Takayasu arteritis (TAK) is a granulomatous large-vessel vasculitis (LVV) that primarily involves the aorta and its major branches[1,2]. Clinical manifestations of the disease include constitutional symptoms and signs of tissue or organ ischemia due to stenosis or occlusion of large arteries[[3], [4], [5]].

Due to the chronicity of the disease and the risk of developing vascular complications, patients with TAK require regular monitoring of disease activity. Relying on clinical follow-up alone is insufficient as symptoms of TAK often are non-specific and patients may be asymptomatic despite progression of vascular disease[6]. Likewise, laboratory markers of inflammation can be unreliable and difficult to interpret under immunosuppressive therapy, particularly when agents blocking the IL-6 pathway are used[[7], [8], [9]]. Vascular imaging can therefore be a valuable tool to objectively quantify disease activity or progression. Conventional angiography has historically been the gold standard for the diagnosis of TAK since it effectively visualizes damage caused by vessel inflammation, such as stenosis, occlusion, and aneurysm formation. However, it is invasive, involves radiation exposure, and lacks the proper ability to assess vessel wall thickening, an early inflammatory feature of TAK[10]. Consequently, non-invasive imaging modalities have become essential for the evaluation and monitoring of TAK. The 2023 update of the European Alliance of Associations for Rheumatology (EULAR) recommendations on LVV imaging endorsed magnetic resonance imaging (MRI) as the preferred modality for diagnosis, with ultrasonography (US), computed tomography (CT) and 18F-fluorodeoxyglucose positron emission tomography (FDG-PET) as alternatives[11]. However, for disease monitoring, the recommendations consider US, MRI, and CT to be equally appropriate. While MRI, CT and FDG-PET offer the advantage of evaluating multiple vessel segments simultaneously, their use in monitoring disease activity is limited by cost, restricted access, and the need for contrast agents. CT and FDG-PET additionally involve significant long-term radiation exposure[[12], [13], [14]]. US, by contrast, presents an attractive alternative for this purpose: it is readily available at the point of care, can be performed by rheumatologists, is cheap, non-invasive and does not involve exposure to radiation or contrast medium[15]. However, to fully maximize the utility of US for monitoring patients with TAK, standardized assessment protocols are needed.

A subgroup of the OMERACT ultrasound working group, which focuses on US outcome measures for LVV and polymyalgia rheumatica (PMR), recently developed a composite US score for giant cell arteritis (GCA)[16] and intends to develop such a score also for TAK. According to the OMERACT Stepwise Approach to Select and Develop Imaging Outcome Measurement Instruments[17,18], several steps are required to develop a new composite score, including the definition of elementary lesions and their assessment in a reliability exercise. We recently conducted a systematic literature review (SLR) identifying candidate definitions for key US-detected lesions in relevant vessel segments in TAK[19].

The aim of the present study was to develop consensus-based definitions for elementary US lesions characteristic of TAK via a Delphi exercise among international experts, to assess their reliability in a web-based scoring exercise on static images, and to reach agreement on the arterial segments that should be assessed for US monitoring of TAK.