Real-ECG extraction and stroke volume from MR-Compatible 12-lead ECGs; testing during stress, in PVC and in AF patients

Abstract

Background: Due to the Magneto-Hydro-Dynamic (MHD) effect, blood flow within the MRI's magnetic field (B0) produces a large voltage during the S-T cardiac segment [1]. The peak MHD voltage (VMHD) can be comparable, in higher-field MRIs, to the R-wave amplitude of the real Electrocardiogram (ECGreal), so that VMHD reduces ECG-gating reliability and prevents ischemia-monitoring during cardiac imaging/interventions. We hypothesized that (1) separation of ECGreal and VMHD from 12-lead ECGs acquired within a 1.5T MRI could be achieved, using adaptive filtering, based on a set of ECG calibration measurements, and (2) a non-invasive beat-to-beat stroke-volume estimation could be achieved from time-integrated systolic VMHD. Methods: Fig. 1 shows 3 sets of 20-sec breath-held ECGs measured at positions (i), (ii) and (iii), utilizing an MRI-compatible Cardiolab-IT digital ECG-recording system [2]. The adaptive filtering procedure was tested in 5 healthy subjects, and 2 patients with Premature Ventricle Contractions (PVCs) and Atrial Fibrillation (AF). Validation was based on comparing the filter-derived ECGreal with ECGs measured periodically outside the MRI. The data processing block diagram (Fig. 2) includes training of adaptive Least-Mean-Square filters with ECGreal input (i), application of the trained filters to ECGs acquired in (ii) and (iii), which separates the VMHD from ECGreal. Results: PVC patient's results (Fig. 3): (a) unprocessed surface- lead V6, (b) extracted ECGreal, and (c) VMHD. In (b) S-T segment voltage is restored, and the R-wave dominates for gating. Aortic-flow vortices (c) generate oscillating-polarity VMHD, with VMHD peaking during S-T segment. Cardiac beat-to-beat stroke volume (d) was estimated from time-integrated systolic VMHD. PVC beats produce substantially lower stroke volume than during sinus-rhythm. AF patient results (Fig. 4): (c) Irregular VMHD and (d) irregular stroke volume are due to ventricular-filling differences at varying heart rates (100-140bpm). Athlete subject results (Fig. 5): Filter tracking of rapid heartrate changes from 44bpm to 87bpm is shown during a treadmill stress test performed inside the MRI. VMHD (b) and stroke volume (c) increase with heart rate, suggesting that the cardiac output matches higher demand. A stroke-volume comparison of all subjects (Fig. 6), derived from time-integrated systolic VMHDs, demonstrates the measurement's sensitivity to pathology. Conclusions: The filtering extracts ECGreal from measured 12-lead ECG, preserving ECGreal for ischemia monitoring and MRI gating. Stroke volume can be non-invasively derived from the time-integrated systolic VMHD(Figure Presented).