DC-MEG Studies: Development of Techniques for Resolving DC Fields

Abstract

Long duration magnetic field changes (DC) [1] in MEG may occur as a result of migraine [2], anoxia [3], epilepsy, closed head injuiy etc. These Field changes are easily masked by changes due to artifacts. It is therefore useful to separate the DC field changes of interest, i.e; arising from physiological phenomena, from those arising from external sources. The ability to measure these DC phenomena may lead to improvements in diagnosis and prognosis for treatment and recovery. With MEG we have successively measured DC field changes over periods of minutes to one hour. For these studies no absolute DC level can be established but shifts in (DC) levels over time can be recorded. However, we are presently developing new techniques to record changes of DC magnetic field amplitude which occur over a period of days or weeks. The problems that must be addressed to obtain meaningful DC measurements include arbitrary neuromagnetometer baseline, non-uniform residual magnetic field within the shielded room and the need to accurately reposition the subjects and control the distance between the cortex and sensor array. In this study, we describe results we have obtained for both experimental DC source models and actual DC field measurements recorded on human subjects on three consecutive days.