Background/Purpose: Dual-Energy computed tomography (DECT) and ultrasound are the two techniques able to identify and measure monosodium urate (MSU) crystal deposition, and both are considered for monitoring crystal depletion during urate lowering therapy (ULT). Particularly, both techniques are able to measure tophi, but it is unknown if their assessment of tophus size are correlated throughout follow-up. The objective of this study was to determine if the evolution of tophus volume measured with DECT and ultrasound were correlated during the first year of ULT.

Methods: Gout patients naïve of ULT and presenting with at least 1 ultrasound tophus of the knees or ankles/feet were recruited in three French referral centers to undergo DECT scans, and were followed-up with ultrasound and DECT scans at months 6 and 12. The largest tophus in ultrasound was considered as the index tophus and measured with both techniques. The primary endpoint was the correlation between the evolution of the volumes of the index tophus measured with DECT and ultrasound assessed by the Spearman coefficient correlation (ρ) and its 95% confidence interval at months 6 and 12.

Results: A total of 41 patients were included; they were mostly males (83.3%) and aged in average 63.7 years (±12.3). Baseline serum urate levels were 515.2 µmol/L (±85) (=8.6mg/dL) and 58.3% had subcutaneous tophi. Most index tophi were localized at the 1^st MTP joint (53.9%) and had a volume of 0.87 cm³ (±1.39). 17 patients were lost to follow-up and 4 patients had index tophi that were not detected by DECT at baseline. The relative decrease of the volume of the index tophi at months 6 and 12 measured with ultrasound was -50% [-79.2%;0%] and -75.6% [-94.2%;-25%], respectively, while the decrease measured with DECT was -32.1% [-85.8%;0%] and -37.1% [-88.5%;0%], respectively.

The correlation between the volumes of the index tophi measured with ultrasound and DECT was poor with ρ=0.29 [-0.22;0.65] at month 6 and ρ=-0.03 [-0.66;0.65] at month 12. The correlation between the decrease of the DECT tophus index volume and the global volume of MSU crystal deposition measured with DECT was very good at both months 6 and 12 with ρ=0.82 [0.55;0.93] and 0.83 [0.5;0.96]. The correlation between the decrease of the volume of the index tophus measured with ultrasound and the decrease of the volume of overall MSU crystal deposition measured with DECT was at best moderate with ρ=0.35 [-0.16;0.75] at month 6 and 0.42 [-0.44 ; 0.82] at month 12.

Conclusion: Ultrasound and DECT do not provide the same assessment of MSU crystal depletion during the first year of ULT. The two techniques are not inter-changeable when considering monitoring gout treatment with imaging, especially when considering the follow-up of a target tophus.

Figure 1. Evolution (in % from baseline) at month 6 of the index tophus measured with dual-energy computed tomography (DECT) and ultrasound.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/change-in-tophus-size-measured-with-dual-energy-ct-and-ultrasound-a-1-year-multicenter-follow-up-study/