Ultrashort echo time spectroscopic imaging (UTESI): an efficient method for quantifying bound and free water.

Biological tissues usually contain distinct water compartments with different transverse relaxation times. In this study, two-dimensional, multi-slice, ultrashort echo time spectroscopic imaging (UTESI) was used with bi-component analysis to detect bound and free water components in musculoskeletal tissues. Feasibility studies were performed using numerical simulation. Imaging was performed on bovine cortical bone, human cadaveric menisci and the Achilles' tendons of volunteers. The simulation study demonstrated that UTESI, together with bi-component analysis, could reliably quantify both T(2)* and fractions of the short and long (2)* components. The in vitro and in vivo studies each took less than 14 min. The bound water components showed a short T(2)* of ~0.3 ms for bovine bone, ~1.8 ms for meniscus and ~0.6 ms for the Achilles' tendon. The free water components showed about an order of magnitude longer T(2)* values, with ~2 ms for bovine bone, ~14 ms for meniscus and ~8 ms for the Achilles' tendon. Bound water fractions of up to ~76% for bovine bone, 50% for meniscus and ~75% for the Achilles' tendon were measured. The corresponding free water components were up to ~24% for bovine bone, 50% for meniscus and ~25% for the Achilles' tendon by volume. These results demonstrate that UTESI, combined with bi-component analysis, can quantify the bound and free water components in musculoskeletal tissues in clinically realistic times.