We describe a 3D+T active surface model for segmentation and tracking of the left ventricular endocardium within 3D MRI sequences of the cardiac cycle. In order to perform segmentation and tracking simultaneously, the surface structure is divided through both time and space, and is therefore handled as an array of planar active contours, interconnected between adjacent slices and frames, providing spatial and temporal consistency. In a given frame, the stacking of slice contours constitute a 3D triangular mesh with a cylindrical topology. Extraction of ventricle border is performed by means of energy minimization, using a combination of boundary-based term and a new computationally efficient region-based term.