BACKGROUND: Gated blood pool tomoscintigraphy has the unique capacity to accurately assess myocardial motion in paced patients. Our goal was to develop a precise radionuclide angiography analysis of cardiac dynamics to evaluate ventricular synchronization in patients undergoing biventricular pacing. METHODS AND RESULTS: On the basis of a 4-dimensional deformable motion estimation algorithm, we developed a protocol allowing estimation of motion fields after gated blood pool tomoscintigraphy. We measured the mechanical activation times for 17 left ventricular (LV) segments and determined the main types of contraction pattern in 10 normal subjects, 17 patients with dilated cardiomyopathy, and 12 resynchronized patients. We analyzed intra-LV dyssynchrony: apex to base, septum to lateral wall, and anterior wall to inferior wall. Three-dimensional measurements of intra-LV activation time (r > .80, P < .001) and LV ejection fraction (r > 0.90, P < .0001) are linearly correlated to 2-dimensional values. LV contraction follows the electrical activation pattern. In normal subjects the anteroseptal and anterior segments are first activated, followed by the apex and inferolateral segments. In resynchronized patients contraction begins with the lateral and apicoseptal segments in correspondence to the LV and right ventricular lead implantation. CONCLUSIONS: By measuring mechanical activation times, this technique allows for the analysis of the regional synchronous contraction. This may help to assess the variation of the activation pattern according to the cardiomyopathy type and the role of septal resynchronization in ventricular functional recovery.