Bush__The_Essential_Physics_for_Medical_Imaging - Biomedical ...

Permanent magnets rely on the ferromagnetic properties of iron, nickel, cobalt, andalloys. Early permanent magnets were extremely large, bulky and heavy, someweighing over 100 tons. Lighter alloy permanent magnets with improved peripheralelectronic equipment are finding a niche in clinical MR!. Operational costs ofthese systems are the lowest of all MR systems (no major electric or cooling requirements).Field uniformity is typically less than a superconducting magnet with similarFOY. An inability to turn off the field in an emergency is a drawback.Ancillary components are necessary for the proper functioning of the MR system.Shim coils are active or passive magnetic field devices that are used to adjust themain magnetic field and to improve the homogeneity in the sensitive central volumeof the scanner. Gradient coils, as previously discussed, are wire conductors thatproduce a linear superimposed gradient magnetic field on the main magnetic field.The gradient coils are located inside the cylinder of the magnet, and are responsiblefor the banging noise one hears during imaging. This noise is caused by the flexingand torque experienced by the gradient coil from the rapidly changing magneticfields when energized.~Matching~JFIGURE 15-41. A: Proximity head coil design and photograph of clinical head coil. The matchingcircuitry provides tuning of the coil for improved sensitivity and response. Two or more tuningcapacitors are required. B: A multiple phased array surface coil detector provides extended coverageand SNR. In this design, a subset of the coils are activated at anyone time (indicated by darkerlines); postprocessing is necessary to combine the images.