Background:Dual energy CT (DECT) has diagnostic potential in gout patients. DECT can automatically colour-code presumed urate deposits based on radiodensity (Hounsfield Units, HU) and DECT ratio (difference in attenuation between high and low kV series) of lesions. However, other materials may imitate properties of urate deposits, most importantly calcium-containing material, dense tendons and artefacts, which may lead to misinterpretations. The characteristics of DECT lesions in gout patients have not yet been systematically investigated.Objectives:To evaluate the properties and locations of colour-coded DECT lesions in gout patients.Methods:DECT were performed in patients with suspected gout. Patients were separated into gout and non-gout patients based on joint fluid microscopy findings. DECT of the hands, knees and feet were performed using default gout settings and colour-coded lesions were registered. Only location-relevant lesions were analysed (e.g. nail bed artefacts excluded). Mean density (mean of HU at 80 kV and Sn150 kV), mean DECT ratio, size and location of each lesion was determined.Subgroup analysis was performed post-hoc evaluating potential differences in properties and locations of lesions. Lesions were separated into groups according to properties (Figure 1, grey box): 1)Size—to separate artefacts characterised by small volume (possible artefacts). 2)DECT ratios—to separate calcium-containing material characterised by high DECT ratio (possible calcium-containing material). 3)Density—to separate dense tendons characterised by low DECT ratio and low HU values (possible dense tendons). Lesion fulfilling all urate characteristics (large volume, low DECT ratio, high density) were labelleddefinite urate deposits. Finally, for non-gout patients, properties ofnon-gout urate-imitation lesions(properties asdefinite urate deposits) were analysed.Results:In total, 3918 lesions (all lesions) were registered in gout patients (n=23), with mean DECT ratio 1.06 (SD 0.13), median density 160.6 HU and median size 6 voxels (Figure 1, blue box). Lesions were seen in all analysed joints, most frequently MTP1 joints (medial side), knee joints and midtarsal joints (Figure 2a). Tendon affections were also common, especially in the knee tendons (patella and quadriceps), malleolus-related tendons (e.g. peroneus and tibialis posterior) and the Achilles tendons (Figure 2a).Subgroup analyses showed thatdefinite urate deposits(figure 2b) were found at the same locations asall lesionin gout patients (figure 2a), with the four most common sites being MTP1 joints, midtarsal joints, and quadriceps and patella tendons (Figure 2b).Possible dense tendonlesions had a mean HU value of 156.5 HU—markedly higher than expected for dense tendons (<100HU)—and lesion-locations were similar todefinite urate deposits(data not shown), indicating that they primarily consisted of true urate deposits. In contrast,possible calcium-containing materialandnon-gout urate-imitating lesionshad distinctly different properties (ratios 1.33 and 1.20, respectively) (Figure 1, yellow and orange box). Furthermore, the locations of these lesions were different fromdefinite urate depositssince they were primarily found in a few weight-bearing joints (knee, midtarsal and talocrural including malleolus regions) and tendons (Achilles and quadriceps), whereas no lesions were found in either MTP1 joints or patella tendons (figure 2c).Conclusion:DECT color-coded lesions in gout patients are heterogeneous in properties and locations. Subgroup analyses found that locations such as MTP1 joints and patella tendons were characterised by almost only showingdefinite urate deposits. A sole focus on these regions in the evaluation of gout patients may therefore improve specificity of DECT scans.Disclosure of Interests:Sara Nysom Christiansen Speakers bureau: SNC has received speaker fees from Bristol Myers Squibb (BMS) and General Electric (GE)., Felix C Müller Employee of: Siemens Healthineers., Mikkel Ǿstergaard Grant/research support from: AbbVie, Bristol-Myers Squibb, Celgene, Merck, and Novartis, Consultant of: AbbVie, Bristol-Myers Squibb, Boehringer Ingelheim, Celgene, Eli Lilly, Hospira, Janssen, Merck, Novartis, Novo Nordisk, Orion, Pfizer, Regeneron, Roche, Sandoz, Sanofi, and UCB, Speakers bureau: AbbVie, Bristol-Myers Squibb, Boehringer Ingelheim, Celgene, Eli Lilly, Hospira, Janssen, Merck, Novartis, Novo Nordisk, Orion, Pfizer, Regeneron, Roche, Sandoz, Sanofi, and UCB, Ole Slot: None declared, Jakob Møllenbach Møller: None declared, Henrik F Børgesen: None declared, Kasper K Gosvig: None declared, Lene Terslev Speakers bureau: LT declares speakers fees from Roche, MSD, BMS, Pfizer, AbbVie, Novartis, and Janssen.
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