Uptake on PET/CT has also been shown to correlate with clinical disease activity in patients with axSpA [66,67]; however, there is conflicting evidence regarding the extent to which tracer uptake correlates with specific inflammatory and structural lesions seen on CT and MRI resulting in uncertainty about its diagnostic utility [64,67-72]. In one study, the interreader reliability of diagnosing sacroiliitis on PET/CT was poor and substantially lower than that for MRI or CT [67]. In a small study using radiographs as the reference standard, F-18-fluoride PET/CT had a sensitivity of 80% and specificity of 77% for the detection of sacroiliitis in AS, although there are inherent limitations in the use of radiographs as the reference standard for diagnosing axSpA [73]. Given the available data, the utility of PET/CT in the diagnosis of axSpA, including its utility in assessment of the spine, is uncertain at this time. MRI Spine Area of Interest Although MRI of the sacroiliac joints has high utility for the diagnosis of axSpA, some patients will not demonstrate imaging features of axSpA in the sacroiliac joints on MRI as indicated by sensitivities of all studies to varying degrees below 100% [22], and therefore some patients may need further imaging evaluation with MRI given that it is the imaging modality of choice for the diagnosis of axSpA [1,3,19,22]. Studies have shown widely varying degrees of isolated spinal involvement on MRI in axSpA ranging from 1% to 49% [19,59-63,114,115]. Therefore, imaging of the spine is beneficial for patients in whom there is suspicion for axSpA but imaging of the sacroiliac joints has been negative. Some authors have shown the utility of using whole-spine or whole-body MRI to identify the burden of disease rather than selecting specific areas of the spine to image because findings may be potentially isolated to any portion of the spine [10,59,63,89,114,120-122].
Inflammatory Back Pain Known or Suspected Axial Spondyloarthropathy PCAs. Uptake on PET/CT has also been shown to correlate with clinical disease activity in patients with axSpA [66,67]; however, there is conflicting evidence regarding the extent to which tracer uptake correlates with specific inflammatory and structural lesions seen on CT and MRI resulting in uncertainty about its diagnostic utility [64,67-72]. In one study, the interreader reliability of diagnosing sacroiliitis on PET/CT was poor and substantially lower than that for MRI or CT [67]. In a small study using radiographs as the reference standard, F-18-fluoride PET/CT had a sensitivity of 80% and specificity of 77% for the detection of sacroiliitis in AS, although there are inherent limitations in the use of radiographs as the reference standard for diagnosing axSpA [73]. Given the available data, the utility of PET/CT in the diagnosis of axSpA, including its utility in assessment of the spine, is uncertain at this time. MRI Spine Area of Interest Although MRI of the sacroiliac joints has high utility for the diagnosis of axSpA, some patients will not demonstrate imaging features of axSpA in the sacroiliac joints on MRI as indicated by sensitivities of all studies to varying degrees below 100% [22], and therefore some patients may need further imaging evaluation with MRI given that it is the imaging modality of choice for the diagnosis of axSpA [1,3,19,22]. Studies have shown widely varying degrees of isolated spinal involvement on MRI in axSpA ranging from 1% to 49% [19,59-63,114,115]. Therefore, imaging of the spine is beneficial for patients in whom there is suspicion for axSpA but imaging of the sacroiliac joints has been negative. Some authors have shown the utility of using whole-spine or whole-body MRI to identify the burden of disease rather than selecting specific areas of the spine to image because findings may be potentially isolated to any portion of the spine [10,59,63,89,114,120-122].