Background/Purpose: PET is a nuclear imaging technique that depicts functional processes within the body based on gamma rays. The biologically active molecule used for PET is the bone-seeking agent ¹⁸F labeled Fluoride (¹⁸F-F). The concentrations of tracer reflect tissue metabolic activity of regional bone perfusion and bone remodeling. We tested the performance of integrated 18F-Fluoride positron emission tomography and magnetic resonance imaging (PET/MRI) of the whole spine and the sacroiliac joints and compared bone marrow edema (BME) and structural (fat deposition, FD) and metabolic findings (18F-F) in patients with active ankylosing spondylitis (AS).

Methods: 13 AS patients (6 male, 7 female, mean age 37.8±11.4 years, all BASDAI>4, no anti-TNF treatment) underwent a 3-Tesla MRI and integrated PET/MRI 40 minutes after injection of a mean dose 157 MBq of 18F-Fluoride of their whole spine and the SIJs. Two readers scored all images independently and the lesions where both readers showed agreement were considered for analysis. Inflammatory activity (bone marrow edema, BME), structural lesions (fat deposition) and focal 18F-Fluoride uptake were recorded on the level of a vertebral quadrant (VQ) or SIJ-quadrant (SQ), where one VQ and SQ consisted by 4 VQs (superior anterior and posterior and inferior anterior and posterior) .

Results: Acquisition of whole-spine 18F-F PET/MRI including the SIJ was successful in all patients. There was excellent agreement in the reading of the two readers. For the SIJ, a total of 104 SQs could be analyzed by MRI and PET/MRI and 44.2% showed BME, while FD 42.3% and 18F-F in 46.2% SQs. BMD without FD was found in 60.9%, the majority with concomitant 18F-F. In comparison, FD alone without BME was found in 59.1% SQs and in those, parallel 18F-F was seen in only 7.7% SQs. The combination of BMD/FD was found in 17.3% SQs and in those, parallel 18F-F uptake was seen 72.2% SQs. For the spine, a total of 1,196 VQs could be analyzed and 9.9% showed BME, while FD was found in 18.2% and 18F-F 5.4% VQs. BME without FD was found in 41.5% and in those, parallel 18F-F was seen in 14.3% VQs. In comparison, FD without BME was found in 68.3% VQs and in those, 18F-F was 8.7% VQs. Finally, the combination BME/FD was found in 5.8% VQs and in those, 18F-F uptake 40.6%. The highest mean SUVmax per lesion was found in both the SIJ (28.1 ±13.5) and the spine (25.7 ±21.1) in in SQs or VQs with a combination of BME and FD.

Conclusion: In this first study on hybrid 18F-F-PET/MRI of active AS patients we show that rather BME than chronic changes is associated with osteoblastic activity. The combination of BME and FD showed the highest 18F-F uptake, confirming our previous finding that this is the strongest predictor of future syndesmophyte formation.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/conventional-magnetic-resonance-imaging-mr-and-hybrid-18f-fluoride-positron-emission-tomography-mri-18f-f-petmri-of-the-spine-and-the-sacroiliac-joints-a-detailed-description-of-pathol/