PubMed · 556498
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H Weigel. 1977-09-01. [The electric pacemaker--structure and function].. https://pubmed.ncbi.nlm.nih.gov/556498/
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It is important to understand the character and the contribution of thermal magnetic noise in designing the measurement site and the instrumentation for biomagnetic measurements. The ultimate limit of the sensitivity is the thermal noise due to the object under study. In the case of the human body, it has been estimated to be about 0.1/square root of Hz. Magnetically shielded rooms are necessary for ultrasensitive biomagnetic measurements of human subject, but they also generate external noise which in some cases may become detectable. This noise problem can be avoided if the innermost walls are constructed of magnetically soft ferromagnetic material. Close to the conducting walls the thermal noise is higher than at the centre. Thus, the shielded room should be relatively large in size. The gantry and other things inside the room may contain metal parts, which can cause excess noise. The intensity depends on the conductivity, geometry, location and movement of these parts. In comparison to bioelectric studies, this inductive noise coupling demands extra attention. In most biomagnetic measurements performed inside a magnetically shielded room, the limiting factor of the sensitivity is the thermal noise caused by electrically conducting thermal shielding used inside the cryogenic measurement dewar. Fortunately, it is possible to reduce the noise contribution arising from the superinsulation in the dewar by careful design. The properties and dynamics of the SQUIDs are well understood nowadays. Studies of the nonlinear character in the coupling between the SQUID and external detection coil have made it possible to reduce the noise contribution of the sensor itself.(ABSTRACT TRUNCATED AT 250 WORDS)
This paper reviews the properties of the major source models which are used in the analysis of observed bioelectric signals in general, and of evoked potentials in particular. The interpretation of such models demands the solution of an associated inverse problem. Emphasis is laid on the possibility of imposing such constraints on the model which render stable inverse solutions and yet allow a physiological interpretation. The nature of this paper is tutorial.
The main purpose of measurements of the magnetic field produced by cerebral electrical activity is to locate this activity or to determine its distribution in the brain. In this paper, methods to locate or otherwise characterize source currents in the brain are briefly discussed. It is emphasized that the optimal source estimation method depends crucially on the availability of prior information and on the questions one wants to resolve with the measurements.