Role of MRI-based synthetic CT and low-dose CT for the assessment of structural damage in axial spondyloarthritis – a review

Structural lesions, including erosions, sclerosis, and pathological new bone formation, are key features of disease progression in axial spondyloarthritis (axSpA) and are associated with irreversible functional impairment. Reliable imaging methods are therefore essential for accurate assessment and monitoring. Radiography remains the recommended method for evaluating structural damage; however, it is limited by poor visualization of the thoracic spine and early sacroiliitis, as well as having low sensitivity to change. Magnetic resonance imaging (MRI) enables early detection of inflammatory lesions but has limitations in assessing cortical bone and subtle structural damage. Computed tomography (CT) is the reference standard for visualizing structural bone changes, but its use has been limited in axSpA due to high radiation exposure. Recent advances in low-dose CT and MRI-based synthetic CT have introduced new opportunities for more sensitive and comprehensive structural damage assessment. Low-dose CT provides high-contrast visualization of structural lesions in both the sacroiliac joints (SIJs) and the entire spine, including regions not adequately visualized by radiography, and has demonstrated superior diagnostic performance and sensitivity to structural progression. Synthetic CT, generated from MRI data using a deep-learning reconstruction method, enables CT-like visualization of bone without ionizing radiation and allows combined assessment of inflammatory and structural lesions within a single examination. Emerging evidence supports its diagnostic accuracy and reliability, particularly in the SIJs, but the method has also shown promise for assessment of spinal structural damage. Together, these modalities are expected to substantially improve assessment of disease progression and treatment effects, although further longitudinal studies are needed to establish their role in clinical practice and research.