Gout is one of the most common inflammatory joint diseases [5] and is characterized by the deposition of monosodium urate crystals in synovial fluid and tissues, with disorders of purine metabolism, hyperuricemia, and excessive activation of inflammatory vesicles as its main pathological basis [6]. Clinical manifestations include recurrent acute arthritis due to urate crystal irritation; chronic arthritis due to long-term recurrent irritation by urate crystals, gout stone formation, and joint deformity; and gouty nephropathy, in addition to hyperuricemia [7]. In recent years, the number of gout patients has increased significantly due to rapid economic development, changes in people's lifestyle and diet, and the utilization of medications, such as diuretics [8]. In addition, gout patients are often accompanied by abnormalities in body metabolism and are prone to the combination of hypertension, obesity, hyperlipidemia, type 2 diabetes, and metabolic syndrome, leading to an increased incidence of cardiovascular diseases, such as heart failure, coronary heart disease, pulmonary embolism, deep vein thrombosis, and stroke [9]. Patients' quality of life gradually decreases as the disease progresses, and gout is also positively associated with depression [10]. Therefore, early diagnosis and timely intervention of gout are particularly important to reduce complications and joint dysfunction [11].
Elevated blood uric acid levels are an important indicator for the diagnosis of gout. However, only 5–12% of hyperuricemia develops into gout, and about 40% of people with acute gout do not have elevated blood uric acid levels. This makes it more challenging to diagnose gout based on hyperuricemia alone. The specific manifestation of gout diagnosis is urate crystal deposition, and the previous gold standard for gout diagnosis was synovial fluid aspiration or gout stone microscopy. The 2018 edition of the EULAR guideline also recommends synovial fluid examination and urate crystal microscopy whenever available or when the diagnosis is unclear [12]. However, this method is an invasive operation with low patient acceptability and requires a high level of physician skill for puncture and microscopy. In addition, some small joints are difficult to sample, and the technique may be impossible to perform, leading to an increase in false negatives and possible bleeding and infection. X-rays, CT, MRI, and ultrasound are also helpful in the diagnosis of gout. However, X-rays can only provide a diagnostic basis for more extensive bone destruction. In the early stages of a gout attack, X-rays only show the swelling of soft tissue around the joints, often with no abnormal relationship between the joints, and they cannot effectively distinguish urate crystals from surrounding tissues. Therefore, X-rays are less sensitive in diagnosing early-stage gout and are only sensitive to nodules with more obvious calcification in middle and late-stage gout [13]. Conventional CT can show subtle changes in bone structure and can clearly show gout stones. However, this technique does not show the structures in the bone marrow cavity and the surrounding soft tissue and has some limitations for early gout diagnosis. MRI has the advantage of high tissue resolution and can detect intra-marrow cavity changes, synovitis, cartilage damage, and swelling of the soft tissue around joints in early-stage patients and can image multiple angles, directions, and sequences. However, its specificity is poor, its display rate of calcification is low, it cannot effectively distinguish gouty arthritis from other inflammatory joint diseases, and it is time-consuming and expensive [14]. Ultrasound can show tiny urate crystals, synovitis, synovial hyperplasia, joint effusion, and bone destruction and can distinguish urate from pyrophosphate crystals, which is important for the differential diagnosis of gout. Ultrasound is radiation-free, inexpensive, and easy to observe dynamically. However, with a more limited field of view, ultrasound is highly dependent on the physician's operating technique. Ultrasound methods also demonstrate somewhat poor reproducibility, inaccurate measurement of gout stone volume, and an inability to accurately assess the relationship between gout stones and surrounding tissues [15].
