McRae line Last edited and medically reviewed by Alaric Steinmetz on 04.09.2023 The McRae line is an imaginary line in radiology that connects the Basion with the Opisthion on a sagittal image. UsesThe McRae line is used for measurement in Basilar invagination or tonsillar descent in Arnold–Chiari malformation[^1] [^2].Imaging Enlarge Visualization of the McRae line on a sagittal CT image.