Volume 12 Supplement 1

Abstracts of the 13th Annual SCMR Scientific Sessions - 2010

Open Access

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

  • Zion Tsz Ho Tse1,
  • Charles L Dumoulin2,
  • Gari Clifford3,
  • Michael Jerosch-Herold1,
  • Daniel Kacher1,
  • Raymond Kwong4,
  • William Gregory Stevenson4 and
  • Ehud Jeruham Schmidt1
Journal of Cardiovascular Magnetic Resonance201012(Suppl 1):O95

DOI: 10.1186/1532-429X-12-S1-O95

Published: 21 January 2010

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

Unprocessed PVC patient ECGs.

Figure 2

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

(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.

Authors’ Affiliations

(1)
Radiology, Brigham and Women's Hospital
(2)
University of Cincinnati College of Medicine
(3)
Health Sciences and Techology, Massachusetts Institute of Technology
(4)
Cardiology, Brigham and Women's Hospital

References

  1. Gupta : IEEE Trans BioMed Eng. 2008Google Scholar
  2. Haberl : 2006, ECG pocket, Borm Bruckmeier PublishingGoogle Scholar
  3. Dukkipati : Circulation. 2008Google Scholar

Copyright

© Tse et al; licensee BioMed Central Ltd. 2010

This article is published under license to BioMed Central Ltd.

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