Relaxation anisotropy of quantitative MRI parameters in biological tissues

Abstract

Quantitative MR relaxation parameters vary in their sensitivity to the orientation of the tissue in the magnetic field. In this study, the orientation dependence of multiple relaxation parameters was assessed in various tissues. Ex vivo samples of different tissue types were prepared either from bovine knee (tendon, cartilage) or mouse (brain, spinal cord, heart, kidney), and imaged at 9.4 T MRI with T1, T2, CW-T1ρ, adiabatic T1ρ and T2ρ, and RAFF2-4 sequences at five different orientations with respect to the main magnetic field. Relaxation anisotropy of the measured parameters was quantified and evaluated. Highly ordered collagenous tissues, cartilage and tendon, presented the highest relaxation anisotropy for T2, CW-T1ρ with spin-lock power < 1 kHz, Ad-T2ρ and RAFF2-4. Maximally, anisotropy was 75% in cartilage and 30% in tendon. In the other measured soft tissues, anisotropy was less than 10% for all the parameters. T1 and Ad-T1ρ exhibit little to no observable anisotropy. The results confirm that highly ordered collagenous tissues have properties that induce remarkable relaxation anisotropy, whereas for the other soft tissues investigated, the effect is less prominent. Quantitative comparison of anisotropies between relaxation parameters highlights the importance of sequence choice and design in MR imaging.