Left-ventricular mechanical activation and aortic-arch orientation recovered from magneto-hydrodynamic voltages observed in 12-lead ECGs obtained inside MRIs: a feasibility study

Abstract

To explore use of the Magnetohydrodynamic Voltage (VMHD), observed in intra-MRI 12-lead electrocardiograms (ECG), to indicate the timing of the onset of left-ventricular mechanical activation (LVMA) and the orientation of the aortic-arch (AAO). Blood flow through the aortic arch during systole, in the presence of the MRI magnetic field (B 0), generates VMHD. Since the magnitude and direction of VMHD are determined by the timing and directionality of blood flow relative to B 0, we hypothesized that clinically useful measures, LVMA and AAO, could be extracted from temporal and vectorial VMHD characteristics. VMHD signals were extracted from 12-lead ECG traces by comparing traces obtained inside and outside the MRI scanner. VMHD was converted into the Vectorcardiogram frame of reference. LVMA was quantified in 1 subject at 1.5T and 3 subjects at 3T, and the result compared to CINE MRI. AAO was inferred for 4 subjects at 3T and compared to anatomical imaging of the aortic arch orientation in the transverse plane. A < 10% error was observed in LVMA measurements, while a < 3° error was observed in aortic arch orientation measurements. The temporal and vectorial nature of VMHD is useful in estimating these clinically relevant parameters.

Figure 1. Gregory, T. Stan

Visualization of the first MHD peak in the VCG domain. (a) MRI Coordinate plane. (b) Body planes shown in MRI coordinate system: X–Y is shown to be the transverse plane, X–Z is the coronal plane, and Y–Z is the sagittal plane for human subjects in the MRI bore. (c) ECG obtained inside the MRI transformed into the VCG domain. Blue and Red curves, show the true VCG (same as outside MRI) and MHD, respectively, as a function of time in the cardiac cycle, with dark blue and dark red lines, respectively, showing the R_peak and MHD1 (d) Pre-cordial ECG leads and the X, Y, and Z VCG leads are shown as a function of time, with R_peak and MHD1 indicated.

Figure 2. Gregory, T. Stan

Figure 2: ECGs (a) obtained inside 1.5T and 3T MRI scanners, with ensuing (b) detection of the onset of mechanical motion (MHD1), from which LVMA was assessed. LVMA was also detected using cine MRI methods (c), with blue arrows denoting the region where wall displacement was evaluated. Both mean (μ) and standard deviation (s) of the LVMA are reported.

Figure 3. Gregory, T. Stan

Assessment of the VMHD vector’s dependence on magnetic field direction and the angle of blood flow using 12-lead ECG monitoring with a single subject placed in various orientations within or outside the MRI bore. (d) Derived MHD vector directions for the orientations shown in a–c.

Figure 4. Gregory, T. Stan

3T Assessment of Aortic Arch Angulation