Pranav Bhagirath

192 Chapter 10 Body surface potential acquisition An MRI scout scan was performed to approximate the position of the heart with respect to the thorax. Subsequently, 62 (+3 limb) electrodes were applied to the subject’s torso, centralized over the heart. Body surface potentials (BSP) were acquired using a 65 channel ActiveTwo system (BioSemi B.V., Amsterdam, The Netherlands). Once the acquisition was completed, the electrode locations were marked with MRI markers enabling accurate identification of the electrode positions. Image acquisition MRI studies were obtained using a 1.5 Tesla Aera scanner (Siemens Healthcare, Erlangen, Germany). Blackblood imaging was performed using a Half-Fourier Acquisition Single Shot Turbo Spin Echo (HASTE) pulse-sequence to acquire three perpendicular stacks (axial, coronal and sagittal) from the neck till lower abdomen. Images were acquired during free breathing using navigator gating (diaphragm) with 1 mm window. ECG gating was used to acquire images during the diastolic phase of the cardiac cycle. Typical imaging parameters were: a spatial resolution of 1.2 × 1.2 × 6 mm, TR/TE 744/42 ms and flip angle= 160°. Volume conductor model and computational algorithm The MRI images were segmented using a custom developed tool. This tool generates a script containing the geometrical description (points, lines and planes and directed line loops) of the segmentation. Subsequently, the script was meshed using GMSH (9) . The Σ for the homogeneous VCM (Model 1) was specified at 0.2 S/m (10) . The inhomogeneous VCM (Model 2) had a similar Σ, but also contained specific conductivity values of the lungs, specified at 0.04 S/m (10) . Cardiac surface potentials (CSP) were calculated from BSP using CSP = (T T T + λ 2 I) -1 T T BSP where T is the transfer matrix and λ is the regularization strength. Electrophysiological study and catheter ablation The electrophysiologists (HR and VD) were blinded for the IPM results. During the electrophysiological study (EPS), femoral venous and arterial access was established. In all cases, a hexapolar catheter (Supreme™, St. Jude Medical) was placed in the right atrial appendage, the proximal electrode located in the inferior vena cava served as the unipolar indifferent electrode. Additionally, a screw-in temporary pacing lead (Medtronic) was placed in the right ventricle as a positional reference for the electro