Why is an MRI
so loud?
and anyone who's curious
Can't play sound? The illustrated version is right below.
Read the transcript
The full narration of the video.
Slide into the big tube of an MRI scanner, and you hear bang, bang, bang, tap-tap-tap. It sounds like someone outside is hammering on a metal bucket. But there's no hammer in there. So who's doing the knocking? The answer is hiding in something you already own: a speaker.
Open up a speaker and there are just two things inside: a magnet and a coil of wire. Send electricity through, and the magnet gives the coil a push, shaking the paper cone. The air shakes with it, and that's the sound you hear. An MRI scanner has the very same two things inside. Everything is just much, much bigger.
The magnet is big enough to fit your whole body. It's always on, and on its own it makes no sound at all. The noise comes from three sets of coils lining the tube: one for front-to-back, one for side-to-side, one for top-to-bottom. Why switch them on at all? To work out exactly where in your body each signal is coming from.
Turn a coil on, and the magnetic field gets a little stronger on one side and a little weaker on the other. Every spot in your body gets its own house number. Those numbers have to be handed out again and again, so the coils switch on and off, thousands of times a second. Every switch, the big magnet shoves the coil hard. Huge current, huge push.
A speaker nudges its cone gently, so it sings. The coil gets slammed all at once, like a hammer hitting an empty bucket: bang! So why does the sound keep changing? Each kind of picture uses its own rhythm. Slower is bang-bang-bang, faster is tap-tap-tap, and the fastest blurs into one long buzz.
At its loudest it can reach a hundred and thirty-one decibels. From about a hundred and twenty, ears start to hurt. That's why the hospital hands you earplugs or headphones. Simple foam earplugs cut out fifteen to twenty-five decibels. Scientists are working on quieter scans, too. One method moves the coils in tiny steps instead of big jumps.
In a 2014 trial, the noise went from about a hundred and five decibels down to sixty-nine, about as quiet as the room itself. Next time you're in there, count them: how many rhythms do you hear? Each new one is the machine finding a new way to look at you. What strange sound have you always wondered about? Tell me in the comments.
Inside the tube,
who's knocking?
Bang, bang, bang. Tap-tap-tap. Like someone outside is hammering on a metal bucket.
But there's no hammer in there. The answer is hiding in something you already own: a speaker.
Open a speaker:
just two things
A magnet, and a coil of wire.
Send electricity through and the magnet pushes the coil, shaking the paper cone. The air shakes with it, and you hear a sound. Change the electricity hundreds or thousands of times a second, and you get a song.
An MRI has
the same two things
A small coil
Three sets of coils lining the tube
The big magnet is always on, and on its own it makes no sound at all. The noise comes from the three sets of coils lining the tube: one for front-to-back, one for side-to-side, one for top-to-bottom.
Switching on a coil
hands out house numbers
The magnetic field gets a little stronger on one side and weaker on the other, so every spot in your body gets its own house number.
That's how the machine can tell where in your body each signal comes from. The numbers have to be handed out again and again, so the coils switch on and off thousands of times a second.
Every switch
is one "bang"
Every time the power goes on or off, the big magnet shoves the coil hard. Huge current, huge push.
A speaker cone is pushed smoothly back and forth, so it sings. The coil gets slammed again and again, and the whole thing shudders, like a hammer hitting an empty bucket.
Bang, tap, buzz:
one rhythm per picture
Each kind of picture uses its own rhythm.
Slower is bang-bang-bang, faster is tap-tap-tap, and the fastest blurs into one long buzz. That's why the sound keeps changing during a scan.
How loud is it?
All in decibels.
That's why the hospital gives you earplugs or headphones. Simple foam earplugs cut out 15 to 25 decibels.
Scientists are working on quieter scans too. One method moves the coils in tiny steps instead of big jumps. In a 2014 trial with ten patients, the noise dropped from about 105 decibels to 69, about as quiet as the room itself.
count the rhythms
finding a new way to look at you.
