TNF and ankylosing spondylitis: In fact, in our previously published 18F-NaF PET-CT study in ankylosing spondylitis patients, we found that in particular PET-positive costovertebral joints and SI joints were responsive to anti-TNF treatment and could distinguish between clinical responders and non-responders, pointing at detection of SpA-related lesions in the axial skeleton [27].