I have had 3 mri's this past year due a colloid cyst. I had my 3 month post op mri, it showed borderline Chiari l malf. Why didn't it show before?

Depends on reader. It probably did but since you're saying that it was called "borderline" by one radiologist that means that it is so close either being called Chiari or NOT BEING called Chiari that who ever the last person was who read it may have chose not make ANY call compared to the current reader who chose to read it using MAXIMUM WAFFLING CAPACITY! Do you have a copy to show? www.healthtap.com/drsaghafi.