The invention refers to providing a system that allows to reduce the computational costs when using an iterative reconstructional algorithm. The system (100) comprises a providing unit (110) for providing CT projection data, a base image generation unit (120) for generating a base image based on the projection data, a modifying unit (130) for generating a modified image, wherein an image value of a voxel of the base image is modified based on the image value of the voxel, and an image reconstruction unit (140) for reconstructing an image using an iterative reconstruction algorithm that uses the modified image as a start image. Since the modifying unit is adapted to modify the base image, the base image can be modified such as to form an optimal start image for the chosen iterative reconstruction such that a faster convergence of the iterative reconstruction can be accomplished.