INTRO: Pars defects can be difficult to see on MRI. Here is an easy way to know if the slip/ spondylolistheisis you are seeing is due to a pars defect or is from degenerative facet disease.
In Degenerative spondylolisthesis, the canal is always small.
In spondylolisthesis due to a Pars Defect, the canal is always large, never small.
So even if you cant visualise the pars defect on MRI (See also our post on how to see the pars defect itself on MRI) you can know if the spondylolisthesis is due to a pars defect or degeneration by looking at the size of the canal.