A joint effusion is an early but nonspecific finding in gout patients [12, 13, 15, 16] . During acute gout, synovial fluid may vary from complete anechogenicity (usually in an early phase of the disease) to aggregates of variable echogenicity (more often after multiple acute attacks) [15, 17–19].

Multiple aggregates of MSU microcrystals are visualized as hyperechoic spots or bright stippled foci and gentle pressure on the skin surface makes them float in the joint cavity, creating a “snowstorm” appearance in some cases [12, 13, 15, 18–22] (Fig. 7.1).

Decrease of the gain could improve the ability to detect hyperechoic structures [13, 14, 18] .

Gout has a predilection for the first metatarsophalangeal (MTP I) joint, with as many as 50–70 % of first gout attacks occurring there [16, 18]. Ultrasound detects more joint effusion than clinical examination especially on dorsal view of the MTP joints [23, 24].

In a recent study, the MTP I as well as the radiocarpal joints were the most frequently involved joints for hyperechoic aggregates in gouty arthritis patients (57.1 % and 38.5 % of patients, respectively). The midcarpal and the knee joints were also frequently involved in patients with gout (28.6 % and 25.3 %, respectively) [16] .

Similar to joint effusion, synovial hypertrophy is not specific to gout, but their association with hyperechoic spots (bright dotted foci) in the synovium is strongly evocative of gout [13, 23] .

Doppler mode (power Doppler, PD; color Doppler, CD) is able to differentiate active, inflamed, from inactive, noninflamed/fibrotic synovial tissue, by estimating the vascular pattern (Fig. 7.2).

While US features of synovial effusion, synovial proliferation, or synovial Doppler signal are not specific for gout, as these US findings are frequently found in any type of inflammatory arthritis, all of them are crucial for the diagnosis of gout and in the monitoring of the patient and therapy. In addition, PD could be a way to monitor acute gouty attacks [25, 26] .

Recent reports have shown that the PD signal disappears with treatment, which suggests that PD could be a useful method to monitor gouty synovitis response to therapy.

Interestingly, other studies reported that the PD signal was even present in asymptomatic joints from gout patients and in asymptomatic hyperuricemia [15, 25, 26], suggesting that subclinical chronic inflammation may be at work [25]. With colchicine therapy, PD signal has decreased and finally disappeared [15] .

The double contour sign (DCS) is a highly specific US feature for gout . This DCS is a result of uric acid (UA) predilection to crystallize forming deposits of MSU microcrystals on the hyaline cartilage surface, which increases the cartilage surface interface giving it a thickness similar to that of subchondral bone [18, 20, 26]. OMERACT MSUS group has given the following definition of the DCS: abnormal hyperechoic band over the superficial margin of the articular hyaline cartilage, independent of the angle of insonation and which may be either irregular or regular, continuous or intermittent and can be distinguished from the cartilage interface sign [28].

Thus, DCS is seen as an echogenic band with an irregular surface detected paralleling the bony contour with an anechoic region between. This anechoic region represents hyaline cartilage [12–14, 16, 23, 32] (Fig. 7.4) .

DCS adheres to the cartilage, moving together with bone and cartilage layer in dynamic moves [13–15].

There is a correlation between the presence of DCS, serum UA level, and the occurrence of UA arthropathy, which suggests that DCS could be a marker of severity [13].

DCS is most frequently detected in the dorsal aspect of the first MT head (61.5 %) and the femoral articular cartilage (41.8 %) and is seen in the majority of joints with intra-articular tophi [16, 23] .

DCS should be searched in the trochleal cartilage of knees (suprapatellar plane in maximal flexion [12, 13, 16, 23, 31] (Fig. 7.5).

