Background/Purpose:   Positron emission tomography (PET) is a high sensitive nuclear imaging technique that depicts functional processes within the body. PET scanners detect annihilation radiation from radioactive decay of a positron-emitting radionuclide labeled to a biologically active molecule (tracer) and introduced into the body. User-defined cross-sectional images of the tracer distribution can then be reconstructed from the acquired 3D data set. ¹⁸F-fluoride (¹⁸F) can be used for PET as a bone-seeking agent reflecting bone perfusion and remodeling. The concentration of the tracer then represents the metabolic activity and regional bone remodeling. Recently, an integrated PET/MR device was introduced with the ability to provide simultaneously acquired images that combine morphological and metabolic information. Based on our long experience with MRIs of patients with axial spondyloarthritis (axSpA) we inaugurated a pilot study with simultaneous PET/MR 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 using a integrated whole-body PET/MR scanner (Siemens Biograph mMR®). 3T-MRIs were scored blinded to patient´s clinical characteristics by two readers (1 rheumatologist and 1 radiologist/nuclear medicine specialist) 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 integrated ¹⁸F 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.

Conclusion: The new technique of integrated PET/MRI provides largely similar imaging signals as conventional MRI. However, we did observe differences between the two modalities – 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 with MRI seems to be of interest.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/integrated-18f-fluoride-positron-emission-tomography-and-magnetic-resonance-imaging-of-the-spine-a-pilot-study-and-comparison-of-signals-in-patients-with-axial-spondyloarthritis/