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Focusing and targeting of magnetic brain stimulation using multiple coils

Article

Abstract

Neurones can be excited by an externally applied time-varying electromagnetic field. Focused magnetic brain stimulation is attained using multiple small coils instead of one large coil, the resultant induced electric field being a superposition of the fields from each coil. In multichannel magnetic brain stimulation, partial cancellation of fields from individual coils provides a significant improvement in the focusing of the stimulating field, and independent coil channels allow targeting of the stimuli on a given spot without moving the coils. The problem of shaping the stimulating field in multichannel stimulation is analysed, and a method is derived that yields the driving currents required to induce a field with a user-defined shape. The formulation makes use of lead fields and minimumnorm estimation from magneto-encephalography. Using these methods, some properties of multichannel coil arrays are examined. Computer-assisted multichannel stimulation of the cortex will enable several new studies, including quick determination of the cortical regions, the stimulation of which disrupts cortical processing required by a task.

Keywords

Coil arrays Lead fields Minimum-norm estimation Multichannel magnetic stimulation Reciprocity Stereotactic targeting 

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References

  1. Barker, A. T., Jalinous, R. andFreeston, I. L. (1985): ‘Noninvasive magnetic stimulation of human motor cortex’,Lancet,1, pp. 1106–1107CrossRefGoogle Scholar
  2. Beckers, G. andZeki, S. (1995): ‘The consequences of inactivating areas V1 and V5 on visual motion perception’,Brain,118, pp. 49–60Google Scholar
  3. Chokroverty, S. (Ed.) (1990): ‘Magnetic stimulation in clinical neurophysiology’, (Butterworth Publications, Boston)Google Scholar
  4. Cohen, D. andCuffin, B. N. (1991): ‘Developing a more focal magnetic stimulator. Part I: some basic principles’,J. Clin. Neurophysiol.,8, pp. 102–111Google Scholar
  5. Eaton, H. (1992): ‘Electric field induced in a spherical volume conductor from arbitrary coils: application to magnetic stimulation and MEG’,Med. Biol. Eng. Comput.,30, pp. 433–440CrossRefGoogle Scholar
  6. Hämäläinen, M. S. andIlmoniemi, R. J. (1984): ‘Interpreting measured magnetic fields of the brain: estimates of current distributions,’ Technical Report TKK-F-A559, Helsinki University of TechnologyGoogle Scholar
  7. Hämäläinen, M. S. andIlmoniemi, R. J. (1994): ‘Interpreting magnetic fields of the brain: minimum-norm estimates’,Med. Biol. Eng. Comput.,32, pp. 35–42Google Scholar
  8. Heller, L. andvan Hulsteyn, D. B. (1992): ‘Brain stimulation using electromagnetic sources: theoretical aspects’,Biophys. J.,63, pp. 129–138CrossRefGoogle Scholar
  9. Himmelblau, D. M. (1972): ‘Applied nonlinear programming’ (McGraw-Hill Book Company, New York)MATHGoogle Scholar
  10. Morioka, T., Yamamotot, T., Mizushima, A., Tombimatsu, S., Shigeto, H., Hasuo, K., Fuji, K. andFukui, M. (1995): ‘Comparison of magnetoencephalography, functional MRI, and motor evoked potentials in the localization of the sensory-motor cortex’,Neurol. Res.,17, pp. 361–367Google Scholar
  11. Nilsson, J., Panizza, M. andGrandori, F. (Eds.), (1996): Advances in magnetic stimulation: mathematical modeling and clinical applications (Salvatore Maugeri Foundation, Pavia, Italy)Google Scholar
  12. Pascual-Leone, A., Rubio, B., Pallardo, F. andCatala, M. D. (1996): ‘Rapid-rate transcranial magnetic stimulation of left dorsolateral prefrontal cortex in drug-resistance depression’,Lancet,348, pp. 233–237CrossRefGoogle Scholar
  13. Ravazzani, P., Ruohonen, J., Grandori, F. andTognola, G. (1996): ‘Magnetic stimulation of the nervous system: induced electric field in unbounded, semi-infinite, spherical, and cylindrical media’,Ann. Biomed. Eng.,24, pp. 606–616Google Scholar
  14. Roth, B. J. (1994): ‘Mechanisms for electrical stimulation of excitable tissue’,Crit. Rev. Biomed. Eng.,22, pp. 253–305Google Scholar
  15. Ruohonen, J. andIlmoniemi, R. J. (1996): ‘Multichannel magnetic stimulation: improved stimulus targeting’,inNilsson, J., Panizza, M. andGrandori, F. (Eds.): ‘Advances in magnetic stimulation: mathematical modeling and clinical applications’ (Salvatore Maugeri Foundation, Pavia, Italy) pp. 55–64Google Scholar
  16. Ruohonen, J., Ravazzani, P., Ilmoniemi, R. J., Galardi, G., Nilsson, J., Panizza, M., Amadio, S., Grandori, F. andComi, G. (1996): ‘Motor cortex mapping with combined MEG and magnetic stimulation’,inArber, C., Celesia, G., Comi, G. andMauguière, F. (Eds.): ‘Functional neuroscience’ (Elsevier Science, Amsterdam) pp. 317–322Google Scholar
  17. Ruohonen, J., Virtanen, J. andIlmoniemi, R. J. (1997): ‘Coil optimization for magnetic brain stimulation’,Ann. Biomed. Engl.,25, pp. 840–849Google Scholar
  18. Ueno, S., Tashiro, T. andHarada, K. (1988): ‘Localized stimulation of neural tissues in the brain by means of a paired configuration of time-varying magnetic fields’,J. Appl. Phys.,64, pp. 5862–5864CrossRefGoogle Scholar
  19. Ueno, S., Matsuda, T. andFujiki, M. (1990): ‘Functional mapping of the human motor cortex obtained by focal and vectorial magnetic stimulation of the brain’,IEEE Trans. Magn.,26, pp. 1539–1544CrossRefGoogle Scholar
  20. Wassermann, E. M., McShane, L. M., Hallett, M. andCohen, L. G. (1992): ‘Noninvasive mapping of muscle representations in human motor cortex’,Electroenceph. Clin. Neurophysiol.,85, pp. 1–8Google Scholar
  21. Yonokuchi, K. andCohen, D. (1991): ‘Developing a more focal magnetic stimulator. Part II: fabricating coils and measuring the induced current distributions’,J. Clin. Neurophysiol.,8, pp. 112–120CrossRefGoogle Scholar
  22. Ziemann, U., Lönnecker, S., Steinhoff, B. J. andPaulis, W. (1996): ‘Effects of antiepileptic drugs on motor cortex excitability in humans: a transcranial magnetic stimulation study’,Ann. Neurol.,40, pp. 367–378CrossRefGoogle Scholar

Copyright information

© IFMBE 1998

Authors and Affiliations

  1. 1.BioMag Laboratory, Medical Engineering CentreHelsinki University Central HospitalHYKSFinland

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