A method of correcting a first CT image of a region of a patient for artefacts, comprising the steps of a) Obtaining the first CT image and a corresponding second CT image of the region as an original first and second CT image, respectively, and letting the original first CT image be the current first CT image, b) Obtaining data representing a registration between the first CT image and the second CT image, c) Obtaining a difference image D reflecting a difference between a value of each voxel of the first image and a corresponding voxel of the second image, d) Defining an image mask M defining which voxels to consider in the difference image when defining the correction map e) Obtaining a final correction map Cf defining which areas of the first CT image should be corrected and by how much, by an iterative method comprising the following steps: i. Defining the correction map C based on the difference image and the image mask M, said correction map indicating which areas of the current first CT image should be corrected for artefacts and by how much ii. Updating the final correction map Cf by adding the correction map C, iii. Updating the current first CT image using the correction map, and taking the updated image as the current first CT image; iv. Updating the image mask M based on information from the correction map C and the difference image or the correction map and the current first and second CT image, v. Calculating a new difference image between the second CT image and the current first CT image, and setting the new difference image as the difference image, f) Applying the final correction map to the original first CT image. a) Obtaining the first CT image and a corresponding second CT image of the region as an original first and second CT image, respectively, and letting the original first CT image be the current first CT image, b) Obtaining data representing a registration between the first CT image and the second CT image, c) Obtaining a difference image D reflecting a difference between a value of each voxel of the first image and a corresponding voxel of the second image, d) Defining an image mask M defining which voxels to consider in the difference image when defining the correction map e) Obtaining a final correction map Cf defining which areas of the first CT image should be corrected and by how much, by an iterative method comprising the following steps: i. Defining the correction map C based on the difference image and the image mask M, said correction map indicating which areas of the current first CT image should be corrected for artefacts and by how much ii. Updating the final correction map Cf by adding the correction map C, iii. Updating the current first CT image using the correction map, and taking the updated image as the current first CT image; iv. Updating the image mask M based on information from the correction map C and the difference image or the correction map and the current first and second CT image, v. Calculating a new difference image between the second CT image and the current first CT image, and setting the new difference image as the difference image, i. Defining the correction map C based on the difference image and the image mask M, said correction map indicating which areas of the current first CT image should be corrected for artefacts and by how much ii. Updating the final correction map Cf by adding the correction map C, iii. Updating the current first CT image using the correction map, and taking the updated image as the current first CT image; iv. Updating the image mask M based on information from the correction map C and the difference image or the correction map and the current first and second CT image, v. Calculating a new difference image between the second CT image and the current first CT image, and setting the new difference image as the difference image, f) Applying the final correction map to the original first CT image.