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 T₂* and fractions of the short and long T₂* components. The in vitro and in vivo studies each took less than 14 min. The bound water components showed a short T₂* 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 T₂* 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. Copyright © 2011 John Wiley & Sons, Ltd.     

Ultrashort TE T1ρ magic angle imaging

An ultrashort TE T₁ρ sequence was used to measure T₁ρ of the goat posterior cruciate ligament (n = 1) and human Achilles tendon specimens (n = 6) at a series of angles relative to the B₀ field and spin‐lock field strengths to investigate the contribution of dipole–dipole interaction to T₁ρ relaxation. Preliminary results showed a significant magic angle effect. T₁ρ of the posterior cruciate ligament increased from 6.9 ± 1.3 ms at 0° to 36 ± 5 ms at 55° and then gradually reduced to 12 ± 3 ms at 90°. Mean T₁ρ of the Achilles tendon increased from 5.5 ± 2.2 ms at 0° to 40 ± 5 ms at 55°. T₁ρ dispersion study showed a significant T₁ρ increase from 2.3 ± 0.9 ms to 11 ± 3 ms at 0° as the spin‐lock field strength increased from 150 Hz to 1 kHz, and from 30 ± 3 ms to 42 ± 4 ms at 55° as the spin‐lock field strength increased from 100 to 500 Hz. These results suggest that dipolar interaction is the dominant T₁ρ relaxation mechanism in tendons and ligaments. Magn Reson Med, 2013. © 2012 Wiley Periodicals, Inc.     

Morphologic characterization of meniscal root ligaments in the human knee with magnetic resonance microscopy at 11.7 and 3 T

Objective

To determine the feasibility of using MR microscopy to characterize the root ligaments of the human knee at both ultra-high-field (11.7 T) and high-field (3 T) strengths.

Materials and methods

Seven fresh cadaveric knees were used for this study. Six specimens were imaged at 11.7 T and one specimen at 3 T using isotropic or near-isotropic voxels. Histologic correlation was performed on the posteromedial root ligament of one specimen. Meniscal root ligament shape, signal intensity, and ultrastructure were characterized.

Results

High-resolution, high-contrast volumetric images were generated from both MR systems. Meniscal root ligaments were predominantly oval in shape. Increased signal intensity was most evident at the posteromedial and posterolateral root ligaments. On the specimen that underwent histologic preparation, increased signal intensity corresponded to regions of enthesis fibrocartilage. Collagen fascicles were continuous between the menisci and root ligaments. Predominantly horizontal meniscal radial tie fibers continued into the root ligaments as vertical endoligaments.

Conclusion

MR microscopy can be used to characterize and delineate the distinct ultrastructure of the root ligaments on both ultra-high-field- and high-field-strength MR systems.