A guide to peripheral physiology measurements using the BrainAmp ExG MR – Part 2: Let’s focus on ECG

by Dr. Robert Stoermer (Brain Products Head of Technical Support)
and Dr. Tracy Warbrick (Brain Products Application Specialist EEG-fMRI)

Abstract

In this part of our peripheral physiology series we introduce ECG measurement using the BrainAmp ExG MR in the MR environment. The safety issues associated with ECG in the MR environment are highlighted. We explain the mechanisms which may limit the signal quality and introduce a methodology to obtain precise heart rate information.

Introduction

The ECG-trace has been part of each EEG-fMRI dataset since the very beginning of EEG recordings in the scanner as it is required for the handling of the cardioballistic artifact. Therefore, all our BrainCaps MR have one electrode that is placed on the participant’s back. If correctly placed, this channel does reliably provide R-peaks as a temporal reference point for the cardioballistic artifact correction and heart rate time series calculations. But how do we obtain this information in experiments where no EEG cap is available because only peripheral physiology is in the focus of the study?

Precise heart rate information is important for countless scientific questions focusing on autonomic nerve system reactions.  Together with respiration data and the galvanic skin response (GSR) it forms the basis for the co-registration of the psychophysiological response during functional brain imaging studies. 

For many MR imagers a dedicated physio kit is available, but the ECG-signal is often not easily accessible, has a dissatisfying sampling rate, and is hard to synchronize with the other signals of interest.


Overview

1. Methodology using BrainAmp ExG MR
     1.1. ECG feature of interest and sources of signal degradation
     1.2. How to prevent typical safety issues by using BrainAmp ExG MR
     1.3. Equipment placement, cable routing and skin preparation
     1.4. ECG electrode placement
     1.5. Recorder workspace settings
2. Recording procedure
3. Offline analysis
Conclusion and ‘top tips’ for successful ECG-fMRI


1. Methodology using BrainAmp ExG MR

In this part of the series, we reveal the secrets of safely recording ECG without an EEG cap, but by using chest electrodes and the BrainAmp ExG MR. As a result, you will get the raw data with a 5 kHz sampling frequency in the well-known vision format for offline processing in BrainVision Analyzer 2 or as a real-time data stream for BrainVision RecView

1.1. ECG feature of interest and sources of signal degradation

Many typical ECG features well known from clinical ECG outside the scanner are strongly distorted and hard to obtain in the MR environment. Here, we will focus only on the precise registration of the R-wave as a basis for inter-beat interval calculation.

ECG registration in MRI has issues in common with sEMG in the MR environment (BrainAmp ExG MR – Part 1: Let’s focus on EMG) while other problems are different. While surface electromyography (sEMG)  reflects the temporal variation of the summation potential of motor units, the ECG mirrors the action of the cardiac stimulus conduction system. As the waveform feature of interest is the R-wave, the arrangement of electrodes and lead wires needs to maximize the voltage of the R-wave while keeping the imaging artifact voltage as low as possible.

Further problems specific to ECG in MR environment are:

  • ECG in the scanner is obscured by the magnetohydrodynamic (MHD) artifact. The MHD artifact is an induced voltage caused by the heart beat related flow of blood in large vessels in the magnetic field. The MHD creates an artifact voltage which superposes the ECG during the ejection phase, mainly during the T-wave (Niendorf, 2012).
  • Less dominant, but still present, are breath synchronous baseline variations which are induced due to the movement of the ECG leads in the static field
  • The electrical heart axis shows individual variation. This is one reason for the different ECG wave forms despite comparable electrode placement.

Perfect imaging artifact subtraction requires a coupling between the gradient system clock and the BrainAmp MR clock. Use of the SyncBox is therefore mandatory for ECG-fMRI (Mandelkow 2006).

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1.2. How to prevent typical safety issues by using BrainAmp ExG MR

ECG belongs to the most safety critical procedures in clinical MR applications.  Contact with ECG electrodes or leads are the second leading cause for thermal injuries in the MR environment (Jana et al 2019, FDA Video). We here introduce a methodology which fundamentally differs from most clinical ECG setups in MRI.

  • We do not use pre-gelled quatrodes
  • We are using the same resistive multitrodes as known from the BrainCap MR. (Figure 1). The skin is carefully prepared, electrodes are individually gelled, and each electrode impedance is individually verified.
  • There are no dried-out gel pads or unknown impedance states as is often seen when disposable electrodes are pasted on insufficiently prepared skin.
  • We are routing the cable path away from the bore walls in the center of the bore.
  • Our B1+rms restriction excludes many structural sequences involved in thermal injuries.

EMG-fMRI Figure 3. The Multitrode MR in detail, a bound pair of multitrodes is shown. A. incomplete ring to avoid induced eddy current, B. Current limiting resistor, C. Resistor value labels, D. MR conditional label, E. Plastic spiral tubing to prevent the lead wire having direct contact with the volunteer and to bind pairs of Multitrodes together.

