"Incomplete ring" sign: a simple method for CT detection of spondylolysis.

Multiple authors have pointed out the difficulty of detecting spondylolysis with lumbar CT. A simple method to detect spondylolysis on lumbar CT is described. Identifying one intact cortical ring for each vertebra absolutely excludes the diagnosis obviating the time-consuming need to search each image looking for positive evidence of the abnormality. If at least one intact cortical ring for each vertebra is not found, a defect in the ring at the appropriate position assures the diagnosis.