Transarterial radioembolization (TARE) is a treatment for liver cancer, during which radioactive microspheres are administered through the hepatic artery. Microspheres containing holmium-166 enable MRI-based dosimetry, based on subtraction of pre- and post-treatment $$$R_2^*$$$ values. This subtraction is performed using a mean pre-treatment $$$R_2^*$$$ value. This does however not take pre-existing differences of $$$R_2^*$$$ values into account, introducing an error in the dosimetry. In this work a voxelwise subtraction method is presented, using deformable registration to transform the pre-treatment $$$R_2^*$$$ map to the post-treatment $$$R_2^*$$$ map, enabling voxel-by-voxel subtraction. This method does take $$$R_2^*$$$ differences into account and improves MRI-based dosimetry.