Why does the MR system make the knocking sound during the exam?
The tapping or knocking noise heard during the MR exam is created when “gradient coils” are switched on and off to measure the MR signal reflecting back out of the patient’s body. Depending on the type of study being performed, the knocking may be loud enough to require ear plugs or head phones for the patient. Please discuss this with the technologist before your examination. During the MR examination, the technologist and patient may usually communicate at any time via intercom or a type of call button. The gradient coil is one of several internal parts of the MR system that you can not see. The gradient coil is made up of loops of wire which are embedded in a hard plastic tube. During the scanning process an electric current is switched on and off through the gradient coil approximately every few milliseconds. Because the switching is so rapid, the wires vibrate within the hard plastic and cause the knocking sound. This knocking is not harmful but the sound can be irritating to some
