Three-dimensional ultrashort echo time (3D UTE) MRI of Achilles tendon at 4.7T MRI with comparison to conventional sequences in an experimental murine model of spondyloarthropathy.

Background: Due to the very short T of its components, the normal anatomy of Achilles enthesis is impossible to define with "conventional" long echo time (TE) T sequences. However, this is a common site affected by rheumatologic disease. Early abnormalities related to inflammatory processes are impossible to detect in this location.

Purpose: To assess the feasibility of a 3D-UTE (ultrashort echo time) sequence to evaluate normal and pathological Achilles entheses, determining both anterior fibrocartilaginous and posterior collagenic portions at 4.7T, in a rat model of spondyloarthropathy (SpA) with histological correlation. To assess whether this sequence detects SpA enthesopathy prior to long TE T sequences, enabling disease monitoring.

Study Type: Prospective case-control study.

Animal Model: Twelve immunocompetent Wistar male rats imaged before (controls); the model was induced in eight rats (16 tendons) imaged at day 6, day 13, and day 21 with regular sacrifice for ex vivo imaging and histological correlation.

Field Strength: 4.7T Bruker Biospec Systems. 3D balanced steady-state free precession (bSSFP) and 3D-UTE sequences, performed at baseline (day 0, n = 12 animals / 24 tendons), day 6 (n = 8/16), 13 (n = 4/8), and day 21 (n = 2/4).

Assessment: Visual analysis and signal intensity measurements (signal to noise ratio, SNR) of both bSSFP and UTE images were performed by two independent musculoskeletal radiologists at different locations of the Achilles enthesis and preinsertional area.

Statistical Tests: Normal and pathological rat values were compared by Wilcoxon signed-rank tests, as well as interobserver differences. MRI findings were compared against histological data.

Results: The 3D-UTE sequence identified the anterior fibrocartilage and posterior collagenic areas of Achilles entheses in all cases. Visual analysis and signal intensity measurements distinguished SpA-affected entheses from healthy ones at days 6 and 13 (P = 0.002 and P = 0.006, respectively). Neither the normal anatomy of the enthesis nor its pathological pattern could be identified on T bSSFP sequences.

Data Conclusion: Unlike bSSFP T sequences, 3D-UTE sequences enable visualization of normal enthesis anatomy and early detection of abnormalities in pathological conditions.

Level Of Evidence: 2 Technical Efficacy: Stage 2 J. Magn. Reson. Imaging 2019;50:127-135.