// file name relative to DICOMDIR base path
var file = image.Get<string>(DicomTag.ReferencedFileID, -1);
// some DICOMDIR files add \ char to beginning of pathfile = file.TrimStart(Path.DirectorySeparatorChar, Path.AltDirectorySeparatorChar);
// combine base DICOMDIR path with relative pathfile = Path.Combine(path, file);
// just to make sure we have complete pathfile = Path.GetFullPath(file);