The tonsils are rounded not peg like and although no scale is present to effect a precise measurement at most. For each mri tp measurement each operator followed a set procedure.
Distances were measured with calipers from the hard copy of the images.
Basion opisthion line mri. It is one of the skull landmarks craniometric points for radiological or anthropological skull measurement. Indicates the presence of basilar invagination atlantoaxial impaction. The posterior margin of foramen magnum should line up with the c1 spinolaminar line.
Various lines and measurements using the opisthion are made in the diagnosis of atlanto occipital dissociation aod injuries platybasia and basilar invagination. Note however that mcrae s line basion to the opisthion needs to be measured a in the midline and b from the tip of the cortical bone and not the fat rich bone marrow. The ratio of basion spinolaminar line of c1 to opisthion posterior cortex of c1 anterior arch normally ranges from 0 6 to 1 0 with the mean being 0 8.
Mcrae line is a radiographic line drawn on a lateral skull radiograph or on a midsagittal section of ct or mri that connects the anterior and posterior margins of the foramen magnum basion to opisthion. The tip of the odontoid process is normally about 5 mm below this line. The opisthion is the median midline point of the posterior margin of the foramen magnum.
B midline sagittal section se 500 40 showing line from basion to opisthion in a nomal patient. As the standard of care for the evaluation of the cervical spine shifts from plain radiographs to multidetector row ct mdct a re examination of the normal anatomic relationships of the occipitovertebral articulations is needed. A line projected downward from the dorsum sellae along the clivus to the basion should point to the dens.
The tip of the dens or odontoid process should be 5 mm below this line. The alar ligament which is attached on each side to the tubercle of occipital condyle divides the foramen magnum into an. We aimed to define the normal anatomic relationships of craniocervical articulations on mdct and address any discrepancies with currently.
Foramen magnum appears normal and no evidence of intramedullary mass at the level of the medulla or anomaly of the opisthion basion or atlantodental interval. Basilar invagination is. Measurements were from this images see discussion and fig.
On the occipital bone the foramen magnum presents two midline cephalometric landmarks the opisthion is the midpoint on the posterior margin of the foramen magnum. 1 a drawing showing basion b opisthion 0 and cerebellar tonsil t in a normal patient. Visualized cervical spinal canal is patent.
The basion is located at the midpoint on the anterior margin of the foramen magnum. Normal appearance of a young person s brain on a 1 5t scanner other than borderline low lying tonsils. Re format the 3d axial image volume in the sagittal plane select the mid sagittal image slice with the maximum caudal extent of tonsillar position and mark the foramen magnum by drawing a line from the basion to the opisthion to most consistently approximate mcrae s line.
C1 c2 junction is anatomically aligned. The mcrae line is a line drawn on a lateral radiograph of the skull or on a sagittal cut from a ct or mri scan that connects the posterior opisthion and anterior basion aspects of the foramen magnum.