Background PET is a nuclear imaging technique that depicts functional processes within the body by detecting pairs of gamma rays emitted indirectly by a positron-emitting radionuclide (tracer), introduced into the body on a biologically active molecule. 3D images of tracer concentration are then constructed by computer analysis. The biologically active molecule used for PET is ¹⁸F labeled Fluoride (¹⁸F-F), a bone-seeking agent reflecting bone perfusion and remodeling. The concentrations of tracer image tissue metabolic activity, in terms of regional bone remodeling. PET scans are increasingly read alongside CT or MRI scans, with the combination (“co-registration”) giving both anatomic and metabolic information, e.g. what the structure is, and what it is doing biochemically.

Objectives Based on our long experience with MRIs of patients with axSpA we have inaugurated a pilot study with simultaneous PET/MRI in order to examine whether the addition of the PET technique may provide different and additional information in comparison to MRI alone.

Methods Eleven axSpA patients, median age 39 years, disease duration range 0.5-10 years, mean BASDAI 5.3, most of them fulfilling the NY criteria for AS, were examined by PET/3-Tesla MRI 40 minutes after injection of a mean dose of 157 MBq of ¹⁸F-F using a hybrid whole-body PET/MRI scanner (Siemens Biograph mMR®). 3T-MRIs were scored blinded to patient’s clinical characteristics by two readers (1 rheumatologist and 1 radiologist) using the Berlin MRI score and also by recording inflammatory lesions on a vertebral edge (VE) level. In a second step PET/MRIs were read blindly by the same readers also based on the VE involvement of individual vertebral bodies.

Results Acquisition of whole-spine hybrid ¹⁸F-F PET/MRI scans was successful in all patients. The resulting mean effective radiation dose per patient was 3.76 mSv. Co-registration of PET/MRI fusion images was highly accurate and allowed a precise comparison of MRI and PET findings. The mean Berlin MRI score was 6.8 (range 0 – 31). In the direct comparison of the MRI and PET signal the two readers saw consistent signals in almost 90% of the sites studied. However, there were some areas where signals differed, for example within existing syndesmophytes where the PET signal was increased but conventional MRIs showed no signal of active inflammation, or the area of sternum and lateral or posterior spinal elements such as facet joints and spinous process.

Conclusions The new technique of combining the PET and MRI provides largely similar imaging signals as conventional MRI. However, we did observe differences between the two techniques - especially in areas with less inflammatory activity where bone metabolism seemed to be active or in areas with blurred resolution on conventional MRI. More studies are needed to answer the question whether the differences between these techniques are pathogenically relevant, whether they can be reliably reproduced and quantified, and, of course, whether they are sensitive to change. Especially the possibility that PET detects osteoblastic activity in areas where no inflammatory signal is detected seems to be of interest.

Disclosure of Interest None Declared