Orientation anisotropy of quantitative MRI relaxation parameters in ordered tissue

Abstract

In highly organized tissues, such as cartilage, tendons and white matter, several quantitative MRI parameters exhibit dependence on the orientation of the tissue constituents with respect to the main imaging magnetic field (B0). In this study, we investigated the dependence of multiple relaxation parameters on the orientation of articular cartilage specimens in the B0. Bovine patellar cartilage-bone samples (n = 4) were investigated ex vivo at 9.4 Tesla at seven different orientations, and the MRI results were compared with polarized light microscopy findings on specimen structure. Dependences of T2 and continuous wave (CW)-T1ρ relaxation times on cartilage orientation were confirmed. T2 (and T2*) had the highest sensitivity to orientation, followed by TRAFF2 and adiabatic T2ρ. The highest dependence was seen in the highly organized deep cartilage and the smallest in the least organized transitional layer. Increasing spin-lock amplitude decreased the orientation dependence of CW-T1ρ. T1 was found practically orientation-independent and was closely followed by adiabatic T1ρ. The results suggest that T1 and adiabatic T1ρ should be preferred for orientation-independent quantitative assessment of organized tissues such as articular cartilage. On the other hand, based on the literature, parameters with higher orientation anisotropy appear to be more sensitive to degenerative changes in cartilage.

3D adiabatic T1ρ prepared ultrashort echo time cones sequence for whole knee imaging

“…Recent studies have shown that AdiabT 1ρ is sensitive to both ex vivo enzymatic cartilage degradation and in vivo articular cartilage degradation in OA patients . More importantly, the AdiabT 1ρ is much less sensitive to the magic angle effect than both conventional CW‐T 1ρ and T 2 as demonstrated in previous bovine cartilage studies . Our human patellar cartilage study extends these findings to 3D UTE‐Cones adapted AdiabT 1ρ sequences.…”

“…Therefore, AdiabT 1ρ prepared sequences will also be preferred over CW‐T 1ρ prepared sequences in high field MRI where these effects are more significant. In addition to the HS1 type of AFP pulse used in this study, other RF types such as HS4 and HS8 can also be designed for AdiabT 1ρ preparation . Different T 1ρ characters can be generated by different types of RF pulses.…”

“…Different T 1ρ characters can be generated by different types of RF pulses. For example, T 1ρ generated by HS4 type pulse trains is slightly more sensitive to cartilage degeneration but also more sensitive to the magic angle effect compared with the T 1ρ generated by HS1 type pulse trains …”

“…In addition, AdiabT 1ρ has other advantages over CW spin‐lock sequences. Most notably, adiabatic pulses are less sensitive to the spatial inhomogeneity of the transmit RF magnetic field compared with CW spin‐lock pulses, and the flexibility of AFP pulse design allows moderation of RF power deposition . Moreover, an extended range of frequencies or correlation times are effectively involved in the spin lattice relaxation when using AFP pulses, which may provide more information on the physicochemical mechanisms underlying pathological changes in tissues.…”

A unique anisotropic R2 of collagen degeneration (ARCADE) mapping as an efficient alternative to composite relaxation metric (R2‐R1ρ) in human knee cartilage study

“…Histologically, articular cartilage could be divided into the superficial (SZ), transitional (TZ), and deep (DZ) zones, where collagen fibers are, respectively, orientated in parallel, randomly, and perpendicularly with respect to the cartilage surface . These highly organized collagen fibers, particularly in the DZ, create an anisotropic environment for the vast amount of water in cartilage, resulting in reported MR relaxation anisotropies …”