Why Does MRI Make All That Noise?



I remember the first time a patient asked me why the MRI machine was so loud. I confidently told them it was the magnetic acoustic effect. They asked me what that was. I stared at them for a moment the shrugged my shoulders. "I don’t know," I told them, feeling a little silly. " No one knows." This is what my boss told me at the time and I rehashed it

to many patients. I later learned that there is no such thing as the magnetic acoustic effect and there is actually an explanation for the noises. I never found out whether he was just messing with me...but it sounded cool. When the MRI scan starts electricity is rapidly pulsed through large coils of metal inside the scanner (AKA. Gradients). This causes them to have a magnetic field that is different from the main magnetic field. In turn the gradients are literally being aggressively pulled upon. The gradients snapping back and forth in place from the rapidly pulsed electric current is what is causing the noise. Keep in mind the electrical

pulses turn off and on in a matter of milliseconds so the clicking sounds blend together to form the banging we hear. This phenomenon increases in 3T where the gradients are stronger and the force exerted on them is higher. The symphony of horrible banging is exemplified by the fact that the patient is in a tube right next to them. This is why hearing damage is a real danger in MRI exams.

So next time your patient asks you why it’s so loud make sure you can give them an intelligent explanation of scientific effect and no one knows why its loud).facts (even if it’s easier to tell them it's the magnetic acoustic

How Loud is an MRI scanner?

How loud is this? Will this damage my hearing? Can you turn this down? We have all heard these questions from our patients. We usually respond with a handful of ear plugs and a calming reassurance that they won't leave the scanner deaf. But do you know how loud your scanner can get? Are you aware of how many decibels it actually produces and what decibel level can cause hearing damage? When you take the time to look into this it’s scarier than you think. Typical conversations are around 50 decibels. On the other end of the spectrum a gunshot from a rifle can approach 170 Decibels. NIHL (Noise Induced Hearing Loss) is a self-describing condition in which repeated exposure to high decibel noise causes damage to small hair cells in the inner ear. The damage can also occur with short exposure to very high decibel noises like gunshots, fireworks, sirens, listening to very

loud music, and... MRI Scanners!

A typical 1.5T Magnet produces an average decibel range of about 90- 110. This is well above the threshold of damage. 3T Magnets can produce up to 125 decibels. At these levels, for scans between 30-60 minutes, permanent damage can occur after a single procedure. As you know, a triple studies with contrast can send you into two hours of this kind of exposure. As an MRI tech, your tools to combat this damage are limited but, when used correctly, can save your patients’ hearing. Ear plugs have been the gold standard for noise reduction in MRI for some time. They have a NRR (Noise Reduction Rating) of

about 30 when used correctly. This means that an MRI scan putting forth 110 decibel sounds would effectively be reduced to 80 decibels.


Make sure you or the patient

squeezes them and places them in the ear correctly; at least half of our patients don't

understand how to properly use them.

Headphones have an NRR of about 25-30.

This can still take a 1.5T down to a safe

decibel level. Be cautious blaring music in the headphones during an MRI. Depending on how loud you play it the decibel reduction can be negated. Combining headphones and earplugs, however, does not produce a decibel reduction equal to the sum of their ratings. Typically, additional reduction is around 10 decibels for another device. For example, if you had the patient wear ear plugs with a NRR of 30, and headphones with a NRR of 25, the combined noise reduction wouldn't be 55 but rather 35. Cushions can also be used in some scans to reduce the decibel level by 10 or so. This is usually only possible in the head or neck coil when you can really get them tight against the patient’s ears. As MRI technologists, we have the responsibility to protect our patients the best we can by utilizing as many forms of protection as we can; this is especially true when utilizing 3T MRI. The ear plugs should be combined with headphones and or cushions around ears to reduce the possibility of damage as much as possible. Keep in mind that with 125 decibel imaging and a NRR of 35 you are still right around the danger zone.

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