MRI Systems - Fringe Fields, Shielding, Shimming and Gradients

Fringe Fields

  • Fringe field is the static magnetic field around the scanner, measured in Gauss.
  • It extends beyond the walls, floor, and ceiling.
  • Different MRI systems have different fringe fields.
  • Vertical open bore magnets have smaller fringe fields compared to closed bore horizontal magnets.
  • Permanent magnets have relatively low fringe fields.
  • Unshielded high field systems can have large fringe fields, extending several feet or meters.
  • Fringe field must be contained, which is a consideration when siting a scanner.

Magnetic Shielding

  • Shielding can decrease the fringe field.
  • Consider potential projectiles when siting a magnet.
  • Two methods of magnetic shielding:
    • Passive shielding: No current or passivity. Achieved by surrounding the magnet or lining the magnet room with steel plates. More about the room around the magnet.
    • Active shielding: Implies current and activity. Is within the magnet itself. More expensive and inconvenient. Can weigh up to 40 tons. Requires specially prepared foundations.
  • Most superconducting scanners use active shielding.
  • Uses an additional solenoid electromagnet around the main magnetic coils at each end of the bore.
  • These are inside the cryostat and are superconducting coils.
  • Exhibit equal but opposite effect to the main magnet which leads to a significant reduction in the size of the fringe field.
  • For an MR system that is not shielded, the scan room would need to be the size of a doubles tennis court to contain the 5 Gauss line.

Shim Coils and Shim System

  • When taking an image, such as of the abdomen, fat suppression is used.
  • Fat suppression should appear uniform across the image.
  • The magnetic field arrives from the factory at 1000 parts per million but needs to be within 4 parts per million for a good image.
  • Spectroscopy studies require better than 1 part per million.
  • Passive shimming: Achieved by placing small ferromagnetic plates in specially constructed nonferrous metal trays around the circumference of the warm bore of the magnet.
    • Refers to the circumference of the inner wall of the cryostat.
  • Anything close to the side of that magnet is not as homogenous.
  • Passive shimming is used to get the magnetic field to a particular level of homogeneity, and then active shimming is used to optimize for each patient exam.

Gradient Coils and Gradient System

  • Found in the warm bore of the magnet is the gradient set.
  • It's a cylindrical structure containing three individual electromagnets.
  • Also contains 18 individual solenoids that make up the active shim system.
  • Gradient coils are supplied by one or two powerful amplifiers.
  • Gradient set is at room temperature, not superconducting with three planes.
  • Gradient coils are used to spatially locate where a signal is within that body and there used for spatial encoding.
  • They are also used to rephase spins to produce echoes.
  • A gradient is a linear slope in the magnetic field strength across the imaging volume in a particular direction.
  • The strength of the magnetic field can be changed by:
    • The current passing through the windings.
    • The number of windings in the coil.
    • The diameter of the wire used in the windings.
    • The distance of spacing between the windings.
  • Altering any of the first three factors would change the amplitude or strength from a low to a higher field.
  • Altering spacing between the loops is possible to slope the magnetic field.
  • The coil has 12 windings uniformly spaced that's attached to an electrical terminal at each end.
  • The current flows in one direction through the coil, resulting in the magnetic field.
  • Each time a gradient is switched on, this produces noise.
  • By changing the field strength in a linear fashion using a gradient, the processional frequency and the phase of magnetic moments are also altered linearly.
  • Faults in gradient coils or gradient amplifiers can result in geometric distortions in the MR images.

Gradient Characteristics

  • Gradient characteristics need to be understood to accomplish the goals of spatial encoding, refocusing, and other tasks during image acquisition.

Definitions

  • Gradient strength/amplitude: Defines how steep or strong a particular gradient is. Measured in milliteslas per meter or Gauss per centimeter.
  • Gradient speed/rise time: Defines the time it takes for the given gradient to reach its maximum amplitude. Measured in microseconds.
  • Slew rate: Defines the time it takes for a given gradient to reach its maximum amplitude and what the amplitude is.
  • Rise time affects how fast a gradient can be switched on an off, affecting scan time.
  • As slew rates increase, the potential for time-varied magnetic field effects increases.
  • As the duty cycle increases, gradient heating can increase, and the number of attainable slices can be reduced.

Acoustic Noise

  • Acoustic noise generated by the scanner is caused by vibrations of the gradient set.
  • Higher amplitude gradient and rapid gradient activation increase acoustic noise.
  • Manufacturers are modifying gradient systems in an attempt to reduce noise.