Failure to direct detect magnetic field dephasing corresponding to ERP generation

Abstract

AbstractfMRI has become the method of choice for mapping brain activity in human subjects and detects changes in regional blood oxygenation and volume associated with local changes in neuronal activity. While imaging based on blood oxygenation level dependent (BOLD) contrast has good spatial resolution and sensitivity, the hemodynamic signal develops relatively slowly and is only indirectly related to neuronal activity. An alternative approach termed magnetic source MRI (msMRI) is based on the premise that neural activity may be mapped by MRI with greater temporal resolution by detecting the local magnetic field perturbations associated with local neuronal electric currents. We used a hybrid ms/BOLD MRI method to investigate whether msMRI could detect signal changes that occur simultaneously at the time of the production of well defined event related potentials, the P300 and N170, in regions that previously have been identified as generators of these electrical signals. Robust BOLD activations occurred after some seconds, but we were unable to detect any significant changes in the T2*-weighted signal in these locations that correlated temporally with the timings of the ERPs. IntroductionThere is continuing interest in the development of improved methods for mapping brain function and organization. Functional MRI (fMRI) is one of the most important and useful of current techniques and is based on the sensitivity of MRI to changes in blood oxygenation, flow, and volume that accompany with changes in brain activation [Ogawa, 1990]. The spatial and temporal resolutions of this technique are limited by the nature of the coupling between neuronal electrical activity and the corresponding hemodynamic response. Spatial resolutions on the order of 1 mm and temporal resolutions on the order of 1 sec are readily achieved. There is considerable interest in developing methods to map neural events with greater temporal resolution.Signal transfer along an axon is based on the ability of the membrane to alter its permeability to Na+ and K+ ions. These changes are caused by the opening of voltage-sensitive channels as a result of an approaching action potential. The action potential can be approximated by two oppositely oriented current dipoles whose separation depends on the conduction velocity. The magnitude of each dipole is about 100fAm. Although the precise natures of neural currents are complex, they generate weak magnetic fields within tissue that in principle may affect NMR signals [Cohen, 1984;Nunez, 2001;Bandettini, 2005]. One approach to detecting these fields is to try to measure the spatial displacement induced by neuronal electrical currents by the Lorentz effect [Song, 2001;Truong, 2006]. In addition, several groups have suggested that neural activity may be detected by MRI by measuring the signal losses and/or phase shifts produced by the local magnetic field perturbations associated with the local electric currents [Bodurka, 2002;Xiong, 2003]. This general approach has been termed magnetic source MRI Publisher's...

Direct neural current imaging in an intact cerebellum with magnetic resonance imaging

“…The literature contains several reports of positive results (Kamei et al, 1999; Xiong et al, 2003; Bianciardi et al, 2004; Liston et al, 2004; Konn et al, 2004; Petridou et al, 2006; Truong and Song, 2006; Chow et al, 2006a; Chow et al, 2006b; Chow et al, 2007; Chow et al, 2008; Xue et al, 2009) which conflict with reports of negative results (Chu et al, 2004; Parkes et al, 2007; Mandelkow et al, 2007; Tang et al, 2008; Luo et al, 2009; Rodionov et al, 2010; Luo, Jiang & Gao 2011; Huang, 2014). …”

Can high-field MREIT be used to directly detect neural activity? Theoretical considerations

“…Methods from direct external impedance measurement or from impedance changes related to blood flow (Bagshaw et al 2003) have also been proposed. Methods that rely on perturbations of the main magnetic field have been found to work in small preparations (Petridou et al 2006, Park et al 2006) but in trials performed in bloodless whole brain preparations (Luo et al 2009), and humans in vivo (Chu et al 2004, Tang et al 2008) significant changes have not been found thus far.…”

Detection of neuronal current MRI in human without BOLD contamination

“…Nonetheless, up to date, a consensus has not been reached on the detectability of ncMRI on human subjects. Multiple groups have reported a successful detection of ncMRI signal in human brain (2)(3)(4)(5)(6)(7)(8)(9)(10), while a failure in ncMRI detection has been claimed in other studies (11)(12)(13)(14)(15). Due to these conflicting results, there is still no conclusion on the question whether ncMRI signal is detectable or not.…”