1. A method for unsupervised cross-modal synthesis of a target modality medical image from a source modality medical image, comprising: receiving the source modality medical image; generating multiple candidate target modality intensity values for each of a plurality of voxels of the target modality medical image based on corresponding voxels in the source modality medical image; and generating a synthesized target modality medical image by selecting, jointly for all of the plurality of voxels in the target modality medical image, intensity values from the multiple candidate target modality intensity values generated for each of the plurality of voxels.