Computation of the gradient-induced electric field noise in 12-lead ECG traces during rapid MRI sequences
Journal of Cardiovascular Magnetic Resonance volume 16, Article number: P151 (2014)
Successful physiological monitoring using a 12-lead ECG during MR imaging is essential for safe conduction of cardiovascular interventions within a MR scanner. However, ECG artifacts induced by magnetic field gradients severely affect the signal quality and fidelity. Previously, the gradient-induced artifacts were reduced by blocking ECG transmissions during all gradient ramps , which has been shown feasible while the method is not suitable for short-TR sequences. Theoretical and experimental studies have shown a linear relationship between electric fields and the temporal derivatives of the magnetic field gradients [2, 3]. We propose an algorithm to restore the true ECG signal by subtracting system response functions, based on the MR gradient signals, from ECG signals distorted by gradient interference.
Data Acquisition: An MRI-conditional 12-lead ECG system  was used to acquire data on two healthy volunteers inside a 3T MRI. Outside the MRI room, high-fidelity ECG traces, along with the x, y and z gradient waveforms were digitally recorded simultaneously at 62kHz. Balanced SSFP sequences with various slice orientations (axial, coronal, sagittal and oblique) were acquired. Data Analysis: The gradient-induced ECG noise was computed as the difference between aligned ECG traces with and without MR sequence running. The noise voltage (Vni) at each electrode (i) was modeled as a linear combination of gradient derivatives and system factors, Vni = αi•dGx/dt+βi•dGy/dt+γi•dGz/dt+Ci, where αi, βi, γi and Ci are position-dependent. These parameters were then used to reconstruct the noise, for comparison with the measured ECG noise, and to further derive the restored ECG.
The recorded ECG traces and low-pass filtered gradient derivatives are displayed in Figure 1a. The computed noise vector (Vni) and the measured noise (Figure 1b) had differences of 21% ± 20% in normalized Euclidean distance. The restored ECG signal was comparable to the clean ECG segments (Figure 2), providing higher signal quality and fidelity relative to low-frequency filtering of the ECG signal. Vectorial display of the fitted parameters (Figure 3) demonstrated systematic changes across the precordial leads, and varied in magnitude between subjects.
The gradient-derivative model closely fit the measured ECG noise, possibly allowing for efficient gradient-noise removal utilizing rapid calibration scans, combined with hardware blocking of extremely high noise intervals.
NIH U41-RR019703, R03 EB013873-01A1, and SBIR-1R43HL110427-01; AHA10SDG261039.
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Zhang, S.H., Tse, Z.T., Wang, W. et al. Computation of the gradient-induced electric field noise in 12-lead ECG traces during rapid MRI sequences. J Cardiovasc Magn Reson 16 (Suppl 1), P151 (2014). https://doi.org/10.1186/1532-429X-16-S1-P151