Tips for buying MRI

  1. When purchasing an MRI system, facilities should consider these important factors: the magnet, gradient system, computer.
  2. As for the MRI magnet, it should produce a highly homogeneous magnetic field covering as wide a field of view as possible and provide as much patient space as possible.
  3. The MRI system image resolution is higher when the gradient system is faster, but than the field of view is smaller.
  4. Keeping up with the magnet, so that images are instantly available while the scan progresses, is the computer system's task in a Magnetic Resonance Imaging system.
  5. Facilities should also consider the site requirements, which are specific to each institution. In this area, three issues should be examined: the extent of the magnetic field, the area occupied by the magnet, and the weight of the magnet.
  6. When selecting the MRI site, the two most important planning considerations are the fringe field and the need for a site free from ambient RF electrical noise. A permanent magnet has a minimal fringe field, but needs careful preconstruction planning because of its tremendous weight.
  7. To contain the magnetic fringe field of resistive and superconducting electromagnet systems, shielding can be used. The fringe field could cause problems at some sites without shielding.
  8. To contain the fringe field, two approaches are used for MRI units: active and passive shielding. Active shielding is a design feature of the magnet, while the passive shielding involves the use of steel around the magnet.
  9. Generally, the size of the MRI unit's controlled-access area increases as the operational field strength increases, for the fringe fields created by superconducting and resistive magnets. The recommended general-public access limit is 5 G. The distance to the 5 G line ranges from approximately 9 meters for a 0.5 T magnet to 13 meters for a 1.5 T magnet. For a 3.0 T magnet, the 5 G line is about one meter further from the isocenter than it is for a 1.5 T magnet, and therefore larger safety boundaries are required.
  10. With MRI active magnet shielding, the distance to the 5 G line can be drastically reduced to less than 4 meters for a shielded 1.5 T magnet and less than 3 meters for a shielded 0.5 T magnet.
  11. Fringe fields are 3-D; so for higher field strengths, areas on the floors above and below the imaging facility may also need controlled access and/or shielding.
  12. Careful site selection is required for MRI units. The operation of gamma cameras, CRT displays, electroencephalogram and electrocardiogram monitors, and image intensifiers - are all affected by the magnetic field; ferromagnetic material in the surrounding area affects the homogeneity of the static magnetic field.
  13. MRI system technicians can partially compensate for the effects of large stationary masses on field uniformity by placing corresponding masses of ferromagnetic material in a symmetric position in the magnet area. A careful site selection for your MRI system can eliminate moving ferromagnetic objects such as elevators, automobiles, or forklifts.
  14. To address the problems associated with the fringe field, active shielding can be used, as well as close-fitting steel shields integral to the magnet, or steel shielding in the walls around the magnet. If MRI system users wish to install large steel sheets of magnetic shielding, they should keep in mind that these are expensive, and require innovative construction techniques because of the weight involved.
  15. Facilities should be sure to include all these extra precautions in early design considerations before the purchase of MRI systems, because they raise the cost of construction. Implementation after installation may be even more costly. Actively shielded magnets have significantly reduced fringe fields and generally do not require steel shielding.
  16. External ambient RF signals can degrade MRI image quality below diagnostically acceptable levels; so even in the most complex MRI system, the magnet assembly must have some type of RF shielding. These shield assemblies usually consist of a complete room of copper or aluminum sheets bonded to a composite plywood support. Physical and visual access to the room is provided by special door assemblies and window coverings shielded with copper screening.
  17. MRI specialists should install, fine-tune, and maintain the equipment since MRI technology is especially complex and sensitive. Experts should train physicians and technologists and answer their questions.
  18. Facilities should choose a supplier whose local MRI service and training resources are extensive and reliable. The availability of such resources should be guaranteed in writing within any contract between the supplier and the buyer.
  19. To facilitate future additions to the network, all newly purchased MRI equipment must be compatible with DICOM 3.0. DICOM conformance statements should be provided by the suppliers and should explain in detail what information objects, service classes, and data encoding are supported by their systems. All statements should share the same format and vocabulary to facilitate Magnetic Resonance Imaging system comparisons among suppliers.
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