Avoiding eddy-current problems in ultra-low-field MRI with self-shielded polarizing coils

J.O. Nieminen (Corresponding Author), P.T. Vesanen, K.C.J. Zevenhoven, J. Dabek, Juha Hassel, Juho Luomahaara, J.S. Penttilä, R.J. Ilmoniemi

    Research output: Contribution to journalArticleScientificpeer-review

    43 Citations (Scopus)


    In ultra-low-field magnetic resonance imaging (ULF MRI), superconductive sensors are used to detect MRI signals typically in fields on the order of 10–100 μT. Despite the highly sensitive detectors, it is necessary to prepolarize the sample in a stronger magnetic field on the order of 10–100 mT, which has to be switched off rapidly in a few milliseconds before signal acquisition. In addition, external magnetic interference is commonly reduced by situating the ULF-MRI system inside a magnetically shielded room (MSR). With typical dipolar polarizing coil designs, the stray field induces strong eddy currents in the conductive layers of the MSR. These eddy currents cause significant secondary magnetic fields that may distort the spin dynamics of the sample, exceed the dynamic range of the sensors, and prevent simultaneous magnetoencephalography and MRI acquisitions. In this paper, we describe a method to design self-shielded polarizing coils for ULF MRI. The experimental results show that with a simple self-shielded polarizing coil, the magnetic fields caused by the eddy currents are largely reduced. With the presented shielding technique, ULF-MRI devices can utilize stronger and spatially broader polarizing fields than achievable with unshielded polarizing coils.
    Original languageEnglish
    Pages (from-to)154-160
    Number of pages7
    JournalJournal of Magnetic Resonance
    Issue number1
    Publication statusPublished - 2011
    MoE publication typeA1 Journal article-refereed


    • Eddy currents
    • magnetically shielded room
    • multipole expansion
    • polarizing coil
    • ultra-low-field MRI


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