3D Radial Knee Imaging at 0.55 T: Simultaneous T₁-T₂ Mapping and Synthetic Contrast Generation in Under 4 min

Quantitative MRI (T₁-T₂) has been shown to be useful for early diagnosis of osteoarthritis (OA) in the knee at 1.5 and 3 T. However, limited research has been performed at 0.55 T. In this study, we aim to develop and evaluate the feasibility of a 3D joint T₁-T₂ mapping estimation and synthetic contrasts radial sequence for quantitative and anatomical water-fat imaging of the knee at 0.55 T. The proposed sequence consists of a free-running, 3D-golden angle radial trajectory preceded with inversion recovery and $T_2$ -preparation pulses. Reconstruction of images is performed with parallel imaging and high-dimensional low-rank patch-based regularization. A water-fat separation technique is employed to obtain water and fat images, and T₁-T₂ maps can be obtained for water and the echo images via Bloch-equation dictionary matching. The sequence was validated on a standardized phantom and in 16 healthy subjects. Phantom results are in good agreement with SE references, with biases of $1.78$  ms for $T_1$ and $-0.23$ ms for $T_2$ . In vivo, the average mean in the articular cartilage was $T_1=460\pm 45$ ms and $T_2=33\pm 4$ ms, compared to $T_1=394\pm 57$ ms, $T_2=44\pm 10$ ms for the reference sequences. Synthetic contrast images produced with the sequence are comparable to the references in terms of contrast to noise ratio (CNR) and relative contrast metrics. The feasibility of 3D joint T₁-T₂ mapping of the knee at 0.55 T in a single scan of ~3 min was demonstrated, showing promising results for efficient knee imaging. Studies in a larger cohort of patients with osteoarthritis at 0.55 T are now warranted.