Giant cell arteritis (GCA) causes inflammation of the arteries and can lead to serious complications such as blindness, necessitating rapid diagnosis and treatment. Although older technology non-digital PET/CT scans are routinely used for the diagnosis of GCA in large arteries, they have not been able to reliably detect inflammation of the small arteries responsible for blindness. Recent technological advances have enabled PET/CT imaging of millimetric disease in the body, which are now able to resolve small arteries. In the proposed research study, patients who are suspected by their doctors to have GCA will undergo an ultrasound of the temporal arteries, and digital PET/CT scan after injection of radioactive glucose. Digital PET/CT scans will be interpreted for the presence of abnormal uptake in the large and small arteries, as well as for the presence of other causes of the patient's symptoms. The diagnostic accuracy of PET/CT and ultrasound will be evaluated with respect to an expert panel diagnosis of giant cell arteritis and compared. Results will be adjusted for lack of a perfect reference test using advanced statistics. The goal will be to see if digital PET/CT can become a single, integrated test to diagnose this disease.

Inclusion Criteria:

• Age ≥ 50 years, capable and willing to provide informed consent and can tolerate PET/CT.
• ESR ≥ 50 mm/h and/or CRP ≥ 10mg/L within 1 week of PET/CT referral.
• New suspected giant cell arteritis according to at least one of the following criteria:

Cranial GCA symptoms (new-onset localized headache, scalp or temporal artery tenderness, ischemia-related vision loss, masseter pain on prolonged mastication) PMR symptoms: shoulder and/or hip girdle pain associated with inflammatory stiffness.

Suspected large-vessel vasculitis based on angiography, MRA, or CTA.

Exclusion Criteria:

• Initiation of corticosteroid or immunosuppressive therapy >4 days before PET/CT.
• Prior TAB or treated GCA with suspected relapse.
• Non-fasting or hyperglycemia (>11.1 mmol/L) resulting in altered FDG biodistribution.

Patients with suspected GCA