Figure 1: The Multitrode MR in detail, a bound pair of multitrodes is shown. A. incomplete ring to avoid induced eddy current, B. Current limiting resistor, C. Resistor value labels, D. MR conditional label, E. Plastic spiral tubing to prevent the lead wire having direct contact with the participant and to bind pairs of Multitrodes together.

All recommendations that we make throughout this support tip apply to measurements in MRI scanners up to 3 T and the safety guidelines and conditions for use should always be followed. You can find further information on our website.

In the MR environment the BrainAmp ExG MR should only be used together with the ExG AUX Box and special MR conditional electrodes and sensors. For bipolar ECG measurements Multitrode MR electrodes should be used because they have some special features that make them suitable for use in the MR environment: they have a current-limiting resistor, they are an incomplete ring to avoid induced eddy currents and are bundled in a spiral tube so that the lead wire cannot come into direct contact with the participant (Figure 1). The Multitrode MR electrodes are intended for surface EMG and ECG and all recommendation that we provide in this article are for ECG (for EMG please refer to the EMG-fMRI support tip).

Local transmit coils always provide a favorable safety profile as compared to a body coil transmitter. They should be used if available. Make sure the imaging sequence meets the requirements for our system: The predicted B1+rms value has to be lower than 1 µT.

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1.3. Equipment placement, cable routing and skin preparation

The placement of the amplifier and PowerPack is similar to the placement introduced for EMG in MR-environment. Please refer to the arrangement shown in Figure 2:

EMG-fMRI Figure 4. The BrainAmp ExG MR in the recommended position at the foot end of the scanner bed. The measurement in the example is sEMG from the lower leg using Multitrode MR with 40 cm lead wires. (Image used with courtesy of Siemens Healthineers

Figure 2: The BrainAmp ExG MR in the recommended position at the foot end of the scanner bed. In order to minimize electrode wire length, amplifier powerpack and ExG AUX input box should be shifted head wards.

The physical cable path consists of a 30 cm ribbon cable, the ExG AUX input box, the bipolar multitrodes, and a third single multitrode which serves as ground electrode. The electrode cables must run straight and without curves or loops close to the bore midline.

Skin preparation:  Dry the skin if it is diaphoretic or moist. Hair that can interfere with electrode placement must be shaved. Sometimes an abrasive gel must be used to remove dead skin cells. Verify the impedances using BrainVision Recorder. The target value is 10 KOhm.

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1.4. ECG electrode placement

In clinical medicine there are various electrode placement schemes for different diagnostic purposes. As mentioned before, our electrode placement needs to minimize imaging artifact voltages while maximizing the ECG component of interest. Usual clinical leads don’t fulfill these requirements. The best results are obtained from a bipolar lead forming a dipole on the left side of the chest.

A guide to peripheral physiology measurements using the Brain Amp ExG MR – Part 2: Let’s focus on ECG

Figure 3: An example body surface potential maps with timepoint of interest visualized. The time signal (bottom) is the RMS of the torso surface signals. The time instances shown are the peak of the RMS QRS, the end of the QRS, and the peak of the T-wave. (Bergquist et al, 2021)

Our electrode placement suggestion makes use of the surface potential distribution as shown in Figure 3. Reliable electrode placement requires orientation on topographical landmarks (Figure 4). Important vertical auxiliary lines are:

  • Mid-clavicular line: A vertical line passing through the midpoint of the clavicle.
  • Sternal line: A vertical line corresponding to the lateral margin of the sternum.

The vertical position is defined by the ribs: When counting down to the 5th intercostal, it’s helpful to know that the 1st intercostal space (ICS) is the space right below the clavicle.

While the placement of the two ECG electrodes is critical, the placement of the ground electrode is arbitrary. The lead wire must be straight and the electrode gelled.

A guide to peripheral physiology measurements using the Brain Amp ExG MR – Part 2: Let’s focus on ECG

Figure 4: This electrode placement avoids excessive gradient artifact amplitudes in whole body 3T systems but provides an pp R-wave of about 2000-3000 µV.

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1.5. Recorder workspace settings

BrainVision Recorder LogoCorrect BrainVision Recorder workspace settings are essential for successful recordings, and for ECG we recommend the same settings as for EEG recordings in the MR environment. A full description of these settings can be found in our support tip “Setting up BrainVision Recorder for simultaneous EEG-fMRI” and we will summarise them below:

  • 5 kHz sampling rate
  • 0.5 µV/bit amplitude resolution for the analogue to digital converter (ADC)
  • AC-coupled acquisition with a low cut-off hardware filter with a time constant of 10 s
  • 250 Hz hardware high cut off filter.

Make sure that the correct value for the resistors in your electrodes are entered in the workspace. This value will be subtracted during the impedance measurement to give accurate impedance values. The correct resistor value can be found on a small, white label close to the electrode and the connector on the Multitrode MR (Figure 1).

