## 2 Materials and Methods  
Patients with suspected GCA referred to the FTA clinic of the Rheumatology Department, IRCCS Policlinico S. Matteo Fondazione, University of Pavia, Italy, from November 2016 until July 2021 were included in this observational prospective study. Patients can be referred to the FTA by general practitioners and primary care or from other specialist within or beyond our hospital facility. Ethical approval (ref. E 2016 0031606) was obtained, and all patients provided written informed consent. Inclusion criteria were: all consecutive patients referred to the FTA clinic for suspected GCA. There were no specific exclusion criteria as long as the inclusion ones were satisfied. New referrals were defined as patients referred for the first time to the FTA clinic to rule out or confirm a diagnosis of GCA. The diagnosis of GCA was confirmed by an expert rheumatologist based on typical symptoms and signs, laboratory tests, and imaging [US, fluorodeoxyglucose 18 (18 FDG) PET-CT] or temporal artery biopsy (TAB). The same rheumatologists (S.M., P.D.) performed the clinical and US assessments. US was performed by the same rheumatologists, with expertise in the assessment of LVV. Information on relapses, including clinical and US findings, were recorded. Relapse was defined as the recurrence of signs and symptoms of GCA, with or without elevation of the inflammatory markers, with the need to increase the dosage of glucocorticoids (GC) or to modify immunosuppressive treatments. Patients with a confirmed diagnosis of GCA were treated according to European recommendations for the management of patients with GCA, with the initial treatment being glucocorticoids 40-60 mg/day, with adjunctive immunosuppressive treatments mainly in relapsing patients. All patients were assessed clinically for signs or symptoms suggestive of LVV, and with US of the TA and AX arteries. US settings and procedures have been described previously [8; 12]. Briefly, a MyLab Seven (Esaote, Genova, Italy) US machine was used with a high-frequency linear transducer (18-6 MHz). Focus was set at 5 mm for TA and 3 cm for AX. A Doppler frequency of 10 MHz was used. Pulse repetition frequency was set at 2-3 kHz for TA and 3-4 KHz for AX. The color box was adjusted to obtain an angle steer correction \(\leq\)60 degrees. The common, parietal, and frontal branches of the TA and the AX were assessed in longitudinal and transverse views. The presence of a halo was defined in accordance with the standard definitions as a homogeneous, hypoechoic wall thickening, well-delineated towards the luminal side, visible in both longitudinal and transverse planes. The compression sign was assessed in transverse views to confirm the findings in case of halo detection [3]. US was defined as positive in case a halo was found at the level of at least one branch of the TA or at least one AX artery. The minimum, routine assessment performed on all patients included TA and AX ultrasound. In selected cases, according to clinical presentation and US findings on TA and AX, other cranial or extra-cranial arteries could be explored to assess for halo signs. Statistical analysis included the description of categorical variables as numbers and percentages; continuous variables were presented as means and standard deviations (SD). For comparison between groups, the independent samples were used. The \(t\)-test for continuous variables and the chi-squared test for categorical variables were applied. The N-1 Chi-squared tests was used; for smaller samples, the Fisher's exact tests was used. Accounting for an expected US positivity of at least 70% in patients with GCA compared to <5% in patients without GCA, a power of 80%, and a 2-sided type I error of 5%, the number of patients included in the study was sufficient to show a difference between the two groups.