This prospective, HIPPA-compliant study received institutional review board approval, and written informed consent was obtained from all participants.
Between 2015 and 2016, 180 patients with a history of gout fulfilling the American Rheumatism Association classification criteria  for gouty arthritis of the hand and/or wrist were included. The initial diagnosis was based on clinical symptoms including swelling of the wrist/hand and/or hotness, redness or tenderness for 1–4 weeks’ duration, as well as serum urate levels. Patients were referred to DECT and US within 1–3 days by the rheumatology department, after being examined by a rheumatologist with 15 years of experience (ASK) .
A single symptomatic hand/wrist was evaluated for each study participant. Patients with history of inflammatory rheumatic disease, hand/wrist trauma or previous hand/wrist surgery were excluded.
All patients were referred to dual-source CT scan (Somatom Definition Flash; Siemens Healthineers) using two different energy levels (80 and 140 kV), with a previously described dual-tube protocol .
Scan parameters included a 2 × 64 × 0.625 mm acquisition at a rotation time of 0.33 s. Transverse sections were reconstructed from the DE data sets, at a resolution of 0.4 mm using the soft tissue kernel (D30) and bone kernel (B60). Tube currents ranged from 100 to 140 mAs for tube A and between 200 to 250 mAs for tube B, based upon automatic care dose CT software. The soft tissue kernel data sets of both tubes were loaded onto a Syngo Multi-Modality Workplace (Siemens Healthineers) and reconstructed with a commercially available software program (DE Gout; Siemens Healthineers). This software provides color-coded images in the transverse, sagittal and coronal image planes, with a slice thickness of 0.75 mm at a slice increment of 0.5 mm. The CT examination extended from the distal forearm to the fingertips.
A commercially available picture archiving and communication system (PACS) was used (IMPAX; Agfa-Gevaert).
Color-coded DECT images were evaluated by a radiologist with 5 years of experience (SS), who was blinded to clinical data and US examination and who classified the findings as positive or negative for the presence of MSU deposits. A positive scan was defined as the presence of color-coded MSU deposits according to Huppertz et al. . In addition, we included a detailed assessment of the location of MSU deposits to differentiate intra- and extra-articular deposits as follows:
Intra-articular: triangular fibrocartilage complex, distal radioulnar joint, radiocarpal joint, intercarpal, carpometacarpal joint, metacarpal joint, proximal interphalangeal joint, distal interphalangeal joint, erosion, and double contour sign. Extra-articular: extensor and flexor tendons, and lymphedema.
According to American College of Rheumatology (ACR)/European League Against Rheumatism (EULAR) guidelines nail bed deposits, sub-millimetre deposits, skin deposits, and deposits obscured by motion, beam hardening and vascular artefact were not classified as positive findings in our study .
Sonographic examination was performed according to European Society of Musculoskeletal Radiology (ESSR) guidelines, using a 15–6 MHz linear array transducer (HI Vision Preirus, Hitachi Aloka Medical) by a musculoskeletal radiologist with 10 years of experience (ASK), blinded to DECT and clinical findings and not allowed to talk to the patient regarding further details.
A positive US diagnosis was provided in the presence of a double contour sign (DCS) or based upon visible aggregates or tophi and tendon depositions according to international consensus OMERACT (outcome measures in rheumatoid arthritis clinical trials) guidelines .
The DCS was characterized by an abnormal hyperechoic band over the superficial margin of the articular hyaline cartilage, independent of the angle of insonation. This band might be irregular or regular, continuous or intermittent, and could be distinguished from the cartilage interface sign and from CPPD deposits located within the cartilage .
A tophus was a circumscribed, inhomogeneous, hyperechoic and/or hypoechoic aggregation (which may or may not generate posterior acoustic shadow), which might be surrounded by a small anechoic rim .
Aggregates (which might be intra-articular or intratendinous) were heterogeneous hyperechoic foci that maintained a high degree of reflectivity even when the gain setting was minimized or the insonation angle was changed and which occasionally generated posterior acoustic shadow .
Intra-articular US findings included DCS, aggregates, tophi and erosions. For extra-articular US findings extensor and flexor tendons were evaluated.
Absence of the above described findings was tabulated as a negative US study, while any detected finding that did not match with the typical criteria of OMERACT guidelines was considered indeterminate (e.g. interrupted or very thin double contour line or hyperechoic unclear deposits, which might represent calcifications in tendons or might be caused by osteoarthritis). Indeterminate US findings were tabulated as missed by US relative to DECT.
Clinical assessment was performed in patients presenting with acute hand/wrist arthritis having a history of gout based on the American Rheumatism Association classification criteria . Serum uric acid levels and CRP were determine at the time of US and DECT examination according to ACR/EULAR guidelines and were rated respectively as elevated above 6 mg/dl  and CRP above 0.5 mg/dl. Both DECT and US operators were blinded to clinical and laboratory findings, as well as to each other’s imaging findings.
Statistical analysis was performed with Stata 12.1 (StataCorp). The presence of positive findings for gout was tabulated for each location in the hand/wrist for each DECT and US. DECT and US findings were observed and recorded independently. The sensitivity and specificity of US were calculated with DECT as the gold standard. Since the US and DECT studies were performed on the same patients (paired study design), the frequency of positive findings for gout based upon US versus DECT was compared with a McNemar’s chi square for symmetry, with a p value of 0.05 indicating a statistically significant result. Three comparisons of frequency of positive findings for gout were performed between DECT and US: frequency of overall diagnosis, frequency of extra-articular diagnosis and frequency of intra-articular diagnosis.