Hi Lizzie and so glad you decided to post your concerns .....
That is why many of us "old-timers" stay active on this forum ..... to help people relax and understand the nature of ANs.
We have several regular posters from the UK and they can tell you better than I about MRIs in the UK. However, if I recall correctly, a recent poster in the UK had difficulty getting her MRI done with contrast. According to her, they typically do not use contrast material for MRIs of the brain, which seems very odd to me.
Some of the newer MRI machines these days are creating clearer pictures so are capable of picking up smaller ANs. Also, here in the US, it depends upon how the doctor orders the MRI as to how thin the slices (number of pictures in sequence) are. Hopefully, if your doctor is suspecting an AN, the order will be the right one to be able to see even a tiny AN.
Let us know what you find out.
Clarice