Note: Scroll over or tap on image to see labels & lines.

Note: Scroll over or tap on image to see labels & lines.

Note: Scroll over or tap on the image to see labels & lines

Note: Scroll over or tap on the image to see labels & lines

McRae Line:

• This 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 and anterior aspects of the foramen magnum.
  
• The tip of the dens (or odontoid process) should be ~5 mm below this line.
• Note:  If the tip of the dens is eroded then the Redlund-Johnell and modified Ranawat methods (normal CT values for men is > 23.7 mm and for women is > 24.2 mm) can be used instead.

Chamberlain Line:

• This is a line drawn from the posterior surface of the hard palate to the tip of the opisthion (posterior aspect of the foramen magnum) and is used to measure the distance of how much the odontoid tip extends above this line. 
  
• If the tip of the dens extends > 3 mm above this line then it helps to recognize the presence of basilar invagination (a craniocervical junction abnormality where the tip of the dens project up into the foramen magnum).
• Note:  When the opisthion (posterior aspect of the foramen magnum) cannot be identified on a plain radiograph use the McGregor line. Draw a line from the posterior edge of the hard palate to the caudal end of the occipital curve. The tip of the dens should lie less than 4.5 cm above this line.
  

Wackenheim Line:

• This is a line drawn from the caudal extension of the dorsal surface of the clivus and is used to measure the distance of space to the tip of the dens (or odontoid process).

When reading any radiograph the clinician should establish a process or order they follow each time.  While in no particular order, consider evaluating the following:

• Make sure you can see all 7 cervical spinous process
  
• Make sure pre-vertebral soft tissue is < 7 mm in thickness in front of C2 (or < 50% of the width of C2 vertebral body) and < 22 mm in front of C6 (or no more than width of C6 vertebral body
  
• Evaluate the orientation of the epiglottis, hyoid bone, tracheal shadow and check for any foreign bodies
  
• Check the anterior vertebral line (anterior longitudinal ligament line)
  
• Check the posterior vertebral line (posterior longitudinal ligament line)
  
• Check the spinolaminal line
  
• Check the spinous process line Inspect each vertebral body, pedicle, lamina and spinous process from C1 - C7
  
• Clivus should be pointing toward the odontoid (the clivus lies at the base of the skull is made from the surface of the occipital and sphenoid bones) 
  
• Check that the intervertebral spaces are uniform at each level
  
• Make sure the atlantodens interval (ADI; or pre-dental space) is < 3 mm in adults or < 5 mm in children
  
• Make sure the basion-dens space is < 12 mm
  
• Make sure the C2 ring is smooth and continuous and that C2 does not appear "fat" (i.e., vertebral body that is wider than C3)
  
• Scan the mandible and base of the skull

To view related content click on an item below:

                
Open Mouth View    C1 Fractures     C2 Fracture

1. Kwong Y et al.  AJR Am J Roentgenol 2011;196(4):W421-5. PubMed
   
2. Cronin CG et al. Clin Radiol 2009;64(1):64-9. PubMed 

Editors: 

• Anthony J. Busti, MD, PharmD, FNLA, FAHA
• Dylan Kellogg, MD
  

Last Updated:  August 2015