Assessment of Tophus Size A Comparison Between Physical Measurement Methods and Dual-Energy Computed Tomography Scanning

Background: Dual-energy computed tomography (DECT) has recently been described as a sensitive method to detect urate deposits in patients with gout. Objectives: The aim of this study was to compare the reproducibility of DECT with various physical measurement methods of tophus size assessment. Methods: Sixty-four tophi from 25 patients were analyzed. Each tophus was assessed by 2 independent observers using Vernier calipers and tape measure. All patients proceeded to DECT scanning of both feet. Urate volume within index tophi was assessed by 2 independent observers using automated DECT volume assessment software (n = 55 tophi). Five patients returned within 1 week for repeat physical assessment of tophus size. Dual-energy computed tomography scans from the returning patients were scored twice by both observers. Intraobserver and interobserver reproducibility was assessed by intraclass correlation coefficient (ICC) and limits-of-agreement analysis. Results: Overall, DECT was more reproducible than the physical methods with interobserver ICCs for DECT of 0.95, for calipers 0.78, and for tape measurement 0.88, and intraobserver ICCs for DECT of 1.00, for calipers 0.75, and for tape measurement 0.91. Vernier caliper and tape measurements correlated highly with each other (r(s) = 0.84, P < 0.0001) but less well with DECT (for index tophi, r(s) = 0.46, P = 0.004 for both). Large variation was observed in the amount of urate deposits documented by DECT in tophi of similar physical size. Conclusions: Dual-energy computed tomography scanning is a highly reproducible method for measuring urate deposits within tophi. This imaging modality reveals the composition of tophi that contain variable urate deposits embedded within soft tissue.