Will a lumbar spine Magnetic Resonance Imaging (MRI) detect the sacroiliac (SI) joint?

From the Guidelines

A lumbar spine MRI can partially visualize the sacroiliac (SI) joints but is not the optimal imaging study for fully evaluating SI joint pathology. While a standard lumbar MRI may capture the upper portion of the SI joints, it typically does not provide complete or dedicated views of these joints. For comprehensive assessment of the SI joints, a dedicated SI joint MRI is recommended, which uses specific protocols and imaging planes designed to fully visualize these joints. The difference lies in the field of view and imaging parameters - lumbar spine MRIs focus on the vertebrae, discs, and neural structures of the lower back, whereas SI joint MRIs specifically target the junction where the sacrum meets the iliac bones. Some key points to consider when evaluating SI joint pathology include:

• The use of MRI to evaluate the sacroiliac joints and/or spine in patients with suspected axSpA, as recommended by the American College of Radiology 1
• The importance of imaging both the sacroiliac joints and the spine to help ensure MRI yields the highest diagnostic utility and establishes the extent of disease burden, particularly in cases where symptoms are referable to the spine 1
• The presence of active inflammatory changes, such as bone marrow edema, and structural changes, such as bone erosion and fatty deposition, which are characteristic findings of axSpA 1 If SI joint pathology is suspected (such as sacroiliitis, inflammatory arthritis, or SI joint dysfunction), it's essential to communicate this concern to your healthcare provider and radiologist so they can order the appropriate imaging study or potentially modify the lumbar MRI protocol to better capture the SI joints. According to the most recent study 1, a dedicated SI joint MRI is the preferred imaging modality for evaluating SI joint pathology, and lumbar spine MRI may not provide sufficient visualization of the SI joints.

From the Research

Lumbar Spine MRI and SI Joint

• A lumbar spine MRI may not always include the sacro-iliac (SI) joint in the imaging process 2.
• The decision to image the SI joint as part of a routine MRI lumbar spine series is not considered cost-effective or a useful use of resources, unless significant clinical findings are demonstrated 2.

SI Joint Pathology Detection

• SI joint pathology can be identified on MRI, but it is not always possible to distinguish between inflammatory and non-inflammatory disease 3, 4.
• The presence of bone marrow edema (BME) is a key requirement for the diagnosis of sacroiliitis, but it can also be found in degenerative disease, athletes, and healthy persons 4.
• Certain patterns of BME and the presence of structural lesions, such as bone erosion or ankylosis, can increase the likelihood of sacroiliitis being present 4.

Alternative Diagnoses

• Alternative diagnoses, such as degenerative changes, diffuse idiopathic skeletal hyperostosis (DISH), osteitis condensans ilii (OCI), and septic sacroiliitis, can be suggested by MRI findings in patients with low back pain 3, 5.
• These alternative diagnoses can be more common in patients with suspected sacroiliitis, particularly in older females 5.
• A substantial proportion of patients with suspected sacroiliitis may have normal SI joints, while others may be diagnosed with other pathologies 5.