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jim.voyvodic
BIAC Faculty
138 Posts |
 Posted - May 20 2003 : 10:29:55 AM
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People have been getting varied results with the goggles on the 4T so we have done some more testing. We currently have 2 sets of stereo goggles, which I will refer to here as sets A and B. For the past several months we have been using setA on the 4T, and set B was supposed to be for the 1.5T. Recently, set A has developed a
problem with vertical red lines appearing in the right eye. It also had the eyetracker camera broken off. In order to send that one back for repair we recently switched to using set B on the 4T, but we didn't send off set A because we wanted to make sure that people could run with set B. On my experiments I definitely found set B to work better than A. It gave a clear image with no red lines, although it may have been somewhat less bright than A. Other users, however, have found set B to have very poor image quality compared to A.
It turns out that set B is more sensitive to head position than set A. Both goggles are designed to function with the video screen perpendicular to the magnetic field, which means that the subject needs to be looking straight up. If the head is tilted forward the video image quality is degraded. For some reason set A is not as sensitive to this tilt than set B. When I put a subject in I routinely try to position them so that they are looking straight up, which is why I had not noticed this problem before.
I have discussed this head position issue with Benjamin. For some subjects getting the proper head position -- and having the subject be comfortable enough to maintain it -- may take a little creative padding. But if you add padding under the body to lift the shoulders up, and perhaps put a cushion under the neck, you should be able to make your subject comfortable, and looking straight up. During setup you want to try to get the top surface of the goggles parallel to the floor (as much as possible). Then when you slide the subject into the bore, ask them to tilt their head back and forth slightly to see which position gives the best image. If the best position is difficult to maintain comfortably, it is probably worth pulling the subject out and adjusting the cushions to help them be comfortable in the optimal position. This process may be a bit easier when using the new head coil because the coil slides back to give you better access. That coil sits higher on the table, however, so getting enough padding under the shoulders will be even more important. I have found that the vacuum bead bag helps.
Getting the goggles properly aligned with the field is important for both sets of goggles, but is especially critical for set B. If you still can't get a good image with set B you can still use set A, which I will not send for repairs until we are sure that people can use set B satisfactorily, or when the 4T goes down for repairs at the beginning of June, whichever comes first.
I hope this helps. Let us (Benjamin and me) know if you are still having problems.
Jim |
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