Clinically, Achilles tendon (AT) rapture accounts for 40% to 60% of all operative tendon repairs, with 75% due to sports-related activities. Conventional MR images exhibit poor contrast and specificity in delineating low-grade tendon injuries. Diffusion tensor imaging (DTI) can be potentially sensitive to the injury-induced changes on the tendons micro-structure (1,2). However, tendons are known to have short T2/T2* relaxation time constant (~15-25 ms for the slow decay component of AT in healthy subjects)(3), so conventional spin-echo based DTI with TE ~80 ms would lead to poor tendon MR signal and difficult diffusion quantification (4,5). In this study, a novel approach of combining stimulated-echo based DTI (6) with readout-segmented multi-shot EPI (7) (ste-RESOLVE) had been developed and evaluated. The values for TE can be as low as 20 ms for typical b value of 800 s/mm2. This enables, for the first time, a robust investigation of Achilles tendon microscopic tissue integrity on clinical MR scanners.

Five healthy volunteers were recruited for this IRB approved study. All experiments were performed on 3T Siemens Skyra and Prisma scanners using a flexible 4-channel coil. To boost AT MR signal from magic angle effect, subjects were instructed to lay side with the tendon positioned ~55 degrees with respect to magnet B0 field.

The stimulated echo RESOLVE (ste-RESOLVE) sequence was modified from Siemens RESOLVE DTI sequence by replacing the bipolar diffusion preparation block with a stimulated echo diffusion prep block. Nulling of eddy current that originated from diffusion gradient was accomplished by placing a compensation gradient at the end of spin evolving period (TM), while the diffusion encoded spins that were temporary stored at the longitudinal axis would not be affected. The sequence parameters for ste-RESOLVE were: FOV of 160×100 mm²; matrix of 80×50; 12 axial slices with thickness of 6 mm and 50% separation; TR/TE of 3500/20 ms; readout segmentation factor of 9 with echo spacing of 0.32 ms; GRAPPA factor of 2; stimulated echo duration (TM) of 200 ms; b value of 400 or 800 s/mm². The total acquisition time was ~6.5 min for 12 diffusion direction. Meanwhile, conventional RESOLVE DTI images were acquired with TR of 1600 ms; TE of 43 (b=400 s/mm²) or 56 ms (b=800 s/mm²); 2 repetitions with total acquisition time of ~5.5 min.

Fig. 1 shows the DWI images acquired by the proposed ste-RESOLVE approach and the conventional RESOLVE sequence. Higher SNR and better image quality for Achilles tendon can be appreciated in ste-RESOLVE images, mainly due to the reduction of TE from 56 ms to 20 ms. In spite of imaging at magic angle, which significantly increases the fraction of slow T2 decay component in tendon and boost the AT MR signal, the AT signal in conventional DWI images were too low for robust DTI quantification. Large deviation on tendon orientation from magic angle can lead to severe SNR degrading (data not shown). For this subject, the mean AT ADC estimated from ste-RESOLVE is 1.1±0.1 mm²/s, mean FA is 0.30±0.1, as illustrated in Fig. 2. Those values are consistent with those of from animal tendon studies (2).

DTI Studio was used to visualize tendon DTI tensor and generate fiber tractography. Fig. 3 presents the result from one subject. The spatial distribution and orientation of DTI fiber tract follows the parallel collagen fiber bundles running along the major axis of the AT. The proximal portion of the AT fiber tractography demonstrated the muscle-tendon junction.

We have developed and evaluated a novel stimulated echo-based DTI technique, ste-RESOLVE, capable of achieving high SNR by significant reducing on TE. Robust estimation on ADC, FA and fiber tractography images have been demonstrated. After further optimization and validation, ste-RESOLVE can be used to evaluate early tendon injuries and monitor the healing process of tendon graft.