Podcast
Questions and Answers
When is high magnetic field homogeneity particularly required?
When is high magnetic field homogeneity particularly required?
What is the function of superconducting shim coils in actively shielded magnets?
What is the function of superconducting shim coils in actively shielded magnets?
What is the most common method for application-specific shimming in conventional MRI systems?
What is the most common method for application-specific shimming in conventional MRI systems?
For higher field systems used in spectroscopy studies, what may be necessary to achieve a higher degree of homogeneity?
For higher field systems used in spectroscopy studies, what may be necessary to achieve a higher degree of homogeneity?
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What is active shimming in the context of magnetic resonance imaging?
What is active shimming in the context of magnetic resonance imaging?
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What is shimming in the context of MRI technology?
What is shimming in the context of MRI technology?
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What is the primary method of shimming performed at the factory?
What is the primary method of shimming performed at the factory?
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How is magnetic field homogeneity often expressed?
How is magnetic field homogeneity often expressed?
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What is the acceptable homogeneity for superconducting magnets over a 20 cm spherical volume?
What is the acceptable homogeneity for superconducting magnets over a 20 cm spherical volume?
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What is typically used to assess the B0 magnetic field homogeneity?
What is typically used to assess the B0 magnetic field homogeneity?
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What describes the dimensions of a conventional imaging volume in an MRI system?
What describes the dimensions of a conventional imaging volume in an MRI system?
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What is commonly required for improving B0 field uniformity at higher magnetic fields (3 T or above)?
What is commonly required for improving B0 field uniformity at higher magnetic fields (3 T or above)?
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Which of the following best describes the homogeneity achieved by a conventional six-coil superconducting magnet?
Which of the following best describes the homogeneity achieved by a conventional six-coil superconducting magnet?
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Study Notes
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.
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Description
Explore the critical concepts of shimming and magnetic field homogeneity in MRI systems. This quiz covers the technical aspects of achieving uniform magnetic fields, including passive shimming techniques and the significance of uniformity measured in parts per million (ppm). Understand how these principles apply during MRI installations and operations.
