MRI Shielding Techniques PDF

Shimming Hayder Jasim Taher PhD of Medical Imaging Outline of my presentation ✓ Shimming. ✓ Shim coils. ✓ RF shielding. ✓ Magnetic field shielding  Shimming Shimming is the process by which the main magnetic field (Bo) is made more homogenous Active...

Shimming Hayder Jasim Taher PhD of Medical Imaging Outline of my presentation ✓ Shimming. ✓ Shim coils. ✓ RF shielding. ✓ Magnetic field shielding  Shimming Shimming is the process by which the main magnetic field (Bo) is made more homogenous Active Shimming – Active shimming uses currents directed through specialized coils to generate a "corrective" magnetic field – Superconducting shims are within the cryostat and resistive shims are outside the cryostat Passive shimming – Passive shimming is a method of field correction involving the use of ferromagnetic materials, typically iron or steel, placed in a regular pattern at specific locations along the inner bore of the magnet  Shim coils One of the requirement for good imaging is a homogenous magnetic field (10ppm) Shim coils are the extra loops of coils over the magnetic coils which are used to reduce magnetic field inhomogeneities Shim coils requires a power supply which is separate from other power supplies within the system  RF shielding RF range used in MRI – 1-200 MHz Radio, TV and other communication system works on ~ 1- 100 MHz In CT scanning the CT room is shielded so that EM radiation (x-rays) do not escape the room but in MRI the scanning room is shielded from EM radiation (RF) that exists outside the room. MRI room is shielded completely by a continuous copper or aluminum sheets or mesh to absorb the external RF, such a design feature is called a RF Cabin or a ‘Faraday Cage’  Magnetic field shielding MRI system needs to be shielded because of the fringe fields which can interfere with other mechanical and magnetically activated devices MR system can be shielded by two processes: – Active shielding – Passive shielding  FRINGE FIELD The stray m.f outside the bore of the magnet is the fringe field. Permanent magnet have relatively low but in solenoid electromagnet it is significant. These must be taken into care so that they do not extend into the areas where potentially contraindicated patients monitoring device and other mechanically and magnetically devices are present. 7  SHIELDING Fringe field can be compensated by the use of magnetic field shielding. They are Active shielding- uses additional coils built into the magnet assembly, they are designed and oriented so that the electrical currents in the coils produce magnetic fields that oppose and reduce the external magnetic field. Passive shielding- Passive shielding: is produced by surrounding the magnet by large pieces of ferromagnetic materials such as iron or by lining the walls of MR scan rooms with steel 8 Active shielding Fringe field