Why are MRI Scanners so Loud?

If you have ever had a magnetic resonance image (I prefer the term NMR image) taken in a hospital, you have undoubtedly heard very loud noises while the instrument collected the data. These noises are the result of the application of magnetic field gradients by way of the gradient coils inside the main magnetic field. The magnetic field gradients are applied in short pulses and are used to spatially encode the sample (person) such that the Larmor frequency of the protons in one region of the sample differs from that in other regions. It is this spatial frequency labelling which allows for the generation of the image. The time dependant magnetic field generated by the pulsed magnetic field gradients interacts with the main magnetic field with a force exactly like the one we are all familiar with when we move two magnets in close proximity to one another. Since the pulsed field gradient coils are held in a rigid form within the main magnet, they are unable to move any great distance however the sudden application of a gradient pulse generates a very strong force which physically slams the gradient coils inside the rigid form making a loud noise from the vibration. The larger the main magnetic field or the stronger the gradient pulses, the greater the force generated and the louder the noise.

These noises can sometimes be heard in high resolution NMR probes equipped with pulsed field gradients. Recently, a student approached me concerned that the NMR spectrometer was making a soft "tick" noise for every scan collected in his gradient COSY experiment. Immediately I thought perhaps the probe was arcing so I turned down the RF power. The noise persisted. Upon closer inspection, I found that the gradient strength was set unnecessarily high for the measurement. When the gradient strength was turned down, the "tick" noise ceased.