18 years back, the resolution of the CT was nowhere NEAR what it is today. Same with the MRI's. Hard to say what it would have shown up as in 1989 depending on the age of the imaging equiptment.
Also, it depends where the malformation is located and what type it is. Today they can do surgeries deeper in the brain than then could do back then. We know a lot more about the brain from mapping and 3D imaging with computers to use graphics. They can now illistrate on computers after conducting a MRI, recreate a map of the brain and illiminate the region of the growth. We couldn't do that before. Using this technology they can decide if surgery is safe or not.
Back when I was operated on they litterally "cored out" a growth like you core out an apple. That was back in 1981. We have come quite a ways since then.