Magnetic Field Stability in MRI: Fringe Fields, Shielding, and Shimming, Slides of Radiology

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Magnetic field Stability

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< .1ppm/hour*

<0.0015 Gauss/hour at 1.5T

*Siemens Harmony/Symphony

Magnets

Fringe magnetic fields:

Fringe magnetic fields can extend great distances from an “unshielded” superconducting MRI magnet

5 Gauss @ 60 feet from magnet isocenter at 1.5 T

The “5 Gauss zone” is the safety zone that persons wearing pacemakers should not enter.

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Magnets

Fringe magnetic fields can be reduced by “active shielding”. An additional set of superconducting coils is added outside of the main magnet coils. Current in the active shielding coils is reversed relative to the main coils. The geometry of the two magnetic fields produced by the two solenoids leads to significant cancellation of fringe magnetic fields.

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Magnets

Refrigeration systems

The boil off of liquid helium can nearly be eliminated by using a cold head and a recondenser to liquify helium gas. In a modern magnet, cryogens have to be replenished every few years.

The refrigeration system causes a persistent “chirp” that is audible in the exam room. 24

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Because the magnet itself is not adequately homogeneous, it is necessary to improve or “shim” the homogeneity of the static magnetic field (Bo).

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Shimming

Shimming

Shimming (or adjustment of the static magnetic field homogeneity) is accomplished by two methods:

(1) Passive shimming (2) Active shimming

The static magnetic field is passively shimmed

by the placement of iron plates in the bore of the magnet. The iron plates offset the effects of external ferromagnetic objects, and other sources of inhomogeneity. 28

Shim coils may be placed inside of the cryostat “superconducting shims” or inside the “bore” of the magnet “resistive shims”. Resistive shims are typically watercooled. Modern magnets use superconducting shims. 30

Shimming

Active shimming requires passage of electric current through coils with unique geometric configurations. The shim coils are designed to correct inhomogeneities of specific geometries. The shim coils are typically named X, Y, XY, XZ, ZY, X^2 Y^2 , Z^2 X, Z^2 Y, Z0, Z1, Z2, Z3, Z

Because the patient’s tissue becomes somewhat magnetized, the nice homogeneity established by shimming is perturbed when a patient is placed in the magnet. The gradient coils can be used to re-establish good homogeneity.

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Shimming

Gradient coils are used for spatial localization. For example in a transverse image, the Z gradient would be used in “selecting” a slice of tissue to image. The X gradient may be used for phase encoding and the Y gradient for frequency encoding.

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Gradient Coils

Gradient coils are generally “pulsed” on very briefly (a few msec). How long a gradient coil must be left on is affected by the strength of the gradient. Stronger gradients pulses can have shorter durations which is important in fast MRI imaging techniques.

The frequent pulsing of the gradients produces the substantial noise associated with an MRI scan.

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Gradient Coils