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.
Highlights
Orientation anisotropy describes the dependence of a measured parameter on the orientation of the measured object or material
The fibres have a more random orientation in the transitional zone (TZ), and in the radial zone (RZ), the orientation has turned perpendicular to the surface
Similar to quantitative polarized light microscopy (qPLM) anisotropy, optical retardation was lowest in the transitional zone and increased towards the deep tissue, indicating increasing anisotropy (Fig. 1b,c)
Summary
Orientation anisotropy describes the dependence of a measured parameter on the orientation of the measured object or material. In the case of magnetic resonance imaging (MRI), nuclear magnetic properties of tissue depend on its chemical characteristics and macroscopic structure, and on the physical organization of macromolecules[1]. Organized tissues, such as skeletal muscle[2], tendon[3, 4], white matter[5, 6] and cartilage[7], exhibit different properties when MRI measurements are conducted at different tissue depths or at different physical orientations with respect to the primary magnetic field[8,9,10]. Orientation anisotropy of MRI parameters in cartilage arises from the structure of the collagenous meshwork in which the fibres have a specific depth-wise orientation and organization[7, 12, 13]. The measurement geometry due to joint shapes cannot be controlled and, orientation-independent MRI methods or detailed understanding of the dependence would be necessary to avoid possible false diagnoses and to allow realistic analysis of the structure and state of the health of cartilage
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