Reproducibility and spatial accuracy of magnetocardiographic source localization: a phantom study in an unshielded laboratory for interventional electrophysiology

Background: Pre-interventional knowledge of arrhythmogenic substrate location may reduce interventional time and risk of cardiac arrhythmias ablation. Magnetocardiographic mapping (MCG) is a contactless method for non-invasive localization of intracardiac sources that has been proposed for three-dimensional (3D) electro-anatomical imaging (EAI) of focal arrhythmogenic substrates. The aim of this study was to assess the repeatability, precision, and accuracy of MCG in localizing dipolar sources when recordings are carried out in an interventional electrophysiology laboratory, without any electromagnetic shielding. The experimental set-up consisted of a geometrically simplified phantom containing multiple artificial current dipoles. Methods: The phantom consisted of a rectangular plastic box filled with 0,9% NaCl saline solution. Multiple artificial current dipoles (6 mm length, 10mA, 50 Hz) were sequantially activated within two amagnetic catheters (ACs). The distance between dipoles was constant (6 mm). 30-seconds MCG (bandwidth DC-200 Hz, 1 KHz sampling rate) were performed, twice, with 36 DC-SQUID sensors coupled to second-order axial gradiometers (pick-up coils 19 mm, baselines 55–70 mm), at distances between sensors plane and dipolar sources (SSD) decreasing from 18 to 9 cm. Orthogonal fluoroscopic imaging, employing lead markers to correct also for x-ray divergence effect, was used to define the 3D physical position of each dipole in respect of the MCG sensors array. MCG repeatability was evaluated calculating the coefficient of variability (CV) ± standard error of the mean (SEM) of each parameter. Localization precision was estimated as the 3D difference between the fluoroscopic position of each artificial dipole and the 3D localization of said dipole provided by MCG inverse solution based on the Equivalent Magnetic Dipole (EMD) model in a semi-infinite half-space with homogeneous conductivity. Localization accuracy was defined as the MCG capability to correctly estimate the distance between two dipoles placed at known distance within the same catheter. The correlation between precision, goodness of fit (GOF) of the EMD model and SSD was also analysed. Results: Overall, optimal repeatability (CV ± SEM = 0,79±0,43%, 3D absolute error = 0,26±0,25 cm), average localization precision (1,13±0,42 cm) and average accuracy (0,20±0,13 cm) were found. Localization precision improved (0,87±0,3 cm) with the GOF of the model increasing above 73%, as observed, when SSD was below 14 cm. Conclusion: Our data demonstrate that contactless MCG, even if performed in an unshielded cardiac catheterization laboratory, provides optimal accuracy in localizing dipolar sources embedded in ACs, with uncertainty below a clinical relevant threshold. Thus, with the development of low-cost non-cryogenic technology, MCG is foreseen as a novel method to be used for both pre-interventional and intraoperative 3D EAI of arrhythmogenic substrates.