Eddy currents
(EC) are induced electric currents that circulate in conductive materials when
exposed to a changing magnetic field. In the context of magnetic resonance
imaging (MRI) and transcranial magnetic stimulation (TMS), eddy currents play a
significant role in influencing the magnetic field distribution and can have
implications for image quality and stimulation accuracy. Here is an overview of
eddy currents and their relevance in MRI and TMS:
1. Generation of
Eddy Currents:
o MRI: In MRI, eddy
currents are commonly generated when gradient coils rapidly switch magnetic
field gradients during imaging sequences. These eddy currents arise due to
Faraday's law of electromagnetic induction, where a changing magnetic field
induces circulating currents in conductive structures, such as the MRI scanner
components or the patient's body tissues.
o TMS: In TMS, eddy
currents can be induced in the brain tissue when the TMS coil generates a
rapidly changing magnetic field to stimulate neural activity. These currents
may affect the distribution and intensity of the magnetic field within the
brain, influencing the efficacy and precision of TMS stimulation.
2. Effects of Eddy
Currents:
o MRI Artifacts: Eddy currents
in MRI systems can lead to image distortions, geometric distortions, and signal
losses. These artifacts can impact the quality and accuracy of MRI images,
affecting diagnostic interpretation and quantitative analyses.
o TMS Stimulation: In TMS, eddy
currents can alter the spatial distribution of the magnetic field generated by
the TMS coil, potentially leading to variations in the targeted brain region's
stimulation intensity and depth. Understanding and mitigating eddy current
effects are essential for ensuring consistent and reliable TMS outcomes.
3. Mitigation
Strategies:
o MRI: To minimize
eddy current artifacts in MRI, various techniques are employed, such as
pre-emphasis gradients, gradient pre-emphasis, and active shimming. These
methods help compensate for the effects of eddy currents and improve image
quality.
oTMS: In TMS, coil
design, orientation, and pulse waveform parameters can be optimized to reduce
eddy current effects and enhance the precision of neural stimulation.
Computational modeling and calibration techniques are also used to account for
eddy current influences on TMS outcomes.
4. Research and
Development:
o Ongoing research
in MRI and TMS focuses on understanding the mechanisms of eddy currents,
developing advanced correction algorithms, and optimizing hardware
configurations to mitigate eddy current-related issues. By addressing eddy
current challenges, researchers aim to enhance imaging accuracy, stimulation
efficacy, and safety in clinical applications.
In summary, eddy
currents are induced electric currents that arise in response to changing
magnetic fields in MRI and TMS systems. Understanding the impact of eddy
currents on image quality, stimulation accuracy, and safety is essential for
optimizing imaging protocols and TMS procedures in research and clinical
settings. Efforts to mitigate eddy current effects through technological
advancements and methodological improvements contribute to the advancement of
MRI and TMS techniques for neuroimaging and neuromodulation applications.
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