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

Abstract

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/mm2 were acquired separately using 43 diffusion gradient directions. The other protocol with b value of 1000 s/mm2 was performed using 147 diffusion gradient directions. The in-plane resolution was 45 µm 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/mm2). 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/mm2) and sufficient angular resolution (>14) with signal-to-noise ratio >10.

Magnetic resonance and diffusion tensor imaging of the adolescent rabbit growth plate of the knee

“…Previous studies have demonstrated that the height of the hypertrophic and proliferative zones increases during periods of increased growth velocity and that linear growth can be expressed as Growth = N * h max 32 (Growth is the 24 h growth in μm; N is the number of chondrocytes created each day in the proliferative zone; h max is the average height of fully mature chondrocytes [μm] in the hypertrophic zone). Similar patterns in the articular and physeal cartilage have been described in experimental DTI studies in rats 19 , 20 as well in human cartilage. 33 The latter study showed that when DTI was performed perpendicular to the transitional zone, type II collagen appeared sideways in the central portion of the articular cartilage.…”

“…On the other hand, the tract length and volume are strongly dependent on the number of directions, especially when they are lower than 40. 20 The number of directions might explain why the ICC was good‐to‐excellent for the DTI metrics (FA, MD, AD, and RD) and lower for tractography (tract number, volume, and tract length). It is noteworthy that with our technical settings, we still see visible tracts and measurable DTI metrics.…”

“…This further strengthens the thesis that DTI can detect tracts in clinical patients with even larger voxels and lower b ‐values than that previously reported. 17 , 20 , 21 , 22 , 37 …”

“…It has been documented that the FA value does not change so much when the number of directions is more than 14. On the other hand, the tract length and volume are strongly dependent on the number of directions, especially when they are lower than 40 20 . The number of directions might explain why the ICC was good‐to‐excellent for the DTI metrics (FA, MD, AD, and RD) and lower for tractography (tract number, volume, and tract length).…”

“… 16 Further, it has been observed that these columns extend into the metaphysis and can be visualized using DTI. 17 However, few studies have addressed this hypothesis in animals 18 , 19 , 20 or humans. 17 , 21 , 22 , 23 …”