Development and validation of a quantitative method for estimation of the urate burden in patients with gouty arthritis using dual-energy computed tomography.

OBJECTIVES

To develop a method that allows approximating the mass of monosodium uric acid (MSU) in a gouty tophus using phantom measurements and including tophus density into the calculation of the dual-energy computed tomography (DECT) tophus volumetry.

METHODS

We prepared specimens of different concentrations of MSU placed in an epoxy-based phantom and an excised porcine foreleg. Density and volumetric measurements were performed in sequential single-source DECT scans acquired with increasingly higher tube currents. We developed a method for estimating the tophus mass by multiplying the detected tophus volume with its mean density and adding a specific gravimetric coefficient k. k was derived from the DECT scans by comparing the approximated MSU masses in the epoxy phantom with the known true MSU masses of the specimens.

RESULTS

Comparison of the approximated MSU masses in the porcine foreleg scans with the true MSU masses of the syringe contents showed similar performance to sole volume measurement while providing additional information on the true uric acid burden: Over 70% of the true urate masses have been detected in MSU concentrations ≥ 45%, while the detection rate was much lower for MSU concentrations ≤ 40%. Retrospective analysis of patients with proven gouty arthritis confirmed the diagnostic potential of the mass approximation technique.

CONCLUSIONS

We successfully established a method to include tophus density measurement for estimation of the uric acid burden in milligrams (instead of ml) in a phantom setting for MSU concentrations above 40%. Future studies should validate its use for follow-up in clinical practice.

KEY POINTS

• Including tophus density measurement in dual-energy computed tomography scans in a phantom setting can be used for estimating the urate burden in milligrams, which might be useful for imaging follow-up. • The mass [mg] of the uric acid burden in a patient with gouty arthritis can be calculated by multiplying volume [ml] with mean density [HU] using a specific gravimetric coefficient. • Retrospective analysis of two patients with gouty tophi showed the relevance of measuring urate mass in addition to urate volume alone.