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See Article by Rodrigo et al A spiral wave is a common macroscopic behavior of excitable media observed in biological, chemical, and physical systems.1,2 In cardiac tissues, spiral wave reentry occurs when a wavefront of electric propagation encounters functionally inexcitable tissue and rotates around it in a vortex-like fashion.3 A rotor of a spiral wave is a rotation center from which a 2-dimensional spiral wave of excitation rotates outward. Phase mapping has been the de facto standard method to identify rotors and to track their trajectories in animal models of fibrillation.4 On phase maps, a rotor is defined as a phase singularity point around which the phase transitions through a complete cycle from −π to +π.3,5 Phase mapping can falsely detect phase singularities in the absence of rotors,6 and spiral waves can exist without phase singularities.7 Rotors and focal impulses have been proposed to be the underlying drivers of atrial fibrillation (AF) in human.3,8,9 This localized source hypothesis has resulted in the utilization of phase mapping in clinical practice, mainly in 2 settings: invasive focal impulse and rotor modulation (FIRM) and noninvasive electrocardiographic imaging (ECGI). FIRM uses phase mapping to locate stable rotors based on endocardial unipolar electrograms acquired by a 64-lead basket catheter.9 ECGI uses phase mapping to locate rotors based on unipolar electrograms acquired by ≈250 body surface electrocardiographic leads, which are back projected onto the epicardial surface, using a technique called inverse solution. FIRM and ECGI have …