Giant cell arteritis (GCA) is the most common vasculitis in the elderly. Accurate diagnosis is of utmost importance in order to then initiate the necessary immunosuppressive therapy. For large-vessel GCA (LV-GCA) involving the aorta and its branches, FDG-PET/CT is the standard in imaging for diagnosis and is recommended by the guidelines. However, this only indirectly visualizes inflammation through vessel wall uptake of glucose. A new PET tracer, 68Ga-pentixafor, is used to visualize the chemokine receptor CXCR4. This receptor is expressed by cells of the immune system. In the context of inflammatory processes, upregulation of CXCL12, the ligand of CXCR4, occurs in affected tissues. The chemotactic effect of this ligand leads to the immigration of CXCR4-positive inflammatory cells into the inflamed area, which can be visualized by PET using the CXCR4-specific tracer 68Ga-Pentixafor. The value of CXCR4-PET should therefore be tested in the context of LV-GCA. This study tests the benefit of CXCR4 in therapy-naïve patients with suspected LV-GCA. For this purpose, patients will receive a FDG-PET and a CXCR4-PET for direct comparison. This is an imaging-only study. Therapy will not be affected by the study. The study is single-arm and not blinded.

Inclusion Criteria:

• New diagnosis of GCA within 5 days of diagnosis or initiation or intensification of therapy.
• Evidence of inflammatory activity based on clinical parameters, elevated serologic markers of inflammation (CRP > 1.0 mg/dl), or a positive imaging finding on vasculitis MRI, CCDS, or FDG-PET/CT
• Ability of the patient to provide information
• Exclusion of contraindications for the performance of a PET/CT examination (see below)

Exclusion Criteria:

• Contraindications for the performance of a PET CT examination
• Pregnancy
• Allergies
• Lack of capacity of the patient to give informed consent