Objectives: Gout is due to monosodium urate (MSU) deposition in joints and soft tissues. Ultrasonography (US) and dual-energy CT (DECT) have been shown to be effective in detecting MSU deposits. Both techniques can examine tophi size. DECT is effective to identify soft-tissue MSU deposits and US can show joint deposition with the double contour (DC) sign. It is unknown if these two techniques provide the same quantification of the extent of urate deposition on a given patient. The main objective of this study is to compare the tophus size measured by US and by DECT. The secondary objective is to evaluate the correlation between the prevalence of the US DC sign and the global volume of urate deposits measured by DECT.
Methodology: This prospective cross-sectional study included patients fulfilling the 2015 ACR/EULAR criteria for gout. Patients underwent US and DECT examinations of their knees and feet. The largest US tophi was selected as the index tophus. US examination of the DC sign was performed on the femoro-patellar joints, talo-crural joints and 1st metatarsophalangeal joints. Total volume of urate deposits of knees and feet was measured by DECT. The primary endpoint was the intra-class correlation coefficient (ICC) of the volume of the index tophus measured by US and DECT [CI 95%].
Findings: A total of 64 patients were included in the study, of which 35 patients presented with at least one US tophus. Patients were in average 64.5±16.3 years old, 84.4% were male, had an average ACR/EULAR score of 13.6±2.5, and disease duration was 12±14.7 years. Overall, 44 patients (68.8%) were currently taking urate lowering therapy and 22 patients (34.4%) had clinical tophi. Out of the 35 US selected largest tophi, 6 tophi were not seen in DECT. Of the tophi identified with both techniques, 21 were localized in the feet and 8 in the knees. The ICC of the tophus volume assessment by US and DECT was 0.45 [0.12-0.69]. The average volume of the largest US tophi was 2.7±6.5 cm3 and 1.5±3.3 cm3 measured by DECT. If the index tophus was localized in the knee, the ICC was 0.36 [0-0.82] and was 0.68[0.37-0.86] if the tophus was in the foot. The Spearman correlation coefficient between the DECT urate volume and the number of joints with a positive DC sign was 0.15.
Significance: US and DECT do not provide the same assessment of tophus size. The correlation is improved when considering tophi localized in the feet. The number of joints with positive DC sign does not correlate to the volume of urate deposition in the soft tissues measured by DECT. These findings have implications both for the assessment of the initial/remaining amount of urate deposition on a given patient which will be different depending on the technique used. The implications regarding follow-up of treated patients are that US and DECT are not interchangeable.