Depiction of achilles tendon microstructure in vivo using high-resolution 3-dimensional ultrashort echo-time magnetic resonance imaging at 7 T

Abstract

Objectives: The objective of this study was to demonstrate the feasibility of depicting the internal structure of the Achilles tendon in vivo using high-resolution 3-dimensional ultrashort echo-time (UTE) magnetic resonance imaging at 7 T.

Materials and methods: For our UTE imaging, a minimum-phase radiofrequency pulse and an anisotropic field-of-view 3-dimensional radial acquisition were used to minimize the echo time and scan time. A fat saturation pulse was applied every 8 spoke acquisitions to reduce blurring and chemical shift artifacts from fat and to improve the dynamic range of the tendon signal. Five healthy volunteers and 1 patient were scanned with an isotropic spatial resolution of up to 0.6 mm. Fat-suppressed UTE images were qualitatively evaluated and compared with non-fat-suppressed UTE images and longer echo-time images.

Results: High-resolution UTE imaging was able to visualize the microstructure of the Achilles tendon. Fat suppression substantially improved the depiction of the internal structure. The UTE images revealed a fascicular pattern in the Achilles tendon and fibrocartilage at the tendon insertion. In a patient who had tendon elongation surgery after birth, there was a clear depiction of disrupted tendon structure.

Conclusions: High-resolution fat-suppressed 3-dimensional UTE imaging at 7 T allows for the evaluation of the Achilles tendon microstructure in vivo.

Figure 1

Pulse sequence for 3D UTE imaging. By using a short minimum-phase RF pulse and 3D radial trajectory, a TE of 229 μs was achieved when the slab was 12 cm thick. A fat suppression pulse was applied every eight spoke acquisitions.

Figure 2

UTE images and standard FSE images from a healthy volunteer. Magnified images of the tendon are shown on the right. (a-b) Sagittally-reformatted slices from fat-suppressed 3D UTE sequence, having TEs of 229 μs and 2.2 ms. In (a), a fascicular pattern is depicted in the tendon. In addition, high signal is observed at the enthesis (tendon insertion), which is consisted of fibrocartilage (dashed arrow). In (b), the tendon is much darker but some of the bright strips shown in (a) (solid arrow) and the enthesis are still visible. (c-d) Matching slices from standard T₁-weighted FSE and fat-suppressed T₂-weighted FSE. In both FSE images, the tendon generates almost no signal but the bright strips at the similar locations in (b) are visible in (c) (solid arrow). Here, the slice thickness for the UTE images is 0.65 mm while this for the FSE images is 3 mm.

Figure 3

Comparison of UTE images with and without fat suppression. Sagittal reformatted slices from 3D UTE imaging with (a) and without (b) fat suppression. Magnified images of the tendon are shown on the right. With fat suppression, blurring and chemical shift artifacts from the fatty tissue are substantially reduced, improving the depiction of tendon microstructure. When fat suppression is not applied, a bright signal occurs at the boundary of the tendon adjacent to Kager’s triangle (arrow) due to chemical shift artifacts from fat.

Figure 4

Axial reformatted UTE slices at different locations along the Achilles tendon. On the sagittal reformatted UTE slice (a), the positions of six axial reformatted slices are denoted. In (b-g), the slices from the inferior to superior locations are shown. At the insertion site (b-c), the enthesis fibrocartilage is depicted as a bright signal (solid arrow). In (c), the sesamoid cartilage is also visible (dashed arrow) and a fascicular pattern is demonstrated in the posterior region of the tendon. At more superior levels, the fascicular pattern is visible throughout the whole Achilles tendon. The tendon is merged to the muscle at the musculotendinous junction (g).

Figure 5

Internal structural analysis of the Achilles tendon. (a-c) Axial, sagittal, and coronal reformatted slices. The positions of the slices are denoted as dashed lines in (a-b). A fascicular pattern is observed in the tendon. In (c), the orientations of the fascicles can be characterized, allowing for distinguishing different groups of fascicles as denoted by solid arrows. Numerous other tendons in the ankle are also well depicted, indicated in (a).

Figure 6

Images from a 32 year old patient who underwent tendon elongation surgery right after birth. (a) UTE coronal reformatted slices. (b-d) UTE sagittal reformatted slices at three different locations denoted in (a). (e-g) T₁-weighted FSE images at the matching locations. On the FSE images, bright signal in the tendon results from fat infiltration (solid arrows). On the UTE images, disrupted microstructure and discontinuous fascicles in the region of the tendon surgical incision (solid arrows) are depicted. The yellow dotted arrow in (b) denotes susceptibility artifacts from a post surgical microscopic remnant.