Correlations of low-field NMR and variable-field NMR parameters with osteoarthritis in human articular cartilage under load

Abstract

NMR experiments carried out at magnetic fields below 1 T provide new relaxation parameters unavailable with conventional clinical scanners. Contrast of T₁ generally becomes larger towards low fields, as slow molecular reorientation processes dominate relaxation at the corresponding Larmor frequencies. This advantage has to be considered in the context of lower sensitivity and frequently reduced spatial resolution. The layered structure of cartilage is one example where a particularly strong variation of T₁ across the tissue occurs, being affected by degenerative diseases such as osteoarthritis (OA). Furthermore, the presence of ¹ H-¹⁴ N cross-relaxation, leading to so-called quadrupolar dips in the ¹ H relaxation time dispersion, provide insight into the concentration and mobility of proteoglycans and collagen in cartilage, both being affected by OA. In this study, low-field imaging and variable-field NMR relaxometry were combined for the first time for tissue samples, employing unidirectional load to probe the mechanical properties. 20 human knee cartilage samples were placed in a compression cell, and studied by determining relaxation profiles without and with applied pressure (0.6 MPa) at 50 μm in-plane resolution, and comparing with volume-averaged T₁ dispersion. Samples were subsequently stored in formalin, prepared for histology and graded according to the Mankin score system. Quadrupolar dips and thickness change under load showed the strongest correlation with Mankin grade. Average T₁ and change of maximum T₁ under load, as well as its position, correlate with thickness and thickness change. Furthermore, T₁ (ω) above 25 mT was found to correlate with thickness change. While volume-averaged T₁ is not a suitable indicator for OA, its change due to mechanical load and its extreme values are suggested as biomarkers available in low-field MRI systems. The shape of the dispersion T₁ (ω) represents a promising access to understanding and quantifying molecular dynamics in tissue, pointing toward future in vivo tissue studies.

Keywords: cartilage; low-field NMR; mechanical load; osteoarthritis; quadrupolar dips; relaxometry.