If you have already purchased a license and lost your serial number, the official RadiAnt Forum and support pages suggest: Check your email
(Invoking related search suggestions for further searches.) Radiant Dicom Viewer Serial Number Free
If a paid license is not feasible, consider these reputable, open-source, and free DICOM viewers that are widely used in the medical community: Unlocking the Full Potential of Radiant Dicom Viewer: