Detailed understanding of the
brain's reaction to sound, language, and music is a critical aspect of
many fields of research. However, the current state-of-the-art for brain
imaging, fMRI, presents a number of significant obstacles to accurate sound
No ferrous metals:
The powerful magnetic field in and around the MRI chamber prohibits
the use of any ferrous metals. At the very least, these objects will disrupt
the uniform magnetic field necessary for the scan; at worst, objects may
be accelerated through the air from surprising distances, causing injury
This rules out any moving-coil dynamic (normal speakers and headphones)
and planar-magnetic transducer design. The only options then, are pneumatic
(sound is produced normally outside of the MRI chamber and sent through
tubes), piezoelectric, and electrostatic transducers. Of those three,
only electrostatic transducers are capable of producing high-fidelity sound.
Noise inside the chamber during scanning measures about 103dB, with
a spectrum between 1kHz and 7kHz (the most sensitive, or "piercing" part
of our hearing). Lower-frequency background machine noise registers about
70dB. EEG studies on subjects listening to a voice recording while
subjected to a recording of fMRI scanning noise showed significantly
less correlated brain activity, compared to subject listening only to
the voice recording.
The best insulated ear protectors provide 30dB of reduction at specific
frequencies, reducing the noise to a bearable level, but not enough to
conduct accurate studies on music or voice cognition.
To reduce noise further, active noise cancellation is required, either
by real-time open/closed loop noise cancellation--or, because scanning
noise is predictably related to scanning parameters, cancellation by
a parametrically generated anti-noise.
fMRI scanning is accomplished in part by sending RF energy at 64mHz
or 128mHz into the chamber and detecting the far weaker response from
the scanned body. Any introduction of outside energy at these frequencies
would disrupt the scan, and any leakage or absorption of the scanned
body's response reduces the imaging resolution.
This affects signal cabling to the headset. Ideally, very high resistance
cables and high-quality RF chokes impervious to energy at 64 and 128mHz
would be used.