MRI Magnetic Field and Shielding Concepts

Shimming

• Shimming is a process to make the main magnetic field (B0) more homogeneous.
• Active shimming uses currents through specialized coils to create a corrective magnetic field.
• Superconducting shims are inside, and resistive shims are outside the cryostat.
• Passive shimming uses ferromagnetic materials (iron/steel) to correct the field, placed in a pattern along the magnet's inner bore.

Shim coils

• A homogeneous magnetic field (10ppm) is required for good imaging.
• Shim coils are extra coils that reduce magnetic field inhomogeneities.
• Shim coils need a separate power supply.

RF shielding

• MRI uses radio frequencies (1-200 MHz).
• Radio, TV, and communication systems use the range of ~1-100 MHz.
• CT rooms are shielded from external electromagnetic (EM) radiation (x-rays), but MRI rooms are shielded from external RF.
• MRI rooms use continuous sheets of copper or aluminum (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 shielding involves active and passive methods.

Fringe Field

• Fringe field is the stray magnetic field outside the magnet's bore.
• Permanent magnets have a relatively low fringe field, while solenoid electromagnets have a higher one.
• Care must be taken to ensure the fringe field does not affect areas with potentially contraindicated patients' monitoring devices or other mechanically and magnetically sensitive devices.

Shielding

• Fringe fields can be compensated using magnetic field shielding.
• Active shielding utilizes additional coils in the magnet assembly to create opposing magnetic fields, reducing the external magnetic field.
• Passive shielding involves surrounding the magnet with large ferromagnetic materials (e.g., iron) or lining the MR scan room walls with steel.

This quiz explores key concepts related to MRI magnetic field homogeneity, shimming techniques, and RF shielding. Understand the differences between active and passive shimming and the importance of shim coils in producing high-quality imaging. Dive into how MRI rooms are shielded from electromagnetic radiation to ensure optimal operation.