DECT is a new noninvasive diagnostic imaging technology developed in recent years, which can distinguish between different substances according to their energy attenuation characteristics. Based on the three-material decomposition algorithm, DECT can scan the examined area simultaneously and rapidly using two sets of bulbs with different energy outputs. The two sets of detectors, corresponding to different tissue attenuation characteristics, will acquire two different sets of data, which will be transferred to the supporting workstation for three-dimensional (3D) reconstruction to obtain a set of tissue-specific images with relatively complete information. Different colored markers are used to distinguish between different substances. In general, the bone cortex is marked in blue, the bone cancellous in purple, and the urate crystals in green. This provides a basis for the qualitative diagnosis of the disease and enables physicians to distinguish the urate crystals from the surrounding tissues and accurately assess the site, quantity, and extent of their deposition. In addition, the combination of dual-energy 3D fusion subtraction images and software measurement tools can measure and calculate the volume of urate crystals, identify the smallest crystals less than 3 mm in diameter [16], and calculate the total amount of urate crystals deposited in a single joint or in the entire scan area. By permitting assessment of the urate crystal load deposited in the joint area, DECT plays a role in quantitative analysis and provides an important basis for early diagnosis and dynamic monitoring of the disease. The above features have led to the necessity and importance of DECT examinations being highlighted in the 2018 EULAR recommendations for gout diagnosis [12].
In this study, among 83 patients with suspected gout, 65 were diagnosed with gout, and 18 were diagnosed with non-gout by synovial fluid aspiration or gout stone microscopy. In comparison, 63 were diagnosed with gout, and 20 were diagnosed with non-gout by DECT imaging. The sensitivity of DECT imaging for the diagnosis of gout was 92.31%, the specificity was 83.33%, the negative predictive value was 75.00%, and the positive predictive value was 95.24%. The results were consistent with those of Yu et al. [17]. They performed DECT and gold standard examinations on 64 patients with suspected gout and confirmed that 58 of them were gout patients, with a sensitivity of 93.1% and a specificity of 100.0%. The possible reasons for false negatives were the initial onset of the disease, the duration of the disease being less than four weeks, and the effect of uric acid-lowering therapy. The possible reason for false positives was advanced knee osteoarthritis in some patients. In this study, the kappa value was 0.73, and the area under the ROC curve was 0.88 (95% CI, 0.77–0.99, P < 0.01), indicating that DECT imaging technology has good sensitivity, specificity, and accuracy in the diagnosis of gout, and is in strong agreement with the gold standard and has a high diagnostic value. As the DECT examination is noninvasive, rapid, and less operator-dependent, it can replace the gold standard to a certain extent. Therefore, DECT can be used as an important noninvasive diagnostic tool for gout. In this study, DECT detected 502 urate crystals, which was 4.40 times more than the number detected by clinical physical examination (114), and was consistent with the findings of Duan [18] and Choi [19]. This indicates that DECT has a stronger and more sensitive ability to detect urate crystals and can identify some urate crystal deposits that are difficult to detect earlier by physical examination, providing a basis for timely clinical intervention. The detection rate of urate crystals and bone destruction in patients with chronic intermittent gout was higher than that in the acute stage, indicating that DECT is more effective in detecting urate crystals in patients with chronic intermittent gout. This may be related to the longer disease duration and larger urate crystal deposits in patients with chronic intermittent gout. In future studies, the sample size can be expanded to verify these results further. It was found that the location of gout stone deposition has a sizable impact on the formation and severity of bone destruction, and the more urate crystals deposited in the bone, the more severe the bone destruction. In general, it is more common for bone destruction to be located outside the joint than inside the joint. Intraosseous gout stones have the greatest impact on the formation of bone destruction, followed by intra-articular gout stones. Intra-soft tissue gout stones have the least impact [20]. Chhana et al. [21] performed pathological examinations and DECT on two cadavers: an 82-year-old gout patient and an 89-year-old patient without gout. The comparative results showed that the DECT findings were consistent with the pathological findings and that there was a high correlation between bone destruction and the location of the gout stones.
The shortcomings of this study are as follows: (1) the sample size was insufficient, and there may be errors between the results and the overall; (2) the follow-up time was short, and the results of urate crystal volume changes were not collected and analyzed; and (3) there was no comparison with other imaging techniques. In the follow-up study, we should strengthen multi-center cooperation, increase the sample size, use multiple methods for comparison or joint examination, and follow up the investigation study for a more extended period.