Long Range Mount Installation
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2005-2010 SR Research Ltd.
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7.9.3
The Head Coil Mirror
The Head Coil Mirror signal needs to be clear and stable for a good eye tracking signal. A first-
surface mirror with good infrared reflecting properties is desirable. The default mirrors
provided by head coil manufacturers is typically not a first-surface mirror, resulting in extra
reflections which can interfere with the image of the pupil and CR, and typically reduces
illumination. This reduces the accuracy of the eye tracker.
SR Research strives to make available good-quality first-surface mirrors (with the silver
reflective coating on the front surface rather than the back surface, and with a high infrared
transmitting coating) for popular MRI systems. Use of such a mirror yields an improved image
signal and cuts down on the number of extra refraction artifacts that can pollute the camera
image. If building your own head coil mirror, try to ensure that a first-surface mirror is used.
Put the desired mirror on the head coil. The goal is to align the illuminator and camera with
the mirror so that reflected infrared from the illuminator lights up the eye-to-be-tracked while
the mirror reflects the image of the eye, back to the camera. To achieve this, direct line-of-sight
from the camera and illuminator via the head coil mirror is required.
7.9.3.1
Replacement First Surface Head Coil Mirrors
SR Research is currently working to replace the standard mirrors on popular systems. Due to
time-consuming and labor-intensive manufacture of these mirrors we cannot easily custom-
make mirrors. If your site uses a custom mirror that has a wider range of tilt and a larger field
of view than manufacturer-supplied head coil mirrors, you may wish to look into obtaining a
front-surface mirror with good infrared reflecting properties. The only head coil pictured in
Table 4 for which SR Research cannot currently supply a front surface replacement mirror is
the 8 channel head coil sometimes used with Siemens scanners and frequently used with the
Philips Achieva.
7.9.3.2
Minimizing Head Coil Mirror Vibration
First surface replacement mirrors typically affix to the head coil in an identical way to the
manufacturer supplied mirrors. It is important that the mirror vibrate as little as possible in
order to minimize the vibration of the stimulus display experienced by the subject and to
minimize movement of the eye image in the mirror from sources other than the eye itself.
Sometimes using thick tape where the mirror assembly contacts the head coil can result in a
tighter connection between the mirror assembly and the head coil as well as dampening the
vibration. Alternatively Velcro may help to secure the mirror in position and assist in damping
vibration.
Minimizing vibration of the head coil itself is important as this will in turn reduce vibration of
the mirror.
