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
Journal of Cardiovascular Magnetic Resonance volume 12, Article number: O95 (2010)
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 . A dominant QRS and undistorted S-T segment are important for MRI gating and physiological monitoring for ischemia during cardiac imaging/interventions .
We hypothesized that adaptive filtering could separate between MHD and ECGreal, and that the MHD signal could non-invasively estimate cardiac output.
MRI-compatible 12-lead ECGs were acquired with a modified ECG-recording system  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.
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.
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.
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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 (Suppl 1), O95 (2010). https://doi.org/10.1186/1532-429X-12-S1-O95