FMRI Facility: Inside the magnet room
The magnet, manufactured by Oxford Magnet Technology, has a bore diameter of 90 cm. It uses passive iron shims which generate a field homogeneity of 3.6 ppm over a 50 cm spherical diameter. The magnet is also stable temporally, drifting less than 0.05 ppm/hr (or less than 8.5 Hz/hr at the proton frequency of 170 MHz). Inside the magnet is located an actively shielded body gradient coil from Tesla Engineering. The maximum gradient along a principal axis is 35 mT/m and this can be achieved in a rise time of 280 ms using 430 A, 700 V amplifiers. The inner diameter of the gradient coil is 61.5 cm, providing sufficient room to position stimulus presentation devices to the subject.
MR signal excitation and detection is achieved using "transverse electromagnetic" (TEM) coils manufactured by HF Imaging. These coils provide good homogeneity of transmission and reception RF fields. The subject's head is positioned centrally inside the coil and is then restrained using a vacuum bead bag and foam padding. For fMRI experiments it is essential to eliminate head motion as far as possible to prevent degradation of image quality and false positive "activations." |
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The Brain Imaging Center's MRI facility houses the first actively-shielded whole-body 4 Tesla (T) magnet installed in the world. The active shielding results in a very small region of stray field and allows peripheral equipment such as required for functional MRI (fMRI) to be positioned in close proximity to the subject being scanned.
The magnet is located in a room which is actually a Faraday shield; an electrically isolating box which prevents spurious radiofrequency (RF) signals from entering the magnet room. Spurious "atmospheric" RF signals arise from many sources, the most obvious being broadcast signals from radio stations which occur between about 88 and 107 MHz. At our operating frequency of 170 MHz these radio signals would create artifactual harmonic signals that would generate noise lines in the images. To maintain the electrical isolation of the magnet in its RF shield, all electrical cables that enter the magnet room are filtered to remove RF signals. Additionally, fiber optic cables and other non-electrical lines (e.g. tubes carrying gases) pass through "waveguides," dielectrically isolated metallic tubes which prevent RF frequencies from passing through. RF filtering is required whether the cables/lines are needed for the MR acquisitions or the fMRI stimulus presentations.