MRI detection of weak magnetic fields due to an extended current dipole in a conducting sphere: A model for direct detection of neuronal currents in the brain

To investigate the feasibility of direct MR detection of neuronal activity in the brain, neuronal current flow was modeled as an extended current dipole located in a conducting sphere. The spatially varying magnetic field induced within the sphere by such a dipole was calculated, including its form close to and within the current source. The predicted field variation was experimentally verified by measurements of the variation in phase of the MR signal in a sphere containing a model dipole. The effects of the calculated magnetic field distributions on the phase and magnitude of the signal in MR images were explored. The minimum detectable dipole strength under normal experimental conditions was calculated to be about 4.5 nAm, which is similar in magnitude to dipole strengths from evoked neuronal activity, and is an order of magnitude smaller than dipole strengths expected from spontaneous activity. This minimum detectable dipole strength increases with increasing spatial extent of the primary current distribution. In the experimental work, the effects of a field of [1.1 ± 0.5] × 10–10 T strength were detected, corresponding to the maximum net field caused by a dipole of 6.3 nAm strength with a spatial extent of 3 × 3 × 2 mm3. Magn Reson Med 50:40–49, 2003. © 2003 Wiley‐Liss, Inc.

[1]  H. Carr STEADY-STATE FREE PRECESSION IN NUCLEAR MAGNETIC RESONANCE , 1958 .

[2]  D. Geselowitz On the magnetic field generated outside an inhomogeneous volume conductor by internal current sources , 1970 .

[3]  R E Ideker,et al.  Eccentric dipole in a spherical medium: generalized expression for surface potentials. , 1973, IEEE transactions on bio-medical engineering.

[4]  E. Haacke,et al.  Identification of vascular structures as a major source of signal contrast in high resolution 2D and 3D functional activation imaging of the motor cortex at l.5T preliminary results , 1993, Magnetic resonance in medicine.

[5]  R. Ilmoniemi,et al.  Magnetoencephalography-theory, instrumentation, and applications to noninvasive studies of the working human brain , 1993 .

[6]  A. Kleinschmidt,et al.  Brain or veinoxygenation or flow? On signal physiology in functional MRI of human brain activation , 1994, NMR in biomedicine.

[7]  J H Duyn,et al.  Inflow versus deoxyhemoglobin effects in bold functional MRI using gradient echoes at 1.5 T , 1994, NMR in biomedicine.

[8]  Xiaoping Hu,et al.  Potential pitfalls of functional MRI using conventional gradient‐recalled echo techniques , 1994, NMR in biomedicine.

[9]  P. Mansfield,et al.  Echo‐Planar Imaging of the Brain at 3.0 T: First Normal Volunteer Results , 1994, Journal of computer assisted tomography.

[10]  H. Gudbjartsson,et al.  The rician distribution of noisy mri data , 1995, Magnetic resonance in medicine.

[11]  A. Grinvald,et al.  Interactions Between Electrical Activity and Cortical Microcirculation Revealed by Imaging Spectroscopy: Implications for Functional Brain Mapping , 1996, Science.

[12]  D. Heeger,et al.  Linear Systems Analysis of Functional Magnetic Resonance Imaging in Human V1 , 1996, The Journal of Neuroscience.

[13]  T. Allison,et al.  Comparison of cortical activation evoked by faces measured by intracranial field potentials and functional MRI: Two case studies , 1997, Human brain mapping.

[14]  W Richter,et al.  Limitations of temporal resolution in functional MRI , 1997, Magnetic resonance in medicine.

[15]  Karl J. Friston,et al.  Nonlinear event‐related responses in fMRI , 1998, Magnetic resonance in medicine.

[16]  D. Noll,et al.  Nonlinear Aspects of the BOLD Response in Functional MRI , 1998, NeuroImage.

[17]  J. Bodurka,et al.  Current-induced magnetic resonance phase imaging. , 1999, Journal of magnetic resonance.

[18]  R J Ilmoniemi,et al.  Spatiotemporal activity of a cortical network for processing visual motion revealed by MEG and fMRI. , 1999, Journal of neurophysiology.

[19]  Dezhong Yao,et al.  Electric potential produced by a dipole in a homogeneous conducting sphere , 2000, IEEE Trans. Biomed. Eng..

[20]  A. Grinvald,et al.  Non-invasive visualization of cortical columns by fMRI , 2000, Nature Neuroscience.

[21]  R. Turner BOLD localization: the implications of vascular architecture , 2001, NeuroImage.

[22]  P. Bandettini,et al.  Spatial Heterogeneity of the Nonlinear Dynamics in the FMRI BOLD Response , 2001, NeuroImage.

[23]  J. Bodurka,et al.  Toward direct mapping of neuronal activity: MRI detection of ultraweak, transient magnetic field changes , 2002 .