferromagnetic objects being pulled into imaging magnets. The most common one is similar to this story. A metal pail on wheels was filled with water and had a mop wringer in it. The pail was located approximately 10 feet from the bore of a 1.5 T magnet. The magnet pulled it across the floor and lifted it up off the ground three feet into the magnet. The wringer caused serious damage to the magnet in that the cryogen boil off rate increased and the magnetic field homogeneity decreased. The head coil located in the bore of the magnet was destroyed.

The most tragic story was the death of a six year old boy in an MRI magnet. This story should serve as an example and reminder of the responsibility that MRI personnel and administrators have to maintain a safe facility. In July 2001 a boy was injured in an MRI and later died when a ferromagnetic oxygen tank was brought into the magnet room and pulled into the magnet where the boy was being imaged.

Another frightening story was of a law enforcement officer being allowed to go near a magnet with a loaded firearm. The handgun was pulled out of its holster, and into the magnet. The force of the impact with the magnet caused the gun to discharge. Luckily, no one was injured in this incident. In addition to the damage to the MRI and the bullet lodged in the scan room wall, the gun was magnetized. Mechanical objects, in general, do not function properly when magnetized. Please, respect the physical laws of nature that cause ferromagnetic objects to be attracted by magnets!

 

[Lisa!]

Thank you very much!