The Retropharyngeal Interspace (RPI) is measured from the anterior inferior part of C2's body to the pharynx. Normal maximum is 7 mm.
The Retrotracheal Interspace (RTI) is measured from the inferior anterior aspect of C6 to the trachea. Normal maximum is 20mm.
Following trauma (like Whiplash) prevertebral hemmorrhage may cccur. This can be appreciated on lateral films as an increased RPI or RTI. This may occur with, or without vertebral fracture or partial disc avulsion.
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