Purpose

To evaluate whole knee joint tractography in the rat, including articular cartilage, ligaments, meniscus, and growth plate using diffusion tensor imaging (DTI) at microscopic resolution.

Introduction

The knee joint, relies on a variety of ligaments, muscles, tendons, bones, and cartilage to maintain flexibility, stability, and strength, is the largest and one of the most complex joints in human body (1). Preclinical studies of these joints are particularly crucial in understanding evolution in disease models and efficacy of experimental drugs. DTI of the knee joint is technically challenging due to the short T2 relaxation times, magic angle effect, complex anatomy, and the presence of multiple tissue types (2,3). Recently, tractography studies in ligaments and tendon demonstrated the feasibility of probing certain types of tissue. However, fiber tracking of the whole knee joint is more challenging and has not yet been reported.

Methods

Animal experiments were carried out in compliance with the Duke University Institutional Animal Care and Use Committee. Three knee joints were harvested shortly after sacrifice from three mature, healthy rats. The specimens were scanned at 9.4 T (Oxford 8.9-cm vertical bore) with maximum gradient strength of 2000 mT/m on each axis. Scan parameters for fully sampled (FS) data using 3D Stejskal-Tanner diffusion-weighted spin-echo pulse sequence were as follows: matrix size = 200 × 128 × 128, FOV = 18 × 11.52 × 11.52 mm³, TE = 9.1 ms, TR = 100 ms, 31 unique diffusion directions with a b value of 1250 s/mm² and 3 non-diffusion-weighted (b0) measurements. A modified 3D Stejskal-Tanner diffusion-weighted spin-echo pulse sequence was also performed by fully sampling the readout dimension and undersampling the phase dimensions by 4 times. Scan parameters for under sampled data were the same as FS scans except the matrix size is 400 × 256 × 256 with 5 b values (250-1250 s/mm²). CS reconstruction was applied to the under sampled k-space data as described in previous studies (4,5). The DTI model was used to calculate the tensor and the scalar indices (FA and MD) using DSI studio software (6).

Results

Figure 1 shows magnified b0 and MD images at 90 µm³ and 45 µm³ isotropic resolution. The images at higher resolution are comparable to the images at 90 µm³ resolution, however, significant resolution improvement is evident in the 45 µm³ isotropic resolution images. The variations of FA and MD at different spatial resolutions are found to be negligible in cartilage, growth plate, ACL and PCL. Figure 2 shows the color FA, fiber tracts and fiber orientation images of patella articular cartilage at 90 µm³ (upper) and at 45 µm³ (lower). The superficial zone (SZ) and radial zone (RZ) are better distinguished in color FA images at higher spatial resolution (white and yellow arrows). Cartilage fiber orientation in the RZ (Fig 2b, 2e) is perpendicular to the cartilage surface, however, most tracts from the RZ are terminated before reaching to the SZ (yellow ellipse) at 90 µm³ resolution, probably due to the fiber orientation sharp turning (almost 90°, Fig 2c, white box) at the transitional zone. The collagen fibers have smaller turning radii because of higher spatial resolution (Fig 2f, white box). Figure 3 reveals the fiber tracts and fiber orientation images of ligaments at 90 µm³ and at 45 µm³. The fiber orientations in ACL and PCL show good agreements between these two different spatial resolutions. Figure 4 illustrates the diffusion tractography results in articular cartilage area (ROIs in green) at different diffusion weightings (250, 750, 1250 s/mm²). The tracts in the radial zone are perpendicular to the cartilage surface, while the tracts in the superficial zone are parallel to the cartilage surface. Well-organized collagen fibers in both radial zone and superficial zone are observed at higher b values (especially at b value of 1250 s/mm²). The tracts at b value of 250 s/mm² did not to show the similar pattern in both SZ and RZ.

Discussion and Conclusion

This study demonstrates the value of diffusion tractography for providing tissue microscopic properties and quantifying integrity of fibril collagen network in knee joint complementary to conventional DTI metrics. The use of CS allowed higher spatial and angular resolution DTI scans to highlight thin connective tissues (e.g. cartilage) with complex collagen fibril orientations. We believe this capacity can provide unique insight in animal studies of degenerative joint disease.

Acknowledgements

This work was supported by NIH/NIBIB P41 EB015897, Office of the Director 1S10ODO10683-01, NIH/NINDS 1R01NS096720-01A1.