Dual-energy computed tomography (DECT) is a recently developed advanced imaging technology with a number of clinical applications. For the rheumatologist, the most exciting application is the ability of DECT to specifically detect deposition of monosodium urate (MSU) crystals.
How It Works
Dual-source DECT differs from single-source, single-detector conventional computed tomography (CT) due to simultaneous scanning using two energy sources of different X-ray spectra (80 kV and 140 kV) and two detectors. Evaluation of the different attenuations of the scanned object allows analysis of the chemical composition of materials, which can be color-coded for visualization and measured using automated volume measurement software.
The ability of DECT to detect urate was initially described in the analysis of kidney stones. This imaging method allows noninvasive analysis of the composition of kidney stones, including uric acid stones, with high accuracy. The first case report of DECT for detection of MSU crystal deposits in gout was described in 2007, with a detailed case series of 20 patients with tophaceous gout reported in 2009.1,2 These first reports highlighted the ability of DECT to produce color displays for MSU crystal deposits and to detect deposits that were not apparent on physical examination (see Figure 1).
Practical Use
In clinical practice, a promising role for this technology is in the diagnosis of gout. Gout is a chronic disease of MSU crystal deposition, and microscopic identification of crystals remains the gold standard for diagnosis. However, at times, microscopic diagnosis may not be possible, particularly when synovial fluid cannot be obtained from joints that are difficult to access or due to contra-indications to joint aspiration. Although some clinical presentations of gout are classical (e.g., podagra or subcutaneous tophi), in the absence of microscopic confirmation, diagnosis can be difficult (particularly in the first presentation of an acute flare, or if presenting with chronic gouty arthritis). In this context, noninvasive imaging methods may be of particular benefit for confirming the diagnosis.
A number of studies have examined the diagnostic properties of DECT for gout; these have demonstrated high inter-reader agreement for the presence of MSU crystal deposition using DECT (inter-reader kappa 0.87), with sensitivity of 78–100% and specificity of 97–100%.3-5 The range of sensitivity and specificity in different studies may relate to a number of factors, including what joints were scanned, the number of joints scanned, the analysis protocols used and, most importantly, the stage of disease.
Gout is a chronic disease of MSU crystal deposition, & microscopic identification of crystals remains the gold standard for diagnosis.
Although excellent diagnostic accuracy has been described in people with tophaceous gout, the clinical relevance in this context is limited because this advanced disease stage would not typically require imaging methods for diagnosis. Most studies have not explored the diagnostic accuracy of DECT in people with their first presentation of acute gouty arthritis, or in the context of primary care, where the majority of gout is managed. However, from the limited available data, it appears that in patients presenting with their first episode of acute gouty arthritis, the sensitivity is lower, in the order of 50%.6 The limits of resolution with DECT are 2–3 mm, and DECT cannot detect MSU crystals in fluid <15–20 vol%; therefore, low concentrations or very small aggregates of MSU crystals will not be detected using this method.7
DECT vs. Ultrasound
Ultrasonography (US) is another novel noninvasive method for making the diagnosis of gout. A number of nonspecific features of gout can be observed on US, including synovitis, bone erosion and joint effusion. The characteristic features of gout on US include the snowstorm appearance of synovial fluid, tophi and the double contour sign. This latter finding presents a hyperechoic band over the surface of articular cartilage, which is thought to represent MSU crystals coating the cartilage surface. There is high inter-reader agreement for the US features of tophi and the double-contour sign when assessing acquired images (inter-reader kappa 0.91–1.0). Compared with other forms of arthritis, the US double-contour sign has a sensitivity of 21–100% and a specificity of 95–100%, and the US tophus has a sensitivity of 23–100% and a specificity of 91–100%.
