The document discusses several lines used in diagnosing basilar invagination, which is an infolding of the base of the skull that occurs when the top of the C2 vertebra migrates upward. The Chamberlain line extends from the posterior end of the hard palate to the midpoint of the foramen magnum. If the structure above is over 3mm, it indicates basilar invagination. The McGregor line is used if the opisthion is not seen, extending from the posterior palate to the most caudal point of the foramen magnum. The digastric and bimastoid lines also indicate basilar invagination if certain structures project above them. The McRae line
3. CHAMBERLAIN LINE
? POSTERIOR END OF HARD PALATE TO
MIDPOINT OF POSTERIOR PART OF FORAMEN
MAGNUM (OPISTHION)
? IF STRUCTURE ABOVE >3MM PRESENT ¨C IT
INDICATES BASILAR INVAGINATON
5. ? IF OPISTHION NOT SEEN ¨C MCGREGAR LINE
? IT EXTENDS FROM THE POSTERIOR PART OF
HARD PALATE TO MOST CAUDAL POINT ON
FORAMEN MAGNUM