Detection of changes in articular cartilage proteoglycan by T1ρ magnetic resonance imaging

The purpose of this work is to demonstrate the feasibility of T1ρ-weighted magnetic resonance imaging (MRI) to quantitatively measure changes in proteoglycan content in cartilage. The T1ρ MRI technique was implemented in an in vivo porcine animal model with rapidly induced cytokine-mediated cartilage degeneration. Six pigs were given an intra-articular injection of recombinant porcine interleukin-1β (IL-1β) into the knee joint before imaging to induce changes in cartilage via matrix metalloproteinase (MMP) induction. The induction of MMPs by IL-1 was used since it has been extensively studied in many systems and is known to create conditions that mimic in part characteristics similar to those of osteoarthritis. The contralateral knee joint was given a saline injection to serve as an internal control. T1ρ-weighted MRI was performed on a 4 T whole-body clinical scanner employing a 2D fast spin-echo-based T1ρ imaging sequence. T1ρ relaxation parameter maps were computed from the T1ρ-weighted image series. The average T1ρ relaxation rate, R1ρ (1/T1ρ) of the IL-1β-treated patellae was measured to be on average 25% lower than that of saline-injected patellae indicating a loss of proteoglycan. There was an average reduction of 49% in fixed charge density, measured via sodium MRI, of the IL-1β-treated patellae relative to control corroborating the loss of proteoglycan. The effects of IL-1β, primarily loss of PG, were confirmed by histological and immunochemical findings. The results from this study demonstrate that R1ρ is able to track proteoglycan content in vivo.