Why are MRIs noisy

Why is an MRI scanner so loud?

Anyone who has ever been in the tube of a magnetic resonance tomograph (MRT) knows how loud such a device is. With an MRI scan of the anatomical subtleties of the body, the noises tend to be deep; slightly higher in functional MRI, which shows brain activity. But where does this noise come from? To answer that, we have to go back a little.

In magnetic resonance imaging, researchers exploit the fact that water molecules - more precisely their atomic nuclei - have special magnetic properties. During the MRI recording, a magnetic field of several Tesla is built up around the body organ to be examined, which corresponds to around 100,000 times the earth's magnetic field. This static field, i.e. constant over time, is generated by a helium-cooled electromagnet.

The liquid helium has a temperature of 4.2 degrees above absolute zero - converted to minus 269 degrees Celsius. At this temperature the metal of the magnet coils becomes superconducting, i.e. the electrical resistance disappears. This is the only way to achieve the high currents that create such a powerful magnetic field.

Egg rotating axis

Soft body tissues contain a lot of water, in the brain it is up to 80 percent. The water molecules behave like small magnets themselves: The intrinsic angular momentum (also called spin) of the hydrogen nuclei is aligned with the static magnetic field. The axis of rotation "wobbles" like a rotating toy top that is not completely vertical. Physicists speak of a precession movement. The frequency with which the spin axis revolves around the direction of the magnetic field depends on the strength of the field. Now you send a radio wave of the same frequency into the organ to be examined. The spin then tips out of its usual precession motion - and returns again. It sends out an electromagnetic wave that is received and evaluated by a separate antenna. Variations in the signal strength of this wave ultimately create the contrast in the MRT image. They stem from the fact that the tissue in the vicinity of the water molecules also has magnetic properties and disrupts the spins.

When the magnetic field pulls the coils towards or away from the center axis of the tube, a sequence of vibrations is created - audible as a hum and roar

In the case of functional MRI, the blood takes the place of the tissue: its magnetic properties change when it transports oxygen to the active areas of the brain. Such changes in the regional blood flow therefore allow conclusions to be drawn about the activity of individual brain areas.

In order to localize where the received signal comes from, you need three additional magnetic coils, the so-called gradient coils. They are aligned with the x, y and z axes. The coils change the otherwise uniform magnetic field in the tomograph: Its exact strength and direction now depend on the respective location in the tissue. This also decides on the alignment and movement of the spins. The position of the spin that emitted the wave can therefore be calculated from the signal frequency.