Italian manufacturers Esaote, who produce dedicated musculoskeletal scanners, allow study of joints and the spine in weight-bearing positions via their G-scan. As the whole unit can be rotated from horizontal to vertical, the scan can be performed at any angle. In their most open system, the C-scan, the patient inserts the joint to be studied while the rest of their body is outside the machine.
The technical challenge
Open and wider-bore MRI machines may seem like an obvious step that should have been taken years ago. However, the technical challenges involved in obtaining a high-resolution image from these systems has been considerable. For a start, in order to get a clear MR image the magnetic field provided by the scanner magnet needs to be uniform – and the shorter the magnet, the more difficult this is to achieve.
“We addressed this by developing a [superconducting] magnet with seven field-generating coils instead of six,” says Blasche from Siemens, who specialise in wide bore scanners. This gave a field uniform enough to image any organ in a single step, despite a short 125cm system length. Siemens also worked on improving the perfor-mance of the radio-frequency (RF) system and RF coils that create the pulse required to obtain an MRI signal, as the more RF channels available, the higher the signal-to-noise ratio and the faster the imaging.
Meanwhile, new types of receiver coil have had to be developed for completely open systems to obtain good image quality. “The rule in MRI is that the axis of symmetry of the receiver coil needs to be perpendicular to the orientation of the main magnetic field. Conventional high-field superconducting MRI systems have a horizontal magnetic field that is parallel to the long axis of the body, so they can use planar (flat) coils to image the spine. The open MRIs operate with a vertical magnetic field and use solenoidal (‘wrap-around’) receiver coils,” explain FONAR.