Effects of Angular Resolution and b Value on Diffusion Tensor Imaging in Knee Joint

Objective To investigate the influences of the diffusion gradient directions (angular resolution) and the strength of the diffusion gradient (b value) on diffusion tensor imaging (DTI) metrics and tractography of various connective tissues in knee joint. Design Two rat knee joints were scanned on a preclinical 9.4-T system using a 3-dimensional diffusion-weighted spin echo pulse sequence. One protocol with b value of 500, 1500, and 2500 s/mm(2) were acquired separately using 43 diffusion gradient directions. The other protocol with b value of 1000 s/mm(2) was performed using 147 diffusion gradient directions. The in-plane resolution was 45 mu m isotropic. 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 The ligament showed higher FA value compared with growth plate and cartilage. The FA values were largely overestimated at the angular resolution of 6. Compared with FA, MD showed less sensitivity to the angular resolution. The fiber tracking was failed at low angular resolution (6 diffusion gradient directions) or high b value (2500 s/mm(2)). The quantitative measurements of tract length and track volume were strongly dependent on angular resolution and b value. Conclusions To obtain consistent DTI outputs and tractography in knee joint, the scan may require a proper b value (ranging from 500 to 1500 s/mm(2)) and sufficient angular resolution (>14) with signal-to-noise ratio >10.

Automated summary

The study found that the direction and strength of diffusion gradients have significant influences on diffusion tensor imaging and fiber tracking in various connective tissues of the knee joint. Ligaments showed higher FA values compared to other tissues, while MD was less sensitive to angular resolution. Fiber tracking failed at low angular resolution or high b values, and measurements of fiber length and volume were highly dependent on angular resolution and b value.