Method for separating multichannel signals produced by AC and DC sources from one another
The present invention relates to a novel manner of measuring DC fields using a multi-channel MEG or MKG measuring instrument; and on the other hand, to a manner of eliminating from the measurement result the interference signals caused by the DC currents. The invention combines the monitoring system of a testee's movement and the method for motion correction of the measured signals so that the signals produced by the DC currents of a moving testee's are visible in the final measurement result as a static signal component in a conventional MEG or MKG measurement. In that case, in the measurement, it is not necessary to beforehand prepare oneself for measuring the DC fields.
1. A method of signal processing which is used to separate from a signal registered using a measuring instrument that measures a dynamic phenomenon, a signal associated with a static source in a measurement object, in which method the measurement object and the measuring instrument move with respect to one another, wherein the measurement object is freely movable, the method further comprising: measuring magnetic fields of the measurement object with a multichannel magnetometer; determining the movement
of the measuring instrument and the measurement object with respect to one another based on the signal registered using the measuring instrument;
modelling the movement of the measurement object as a movement of the measuring instrument around the measurement object;
presenting the signal registered as elementary fields in a signal space basis whose basis vector coefficients have been attached to the co-ordinates of the measurement object based on a known geometry between the measurement object and the measuring instrument, whereby the signal produced by a static source is detected as a static signal; and
separating the aforementioned static signal from the signal registered; wherein the measurement object is a brain.
2. The method as defined in claim 1 , characterised in that the movement of the measuring instrument and the measurement object is determined in real time when registering a signal.
3. The method as defined in claim 2 , characterised in that at least a portion of external interference fields is eliminated within the presentation of the elementary fields.
4. The method as defined in claim 3 , characterised in that the elementary fields are calculated using spherical harmonic functions.
5. The method as defined in claim 1 , characterised in that the determined movement is corrected by using the minimum norm estimate of the current distribution of the measurement object.
6. The method as defined in claim 1 , characterised in that the static signal is separated from the measurement signal by a high-pass filter.
7. The method as defined in claim 1 , characterised in that
dividing the signal registered into two periods of time;
separating the aforementioned static signal over either one of the periods of time;
calculating the difference between the signal registered and the separated static signal over the entire period of time.
8. The method as defined in claim 1 , characterised in that the movement of the measuring instrument and the measurement object with respect to one another is achieved so that a person being monitored moves his or her intentionally.
9. The method as defined in claim 1 , characterised in that
measuring the signal caused by magnetic pieces attached to the measurement object, whose location in the co-ordinates of the measurement object is known; and
determining the location of the measurement object in relation to the measuring instrument using the signal registered.
10. The method as defined in claim 1 for reducing an interference caused by the movement of a static magnetisation in a biomagnetic signal, characterised in that the signal registered is high-pass filtered both prior to presenting the signal registered in the co-ordinates attached to the measurement object and after the presentation.