Shimming & Magnetic Field Homogeneity in MRI

Shimming & Magnetic Field Homogeneity

• Shimming is the process of making the B0 field uniform throughout the imaging volume.
• Shimming is often achieved by strategically placing steel bars in the magnet at specific positions within trays located along the inside of the magnet bore.
• Primary shimming of a magnet is generally done during factory production, while the MRI system installation engineer performs fine-tuning at the installation site. This is referred to as passive shimming.
• Room temperature coils can also be used to improve B0 field uniformity for specific procedures requiring high magnetic field homogeneity.
• Magnetic field homogeneity is referred to as uniformity and is typically expressed in parts per million (ppm).
• A 1.0 T magnet with perfect homogeneity throughout the imaging volume would have 1 ppm, indicating a variation of 1 mT across the imaging volume.
• Homogeneity is specified over a specific spherical volume, and acceptable homogeneity for MR systems varies. A 20 cm spherical volume typically has about 20 ppm with permanent magnets and only 1 ppm with superconducting magnets.
• The imaging volume in an MRI system is usually a cylinder with a diameter of approximately 60 cm and a length of 50 to 70 cm.
• A conventional six-coil superconducting magnet typically achieves a magnetic field homogeneity of approximately 5 ppm.
• The installation engineer assesses magnetic field homogeneity by using a small probe with a tiny sample of water to measure the resonance frequency and map the magnetic field at precise locations within the magnet bore.
• Shimming adjusts the homogeneity by placing steel shims in trays within the magnet bore.
• Higher field systems require better B0 homogeneity for high-speed functional imaging techniques and for chemical shift–sensitive fat suppression.
• Active shimming involves incorporating superconducting shim coils into the cryostat with the primary magnet. These coils significantly enhance homogeneity with achievable values below 1 ppm.
• Conventional MRI systems often achieve desired homogeneity by adjusting baseline currents of the X, Y, and Z gradient coils. This technique leverages the superior homogeneity of modern superconducting magnets.
• However, higher-field systems used for spectroscopy studies, which demand even greater homogeneity, might require independent sets of room temperature shim coils.