Eddy Currents

I've heard that "eddy currents" cause problems for imaging.  What are they?  

The Faraday-Lenz principle of magnetic induction. A voltage (V) is generated in the coil proportional to the rate of change of the magnetic field (dB/dt). The induced current opposes the applied field.

The source of the changing magnetic fields in MRI may either be the imaging gradients or radiofrequency (RF) coils.  The conductive material in which eddy currents are induced may be any metallic component of the MR scanner (other coils, shields, tubes, and housing), wires or devices within or on the patient, and the patient as a whole (in the final analysis, people are just big bags of saline!)

Electrolux induction cooktop. An oscillating magnetic field in the cooktop induces eddy currents in the pan, heating and melting the chocolate. The cooktop itself does not get hot.

The Faraday-Lenz Law of electromagnetism states that electrical currents are induced in nearby conductors by a changing magnetic field. Since MR uses rapidly changing magnetic fields to generate and spatially define the signal, swirling ('eddy') currents are always produced whenever imaging is performed.

Currents are generated in a nearby conductor whenever a changing magnetic field is present. Because they swirl like eddies in a river, they are called "eddy currents".

Eddy currents within the patient may produce important biological effects such as tissue heating or peripheral nerve stimulation. These concepts deserve several Q&A's of their own and are considered in detail in the Quality and Safety section of this website. The present discussion deals with eddy currents induced in the MR scanner itself.

Within the MR scanner, eddy currents are induced any any nearby conducting media, which include within the gradient coils themselves, the main magnet and shim coil windings, cryoshields, liquid helium vessel, and RF shields. Eddy currents create two undesired phenomena: unwanted time-varying gradients and shifts in the main magnetic field (Bo). 

Nyquist N/2 ghost artifact

As per Faraday's Law, the magnitude of eddy currents depends on the rate of change of the inciting magnetic field. Thus fast imaging sequences (where gradients are pulsed on and off quickly) produce the largest and most severe eddy current problems.  Echo-planar imaging sequences are especially affected where a characteristic eddy current artifact (Nyquist N/2 ghost) is commonly seen.  Other sequences that may be most affected by eddy-currents include diffusion-weighted imaging, MR spectroscopy, MR angiography, balanced gradient echo sequences, and any sequence with a very short TE.

Eddy currents degrade the speed and efficiency of gradient switching; electrical pulses driving the gradients become distorted, producing a wide range of image artifacts, including shearing, shading, scaling, blurring, and spatial misregistration.  Eddy currents within the cryostat produce Joule heating, degrading the quality of the magnetic field and resulting an increased boil-off of cryogens. 

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Several techniques are available to minimize the effects of eddy currents:
     1) Design methods that interrupt potential current loops (use of slotted coils and shields)
     2) Active shielding of gradients (secondary coils outside to constrain magnetic flux changes induced by primary gradients);
     3) Pre-emphasis (modifying the input current to the gradients to account for expected eddy-current distortions); and
     4) Image post-processing (to correct for spatial nonlinearities and frequency/phase shifts due to eddy currents) 

These methods will be described in more detail in subsequent questions.

Advanced Discussion (show/hide)»

A few additional notes concerning adverse effects of eddy currents. . . .

Multiple eddy currents with different time constants always coexist. The most rapid eddy currents (with time constants on the order of a few hundred micro(μ)-seconds) are present in the RF coil and shield. These are responsible for the Nyquist N/2 artifact described above. Intermediate term eddy currents (on the order 1-100 milliseconds) typically originate in the walls of the cryostat or outer radiation shields and produce shading artifacts on fast (turbo) spin echo or gradient echo images. Eddy currents with long time constants (hundreds of milliseconds) often derive from the inner cryogen shields and produce frequency drifts in the main field. This may be manifest on images as insufficient fat or water suppression.

Eddy currents are particularly problematic in localized spectroscopy, where they cause broadened resonances and distorted line shapes, especially in short TE experiments. At times they may completely prevent the acquisition of localized MRS measurements.

In diffusion-weighted imaging, eddy currents can result in spuriously high ADC values.

In all forms of echo-planar imaging, eddy current gradients and field shifts produce distinctive geometric image distortions. These include: 1) image shearing artifacts (from induced gradients in the frequency-encode direction); 2) image scaling artifacts (from induced gradients in the phase-encode direction), and 3) global position shifts (from homogenous shifts in the B_o field).

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References
     Ahn CB, Cho ZH. Analysis of the eddy-current Induced artifacts and the temporal compensation in nuclear magnetic resonance imaging.  IEEE Trans Med Imaging 1991; 10:47-52.     
     Doty FD. MRI gradient coil optimization. In: Blumler P, Blumich B, Botto R, Fukushima E (eds), Spatially Resolved Magnetic Resonance. Wiley; 1998, pp 647-674.      
     Schmitt F. The gradient system. Understanding gradients from an EM perspective. Proc Intl Soc Mag Reson Med 2013; 21:1-13.   
     Spees WM, Buhl N, Sun P et al.  Quantification and compensation of eddy-current-induced magnetic-field gradients. J Magn Reson 2011; 212:116-23.
     Trakic A, Liu F, Sanchez Lopez H et al. Longitudinal gradient coil optimization in the presence of transient eddy currents. Magn Reson Med 2007; 57:1119-1130.

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Related Questions
     Why do the scanner walls get so warm?      
     What is pre-emphasis and how does it reduce eddy currents? 
     What are actively shielded gradients?   

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