Background/Purpose: Fat metaplasia
in the bone marrow is an indicator of disease progression in spondyloarthropathy
(SpA). This study aimed to evaluate the bone marrow fat content in patients
with SpA using advanced magnetic resonance imaging (MRI) techniques to measure
the proton density fat-fraction and identify fat-fraction changes according to
disease progression.

Methods: A total of 129 patients with SpA
who underwent a pelvis MRI at Hanyang University Hospital for Rheumatic
Diseases from September 2014 to March 2015 were retrospectively evaluated. Quantitative
fatty infiltration was assessed on the fat-signal fraction maps using the Dixon
technique. Fat fractions were quantified after measuring the signal intensity
within the region of interest (ROI) over the periarticular bone marrow where
there was no contamination from the bony cortex or joint spaces. The ROIs were
drawn at each of two sites of the right and left ilium. MRI was used to examine
the fat
fraction at inflammatory regions (inflammatory fat) and noninflammatory regions
(noninflammatory fat) in the bone marrow. Clinical characteristics and the correlation
between the sacroiliac (SI) joint grades and fat fractions were evaluated.

Results: Patients had a mean age of
32.2 ± 11.6 years, and 84 were male (65%). Mean symptom duration was 4.2 ± 6.0
years, and 84 out of 116 patients (72.4%) had tested positive for HLA-B27. Mean
right and left SI joint grades on the radiograph were 1.7 ± 1.3 and 1.8 ± 1.3,
respectively. Mean fractions of inflammatory fat were 82.6% ± 9.8% and 82.0% ±
11.1% (right) and 82.4% ± 10.2% and 82.2% ± 10.3% (left), while the mean
fractions of noninflammatory fat were 51.1% ± 9.6% and 53.2% ± 9.6% (right) and
52.8% ± 9.5 % and 53.4% ± 11.0% (left). A significant correlation was found
between right and left inflammatory fat fractions and right (p = 0.003 and p = 0.005)
and left (p = 0.000 and p = 0.000) SI joint grades on the radiographs,
respectively (Spearman correlation analysis), although no correlation was found
between the total noninflammatory fat fraction and each SI joint grade on the
radiographs (Figure 1). A significant correlation was
also found between inflammatory fat fractions and modified Stoke Ankylosing
Spondylitis Spinal Score.

Conclusion:
The inflammatory fat fraction of the bone marrow increased in SpA patients who showed
severe radiographic SI joint changes. Quantitative MRI assessment of the fat
fraction may represent another useful imaging technique to evaluate the progression
of SpA.

Figure
1. Correlation between sacroiliac joint grade and fat fraction of the bone
marrow in the ilium. (A) Right sacroilium. (B) Left sacroilium.