Author(s): Walter P Maksymowych 1
Clinicians and researchers have turned to imaging to address several major challenges in the clinical evaluation and treatment of ankylosing spondylitis (AS). The advent of more effective therapies targeting the pro-inflammatory cytokine tumor necrosis factor (TNF)-[alpha] has provided a more justifiable need to establish a diagnosis early in the disease course, particularly if it can be shown that such therapies have disease-modifying potential. Unfortunately, symptom duration prior to diagnosis of AS remains stubbornly at 8-9 years in most advanced countries  . This reflects the low discriminant value of the history in distinguishing between inflammatory and mechanical causes of back pain  , the lack of physical signs related to spinal and sacroiliac joint inflammation in early disease, and the low sensitivity and specificity of laboratory abnormalities that are confined to acute-phase reactants  . The same limitations preclude objective evaluation of disease activity in patients with established disease. The advent of magnetic resonance imaging (MRI) has proven to be a milestone in the field, through its ability to permit direct visualization of inflammatory lesions in the spine and sacroiliac joints. Scoring systems that permit quantification of the degree of inflammation on MRI scans have also been developed, which now allow the objective analysis of disease severity in longitudinal studies and in clinical trials evaluating the efficacy of new anti-inflammatory agents. Advances in the use of other imaging modalities have been more limited and primarily confined to the development of a scoring tool to quantify structural damage on plain radiography of the spine. This tool is now being used to assess the disease-modifying potential of standard therapies, such as nonsteroidal anti-inflammatory agents, as well as anti-TNF-[alpha] therapies. Although it is now undeniable that advances in imaging have enhanced their value to both the clinician and the researcher, there has been insufficient awareness of the pitfalls inherent to the use of these imaging modalities in the setting of AS.
The primary advantage of MRI is its ability to visualize lesions within soft tissues and bone in 3D. T1-weighted sequences primarily detect the signal from fat, and the contrast with bone, which is dark, enhances anatomical delineation of joint structures. T2-weighted sequences suppress the signal from fat that is present in bone marrow, allowing visualization of an underlying water signal that may be related to inflammation, cyst, tumor and other pathologies associated with increased vascular permeability. The two images should be analyzed simultaneously as they provide complementary information. For example, loss of the fat signal in subchondral bone marrow on the T1 image of the sacroiliac joint, accompanied by a corresponding water signal on the T2 image, typically denotes inflammation.
The presence of subchondral bone marrow edema in the postero-inferior region of the synovial portion of the joint is one of the earliest features of sacroiliitis  . By contrast, loss of the fat signal on the T1 image in subchondral bone marrow without the corresponding presence of a water signal on the T2 image suggests an alternative explanation for the fat...