Carmel G Cronin MBBCh, Presenter: Nothing to Disclose

Derek Gerard Lohan MBBCh, Abstract Co-Author: Nothing to Disclose

Conor P Meehan MBBCh, Abstract Co-Author: Nothing to Disclose

Joseph Murphy MBBCh, Abstract Co-Author: Nothing to Disclose

Clare Roche MBBCh, Abstract Co-Author: Nothing to Disclose

The distance of the tip of the normal odontoid peg from Chamberlain‘s line was established on plain radiographs in 1943. However, this measurement has never since been quantified, or evaluated on current modalities used in skull base pathology assessment- MRI and CT. To evaluate the range of normal measurements of the odontoid-base line distance in the current population on MRI. To establish a range of normality of Chamberlain's, McGregor’s and McRae’s line on MRI imaging.

We retrospectively evaluated midline sagittal MR images of 200 adults (100 males and 100 females). Patients did not have symptoms suggestive of basilar impression, spinal trauma, vertebral collapse or disease. Using Monte Carlo Simulation Histograms, mean and standard deviation, median and range values were calculated. These findings were then compared with previous plain radiograph measurements.

The mean position of the odontoid peg was 1.36mm below Chamberlain’s line with a standard deviation (SD) of 2.66mm and the 95% confidence interval ranging from 0.5 to 2.25mm below this line. Saunders in 1943 on plain film assessment found this mean distance to be 0.94mm below chamberlain’s line and standard deviation of 3.6mm. The odontoid peg was 1.2mm (SD=2.8mm) below McGregor’s line. Previously Hinck et al found this measurement to be 3.67mm (SD=1.69mm) for females and 0.33mm (SD= 3.81mm) for males. The odontoid peg was 5.7mm (SD=2.7mm) below McRae’s line.

These results provide the mean and range of normal distance of the Odontoid Peg from the most frequently used skull base lines in the current normal population on MRI imaging. We have established a range of normal measurement values for MRI usage outlined above.

Knowledge of the normal values will be of benefit to each radiologist in their everyday diagnosis and assessment of the skull base and aid establishment of the diagnosis of Basilar Impression.

Cronin, C, Lohan, D, Meehan, C, Murphy, J, Roche, C, MRI Measurement and Evaluation of the Normal Odontoid Position.  Radiological Society of North America 2006 Scientific Assembly and Annual Meeting, November 26 - December 1, 2006 ,Chicago IL. http://archive.rsna.org/2006/4429487.html