Why MRI machines make that loud noise?


The banging is the vibration of metal coils in the machine caused by rapid pulses of electricity, said Dr. Keith Hentel, chief of emergency/musculoskeletal imaging at NewYork-Presbyterian Hospital/Weill Cornell Medical Center.
Magnetic resonance imaging produces images of the body by causing shifts in a very strong magnetic field and measuring how tissues react, Dr. Hentel said. The principal magnet of an M.R.I. scanner commonly is strong enough to produce fields 60,000 times the strength of the earth’s natural magnetic field.
Inside the scanner are coils of metal wire called gradient coils. When electricity is passed through such a coil, a magnetic field is created. Rapid pulses of electricity cause predictable changes in the field, resulting in tissue changes that can be measured and transformed into anatomic images.
The pulses cause not just the desired changes but undesired vibrations of the gradient coils, resulting in the banging heard during an M.R.I. examination.
As stronger magnets result in stronger vibrations, the higher the field strength of the M.R.I. scanner, measured in teslas, the louder the banging, Dr. Hentel said.
In a three-tesla system, a strength common in clinical practice, he said, these sounds may be as loud as 125 decibels, equivalent to a rock concert or a balloon popping near the ear. That is why ear protection is recommended.

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