It has been reported that the frequency of the “double contour” sign in the knee is higher in clinically affected versus unaffected knees of gout patients, although the difference does not achieve statistical significance [21] . DCS has been noted in patients with an acute gout flare-up, with a history of prior gout attacks, in clinically unaffected joints, and also in patients with asymptomatic hyperuricemia. The sensitivity of this finding ranges from 25 to 95 % in patients with gout [12–18, 20, 21, 23, 25, 26, 30, 33]. False-positive results for DCS are possible in the following circumstances [13, 15, 19, 20, 23, 30, 31] :

Responsiveness of US to cartilage changes in gout has been demonstrated by the disappearance of the DCS in response to urate-lowering therapy (ULT) in gouty patients whose serum UA levels were lowered to 6 mg/dl for 7 months or more, but not in patients whose levels remained above 6 mg/dl [34].

A wide range of sensitivity (e.g., 21–92 %) with very high specificity (98–100 %) for US-detected DCS in gouty arthritis patients have been reported in the literature [12–14, 16, 23, 30–32]. Statistically, The was found to be present in subjects with gout significantly more often than in healthy subjects [14, 20, 23]. Inter-reader reliability assessing DCS was found to be excellent [13, 14, 21, 23, 31] .

Tophi have unique diagnostic importance to gout . They represent deposits composed of MSU crystals core, encased by dense connective tissue [35]. Earlier studies comparing US to MRI in the detection of gouty tophi revealed that MRI confirmed 81 % of the tophi reported by US and US detected 90 % of the reported tophi on MRI [7, 36].

The OMERACT MSUS group defined tophus and aggregations as follows [28]:

Tophus is a circumscribed, inhomogeneous, hyperechoic, and/or hypoechoic aggregation (that may or may not generate posterior acoustic shadow) which may be surrounded by a small anechoic rim. Aggregates are heterogeneous hyperechoic foci that maintain their high degree of reflectivity even when the gain setting is minimized or the insonation angle is changed and which occasionally may generate posterior acoustic shadow. Most commonly, tophi are hyperechoic and always heterogeneous [12–14, 16, 19, 23].

Tophi have been also described by US as “wet sugar clumps” with an oval or irregular shape [14].

Intra-articular and intrabursal tophi have been defined as heterogeneous hyperechoic (relative to subdermal fat) aggregates with poorly defined margins with or without areas with acoustic shadowing within the synovial recesses or bursae, respectively [16] .

Tophi are walled off and their contour is poorly defined [14, 37]. They could be surrounded by a thin anechoic rim. The histopathological equivalent of the anechoic rim is formed by macrophages, lymphocytes, and large foreign body giant cells [37]. This rim allows distinction of tophaceous material from proliferative synovial tissue and hyperechoic joint capsule. At the same time, this halo would be in close contact with subchondral bone in the case of pannus invasion of an erosion [14].

According to the degree of compaction of the deposits, tophi could be divided into soft/inhomogeneous echogenicity and soft to palpation/ hard/ hyperechoic aggregate generating a posterior acoustic shadow and hard to palpation/and mixed/ features of both soft and hard tophus. Tophi that are sonolucent have been termed as “soft tophi.” Hard tophi occur in longstanding tophaceous gout, and generation of posterior acoustic shadow is due to calcification within the tophus/calcified tophi [15, 17, 18, 20] (Fig. 7.6) .

The Doppler US can make the difference between active/”hot” tophi and inactive/”cold” tophi by the presence or absence of the Doppler signal.

Nevertheless, there is a frequent presence of persistent low-grade inflammation in asymptomatic chronic tophaceous gouty arthritis on US [29]. Main sonographic features of tophi are presented in Table 7.2 .

US is able to detect tophi both inside and around the erosions [19]. Naredo et al. found that the assessment of one joint (i.e., radiocarpal joint), two tendons (i.e., patellar tendon and triceps tendon), and three articular cartilages (i.e., first metatarsal dorsal and plantar cartilage, talar cartilage, and either second metacarpal cartilage (dorsal aspect) or femoral condyle cartilage) showed the best balance between sensitivity and specificity (84.6 and 83.3 %, respectively), positive predictive value and negative predictive value (91.7 and 71.4 %, respectively) [16] .