Shimming & Magnetic Field Shielding Lecture & Lab 5 PDF

Shimming & magnetic field shielding (lecture & lab. 5) Dr. HAIDER ABDULHAMEED MBCHB/CABMS (Diagnostic Radiology)  Define Shimming? it is the process of making the B0 field uniform throughout the imaging volume. What does The shimming process often involve? it involves placing steel bars...

Shimming & magnetic field shielding (lecture & lab. 5) Dr. HAIDER ABDULHAMEED MBCHB/CABMS (Diagnostic Radiology)  Define Shimming? it is the process of making the B0 field uniform throughout the imaging volume. What does The shimming process often involve? it involves placing steel bars in the magnet at precise positions in trays located along the inside of the magnet bore. Primary shimming of a magnet is usually performed at the factory. The MRI system installation engineer will likely do some fine tuning of the magnet’s shim at the site. This is called passive shimming. Room temperature coils may also be used to improve the B0 field uniformity within the patient for particular procedures that require high magnetic field homogeneity. On systems operating at 1.5 T and below, this procedure only requires a DC offset of the current applied to the gradient coil. At 3 T and higher, separate coils are installed that adjust the higher functional orders of the magnet field, These are the shim coils. Magnetic field uniformity is termed homogeneity. The Parts per Million Scale How is the magnetic field homogenecity is usually expressed? Its expressed in parts per million. If a 1.0 T magnet were perfectly homogeneous throughout the imaging volume, it would be stated as homogeneity of 1 ppm is a variation of 1 mT throughout the imaging volume. Magnetic field homogeneity is specified as the ppm over a specific spherical volume. What is the acceptable magnetic field homogenecity for MR system? Acceptable homogeneity over a 20 cm spherical volume is approximately 20 ppm with permanent magnets but only 1 ppm with superconducting magnets. Shimming the Magnet What are the usual specifications of The imaging volume in an MRI system ? It generally consists of a cylinder with a diameter of approximately 60 cm and a length along the Z-axis of 50 to 70 cm. A conventional six-coil superconducting magnet will normally produce a magnetic field with homogeneity of approximately 5 ppm. How is the magnetic field homogenecity is technically assessed? When determining the B0 magnetic field homogeneity, the installation engineer will typically use a small probe containing a tiny sample of water to measure the resonance frequency and map the magnetic field. A special jig is used to position the small probe at precise locations within the magnet bore. Once the measurements are performed, then the homogeneity can be modified by placing steel shims in trays located in the bore of the magnet.  When is high magnetic field homogenecity required ? With high-speed functional imaging techniques and chemical shift–sensitive fat suppression, better B0 homogeneity is required. Current superconducting magnets are designed and manufactured according to precise specifications. What is active shimming? An actively shielded magnet incorporates superconducting shim coils in the cryostat with the primary magnet. This seems to be the most effective shim design of all. Field homogeneity of less than ― 1 ppm is attainable with super conducting shim coils.  What is the most common method for application specific shimming in conventional MRI systems ? is to offset the baseline currents of the X, Y, and Z gradient coils. This method can usually achieve the desired degree of field homogeneity with the standard hardware. Shimming using only the gradient coils produces adequate results partly due to the superior homogeneity of modern superconducting magnets. However, for higher field systems that are used for spectroscopy studies, in which a higher degree of homogeneity is required, an independent set of room temperature shim coils may be necessary. References Stewart Carlyle Bushong, Geoffrey Clarke, “Magnetic resonance imaging : physical & biological principles”, Mosby, Inc., an affiliate of Elsevier Inc., 4th edition, 2015.