Method and apparatus for reducing contamination of an electrical signal

• US 20040097802A1
• Filed: 02/18/2003
• Published: 05/20/2004
• Est. Priority Date: 08/15/2000
• Status: Active Grant

• Current Assignee: Regents of the University of California (University of California)
• Original Assignee: Regents of the University of California (University of California)
• Inventors: Cohen, Mark S
• Granted Patent: US 7,286,871 B2
• Time in Patent Office: Days
• US Class Current: 600/411
• CPC Class Codes: A61B 5/30 Input circuits therefor
• CPC Class Codes: A61B 5/316 Modalities, i.e. specific diagnostic methods
• CPC Class Codes: A61B 5/4094 Diagnosing or monitoring seizure diseases, e.g. epilepsy

Abstract

The method of reducing contamination of electrical signals recorded in the presence of repeated interference contamination comprises obtaining an electrical signal recorded in the presence of a contaminating signal, and detecting a timing signal that occurs at a fixed time point during the electrical signal relative to the onset of the contaminating signal. The electrical signal is digitized, wherein the digitizing begins with the timing signal. A plurality of digitized electrical signals is analyzed, wherein the electrical signals are synchronized with respect to the timing signal, to obtain an estimated contaminating signal that is subtracted from the digitized electrical signal. This method can be used with electrophysiological signals, such as EEG, ECG, EMG and galvanic skin response, and for elimination of noise associated with concurrently used methods such as MRI. The method of noise reduction is applicable to recordings of other electrical signals, including audio recordings.

Claims

1. A method of reducing contamination of electrical signals recorded in the presence of repeated interference contamination comprising:

(a) obtaining an electrical signal, wherein the electrical signal was recorded in the presence of a contaminating signal;

(b) detecting a timing signal that occurs at a fixed time point during the electrical signal relative to the onset of the contaminating signal;

(c) digitizing the electrical signal, wherein the digitizing begins with the timing signal;

(d) analyzing a plurality of digitized electrical signals, wherein the electrical signals are synchronized with respect to the timing signal to obtain an estimated contaminating signal; and

(e) subtracting the estimated contaminating signal from the digitized electrical signal, thereby reducing contamination of the electrical signal

2. The method of claim 1, wherein the analyzing to obtain the estimate of the contaminating signal comprises averaging the electrical signals.

3. The method of claim 1, wherein the analyzing to obtain the estimate of the contaminating signal comprises calculating a weighted average of the electrical signals.

4. The method of claim 3, wherein the estimate of the contaminating signal is biased towards recent events.

5. The method of claim 4, wherein the estimate of the contaminating signal comprises a first sum consisting of the nth electrical signal added to a scalar multiple, w, of a prior estimate of the contaminating signal, divided by a second sum obtained by adding a series 1+w²+w³+w⁴ . . . + . . . wⁿ.

6. The method of claim 1, 2 or 3, wherein the electrical recording comprises an electrophysiological signal.

7. The method of claim 1, wherein the estimate of the contaminating signal is multiplied by a scalar prior to the subtracting of step (e).

8. The method of claim 4, wherein the electrophysiological signal comprises an electroencephalographic recording, an electromyelographic recording, an electrocardiographic recording or a measure of galvanic skin resistance.

9. The method of any one of claims 1-8, wherein the interference comprises interference arising from inductively coupled magnetic fields.

10. The method of any one of claims 1-9, wherein the interference comprises interference arising from alternating current (AC) line noise.

11. The method any one of claims 1-10, wherein the digitizing is performed at a sampling rate below the Nyquist rate for the contaminating signaL

12. The method of any one of claims 1-11, wherein the electrical signal obtained in step (a) is passed through a low pass filter prior to the digitizing, at a frequency of approximately one half of the frequency at which the electrical signal is sampled.

13. The method of claim 12, wherein the low pass filter passes signal frequencies of less than about 200 Hz.

14. The method of any one of claims 1-13, wherein the method is performed concurrently with Magnetic Resonance Imaging of the subject.

