The development of biomagnetic systems : planar gradiometers and software tools
This thesis is concerned with two aspects of the design and construction of biomagnetic systems. Firstly, it considers the optimum design of planar gradiometers. The modelling of gradiometers is discussed and an algorithm for optimising the sensitivity of a specific type of gradiometer is presented. A test thin-film procedure for the manufacture of a planar gradiometer is outlined. The performance of three different types of gradiometer in recovering test current distributions, using a distributed current analysis technique, is assessed. Secondly, four major software tools that are essential in the analysis of data from large multi-channel biomagnetic systems are presented. These tools are then used to analyze data from a visual evoked response experiment. The system used to collect data was the Helsinki multi-channel system which consists of 24 planar gradiometers. The results confirm the retinotopic mapping of visual field information, and suggest that the time evolution of activity in different parts of the visual cortex is similar for early latencies.