Entheses are sites where tendons, ligaments, and joint capsules attach to bone 1. They may be fibrous or fibrocartilaginous. Entheses are adapted at both the macroscopic and the microscopic level to distribute stress over a wide area or volume. At a macroscopic level, tendons and ligaments often flare out at their attachment sites as an adaptation to secure greater skeletal anchorage and distribute force over a larger region. The shape of an enthesis is often matched to that of the tendon. The jig-saw interlocking of calcified fibrocartilage with bone in the subchondral bone plate, is a microscopic adaptation to force transfer that is of critical importance in providing strong union between these two tissues. Entheses are very commonly involved in many diseases including the overuse syndromes 2, the early stages of osteoarthritis (OA) 3 and psoriatic arthropathy (PsA) 4. They are the primary target of disease in the seronegative spondyloarthropathies 5. In this study we aim to study the MR properties of cadaveric Achilles tendon enthesis using ultrashort echo time (UTE) sequences and correlate them with biomechanics.Fig 2 Two-pool MT modeling of an ankle specimen yields maps of water proton fraction (A), collagen proton fraction (B), water proton T2 (C), collagen proton T2 (D), exchange rate (E) and collagen proton T1 (F). Compared to the tendon, the enthesis has a higher water proton fraction, a lower collagen proton fraction, a longer water proton T2, a higher exchange rate and a longer collagen proton T1, but a similar collagen proton T2. Six Achilles tendon enthesis slices (~3 mm thickness) were sectioned from six cadaveric ankle specimens (n=6), and were imaged on a GE 3T Signa TwinSpeed MR scanner (GE Healthcare Technologies, Milwaukee, MI) and then a 11.7T Bruker imaging system. Both 2D UTE and 3D UTE Cones sequences were employed for morphological imaging and quantification of T1, T2* and magnetization transfer ratio (MTR) 6-8. T1 was measured using 3D UTE Cones acquisitions with variable TRs (TR = 7.6, 10, 15, 20, 30, 40ms). T2* was measured with dual-echo 3D UTE Cones acquisitions with variable TEs (TE = 0.03/4.4; 0.2/6.6; 0.4/8.8; 0.8/11; 2.2/13 ms). UTE-MTR was measured with a series of MT frequency offsets (Df = 2, 3, 5, 7, 10 kHz) and MT powers (q = 200º, 400º, 600º, 870º). Other imaging parameters included a flip angle of 20o, a bandwidth of 90 kHz, a FOV of 6 cm, a slice thickness of 2 mm (3D Cones), reconstruction matrix of 256×256. A two-pool MT-modeling was also performed on the UTE-MT dataset 9,10. Indentation testing is a non-destructive method sensitive to the compressive properties of tissues, and was performed on each enthesis slice. Correlation between UTE MRI measures and biomechanics was analyzed.Fig 1 shows selected 3D UTE Cones images of a cadaveric ankle specimen together with clinical T1w and PDw images at 3T, as well as high resolution imaging at 11.7T. Clinical sequences show near zero signal for the enthesis and tendons, while 3D Cones show high signal. UTE images at 11.7T show much greater detail because of the increased SNR and spatial resolution. Fig 2 shows UTE-MT modeling of an ankle specimen. For simplicity, a two-pool model was used to access collagen protons and water protons. Maps of T2* and fractions of water and collagen protons, as well as exchange rate and T1 relation times were generated. These values are generally consistent with values reported in the literature, suggesting the clinical feasibility of this technique.Fig 3 shows correlation between UTE MRI measures and biomechanics of two representative enthesis slices. Indentation stiffness varied significantly with site (p<0.001) and abnormality (p<0.001). The results suggest that increased T2* and free water fraction may serve as biomarkers of Achilles tendon enthesis degeneration. The UTE sequences provide high resolution morphological imaging of the Achilles tendon enthses, as well as quantitative measures of T2*, T1, MTR and MT modeling. The UTE measures can be used as biomarkers of biomechanical degradation of this tissue. Clinical applications of these measures remain to be investigated in future studies.