We present a volumetric electrocardiographic imaging (ECGI) method for the 3D reconstruction of cardiac electrical activity. Typically, volumetric methods estimate transmembrane potential (Vm), which depends explicitly on the intracellular conductivity tensor. To overcome this, we propose Volumetric Source Imaging (VSI), an approach that estimates cardiac sources to isolate the inverse solution from intracellular uncertainties. We compared Local Activation Times (LATs) derived from both formulations. In numerical simulations of 10 ventricular ectopic beats, Vm-based LAT accuracy degraded (CC dropped from 0.66 ± 0.09 to 0.60 ± 0.07 and MAE increased by ≈ 1.5ms ± 0.4ms) under controlled intracellular anisotropy perturbations, whereas VSI maintained a stable error profile (CC = 0.86 ± 0.04; MAE = 0.24 ± 0.08 ms). Furthermore, in four diverse clinical cases, VSI mitigated structural artifacts and provided more coherent 3D activation patterns, enabling accessible global arrhythmia characterization without requiring prior patient-specific intracellular data.