Measurement of Fast T2* of the ACL Using 3D UTE Imaging
Kenneth Wengler1,2, Mingqian Huang 2, Elaine Gould2, Mark Schweitzer2, Seth Korbin3, James Paci3, and Xiang He2

1Biomedical Engineering, Stony Brook University, Stony Brook, NY, United States, 2Radiology, Stony Brook University School of Medicine, Stony Brook, NY, United States, 3Orthopaedic Surgery, Stony Brook University School of Medicine, Stony Brook, NY, United States

### Synopsis

3D-UTE imaging was utilized to estimate the fast T2* component of ACL after graft repair surgery. The average fast T2* component in the grafted ACL 3 months post-surgery for eight subjects is 1.66 +/- 0.4 ms. Two subjects currently have both 3 and 6 month post-surgery images. The ACL average fast T2* component went from 2.55ms to 1.81ms at three and six months post-surgery respectively for the first subject, and 1.39ms to 1.24ms for the second subject. The results demonstrate promise for 3D-UTE T2* mapping of bound water to evaluate ACL repair.

### Purpose:

The anterior cruciate ligament (ACL) is an important structure in maintaining the normal biomechanics of the knee and is the most commonly injured knee ligament. ACL tears are common injuries in all levels of the athletic population1. Most ACL injuries (~80%) are complete and require surgical repair. A major concern has been raised regarding the difficulty of assessing reconstructed ACL grafts using MRI. Methods based on relative ACL image intensity or proton density is often inclusive. Graft MRI intensity changes might reflect not only graft damage, but also neovascularization and matrix synthesis within the graft.

T2*-weighted images has been shown to be a good indicator of graft function2. However, similar to other tendons and ligaments, the ACL also exhibits a two-compartment T2* decay profile. The estimated T2* from single compartment model, or slow decay component from two-compartment model, is sensitive to effect such as edema or other factors unrelated to ACL function3,4. The fast T2* (~1 ms) faction originating from water tightly bound with collagen fibers and other tendon structure, is potentially a more sensitive indicator of the intrinsic graft tissue metamorphism. Therefore, an ultrashort echo time (UTE) MRI technique was used in this ACL study to quantify the changes on the T2* of bound water during ACL graft incorporation.

### Methods:

MRI of the ACL was performed on a Siemens 3T PET/MRI system with a twelve-channel knee coil. A total of eight patients undergoing ACL graft surgery were recruited for this IRB approved study. At this moment, all eight had their ACL imaged at three months post-surgery and two subjects had their ACL imaged at three months and six months post-surgery. 3D double echo UTE imaging was performed with the first echo formed at 100, 270, and 500 µs after non-selective excitation. A spiral out radial projection k-space sampling strategy was used. Other imaging parameters were: FOV of 160 x 160 x 160 mm3; voxel size of 1 x 1 x 2 mm3; TR of 13 ms; flip angle of 15°; total acquisition time of 2.5 mins for each echo.

### Results & Discussions:

3D-UTE images were reconstructed using a regridding procedure5 adapted for a non-isotropic voxel size. For fast T2* component calculation an ACL region of interest (ROI) was drawn and the signal was averaged within the ROI for each UTE image. Figure 1 displays the T2* decay at different TE's, clearly demonstrating the two-compartment feature. However, due to uncompensated eddy current effect, and potential delay of readout gradient and ADC timing, it is un-reliable to combine the data from the first echo (acquired with half readout k-space sampling) and second echo (acquired with full readout k-space sampling) for a complete two-compartment model curve fitting. Therefore, the average signal at each first echo was then fit with a mono-exponential function to calculate the fast T2* component of the ACL using the equation $S(TE)=S(0)\cdot e^{-TE\diagup T2_2^*}$.

Since the T2* of ligaments is dependent on its relative orientation with respect to the scanner B0 field, only the middle sections of the ACL graft were used for ROI. Across all eight subjects, the mean ACL T2* of the fast decay component was 1.66 +/- 0.4 ms at their first imaging visit (~3 month after ACL transplant). Figure 2 shows the three UTE images acquired six months post-surgery for two subjects. The average fast T2* component for the ACL went from 2.55ms at three months post-surgery to 1.81ms at six months post-surgery for the first subject, and 1.39ms to 1.24ms for the second subject. This confirms the hypothesis that tissue metamorphism of ACL graft may reduce T2* of the ACL bound water. It has been studied that the ACL graft undergoes the process of peripheral synovial vascularization and is on the way to ultrastructural incorporation6-8.

### Conclusions:

In this study, the T2* of the bound water in ACL graft was estimated utilizing a 3D-UTE technique. Based on limited data, the T2* for the fast component decreases between 3 month to 6 month post-surgery. The results demonstrate the promise of establishing the use of UTE and T2* mapping of bound water as a biomarker for the ACL graft incorporation process and prediction of patient outcome.

### Acknowledgements

No acknowledgement found.

### References

1. Miyasaka, K. C., et al. "The incidence of knee ligament injuries in the general population." Am J Knee Surg 4.1 (1991): 3-8.

2. Hakozaki, Akihiro, et al. "Clinical significance of T2*-weighted gradient-echo MRI to monitor graft maturation over one year after anatomic double-bundle anterior cruciate ligament reconstruction: A comparative study with proton density-weighted MRI." The Knee 22.1 (2015): 4-10.

3. Du, Jiang, et al. "Ultrashort echo time imaging with bicomponent analysis."Magnetic Resonance in Medicine 67.3 (2012): 645-649.

4. Chang, Eric Y., Jiang Du, and Christine B. Chung. "UTE imaging in the musculoskeletal system." Journal of Magnetic Resonance Imaging 41.4 (2015): 870-883.

5. Zwart, Nicholas R., Kenneth O. Johnson, and James G. Pipe. "Efficient sample density estimation by combining gridding and an optimized kernel." Magnetic resonance in medicine 67.3 (2012): 701-710.

6. Biercevicz, Alison M., et al. "T2* MR relaxometry and ligament volume are associated with the structural properties of the healing ACL." Journal of Orthopaedic Research 32.4 (2014): 492-499.

7. Zaffagnini, S., et al. "Neoligamentization process of BTPB used for ACL graft: histological evaluation from 6 months to 10 years." The Knee 14.2 (2007): 87-93.

8. Unterhauser, Frank N., et al. "Endoligamentous revascularization of an anterior cruciate ligament graft." Clinical orthopaedics and related research 414 (2003): 276-288.

### Figures

Fig 1: The signal decay over extended echo times for a typical ACL imaging voxel. The two-compartment behavior is apparent and depicts bound and free water within the ACL.

Fig 2: Representational slices of 3D-UTE images at each of the three TEs acquired for two subjects at 6 months post-surgery. The red arrow marks the ACL.

Proc. Intl. Soc. Mag. Reson. Med. 24 (2016)
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