Key publications
Books:
Riitta Hari
Magnetoencephalography in Clinical Neurophysiological Assessment of Human Cortical Functions.
In: E. Niedermeyer & F. Lopes da Silva (Editors); Electroencephalography – Basic principles, clinical applications and related field.
Philadelphia: Lippincott William & Wilkins, S. 1165-97; 2005. ISBN 0-7817-5126-8
Journal Articles:
Matti Hämäläinen, Riitta Hari, Risto J. Ilmoniemi, Jukka Knuutila, Olli V. Lounasmaa
Reviews of Modern Physics 65, 413–497; DOI: 10.1103/RevModPhys.65.413
Abstract:
Magnetoencephalography (MEG) is a noninvasive technique for investigating neuronal activity in the living human brain. The time resolution of the method is better than 1 ms and the spatial discrimination is, under favorable circumstances, 2-3 mm for sources in the cerebral cortex. In MEG studies, the weak 10 fT-1 pT magnetic fields produced by electric currents flowing in neurons are measured with multichannel SQUID (superconducting quantum interference device) gradiometers. The sites in the cerebral cortex that are activated by a stimulus can be found from the detected magnetic-field distribution, provided that appropriate assumptions about the source render the solution of the inverse problem unique. Many interesting properties of the working human brain can be studied, including spontaneous activity and signal processing following external stimuli. For clinical purposes, determination of the locations of epileptic foci is of interest. The authors begin with a general introduction and a short discussion of the neural basis of MEG. The mathematical theory of the method is then explained in detail, followed by a thorough description of MEG instrumentation, data analysis, and practical construction of multi-SQUID devices. Finally, several MEG experiments performed in the authors' laboratory are described, covering studies of evoked responses and of spontaneous activity in both healthy and diseased brains. Many MEG studies by other groups are discussed briefly as well.
Anti Ahonen, Matti Hämälainen, Matti Kajola, Jukka Knuutila, P. Laine, Olli V.Lounasmaa, Lauri T. Parkkonen, Juha T. Simola, Claudia D. Tesche
122-channel SQUID instrument for investigating the magnetic signals from the human brain.
Physica Scripta 1993; T49: 198±205; DOI: 10.1088/0031-8949/1993/T49A/033
Abstract:
A 122-channel d.c. SQUID magnetometer with a helmet-shaped detector array covering the subject's head has been operational in the Low Temperature Laboratory of the Helsinki University of Technology since June 1992. The new system allows simultaneous recording of magnetic activity all over the head. The probe employs 122 planar first-order thin-film gradiometers in dual units with two exactly orthogonal channels at 61 measurement sites. The performance of the device is analyzed and compared with more conventional axial gradiometer arrays by considering signal-to-noise ratios, spatial sampling theory, confidence intervals for the estimated equivalent current dipole positions, and information-theoretical channel capacity. The signal-to-noise ratio and the resolution of the planar and axial arrays with the same number of channels are found practically equal. The number of channels and their spacing in our new Neuromag-122 system are found fully adequate for neuromagnetic measurements. An example of whole cortex recordings of auditory evoked brain activity is presented and analyzed.