Dual-phase hybrid 18F-Fluoride Positron emission tomography/MRI in ankylosing spondylitis: Investigating the link between MRI bone changes, regional hyperaemia and increased osteoblastic activity

IntroductionFocal F-18-Fluoride uptake on blood-pool phase PET represents regional hyperaemia, while it indicates osteoblastic activity on mineralization phase PET. This study investigates the link between regional hyperaemia and osteoblastic activity in inflammatory and chronic lesions of ankylosing spondylitis (AS) of the sacroiliac joints (SIJ) using dual-phase F-18-Fluoride PET/MRI. MethodsThirteen patients (six men, seven women, age: 3710years) with active AS prospectively underwent dual-phase F-18-Fluoride PET/MRI. Blood-pool phase PET was acquired 6min and mineralization phase PET 40min after injection of 1588 MBq F-18-Fluoride. SIJ quadrants (SQ) were assessed regarding inflammatory lesions represented by bone marrow oedema (BME), chronic AS lesions such as erosion, fat deposition (FD), sclerosis and ankylosis on MRI, and regarding focal F-18-Fluoride uptake on both PET datasets. Image quality (IQ) of both PET datasets and MRI was evaluated using a 4-point Likert scale. ResultsOf 104 SQ, there were 63.4% SQ with FD, 42.3% SQ with BME, 26.9% SQ with erosions, 26% SQ with sclerosis and 10.6% SQ with ankylosis. BME alone was associated with focal F-18-Fluoride uptake in 63.6% SQ on blood-pool phase and 90.9% SQ on mineralization phase F-18-Fluoride PET/MRI. Instead, FD, erosion, sclerosis, ankylosis were not associated with focal F-18-Fluoride uptake on either blood-pool or mineralization phase F-18-Fluoride PET/MRI. SQ showing BME alone or a combination of BME and chronic AS lesions had a significantly higher percentage of focal F-18-Fluoride uptake on blood-pool phase and mineralization phase PET/MRI than SQ showing AS lesions without BME (P<0.001). Both F-18-Fluoride PET datasets provided high IQ, albeit IQ of mineralization phase PET was superior to blood-pool phase PET (P<0.001). ConclusionDual-phase F-18-Fluoride PET/MRI of the SIJ showed that inflammatory rather than chronic AS lesions are associated with regional hyperaemia and osteoblastic activity.