Orientation dependence for $$$T_2$$$ relaxation time in articular cartilage has been ascribed to residual dipolar interactions of water protons due to their restricted spatial arrangement within the collagen fibrils1. For continuous-wave (CW)-$$$T_{1\rho}$$$, reduction of orientation sensitivity in cartilage has been reported for increasing spin-lock powers2. Recently, other rotating frame relaxation (RFR) methods including adiabatic $$$T_{1\rho}$$$, adiabatic $$$T_{2\rho}$$$ and $$$T_{\rm RAFF}$$$ have been reported sensitive to cartilage degeneration3,4, and briefly investigated for their orientation sensitivities5,6. The purpose of this study was to investigate the orientation dependence of these parameters and evaluate against collagen fibril orientation as measured by polarized light microscopy (PLM).Cylindrical osteochondral plugs ($$$n=2$$$, diameter=6mm) from bovine patella were prepared. The samples were placed inside a custom-built holder, which allowed rotation of the specimens with respect to the $$$B_0$$$ from outside the scanner, and immersed in perfluoropolyether. MRI was performed at 9.4 T using a 19 mm quadrature RF volume transceiver and VnmrJ3.1 Varian/Agilent DirectDrive console. The samples were imaged at seven different orientations, spanning 0-90 degrees with respect to $$$B_0$$$. The orientation was confirmed from scout images and later precisely measured from 3-D gradient echo (GRE) data acquired for every orientation. Relaxation time measurements were realized using a global preparation block coupled to fast spin echo (FSE) readout (TR=5s, ESP=5.5ms, ETL=8, matrix=256x64, FOV=16x16mm, 1mm slice, resolution along cartilage depth 62.5µm). The measurements included $$$T_1$$$ relaxation time with inversion recovery, $$$T_2$$$ with spin echo preparation, CW-$$$T_{1\rho}$$$ with four spin-lock amplitudes ($$$\gamma B_1$$$=250, 500, 1000 and 2000Hz), adiabatic $$$T_{1\rho}$$$ with HS1, HS4 and HS8 pulses ($$$\tau_{\rm p}$$$=4.5ms, and $$$\gamma B_{\rm 1,max}$$$=2.5, 1.2 and 1.04kHz, respectively, to match RMS power between the pulse shapes), adiabatic $$$T_{2\rho}$$$ with HS1-train embedded between adiabatic half passages and $$$T_{\rm RAFF}$$$7 ($$$\gamma B_1$$$=625Hz, $$$\tau_{\rm p}$$$=9ms). Finally, $$$T_2^*$$$ relaxation time was measured in the same slice using multi-echo-GRE sequence. All measurements were repeated for every orientation. After the MRI studies, the samples were fixed in 10% formalin for 48 hours and then decalcified in EDTA. After decalcification, unstained histological sections were prepared from the location of the MRI slice and digested with hyaluronidase enzyme for 18 hours to remove proteoglycans before PLM measurements. The collagen fibril orientation in the sections was measured using quantitative Abrio PLM imaging system. Finally, depth-wise profiles of relaxation times and collagen fibril orientation in the cartilage were obtained.$$$T_1$$$ relaxation time profiles showed the least variation with respect to tissue orientation. Similarly, adiabatic $$$T_{1\rho}$$$ with HS1 pulse modulation and CW-$$$T_{1\rho}$$$ at 2 kHz spin-lock amplitude demonstrated minimal orientation dependence (Figure 1). On the other hand, by using HS4 or HS8 pulses for adiabatic $$$T_{1\rho}$$$, or decreasing the spin-lock power of CW-$$$T_{1\rho}$$$, the orientation dependence was increased (Figure 2). $$$T_2$$$, $$$T_2^*$$$, $$$T_{\rm RAFF}$$$ and adiabatic $$$T_{2\rho}$$$ demonstrated significant orientation dependence (Figure 3). PLM revealed typical tri-laminar collagen orientation in the samples, starting with fibrils oriented along the cartilage surface and arching towards radial orientation at the cartilage-bone interface (Figures 1 and 2).Orientation dependence of several quantitative traditional and RFR MRI parameters in articular cartilage was investigated. The findings confirmed earlier reports on the sensitivity of $$$T_2$$$ relaxation time1, and the reduction of the orientation sensitivity with increasing spin-lock power for CW-$$$T_{1\rho}$$$2,8. The orientation dependencies of adiabatic $$$T_{1\rho}$$$, adiabatic $$$T_{2\rho}$$$ and $$$T_{\rm RAFF}$$$ have been investigated previously for articular cartilage5; however, no comparison with the actual gold standard, i.e, PLM has been reported. The present results confirmed the orientation dependence of $$$T_{\rm RAFF}$$$ and adiabatic $$$T_{2\rho}$$$ to be very similar with that of $$$T_2$$$ or $$$T_2^*$$$. Furthermore, an increase in the dependence for adiabatic $$$T_{1\rho}$$$ was observed with increasing pulse stretching factor, although the dependence was relatively low for all pulse shapes. Native $$$T_1$$$ relaxation and adiabatic $$$T_{1\rho}$$$ with HS1 pulses were found to be essentially independent of the tissue orientation. Similarly, CW-$$$T_{1\rho}$$$ at 2 kHz spin-lock power was found to be nearly independent of the orientation. However, increasing the spin-lock power of CW-$$$T_{1\rho}$$$ quickly leads to SAR values that are not clinically feasible (typically up to ~500Hz clinically applicable). In conclusion, adiabatic $$$T_{1\rho}$$$, also reported sensitive to cartilage degeneration3,4,9, appeared as a promising, minimally orientation-dependent quantitative MRI parameter for the articular cartilage, as it is more flexible with respect to overcoming SAR issues. For native $$$T_1$$$ relaxation time, sensitivity (although somewhat variable) to degeneration has been reported, but in terms of SAR and orientation dependence it appeared as the most promising parameter.