IntroductionCompared with clinical examination (CE), ultrasonography (US) provides additional and more accurate assessment of inflammation and visualization of structural damage. To better understand the effectiveness of US in metatarsophalangeal joints (MTPJs), we compared disease activity in MTPJs 2–5 assessed by CE and US, with magnetic resonance imaging (MRI) as reference standard.MethodTreatment-naïve adult patients with early RA (ACR criteria, disease duration < 2 years) were consecutively recruited. MTPJs 2–5 were assessed for swelling and tenderness, and imaged by US (Esaote MyLab70). The most symptomatic foot was imaged by peripheral MRI (1.0 Tesla). US was semiquantitatively graded for synovial thickening (ST) and power Doppler (PD) (0–3), and erosions (yes/no). MRI was semiquantitatively graded for bone marrow edema (BME), synovitis, and erosions (OMERACT). Kappa agreement, sensitivity, specificity, and predictive values were analyzed using cut-offs at ST ≥ 2, PD ≥ 1, and MRI synovitis and BME at both ≥ 1 and ≥ 2.ResultsThis study included 39 patients (85% female, mean (SD) age = 51.6 (10.3)). Using MRI synovitis and BME grade ≥ 2 as the reference, PD had superior sensitivity (82%) and kappa agreement (k = 0.43) than swollen joint count (55%, k = 0.20), but similar high specificity (88%, 83%). ST and PD were often observed in clinically asymptomatic MTPJs. US detected very few MRI erosions, but several observed erosions corresponded to grade ≥ 2 MRI erosions.ConclusionClinical swelling and PD are highly specific for active inflammation in the MTPJs. US supplemented CE by allowing observation of subclinical inflammation and structural damage.Key Points• Ultrasonography detected many subclinical synovial inflammations in metatarsophalangeal joints (MTPJs), many confirmed by MRI• Ultrasonography may best be used clinically to supplement clinical examination by assessing non-swollen joints• Ultrasonography provided quick method of visualizing bone erosions that were grade ≥ 2 on MRI