Auditory; Eye Tracker; Physiological Monitoring - Siemens fMRI User Manual

Table of Contents

fMRI User Guide

4.3 Auditory

Auditory stimulus can be presented by means of special MR compatible headphones. A pair of headphones would

usually require some kind of amplification to deliver an audio signal over a large distance. This amplification could

either be done inside or outside the scanner room. If the amplification is done inside the room, care must be

taken in delivering power to the amplification unit. Both DC and AC power can be easily supplied via a filter in the

penetration panel. An extra AC power filter is available in the Siemens panel, but a custom penetration panel can

be installed for other filtering needs. Inexpensive DC power filters can be purchased from any large electronics

supplier. The audio signal itself could be delivered to the headphones in two ways. Optically, via a fiber optic

cable through a waveguide, or electrically, through a filter in the penetration panel. Usually, well-shielded co-ax

cables are sufficient for shielding the audio signal from external RF noise. However, it is still a good idea to use

a low pass filter to take away any noise that might be coming from the sound control unit. Please be aware that

depending on the construction of the headphones, any RF noise present in the scanner room could easily use the

headphone wires as a conduit and introduce artifact in your images.

An optical microphone is a perfect candidate for recording subject voice responses. If recording must be done

during a scan, a noise canceling option must be available for voice recording.

An eye tracker can be used to monitor subject eye movements in response to a given visual stimulus. In order

to reduce the possibility of introducing artifacts into MR images, the eye tracker must be placed away from the

scanner. Placing a unit at the back of the bore is ideal for scanner setup. However, due to the large distance from

the eye, the tracking accuracy is not as good as one could achieve outside of the scanner. Again, care must be

taken to filter both power and video signal. However, one must be careful not to over-attenuate the video signal

as not to affect eye-tracking quality.

A few different physiological signals may be monitored during an fMRI exam. Signals such as heart rate,

breathing, and pulse may be measured and correlated with data acquisition and stimuli. The galvanic skin

resistance can also be measured as a form of subject response to stimuli. These physiological recordings can then

be used as regressors in the fMRI analysis to remove signal changes extraneous to the presented stimulus.

ECG, respiration, and pulse monitoring are available through the Siemens scanner, while GSR monitoring

(operating within a magnetic field) is available commercially as an external unit. For details on how to set up the

Siemens wireless ECG, respiratory cushion, and pulse oximeter, please refer to the System manual of the scanner.

The signals from these devices may be saved in the form of text files by using specialized sequences. This enables

easy co-registration of the physiologic signals with data acquisition. All signals may be logged simultaneously.

Table of Contents