title: An advanced phantom study assessing the feasibility of neuronal current imaging by ultra-low-field NMR.
abstract: In ultra-low-field (ULF) NMR/MRI, a common scheme is to magnetize the sample by a polarizing field of up to hundreds of mT, after which the NMR signal, precessing in a field on the order of several μT, is detected with superconducting quantum interference devices (SQUIDs). In our ULF-NMR system, we polarize with up to 50mT and deploy a single-stage DC-SQUID current sensor with an integrated input coil which is connected to a wire-wound Nb gradiometer. We developed this system (white noise 0.50fT/√Hz) for assessing the feasibility of imaging neuronal currents by detecting their effect on the ULF-NMR signal. Magnetoencephalography investigations of evoked brain activity showed neuronal dipole moments below 50nAm. With our instrumentation, we have studied two different approaches for neuronal current imaging. In the so-called DC effect, long-lived neuronal activity shifts the Larmor frequency of the surrounding protons. An alternative strategy is to exploit fast neuronal activity as a tipping pulse. This so-called AC effect requires the proton Larmor frequency to match the frequency of the neuronal activity, which ranges from near-DC to ∼kHz. We emulated neuronal activity by means of a single dipolar source in a physical phantom, consisting of a hollow sphere filled with an aqueous solution of CuSO4 and NaCl. In these phantom studies, with physiologically relevant dipole depths, we determined resolution limits for our set-up for the AC and the DC effect of ∼10μAm and ∼50nAm, respectively. Hence, the DC effect appears to be detectable in vivo by current ULF-NMR technology.
authors.affiliation: Physikalisch-Technische Bundesanstalt (PTB), Abbestrasse 2-12, 10587 Berlin, Germany. Electronic address: rainer.koerber@ptb.de.
authors.forename: Rainer
authors.initials: R
authors.lastname: Körber
authors.affiliation: Physikalisch-Technische Bundesanstalt (PTB), Abbestrasse 2-12, 10587 Berlin, Germany; Department of Biomedical Engineering and Computational Science, Aalto University School of Science, P.O. Box 12200, FI-00076 AALTO, Finland.
authors.forename: Jaakko O
authors.initials: JO
authors.lastname: Nieminen
authors.affiliation: Physikalisch-Technische Bundesanstalt (PTB), Abbestrasse 2-12, 10587 Berlin, Germany.
authors.forename: Nora
authors.initials: N
authors.lastname: Höfner
authors.affiliation: Institute of Mathematics, Physics and Mechanics, Jadranska 19, 1000 Ljubljana, Slovenia.
authors.forename: Vojko
authors.initials: V
authors.lastname: Jazbinšek
authors.affiliation: Physikalisch-Technische Bundesanstalt (PTB), Abbestrasse 2-12, 10587 Berlin, Germany.
authors.forename: Hans-Jürgen
authors.initials: HJ
authors.lastname: Scheer
authors.affiliation: Korea Research Institute of Standards and Science, Daejeon 305-340, South Korea.
authors.forename: Kiwoong
authors.initials: K
authors.lastname: Kim
authors.affiliation: Physikalisch-Technische Bundesanstalt (PTB), Abbestrasse 2-12, 10587 Berlin, Germany.
authors.forename: Martin
authors.initials: M
authors.lastname: Burghoff
mesh_terms: D000328:Adult; D000465:Algorithms; D001921:Brain / Q000502:physiology; D004558:Electric Stimulation; D004566:Electrodes; D004574:Electromagnetic Fields; D006801:Humans; D008279:Magnetic Resonance Imaging / Q000379:methods*; D015225:Magnetoencephalography; D008297:Male; D015233:Models, Statistical; D009474:Neurons / Q000502:physiology*; D019047:Phantoms, Imaging; D059629:Signal-To-Noise Ratio
keywords: Current dipole phantom; MEG; Neuronal currents; Ultra-low-field NMR