MRI-based quantification of knee synovial fluid volume validated against an improved reference standard incorporating intraoperative aspiration.

PURPOSE: Synovial fluid volume is an important indicator of intra-articular pathology. Magnetic resonance imaging (MRI) enables noninvasive three-dimensional visualization of joint effusion, but its accuracy has been difficult to validate because conventional aspiration may not completely recover intra-articular synovial fluid. This study developed an MRI-based system for quantifying knee synovial fluid volume and evaluated its accuracy using an improved reference standard incorporating intraoperative aspiration. METHODS: This prospective cohort study enrolled 25 patients undergoing knee arthroplasty. After excluding one patient with motion artifacts and five with blood contamination, 19 patients were analyzed. Synovial fluid volume was quantified from three-dimensional MRI using a threshold defined as the mean signal intensity of femoral cartilage plus three standard deviations. Synovial fluid was aspirated preoperatively and intraoperatively, and the combined volume was used as the reference standard. The relationship between MRI-derived and aspirated volumes was assessed using linear regression analysis. RESULTS: Nineteen patients, including 15 women, with a median age of 74 years were analyzed. Additional synovial fluid was recovered intraoperatively in 17 of 19 patients, including cases with little or no preoperative aspirate. MRI-derived synovial fluid volume showed excellent agreement with total aspirated volume (y = 0.81x - 0.43; R2 = 0.93; p < 0.0001). CONCLUSION: MRI-based synovial fluid quantification, validated against an improved reference standard incorporating intraoperative aspiration, demonstrated excellent agreement with total aspirated volume. This method provides a practical, noninvasive tool for quantitative three-dimensional assessment of knee joint effusion.