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Removing MRT-caused artifacts from electrooculographic signal by subtraction of the average artifact pattern
International Congress Series 1230 (2001) 1180 – 1181
Removing MRT-caused artifacts from electrooculographic signal by subtraction of the average artifact pattern C.M. Krick*, B. Schmitz, W. Reith Department of Neuroradiology, Saarland University Clinics, Kirrberger Strasse, D-66421 Homburg, Germany
Electrooculographic signal was simultaneously measured during a fMRI-paradigm concerning eye movement. Inside the bore of the Siemens Vision MRT, a low frequency artifact between 0.3 and 2.1 Hz overlays the record. In addition, the fMRI-Epi-Sequence induces a high frequency artifact around 830 Hz caused by the gradient switching (Fig. 1A). We performed an electrooculogram using self-adhesive ‘‘blue sensor’’ carbon fibre cable Ag/AgCl-electrodes. These were directly connected to a notebook outside the Faraday shield over a shielded and grounded cable and a National Instruments low-cost DAQ-Cartridge. The ADC input voltage range was set at ± 50 mV. The sampling rate was set at 5000 Hz. First, the mean volume-period high frequency artifact was subtracted from the signal. After that, the mean artifact pattern in the low frequency range was separated, averaged and subtracted within 40-s periods, because the low frequency artifacts were not homogeneous over the whole record. Finally, residual error noise was treated by a median filter, by smoothing and by a bandpass filter. The filtered electrooculogram is presented in Fig. 1B. The method was able to sufficiently remove low and high frequency MRT-artifacts from a simultaneously recorded electrooculogram to calculate the average eye activity (Fig. 1C).
*
Corresponding author. Tel.: +49-6841-164-313; fax: +49-6841-164-310. E-mail address: [email protected] (C.M. Krick).
0531-5131/01/$ – see front matter D 2001 Elsevier Science B.V. All rights reserved. PII: S 0 5 3 1 - 5 1 3 1 ( 0 1 ) 0 0 2 3 2 - 1
C.M. Krick et al. / International Congress Series 1230 (2001) 1180–1181
Fig. 1. Sample electrooculogram that was simultaneously recorded during fMRI-paradigm. (A) Original data cutting; (B) treated data; paradigm block design is inserted as dotted square curve; (C) average eye activity over the on – off cycle. Abscissas in seconds; ordinates in millivolts.
1181
Removing MRT-caused artifacts from electrooculographic signal by subtraction of the average artifact pattern C.M. Krick*, B. Schmitz, W. Reith Department of Neuroradiology, Saarland University Clinics, Kirrberger Strasse, D-66421 Homburg, Germany
Electrooculographic signal was simultaneously measured during a fMRI-paradigm concerning eye movement. Inside the bore of the Siemens Vision MRT, a low frequency artifact between 0.3 and 2.1 Hz overlays the record. In addition, the fMRI-Epi-Sequence induces a high frequency artifact around 830 Hz caused by the gradient switching (Fig. 1A). We performed an electrooculogram using self-adhesive ‘‘blue sensor’’ carbon fibre cable Ag/AgCl-electrodes. These were directly connected to a notebook outside the Faraday shield over a shielded and grounded cable and a National Instruments low-cost DAQ-Cartridge. The ADC input voltage range was set at ± 50 mV. The sampling rate was set at 5000 Hz. First, the mean volume-period high frequency artifact was subtracted from the signal. After that, the mean artifact pattern in the low frequency range was separated, averaged and subtracted within 40-s periods, because the low frequency artifacts were not homogeneous over the whole record. Finally, residual error noise was treated by a median filter, by smoothing and by a bandpass filter. The filtered electrooculogram is presented in Fig. 1B. The method was able to sufficiently remove low and high frequency MRT-artifacts from a simultaneously recorded electrooculogram to calculate the average eye activity (Fig. 1C).
*
Corresponding author. Tel.: +49-6841-164-313; fax: +49-6841-164-310. E-mail address: [email protected] (C.M. Krick).
0531-5131/01/$ – see front matter D 2001 Elsevier Science B.V. All rights reserved. PII: S 0 5 3 1 - 5 1 3 1 ( 0 1 ) 0 0 2 3 2 - 1
C.M. Krick et al. / International Congress Series 1230 (2001) 1180–1181
Fig. 1. Sample electrooculogram that was simultaneously recorded during fMRI-paradigm. (A) Original data cutting; (B) treated data; paradigm block design is inserted as dotted square curve; (C) average eye activity over the on – off cycle. Abscissas in seconds; ordinates in millivolts.
1181