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Eddy Current (EC)

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Dr. Rishabh Pathak

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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