Though seemingly affected only by its immediate surrounding, the sphere of external influence extends to infinite distance. Nikola Tesla
Table of Contents |
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Prerequisite
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This tutorial teaches how to use the Maxfilter command line tool to remove artifacts on MEG signals coming from outside of the sphere of all MEG channels. This tool was build by Elekta, the vendor of our MEG.
It is also at this step that bad channels are marked and discarded (interpolated).
Step-by-step guide
- Choose a reference head position
- Run Maxfilter on the raw data
- Check correction with Muse
- Iterate to fine tune correction parameters
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Code Block |
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maxfilter_cenir -f run01.fif -st 20000 -corr 0.98 -bad 0221 0332 -trans meanmeanhp.fif -o run01_tsss.fif |
Brief description of the options (the user is referred to the pdf manual for more information):
-f
: the input file-o
: the output file-tsss
: perform temporal source space separation (the standard at CENIR). The option is followed by a buffer length (in seconds), which should always be larger than your individual recordings.-corr
: amount of correlation between data and noise that should be rejected (see manual for more explanation). Lowering the value below the default 0.98 may sometimes help getting rid of artifacts but can also lead to removing interesting signal from the data. Careful inspection is recommended (see below).-bad
: which channels should be considered bad, i.e. removed prior to correction and later reinterpolated. The -autobad command is not recommended because it leaves no trace of which channels are considered bad. Rather use xscan (see below).-trans
: transforms the MEG data into the sensor array defined by the channel info in a specified file (heremean.fif
).
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Be sure to use the |
Info | ||
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On rare occasions, we experienced some trouble with tsss processing: when large dc-offset is present in the signal at the beginning of the file a bug can occur, leaving the resulting file with strong artifacts at the beginning of the file and a lot of noise above 80Hz on a number of channels. If you encounter this kind of behavior, the solution is simple, before running tsss you need to remove the large dc-offset with dataHandler: dataHandler -rbr 1 1000 -new run02_dc_offset run02.fiff and then your maxfilter command: (for instance) maxfilter_cenir -gui -st 20000 -trans meanhp.fif -f run02_dc_offset.fif This problem is quite rare. A possible cause is large low frequency artifacts happening during recording (for those working with patients having big metallic part, be careful!). |
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#!/bin/bash
function display_usage() {
cat <<EOF
Usage: $0 file1.fif file2.fif file3.fif ...
$0 file1.fif file2.fif file3.fif ... -trans fileX.fif
This script takes any number of .fif files as input and generates up to three outputs per file.
For file1.fif:
1/ a maxfiltered version file1_tsss.fif (using maxfilter -st)
2/ a list of bad channels file1.bad (using xscan). If a file file1.bad already exists,
this step is skipped and the content of file1.bad will be used for step 3 below.
3/ a maxfiltered version excluding these bad channels file1_tsss_nobad.fif (using maxfilter -bad )
If the -trans fileX.fif option is passed (often recommended), the head position in each file will be
set to that in fileX.fif
NOTE: this script will never overwrite any file. Remove previous versions of files if needed.
EOF
}
# first display usage if needed and parse trans arguments
if [ -z "$1" ]
then
display_usage
exit 1
else
POSITIONAL=()
while [[ $# -gt 0 ]]
do
key="$1"
case $key in
-trans)
TRANS="$2"
shift # past argument
shift # past value
;;
*) # unknown option
POSITIONAL+=("$1") # save it in an array for later
shift # past argument
;;
esac
done
set -- "${POSITIONAL[@]}" # restore positional parameters
toprocess=$@
fi
# for each input file
for f in $toprocess
do
dn=$(dirname "${f}")
# make sure we use sss_config if next to the data
if [ -d "${dn}/sss_config/" ]
then
xscan_cenir="xscan -ctc ${dn}/sss_config/ct_sparse.fif -cal ${dn}/sss_config/sss_cal.dat -st 20000 -corr 0.98 "
maxfilter_cenir="maxfilter -ctc ${dn}/sss_config/ct_sparse.fif -cal ${dn}/sss_config/sss_cal.dat -st 20000 -corr 0.98 -trans $TRANS "
else
printf "Warning !\n No 'sss_config' directory next to the data in ${dn}\n\n maxfilter results may be inaccurate...\n\n"
xscan_cenir="xscan -st 20000 -corr 0.98 "
maxfilter_cenir="maxfilter -st 20000 -corr 0.98 -trans $TRANS "
fi
# start
echo "processing $f"
nf=`echo $f | sed -e 's/.fif$//g'`
# if output _tsss file does not exist
if [ ! -f ${nf}_tsss.fif ]
then
# run maxfilter -st
echo "$maxfilter_cenir -f $f -o ${nf}_tsss.fif"
$maxfilter_cenir -f $f -o ${nf}_tsss.fif
else
# if file exists, then skip
echo "${nf}_tsss.fif exists. Skipping."
fi
# if a list of bad channels does not exist
if [ ! -f ${nf}.bad ]
then
# create a list of bad channels
echo "detecting bad channels in $f"
echo "$xscan_cenir -f $f"
BAD_CHANNELS=`$xscan_cenir -f $f`
echo $BAD_CHANNELS > ${nf}.bad
else
# else use existing list
echo "${nf}.bad exists. Skipping automatic detection."
BAD_CHANNELS=`cat ${nf}.bad`
fi
# if there is no bad channel
if [[ -z "$BAD_CHANNELS" ]]
then
echo "No bad channels for $f"
else
# else create a _nobad.fif file where bad channels are excluded (interpolated)
if [ ! -f ${nf}_tsss_nobad.fif ]
then
echo "Maxfilter ignoring bad channels..."
echo "$maxfilter_cenir -f $f -o ${nf}_tsss_nobad.fif -bad $BAD_CHANNELS"
$maxfilter_cenir -f $f -o ${nf}_tsss_nobad.fif -bad $BAD_CHANNELS
else
echo "${nf}_tsss_nobad.fif exists. Skipping."
fi
fi
done
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If we call the script above "maxfilter.script
", we run it as follows:
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After Maxfilter has finished, it is important to check signal quality visually. We use Muse to do this.
Code Block |
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muse run01_tsss.fif |
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