Gradient-induced voltages on 12-lead ECGs during high-duty-cycle MRI sequences and a theoretically-based method to remove them
Journal of Cardiovascular Magnetic Resonance volume 17, Article number: P243 (2015)
An MRI-compatible 12-lead ECG platform, equipped with MRI-gradient induced-voltage removal hardware and magneto-hydrodynamic voltage removal software , was previously applied to physiological monitoring and synchronization of cardiac imaging of patients inside MRI. This approach had limited success for high-duty-cycle [(total Gradient-ramp-time per R-R)/(R-R time) >20%] MRI sequences, such as Steady State Free Precession (SSFP), Short-TR Gradient Echo (GRE), and Short-TR Fast Spin Echo. The study objective is to measure and develop a method to remove gradient-induced voltages on 12-lead ECGs during high-duty-cycle MRI sequences.
A modification of the hardware developed in  enabled measuring the gradient-induced voltage over a 24 kHz frequency-range and +/-10V, together with the x, y, and z gradient waveforms. ECGs were measured in 9 volunteers at 3T (Siemens Skyra). A theoretical equation for the gradient-induced voltages on each ECG electrode (Vi, where i=1,2,3..9 was derived, based on Maxwell's equations  and concomitant fields  [Fig. 1A]. It includes 1st and 2nd order gradient waveform terms and estimates induced voltages even on ECG electrodes positioned farthest from magnet iso-center, such as the limb leads.
The 19 equation coefficients were obtained during 6-8 second training sequences, consisting of highly accelerated (3-4sec, GRAPPA=6-8) versions of each sequence, followed by non-imaging segments. The non-imaging segments were used to obtain the shape of the true ECG traces over the entire R-R cycle ("template"). This template was subtracted from ECG traces acquired during imaging, resulting in the net gradient-induced voltages, which was then fit to the equation, providing the 19 coefficients for each electrode.
Multi-slice imaging was then performed, with real-time subtraction of the gradient-induced voltages from each acquired ECG trace, utilizing the computed Vi.
Measured limb-lead ECG voltages during SSFP imaging, with the heart at iso-center (Fig. 1B), were 0.7-1.0 Volt PTP, with frequency components up to 20 KHz (Fig. 1C). Applying the equation for gradient-induced voltage removal during GRE (Fig. 1D-E), and multi-slice SSFP imaging (Fig. 1F), resulted in a normalized-difference between the recovered ECG trace and the true ECG of <20%. Equation coefficients varied by subject, sequence, sequence parameters, and slice orientation (Fig. 1G).
An equation was derived for the strong gradient-induced voltages observed in 12-lead ECGs during high-duty-cycle MRI sequences. A rapid training sequence permitted computing equation-coefficients, followed by real-time gradient-induced voltage removal during imaging.
NIH U41-RR019703, R03-EB013873-01A1, AHA 10SDG261039.
Tse : MRM. '13.
Bowtell : MRM. ‘00.
Bernstein : MRM. '89.
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Zhang, S.H., Tse, Z.T., Dumoulin, C.L. et al. Gradient-induced voltages on 12-lead ECGs during high-duty-cycle MRI sequences and a theoretically-based method to remove them. J Cardiovasc Magn Reson 17 (Suppl 1), P243 (2015). https://doi.org/10.1186/1532-429X-17-S1-P243