Gout is caused by the deposition of MSU crystals in the joints or surrounding tissues, among which, the MTP1st joint is the most common joint involved in gout . The incidence of gout has been increasing annually with the improvement of living standards and changes in diet . Long-term urate deposition is closely related to diabetes mellitus, coronary heart disease, metabolic disorder, and other diseases, which result in serious complications, including joint destruction . Therefore, the early diagnosis and timely treatment of gout are important for the prognosis of patients with gout. Most of the experimental studies on urate deposition were conducted using dual-source devices [12,13,14, 22]. The current study used single-source systems (DECT GSI: Discovery CT750HD and Revolution CT) to explore the diagnostic performance of DECT in patients with different disease durations and correlate urate deposition score with clinical characteristics. Furthermore, whether artifacts have impacts on gout diagnosis was also investigated.
Dual-source DECT has been reported as a highly accurate tool for gout diagnosis [6, 7, 9]. However, the studies regarding the diagnostic performance of single-source DECT were rare. Glazebrook et al.  conducted a retrospective study on 12 patients with aspiration-proven gout, and reported that the sensitivity and specificity of dual-source DECT are 100 and 89%, respectively. Jia et al.  concluded that the sensitivity of dual-source DECT is lower by 35.7% in patients with onset gout. Zhang et al.  compared the diagnostic accuracies of dual-source DECT and ultrasonography in patients with different gouty disease durations and reported that the sensitivities of DECT for gout within 1 year, 1–3 years, and more than 3 years are 26.6, 66.6, and 90%, respectively. They also revealed that the sensitivity of ultrasonography is remarkably higher than that of dual-source DECT in early gout and suggested ultrasonography as the first choice for the diagnosis of early-stage gout. By contrast, Bongartz et al.  reported a higher sensitivity of 80% using dual-source DECT in 20 gout patients with symptom duration of < 6 weeks. In the present study, the sensitivities of single-source DECT in middle- and late-stage gout were remarkably higher than that in early-stage gout. The outcomes demonstrated that disease duration strongly affects the diagnostic accuracy of single-source DECT, and DECT has limited diagnostic value in early-stage gout.
The results could be explained as follows. First, DECT could only detect the smallest size of 2 mm in diameter and a minimum volume concentration of 15–20% [16, 23]. However, the volume of MSU crystals in patients with short disease course was small and undetected. Thus, the sensitivity of single-source DECT is decreased in the early stages of gout. Wu et al.  and Jia et al.  found a strong relationship between disease duration and MSU crystal volume, which can partly explain the higher diagnostic effect of DECT in patients with late-stage gout. Most studies on gout presented a relatively long disease duration, and the overall high diagnostic accuracy is largely attributed to the high MSU crystal volume in the late stage of gout [12, 14, 24]. Moreover, the deposition of MSU crystals in the early stage of gout is also affected by the active chemotaxis and phagocytosis of leukocytes. The detection of single-source DECT for MSU crystals might be adversely affected by these chemical compositions . Future researches should test the reliability of single-source DECT in early gout and find an optimal imaging approach for the identification of MSU deposits in early-stage disease.
Besides, growing evidence shows that DECT can determine disease activity and therapy efficacy in gout . Bayat et al.  developed a semi-quantitative DECT scoring method for the evaluation of MSU crystal deposits at specific sites in the feet/ankles during therapy, and the scores were highly correlated with urate volumes. Dalbeth et al.  measured the MSU crystal deposition in patients with gout who received stable-dose allopurinol and demonstrated that the higher crystal deposition on DECT was associated with higher SUA and lower allopurinol dose. However, the results of our study showed that SUA level at time of DECT.
