As truly open scanners have a gap between two magnet pole-pieces, a new ‘iron-frame’ technology was also needed to provide a suitable magnetic field. This forms the basis for all open MRI scanners. Iron-frame technology uses iron to provide a conduit for the magnetic flux to complete its circuit from the north pole of the magnet to the south pole. In the FONAR 360° scanner, it is partly built into the scanner room walls.
The surgery of the future?
Open scanners lend themselves to MRI-guided surgical procedures, known as ‘interventional MRI’ – something currently in its infancy. The hope is that complex surgery, including the removal of cancerous tumours, could be performed under MR guidance making it more accurate. But there is a catch.
Many metals are strongly attracted by the large magnetic field generated by MRI scanners, so surgeons cannot use conventional surgical instruments. “A scalpel must be sharp, but if it cannot be made from a ferromagnetic or conductive material, what material do you use? Interventional MRI is not just an engineering challenge, but a challenge for materials scientists too,” explains Steve Keevil, senior lecturer in Imaging Sciences at St Thomas’ Hospital in London.
Guy’s and St Thomas’ Hospital has carried out MRI guiding of cardiac catheters since 2001 using a conventional scanner. “We have been restricted to using devices that are MR safe by chance,” says Keevil. Meanwhile, the NOC have conducted preliminary research into interventional procedures using their open scanner. “However, there have been limitations including the supply of MR-safe needles suitable for interventional musculoskeletal work,” says McDonnell.