While 18F-fluorodeoxyglucose (FDG) positron emission tomography (PET) imaging is often included in the diagnostic work-up of patients with large-vessel vasculitis (LVV), 18F-FDG lacks specificity for inflammatory cells and has limited ability to track therapy response. Moreover, high background 18F-FDG uptake in the brain and myocardium largely precludes imaging temporal arteritis in giant-cell arteritis (GCA) and coronary artery involvement in Takayasu arteritis respectively. These limitations of 18F-FDG for imaging LVV highlight important unmet clinical needs, which might be overcome by using a somatostatin receptor subtype-2 (SST2) PET tracer.

Up-regulation of SST2 in activated macrophages represents a novel imaging target for measuring vascular inflammation, which has been previously examined in atherosclerosis using 68Ga-DOTATATE. To test the hypothesis that SST2 PET imaging can accurately identify LVV, patients with active GCA or Takayasu arteritis will undergo vascular 68Ga-DOTATATE or 18F-fluoroethyltriazole-(Tyr3)-octreotate (FETO) PET-MRI at baseline, with repeat imaging after 6 months of treatment. A group of individuals with LVV in clinical remission will also undergo SST2 PET imaging. Data from patients with clinically inactive disease will serve to confirm tracer specificity for active disease, as well as signal reproducibility. 18F-FETO is an alternative SST2 tracer to 68Ga-DOTATATE; the longer half-life and shorter positron range of 18F compared to 68Ga may offer several advantages, including wider tracer availability and improved spatial resolution when imaging small arteries. All patients will also undergo 18F-FDG imaging before treatment, where clinically indicated.

Inclusion Criteria:

• Male or female participants >18 years old

• Able to give written, informed consent and to lie flat

• Either:

  1. New clinical diagnosis or acute flare of LVV (Giant-cell arteritis or Takayasu's arteritis) within ~1 week of treatment initiation, and
  2. Clinical indication for 18F-FDG PET-CT scan determined by the referring physician, or
  3. Undergoing surgery for LVV, or
  4. Diagnosis of LVV in remission

Exclusion Criteria:

• Women of child bearing potential not using adequate contraception
• Contra-indication to MRI scanning
• Contrast allergy or contrast-nephropathy
• Chronic kidney disease (eGFR <30 mL/min/1.73 m2)
• Any medical condition, in the opinion of the investigator, that prevents the participant from lying flat during scanning, or from participating in the study
• History of recent malignancy deemed relevant to the study by the investigator

Patients with LVV