Feasibility of a UTE Stack-of-Spirals Sequence for T_1ρ Mapping of Achilles Tendinopathy

We analyzed the feasibility of using a UTE stack-of-spirals turbo FLASH (STFL) sequence to measure T_1ρ relaxation in the Achilles tendon. Six HS (25-31 years) and five AT patients (32-47 years) participated. The study evaluates the clinical utility of the STFL sequence to generate T_1ρ maps using mono-exponential (ME) and bi-exponential (BE) fitting models. In a phantom experiment, ME-T_1ρ values and SNR estimated from the STFL sequence are compared with those of the Cartesian turbo FLASH (CTFL) sequence. In human subjects, we evaluate differences in estimated ME (ME-T_1ρ) and BE parameters (short T_1ρ, long T_1ρ, and short fraction) between AT and HS groups along with repeatability of STFL. The agarose phantom demonstrates biases of 2.89% (3% agarose), -1.88% (5%), and -0.92% (7%) between ME-T_1ρ values from STFL and CTFL. In the bovine Achilles tendon, STFL shows a large bias of -58.6%, with a lower median ME-T_1ρ (2.9 ms) than CTFL (4.6 ms). SNR is higher in STFL (77.05-80.72 for 3%-7% agarose; 24.43 for bovine tendon) than CTFL (66.73-58.97 for agarose; 3.21 for bovine tendon). ME and BE parameters were averaged over the entire Achilles tendon, and none showed significant group differences (p > 0.05; effect size = 0.05-0.22). Subregional analysis showed that in the mid-Achilles tendon, short and long BE-T_1ρ components were 26% and 37% lower in AT than HS, though not statistically significant. The LDA-combined BE parameter showed significant group separation in the midtendon region (p = 0.016; effect size = 1.53). In HS, the long BE-T_1ρ component showed subregional variation (p = 0.006), increasing 58% from calcaneal to midtendon, and then decreasing 23% toward the intramuscular region. ME and BE fitting showed high repeatability with scan-rescan variations of 2.64% (T_1ρ), 3.38% (short T_1ρ), 3.0% (long T_1ρ), and 0.21% (short fraction). We demonstrated the feasibility of using STFL for T_1ρ quantification in the Achilles tendon.