Different approaches to synovial membrane volume determination by magnetic resonance imaging: manual versus automated segmentation.

Automated fast (5-20 min) synovial membrane volume determination by MRI, based on pre-set post-gadolinium-DTPA enhancement thresholds, was evaluated as a substitute for a time-consuming (45-120 min), previously validated, manual segmentation method. Twenty-nine knees [rheumatoid arthritis (RA) 13, osteoarthritis (OA) 16] and 17 RA wrists were examined. At enhancement thresholds between 30 and 60%, the automated volumes (Syn(x%)) were highly significantly correlated to manual volumes (SynMan) (knees: rho = 0.78-0.91, P < 10(-5) to < 10(-9); wrists: rho = 0.87-0.95, P < 10(-4) to < 10(-6)). The absolute values of the automated estimates were extremely dependent on the threshold chosen. At the optimal threshold of 45%, the median numerical difference from SynMan was 7 ml (17%) in knees and 2 ml (25%) in wrists. At this threshold, the difference was not related to diagnosis, clinical inflammation or synovial membrane volume, e.g. no systematic errors were found. The inter-MRI variation, evaluated in three knees and three wrists, was higher than by manual segmentation, particularly due to sensitivity to malalignment artefacts. Examination of test objects proved the high accuracy of the general methodology for volume determinations (maximal error 6.3%). Preceded by the determination of reproducibility and the optimal threshold at the available MR unit, automated 'threshold' segmentation appears to be acceptable when changes rather than absolute values of synovial membrane volumes are most important, e.g. in clinical trials.