India ink artifact on Dixon out-of-phase images can be used as a landmark to measure joint space width at MRI.

The purpose of this study was to test the feasibility of joint space width (JSW) measurement on Dixon MR images with the "India ink" artifact between cartilage and bone marrow as a landmark for the subchondral plate and to correlate it with radiographic JSW. Both hands of six volunteers (three women, three men; mean age, 36.7 ± 10.4 [SD] years) and 24 patients with early rheumatoid arthritis (16 women, 8 men; mean age, 45.7 ± 14.5 [SD] years) were imaged with MRI Dixon sequences and radiographs. Two radiologists (R1, R2) separately measured JSW in 11 joints per hand on all Dixon images in volunteers, on contrast-enhanced T1-weighted out-of-phase images in patients and on radiographs in both groups. Inter-technique, intra-observer and inter-observer agreements were assessed using intraclass correlation coefficient (ICC) and Bland Altman analysis. In volunteers, agreement between JSW measurements on MRI and radiographs was the highest with T1-weighted Dixon out-of-phase images (mean ICC ranging from 0.69 to 0.76 for R1 and 0.65 to 0.74 for R2). In patients, median bias between JSW measurements at first and second readings was not statistically significantly different from 0 on T1-weighted Dixon out-of-phase images (mean bias of 0.00 and + 0.01 mm) and radiographs (mean bias of 0.00 and +0.01 mm). Median bias of the difference between measurements of R1 and R2 was statistically significantly different from 0 on T1-weighted Dixon out-of-phase images (mean bias of -0.11 and -0.09 mm; P < 0.039) and radiographs (mean bias of -0.24 and -0.20 mm; P < 0.035). Measurement of hand JSW on T1-weighted Dixon out-of-phase images using India ink artifact as a landmark for the subchondral plate is repeatable and reproducible.

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