Synopsis

Diffusion MRI and tractography in the musculoskeletal system has recently attracted more and more attention to investigate the connective tissue microstructure, local collagen fiber alignment, and the 3D collagen fiber network. Although diffusion tractography was successfully performed in some individual regions of knee, investigating the effects of the acquisition parameters for the whole joint are still lack.

Purpose

To investigate the influences of the diffusion gradient directions (angular resolution) and the strength of the diffusion gradient (b value) on DTI metrics and tractography of various connective tissues in knee joint.

Introduction

The knee joint is the largest and one of the most complex joints in human body¹. It includes various types of tissues, including articular cartilage, meniscus, ligament, muscle, tendon, etc². Diffusion tensor imaging (DTI) allows to quantify the water molecular diffusion properties and calculate the orientation of the principal eigenvector³. Fiber tractography has opened an entire new window to investigate the anatomical connections in brain, kidney, spinal cord^4-6. We systematically investigated the influences of two important experimental factors on the DTI metrics and tractography in knee joint. We aimed to provide a coherent baseline for the different factors that can influence the measurement of DTI in knee joint.

Methods

Two rat knee joints were scanned on a preclinical 9.4T system using a 3D diffusion-weighted spin echo pulse sequence. The scan parameters: TR = 100 ms, matrix size = 400 × 256 × 256, FOV = 18 × 11.52 × 11.52 mm³, 45 µm isotropic spatial resolution. One protocol with b value of 500, 1500, and 2500 s/mm² were acquired separately using 43 diffusion gradient directions. The other protocol with b value of 1000 s/mm² were performed using 147 diffusion gradient directions. Fractional anisotropy (FA) and mean diffusivity (MD) were compared at different angular resolution. Tractography was quantitatively evaluated at different b values and angular resolutions in cartilage, ligament, meniscus, and growth plate.

Results

Figure 1 showed the FA maps of ligament, growth plate, and cartilage at different angular resolution. The FA values were largely overestimated at the angular resolution of 6 for all three types of tissues. Figure 2 illustrated the FA (a) and MD (b) profiles of different connective tissues as the function of angular resolution. FA values reached a plateau when the angular resolution is higher than 14 for ligament, growth plate, and cartilage, while higher than 40 for meniscus. Compare to FA, MD showed less sensitivity to the angular resolution. Figure 3 showed the tractography results in growth plate and meniscus at different angular resolution. Tractography was failed in all the regions with 6 diffusion gradient directions. Tractography with higher angular resolution showed more intact and organized fiber tracts. The similar results were found in articular cartilage and ligament (data not shown). Figure 4 showed the quantitative tractography metrics of different connective tissues in knee joint, including cartilage, growth plate, ligament, and meniscus. Both tract length and volume rose steeply when the angular resolution reached to 40, and then increased mildly when the angular resolution was higher than 40. To investigate the effect of b value, diffusion tractography of ligament at b value of 500 s/mm², 1500 s/mm², and 2500 s/mm² were illustrated in Figure 5, respectively. The tracts and collagen fiber directions were visually comparable at b value of 500 s/mm² and 1500 s/mm². The fiber orientation estimation and the fiber tractography failed at b value of 2500 s/mm², probably due to the lower single-to-noise ratio (SNR).

Discussion and Conclusion

We have systematically explored the effects of b value and angular resolution on DTI performance at different connective tissues in knee joint. The tract length and volume are sensitive to both the b value and angular resolution, which demands care in interpretation. In order to obtain consistent DTI outputs and tractography in knee joint, the scan may require a proper b value (ranging from 500 s/mm2 to 1500 s/mm2) and sufficient angular resolution (14 or higher) with SNR (>10). Please note that changing b values normally will affect the SNR, which should be considered for the setup of DTI acquisitions.

Acknowledgements

This work was supported through the NIH/NIBIB National Biomedical Technology Resource Center P41 EB015897. The work is also supported by Charles E. Putman MD Vision Award of the Department of Radiology, Duke University School of Medicine (Nian Wang and Charles E. Spritzer).

References

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