1. An image processing apparatus for filtering an image, said apparatus comprising an image input for obtaining a first and a second image of the same object, the first and second images comprising a plurality of voxels and being interrelated by a noise covariance, each voxel having a voxel value including a signal value and a noise value, a joint bilateral filter for filtering the first image, wherein a first voxel of the first image is filtered by a filter function including a relative voxel-specific weight, said weight including a likelihood of obtaining the voxel values of said first voxel and a second voxel in the first image and of a first voxel and a second voxel in the second image, assuming that the signal value of said first voxel of the first image is identical to the signal value of a second voxel of the first image and that the signal value of the first voxel of the second image is identical to the signal value of a second voxel of the second image, wherein said filter is configured to use a likelihood including a noise covariance matrix describing said noise covariance, an image output for providing said filtered image.