A 4-year-old boy comes to the ED with a severe sore throat and history of “refusing to eat”. He has severe pharyngitis on exam an you are concerned that he may have a retro pharyngeal abscess.

What technique should be used to most accurately assess the prevertebral space on lateral neck X-ray?
To avoid taking X-rays demonstrating showing a falsely widened retro pharyngeal space, lateral neck X-rays should be taken in full extension and during inspiration.
The prevertebral space should not be wider than the width of the vertebral body behind it.
If you want numbers: the prevertebral soft tissue should be <7mm at the level of c2 ( from the anterior-inferior)border.
See example at: http://radiology.otolaryngology.co.in/node/19