1. A method for reconstructing an image of an object from image data, comprising: a processor receiving non-contrast-enhanced magnetic resonance image data of the object containing a first and a second dataset in k-space obtained with a medical imaging device which is acquired in an undersampled manner; the processor generating a sparsified third dataset by subtracting the first from the second dataset in k-space; and the processor iteratively minimizing an objective function under a constraint related to the third dataset to reconstruct the image of the object.