1. A medical image conversion apparatus comprising: an image obtainment unit that obtains a series of temporal medical images about a specific organ in different phases; a registration unit that performs registration of corresponding voxel positions of the specific organ in each of the temporal medical images of the series of temporal medical images to thereby register the corresponding voxel positions of the specific organ between each of the temporal medical images of the series of temporal medical images; and a conversion unit that, from the series of temporal medical images of the specific organ that includes a base medical image and additional medical images, the additional medical images including at least a first medical image and a second medical image, i) for each of the additional medical images, converts signal values at each of the registered voxel positions of the specific organ in each of the additional medical images of the series of temporal medical images into a voxel value at a respective corresponding registered voxel position of the specific organ in the base medical image to thereby form a converted series of temporal medical images, where the signal value at each corresponding registered voxel position of the specific organ, includes a same voxel value in the base medical image and each of the additional medical images, and ii) further converts the signal values at each of the corresponding registered voxel positions in the additional medical images of the converted series of temporal medical images into a same display voxel value as in the base medical image such that each respective one of the corresponding registered voxel positions of the specific organ in the base medical image and the additional medical images of the converted series of temporal medical images have a respective same display voxel value.