Though seemingly affected only by its immediate surrounding, the sphere of external influence extends to infinite distance. Nikola Tesla

Prerequisite

Maxfilter is applied to raw MEG data just after recording on .fif files. It is usually the very first preprocessing step applied (except perhaps merging several data modalities).

Goal of this tutorial

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.

Step-by-step guide

1. Choose a reference head position 
2. Run Maxfilter on the raw data
3. Check correction with Muse
4. Iterate to fine tune correction parameters

Choose head position

Even though not mandatory, this step is important because it facilitates merging different runs in subsequent steps, especially if you compute evoked fields over several runs. Typically, one (the first) run or the mean position of the session will be used as reference. This head position is passed to maxfilter via the -trans option.

If you choose to use a mean head position to realign during the tsss processing, it can be computed as follows:

dataHandler -r -avg run01.fif run02.fif run03.fif run04.fif -hp mean -time 0 -.1 meanhp.fif

Run Maxfilter

Here is a generic example of a Maxfilter command at CENIR:

maxfilter_cenir -f run01.fif -st 20000 -corr 0.98 -bad 0221 0332 -trans mean.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 (here mean.fif).

dataHandler -rbr 1 1000 -new run02_dc_offset run02.fiff

maxfilter_cenir -gui -st 20000 -trans meanhp.fif -f run02_dc_offset.fif

Automated bad channel detection

Maxfilter has an -autobad option to detect noisy or defect channels. We do not recommend using this option because it does not log which channels were considered bad and thus prevents tracability. We rather recommend using Elekta's xscan command which allows listing bad channels. The usage of this command is very similar to that of maxfilter. Run xscan -help for more information. The script below uses xscan to list bad channels.

Full script

The script below takes any number of .fif files as input and generates up to three outputs per file.
For instance, for a file called file1.fif, it creates:
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.
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

Copy and paste the code below in a text file (following instructions here) to create a script.

#!/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

If we call the script above "maxfilter.script", we run it as follows:

./maxfilter.script /path/to/mydata/file1.fif /path/to/mydata/file2.fif -trans /path/to/mydata/file1.fif

This will process two files file1.fif and file2.fif in the folder called /path/to/mydata and set the position of the head to that in file1.fif

Inspect with muse

After Maxfilter has finished, it is important to check signal quality visually. We use Muse to do this.

muse run01_tsss.fif

Important aspects of the data to pay attention to:

• "ticks" on all sensors → one electronic artifact happened on a sensor that hasn't been treated as bad. Usually this is the sensor on which the amplitude of the "ticks" are maximal.
• Whether or not large eye artifacts have been removed. The eyes are at the limit of what Maxfilter considers the head sphere. It may consider that blinks originate from outside the sphere and discard them. Whether you want that or not is your call.

Below is an example of a file that has been processed without removing a bad channel. The very brief artifact highlighted in red occurred on only one sensor. Maxfiltering has spread it to several others. (Click on the image for a larger version).

Below is the same data before any maxfiltering. The artifact occurs on only one channel (1142, zooming with Ctrl+mouse wheel would reveal that only one channel is involved). The artifact that is present above on channels 2232 and 1112 is not visible here. Not discarding it during maxfilter above "spread" this artifact to other channels.

Below is the same data where channel 1142 has been marked as bad (with the command -bad 10142). Note that this bad channel was detected with the automatic xscan command included in the script above.

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