are not correlated with the total urate deposit score, which might be attributed to the fact that for some patients with gout, the SUA levels are in the normal range, or even lower . Urano et al.  concluded that the decrease in SUA during acute gouty arthritis was associated with increased urinary excretion of uric acid. In addition, we also found that there was no correlation of use of urate-lowering treatment with the total urate deposit score. Dehlin et al.  showed the suboptimal treatment using urate-lowering treatment in gout and suggested that the efficiencies of urate-lowering treatment with long-term periods were limited for patients who had difficulties in compliance to the advice of doctor, thereby contributing to a health care problem of urate-lowering treatment management. Moreover, well-treated patients are slow “MSU deposit depletors” and still have substantial urate volumes even after 2 years. Patients unevenly reduce their MSU burden after urate-lowering treatment. Thus, adding a density measurement of MSU crystal deposition to the apparent volume assessment may help understand the varying kinetics of MSU burden depletion [29, 30]. In addition, some of the patients enrolled in our study used urate-lowering treatment for a short time after gout diagnosis. These factors above could explain the unrelatedness of urate-lowering treatment use and the total urate deposit scores. Furthermore, the results of our study also showed that the total urate deposit score was higher in patients with longer disease duration and correlated strongly to the presence of tophus, bone erosion, and disease duration. Similarly, Svensson et al.  applied this urate scoring method and found that the amount of MSU deposits is associated with the presence of tophus and disease duration. Dalbeth et al.  found that higher urate deposits are correlated with tophi and bone erosion. The reason why the presence of tophus, bone erosion, and longer disease duration caused higher amount of urate deposition could be explained by the following reasons. First, disease duration is the major contributor to urate deposition; urate deposition increases with disease duration [9, 14]. Second, the presence of tophus is a dominant factor for bone erosion in gout . Moreover, the presence of tophus and bone erosion result in a longer disease duration in patients with gout.
There are differences between single- and dual-source DECT. As for the dual-source DECT, two tube-detector pairs are employed and the tube voltages can be adjustable with the advantage of fast single energy combinations. However, the two separate tubes are offset by approximately 90° to each other, thereby contributing to the material decomposition that is required to be performed only on the image domain because of the spatial offset between acquisitions . In terms of the single-source rapid kilovoltage switching scanners (Discovery CT750 HD and Revolution CT), there are almost no-temporal mismatch and full feilds of view with the X-ray tube switching between 80 and 140 kVp in less than 0.2 ms.  Thus, multiple spectral images are generated by projection-space decomposition. As opposed to dual-source DECT systems, projection-space decomposition has the advantages of greater flexibility in the types of materials that can be used of data to minimize beam hardening artifacts .
DECT has limitations in gout diagnosis. First, the artifacts are commonly observed in the feet and ankles and might interfere with radiologists’ performance and experience. Some scholars believed that tiny scattered green pixelation within tendons may be the result of the subclinical deposition of MSU crystals [6, 18]. Besides, Chen et al.  demonstrated that nail urate could be a proxy for the burden of MSU deposition. We found no statistical difference in the positive detection of nail artifact, skin artifact, vascular calcification, and noise artifact between the case and control groups. Furthermore, the artifacts caused by noise and motion were not seen in the present study, which is probably attributed to the ultrafast reconstruction algorithms by the scintillator of the DECT GSI equipment with fast sampling capabilities (~ 50 μs) . Moreover, the ionizing radiation of DECT is harmful to patients, although the radiation doses of DECT are comparable to or even lower than the dose reference level of body CT . In addition, DECT has been described as a highly accurate tool for the detection and measurement of disease burden and is thus well-suited to evaluate the treatment response of gout. However, the presence of MSU deposits is not necessarily associated with gout, although they increase the risk of its occurrence. Meanwhile, DECT cannot be utilized for the establishment of inflammation. Thus, DECT can diagnose these deposits that contribute to the diagnosis of gout instead of directly diagnosing gout. In the criteria of ACR /EULAR 2015, DECT is one of the criteria, not the only and sufficient one. Moreover, DECT is not widely available and only restricted to certain radiology centers. Nevertheless, DECT provides material characterization via two or more X-ray photon energy-dependent attenuation, which allows the qualitative and quantitative determination of MSU deposition in joints and tissues. Furthermore, DECT performs an excellent visualization of deeper or complex structures and display the anatomic extent of gouty deposits.
This study has several limitations. First, this study was a cross-sectional study and therefore cannot determine the relationship between changes in urate deposition and urate-lowering treatment. Second, no reliable information on the characteristics of gout attack, such as the severity and frequency of attacks, was obtained in the medical records. Third, we did not compare our method with other types of DECT techniques to determine any difference in gout diagnosis. Finally, a few cases were confirmed by arthrocentesis, which is the gold standard for gout diagnosis. Alternatively, we utilized the 2015 EULAR/ACR classification criteria as reference instead of the invasive method.