# The impact of MRI slice thickness on the detection of spinal syndesmophytes in axial spondyloarthritis

## TL;DR

Thinner MRI slices detect more spinal bony growths than X-rays, but thicker slices align better with X-ray results.

## Contribution

This study evaluates how MRI slice thickness affects detection of spinal syndesmophytes compared to radiography in axial spondyloarthritis.

## Key findings

- MRI detected more syndesmophytes than radiography, but missed 21%-31.3% of those seen on X-rays.
- Thicker MRI slices showed better agreement with radiography for syndesmophyte detection.
- Fat lesions on MRI were associated with improved agreement between MRI and radiography findings.

## Abstract

Radiography is commonly used in clinical practice for detecting syndesmophytes in radiographic axial Spondyloarthritis (r-axSpA), while the ability of magnetic resonance imaging (MRI) to detect such bony structures is questionable due to its slicing technique. We aimed to assess the ability and performance for detection of syndesmophytes on MRI using different slice thicknesses and compare them with radiography in r-axSpA.

MRI (T1-weighted (T1W) sequences) with slice thicknesses of 1–6 mm of the lower thoracic and lumbar spine were prospectively performed in patients with available radiographs. Each vertebral corner (VC) (anterior and posterior) from thoracic (Th11) to lumbar (L5) was assessed for presence/absence of syndesmophytes and/or fat lesions (FL, MRI only) by two experienced readers in independent MRI and radiography sessions and agreement was then reached in consensus.

A total of 1.204 VCs were assessed from 43 r-axSpA patients. Syndesmophytes were recorded in 19.3% VCs on radiography and in 38.3%, 37.5%, 34.8%, 33.7%, 31.4%, 28.7% VCs on MRI slice thicknesses of 1–6 mm, respectively (all p ≤ 0.001 vs. radiography). Although more syndesmophytes were recorded on MRI than radiography, MRI also missed 21%-31.3% syndesmophytes detected in radiography. Agreement with radiography was found in 72.6%, 73.8%, 75.9%, 76%, 77.3% and 78.5% on MRI slice thicknesses of 1–6 mm, respectively. FL were detected in 38.2%-39.2% in slice thicknesses 1–6 mm. Occurrence of FL was associated with better agreement between MRI and radiography findings.

The thinner the MRI slices, the more syndesmophytes were detected compared to radiography, but the best agreement with radiography was found in the thicker slices. The presence of fat lesions on MRI was associated with better agreement with radiography for syndesmophyte detection.

The online version contains supplementary material available at 10.1186/s13075-025-03665-x.