is anterolisthesis the same as retrolisthesis (slippage towards the back) or spondylolisthesis (which is slippage towards the belly)?
Grades of spondylolisthesis are calculated based on the percentage of movement (e.g. one vertebra slips forward 20% of the length of the vertebra below). So unless you know the length of the vertebrae aswell as the amount of displacement I think its hard to calculate yourself. I'm sure you could ask the radioologist or a doctor to calculate it for you from the MRI.
As far as I can tell, there is no grading system for retrolisthesis.