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12-lead ECG in a 1.5 Tesla MRI: Separation of real ECG and MHD voltages with adaptive filtering for gating and non-invasive cardiac output

Introduction

The Magneto-Hydro-Dynamic (MHD) effect arises when conductive blood flows in the MRI magnetic field (B0). MHD generates a voltage which distorts the real electrocardiogram (ECGreal), especially during the S-T segment where flow from the left ventricle (LV) into the aorta contributes to a large MHD voltage [1]. A dominant QRS and undistorted S-T segment are important for MRI gating and physiological monitoring for ischemia during cardiac imaging/interventions [2].

Purpose

We hypothesized that adaptive filtering could separate between MHD and ECGreal, and that the MHD signal could non-invasively estimate cardiac output.

Methods

MRI-compatible 12-lead ECGs were acquired with a modified ECG-recording system [3] from three healthy volunteers and one patient with idiopathic outflow tract Premature Ventricle Contractions (PVCs) (Ejection Fraction 20-25%, LV wall thickening, mitral regurgitation). Three sets of 20-sec breath-held ECGs (Fig. 1), were measured in a 1.5 scanner with subjects placed (i) outside the scanner with their head-in (ECG = ECGreal), (ii) at iso-centre with their head-in (ECG = ECGreal+MHDhead-in), and (iii) at iso-centre with their feet-in (ECG = ECGreal+MHDfeet-in), which reverses B0 polarity (MHDfeet-in~-1 × MHDhead-in). Data processing (Fig. 2) involved application of an adaptive Least-Mean-Square filter to (ii) and (iii), whilst (i) was used to train the filter to decouple the MHD signal from ECGreal.

Figure 1
figure1

Unprocessed PVC patient ECGs.

Figure 2
figure2

Adaptive filtering procedure.

Results

Fig. 3(a-d) show processing of the patient's V6 ECGs in positions (ii) and (iii). MHD signals are effectively removed in Fig. 3(c-d), showing ECGreal with the S-T segment preserved. The MHD signals, Fig. 3(e-f), are maximal during the S-T segment. Oscillating positive and negative MHD voltages during systole in each PVC cycle can be explained by flow eddies, consistent with the patient's mitral regurgitation. Fig. 3(g-h) show the cardiac output, calculated from the systolic time-integrated MHD. Cardiac output during PVC cycles is much smaller than during normal beats. Fig. 3(i) indicates that the PVC patient's average cardiac output is 44-54% of the healthy volunteers', due to less effective PVC beats.

Figure 3
figure3

(a-h) PVC patient ECGs at iso-centre. (i) Cardiac outputs of healthy volunteers versus the patient.

Conclusion

The filtering procedure separates the ECGreal and MHD signals in 12-lead ECGs acquired within the MRI. The QRS complex becomes dominant, as required for good MRI gating, while preserving S-T segment fidelity for physiological monitoring during imaging/interventions. MHD signals allow for non-invasive monitoring of beat-to-beat cardiac output.

References

  1. 1.

    Gupta : IEEE Trans BioMed Eng. 2008

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    Haberl : 2006, ECG pocket, Borm Bruckmeier Publishing

  3. 3.

    Dukkipati : Circulation. 2008

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Correspondence to Zion Tsz Ho Tse.

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Open Access This article is published under license to BioMed Central Ltd. This is an Open Access article is distributed under the terms of the Creative Commons Attribution 2.0 International License (https://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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Tse, Z.T.H., Dumoulin, C.L., Clifford, G. et al. 12-lead ECG in a 1.5 Tesla MRI: Separation of real ECG and MHD voltages with adaptive filtering for gating and non-invasive cardiac output. J Cardiovasc Magn Reson 12, O95 (2010). https://doi.org/10.1186/1532-429X-12-S1-O95

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Keywords

  • Cardiac Output
  • Left Ventricle
  • Mitral Regurgitation
  • Left Ventricle Wall
  • Physiological Monitoring