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2. Recording procedure

Always verify the signal quality in BrainVision Recorder using monitoring mode when the scanner table along with the participant and equipment are in place.  The ECG must be clearly visible with R-wave amplitude in the order of 3 mV (Figure 5).

A guide to peripheral physiology measurements using the Brain Amp ExG MR – Part 2: Let’s focus on ECG

Figure 5: Single lead chest wall ECG with recommended electrode positions in a 3T static field.The magnetohyrodynamic (MHD) effect is indicated.

After starting the imaging sequence (Figure 6), the ECG can be continuously monitored using BrainVision RecView. For more details on this, please refer to chapter 7 of the BrainVision RecView user manual.

A guide to peripheral physiology measurements using the Brain Amp ExG MR – Part 2: Let’s focus on ECG

Figure 6: Ten seconds of ECG data during a continuous EPI scan. R128 marker indicate the onset of volumes (TR=2000 ms). The peak-to-peak amplitude of the imaging artifact is underestimated in the figure. The actual peak-to-peak artifact amplitude for the recommended electrode placement is in the order of 25 mV.

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3. Offline analysis

BrainVision Analyzer 2Gradient artifact correction should be performed using the MR Correction transformation in BrainVision Analyzer 2. A comprehensive demonstration of how to use the MR Correction transformation in Analyzer 2 is provided in our webinar on handling scanner related artifacts as well as in the BrainVision Analyzer 2 user manual.

We also provide some key points in the first article of this support tip series. So to avoid repetition, please refer to the EMG-fMRI article.

Once the gradient artifact has been removed you can analyse the signal of interest. In this case, we want to identify the R-peak and then calculate the inter-beat interval. The CB Correction transformation can be used in semi-automatic mode to detect and mark the R-Peaks. The IBI Export (Solutions > Export > IBI Export) can then be used to export the inter-beat intervals, you can then analyse or plot these values as needed. You can find more information about processing the ECG data offline in our support tip on analysing data from peripheral physiology sensors.

A guide to peripheral physiology measurements using the Brain Amp ExG MR – Part 2: Let’s focus on ECG

Figure 7: Top row: ECG after artifact average subtraction in BrainVision Analyzer (Allen et al, 2000) and decimation to 500 Hz, R-wave identified by the cardioballistic correction transform. Bottom row: calculated inter-beat interval time series

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Conclusion

Getting highly reliable heart rate data is easy if you use the described methodology. It allows a multimodal monitoring of autonomous nerve functions during functional MRI.

The BrainAmp ExG MR is at the core of successful acquisition of peripheral physiology signals.

Our ‘top tips’ for successful ECG-fMRI

  For participants of different heights, tailored cable kits are needed.

  It is wise to keep the electrode polarity constant within a study.

  Pilot tests must include the entire ECG processing pipeline.

  While rare, dextrocardia is estimated to occur in approximately 1 in 12,019 people, so if you fail to obtain an ECG with the recommended electrode arrangement on the left side, try the right side.

  Always give the participant an alarm bell and explain how to use it.


We hope you found this support tip helpful and if you have any further questions on this topic, please feel free to contact our Technical Support team.

References

MRI-related FDA adverse event reports: A 10-yr review
Jana G. Delfino, Daniel M. Krainak, Stephanie A. Flesher, Donald L. Miller
First published: 16 August 2019

Body Surface Potential Mapping: Contemporary Applications and Future Perspectives
Jake Bergquist, Lindsay Rupp, Brian Zenger, James Brundage and Anna Busatto, and Rob S. MacLeod
Hearts 2021, 2, 514–542. https://doi.org/10.3390/hearts2040040

Electrocardiogram in an MRI Environment: Clinical Needs, Practical Considerations, Safety implications, Technical Solutions and Future Directions
Thoralf Niendorf, Lukas Winter and Tobias Frauenrath
Advances in Electrocardiograms – Methods and Analysis, PhD. Richard Millis (Ed.), ISBN: 978-953-307-923-3, InTech, DOI: 10.5772/24340.

Cables and electrodes can burn patients during MRI
Susan Lange, Quynh Nhu Nguyen
Nursing 2006 Nov;36(11):18.

A Method for Removing Imaging Artifact from Continuous EEG Recorded during Functional MRI
Allen et al.
September 2000 NeuroImage 12(2):230-9 DOI:10.1006/nimg.2000.0599

Hazardous situation in the MR bore: induction in ECG leads causes fire.
Kugel H, Bremer C, Pueschel M, Fischbach R, Lenzen H, Tombach B, Aken H, Heindel W.
Eur Radiol 2003;13:690–4.

Synchronization facilitates removal of MRI artefacts from concurrent EEG recordings and increases usable bandwidth.
Mandelkow, H., et al. (2006)
Neuroimage 32(3): 1120-1126.

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