Evaluation of normal cadaveric Achilles tendon and enthesis with ultrashort echo time (UTE) magnetic resonance imaging and indentation testing.

Entheses are regions where tendons and ligaments attach to bone, and are the primary target in seronegative and other diseases of the musculoskeletal (MSK) system. MRI has been widely used for visualizing features of inflammatory and degenerative MSK disease; however, normal tendons and entheses have short transverse relaxation times (T2 ), and show little or no signal with conventional clinical MRI pulse sequences, making it difficult to investigate their MR properties. In this study we examined the normal MR morphology of the cadaveric Achilles tendon and enthesis at 3 T using novel three-dimensional ultrashort echo time (3D UTE) Cones sequences, and at 11.7 T using conventional MRI sequences. We also studied the MR properties of the Achilles tendon and enthesis including T2 *, T1 , and magnetization transfer ratio (MTR). In addition, MT modeling of macromolecular proton fractions was investigated using 3D UTE Cones sequences at 3 T. Indentation testing was performed to investigate the mechanical properties of the tendons and entheses, and this was followed by histological examination. In total five specimens (<50 years) were investigated. On average, tendons and entheses respectively had T2 * values of 0.93 ± 0.48 ms and 2.77 ± 0.79 ms, T1 values of 644 ± 22 ms and 780 ± 55 ms, MTRs of 0.373 ± 0.03 and 0.244 ± 0.009 with an MT power of 1000° and frequency offset of 2 kHz, and macromolecular proton fractions of 18.0 ± 2.2% and 13.9 ± 1.9%. Compared with the tendon, the enthesis generally had a longer T2 *, a longer T1 , a lower MTR, and a lower macromolecular proton fraction as well as both a higher Young's modulus and stiffness. Results from this study are likely to provide a useful baseline for identifying deviations from the normal in seronegative arthritis and other disease of the entheses.