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Wed, July 10, 2013
Functional and structural information about the human brain can be obtained noninvasively with magnetoencephalography (MEG) and magnetic resonance imaging (MRI), respectively. MEG, which is based on the recording of the extracerebral magnetic fields, gives a direct measure (projection) of neuronal currents. On the other hand, MRI, in which polarized spin populations are manipulated and observed magnetically, provides 3-dimensional images of proton density and relaxation times. It was demonstrated recently (McDermott et al., PNAS 21, 78577861, 2004) that high-quality MRI is possible at magnetic fields as low as 100 microteslas if the sample is first polarized in a higher field and if SQUID sensors are used to detect the spin precession. Subsequently, it was demonstrated (Zotev et al., J. Magn. Reson. 194, 115120, 2008; Vesanen et al., Magn. Reson. Med. 2012, DOI 10.1002/mrm.24413) that MEG and ultra-low-field MRI (ULF MRI) can be performed with same SQUID sensor array; this guarantees that the coordinate systems of MEG and MRI are the same. In addition, the combination of the two techniques improves workflow. Advantages of ULF-MRI include high T1 contrast, the absence of susceptibility artifacts, quiet and safe operation, open structure, and relatively low cost when added to an MEG system. ULF-MRI may also be suitable for performing electrical impedance tomography (EIT) of the head; better knowledge of the conductivity structure, together with the error-free registration, could dramatically improve the accuracy of locating neuronal sources. The challenge in making combined MEG and MRI practical is to improve the data rate sufficiently. We need to improve SQUID sensitivity by a factor of 510 (to about 0.5 fT/sqrt[Hz]) and increase the pre-polarization field strength by a factor of 5 (to 100 mT or more). If we succeed in doing this in an array of several hundred sensors, clinically and scientifically useful MEG-MRI systems may be available within a few years.
Hybrid Magnetoencephalography (MEG) and Magnetic Resonance Imaging (MRI): Techniques and Benefits