I'm not sure my reply is going to help you much, but I did have 2 MRIs to confirm/rule out AS and both came back negative. They haven't used any chemical (contrast or else). My MRIs were full spine and SI joints.
I have debilitating pain (back + several other joints) when I'm not taking NSAI, but since we don't see anything on the MRI and I'm HLA B27 negative, I don't "officially" have AS (and I'm not allowed to receive the biologics for example).
So, to answer to your question, it's possible to have AS and have clear MRIs.
When I got my MRI results, I've had mixed feelings... On one hand, I was relieved that there was no damage (for the moment) on my joints. But on the other hand, I wanted a "proof" that my pain was real...
Have you had the HLA B27 genetic test?