15. The method of claim 14, wherein the electrical signal comprises an electrophysiological signal and the contaminating signal comprises gradient activity.

16. The method of claim 15, wherein the electrical signal comprises an electrophysiological signal and the contaminating signal comprises radio frequency transmitter activity.

17. The method of any one of claims 1-16, wherein the digitizing is performed at a rate of about 200 to about 5000 samples pet second.

18. The method of claim 1, further comprising removing a DC offset from the electrical signal by analog subtraction prior to the digitizing.

19. The method of claim 18, wherein the DC offset is measured and subtracted from the electrical signal using a differential amplifier.

20. The method of claim 19, wherein the DC offset is measured by analog to digital conversion, and averaged over a time period long compared to lowest frequencies of interest in the electrical signaL

21. The method of claim 20, wherein the time period is approximately 10 times longer than the lowest frequencies of interest in the electrical signal.

22. The method of claim 20 or 21, wherein the analog subtraction comprises converting the averaged signal to an analog voltage and electrically subtracting the averaged signal from the electrical signal through differential amplification.

23. The method of claim 19, wherein the DC offset is measured in an analog integrator having a time constant long compared with lowest frequencies of interest in the signal.

24. The method of claim 23, wherein the time constant is approximately 10 time the lowest frequencies of interest

25. The method of claim 7, wherein the electrophysiological recording comprises an electroencephalogram and is recorded concurrently with magnetic resonance image acquisition.

26. The method of claim 25, wherein the electrophysiological recordings are used to inform interpretations of the magnetic resonance images.

27. The methods of claim 25 or 26, wherein the electrophysiological recordings are used in a statistical analysis of change in intensity of the magnetic resonance signal.

28. The method of any one of claims 25-27, further comprising determining a correlation between change in intensity of the magnetic resonance signal and a feature of the electrophysiological recording.

29. The method of claim 28, wherein the correlation is used to make statistical images that represent an association between the electrical signals and the intensity of the magnetic resonance signal intensity.

30. The method of claim 28, wherein the feature of the electrophysiological recording comprises a time course of signal intensity change in defined frequency bands contained in the electrophysiological recording.

31. The method of claim 30, wherein the defined frequency bands correspond to standard ranges used for clinical interpretations of the electroencephalogram.

32. The method of claim 31, wherein the standard ranges are selected from the group consisting of from 0 to approximately 4 Hz (the Delta band), from approximately 4 to approximately 8 Hz (the Theta band), from approximately 8 to approximately 12 Hz (the Alpha band), from approximately 12 to approximately 30 Hz (the Beta band), and from approximately 30 Hz and greater (the Gamma band).

33. The method of any one of claims 25-32, further comprising convolving the time course of the electrophysiological signal with an estimate of the magnetic resonance hemodynamic impulse response function.

34. A method of reducing magnetic interference during electrophysiological recording from a subject by measuring an electrical potential difference between a pair of electrodes, wherein the pair of electrodes communicate with a differential amplifier via electrical connections, the method comprising twisting the electrical connections together, thereby reducing magnetic interference.

36. The method of claim 34 or 35, wherein the electrophysiological recording comprises an electroencephalographic recording.

37. The method of any one of claims 34-36, wherein the method is performed concurrently with Magnetic Resonance Imaging of the subject.

35. A method of reducing magnetic interference during electrophysiological recording from a subject by measuring an electrical potential difference between a pair of adjacent electrodes, wherein each electrode comprises two leads, the method comprising twisting each lead together with a lead of an adjacent electrode, thereby reducing magnetic interference.

38. An apparatus for processing digitized electrical signals in the presence of a repeated contaminating signal comprising

(a) a signal processor 90 adapted to receive a recording of an electrical signal;

(b) a detector 92 adapted to detect a tiring signal that occurs at a fixed time point during an electrical signal relative to the onset of a contaminating signal;

(c) a signal accumulator 94; and

(d) a processor 96 adapted to subtract averaged waveforms from an electrical signal.