Podcast
Questions and Answers
What is the main goal of shimming in MRI technology?
What is the main goal of shimming in MRI technology?
Which of the following describes active shimming?
Which of the following describes active shimming?
What is the typical composition of shim coils in MRI systems?
What is the typical composition of shim coils in MRI systems?
How does RF shielding work in an MRI room?
How does RF shielding work in an MRI room?
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What is the purpose of magnetic field shielding in MRI systems?
What is the purpose of magnetic field shielding in MRI systems?
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Which type of shimming involves the use of currents through coils?
Which type of shimming involves the use of currents through coils?
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What materials are typically used in passive shimming?
What materials are typically used in passive shimming?
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What is the frequency range typically used in MRI RF shielding?
What is the frequency range typically used in MRI RF shielding?
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Study Notes
Shimming
- Shimming is a process to create a more homogeneous main magnetic field (B0)
- Active shimming uses currents in specialized coils to generate a corrective magnetic field
- Superconducting shims are within the cryostat, resistive shims are outside
- Passive shimming utilizes ferromagnetic materials (iron or steel) in a pattern along the magnet's inner bore for field correction
Shim Coils
- A homogenous magnetic field (10 ppm) is required for good imaging
- Shim coils are extra coils placed over the main magnetic coils to reduce inhomogeneities
- Shim coils require a separate power supply
RF Shielding
- MRI operates in the 1-200 MHz range
- Radio, TV, and other communication systems operate in the ~1-100 MHz range
- CT rooms shield EM radiation (x-rays); MRI rooms shield external EM radiation (RF)
- MRI rooms have continuous copper or aluminum sheets or mesh (Faraday Cage) to absorb external RF
Magnetic Field Shielding
- MRI systems need shielding due to fringe fields that can interfere with other mechanical and magnetically activated devices
- MRI systems can be shielded actively or passively
Fringe Field
- The stray magnetic field outside the magnet's bore is the fringe field
- Permanent magnets have relatively low fringe fields; electromagnets have significant fringe fields
- Fringe fields must be managed to avoid interference with patients' monitoring and other devices
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