Is this image good enough for OCR?
It looks like using the polartocartesian transformation is destroying the characters (look at the "M", "E" before and after...). I think it will be a better idea to think about some rotations mechanism using imrotate.

Any way, I'd try the following additional pre-proc:
1. you might want to perform some morphological operations on the image after the adaptive threshold. 
2. I would also try labeling the image (bwlabel) and removing too large objects (most of the background noise in your image is connected so it will be a huge object - in terms of pixel counting)

Have fun!