The aim of this study is to compare the use of FDG PET/CT to Ga-68 HA-DOTATATE (abbreviated DOTATATE) PET/CT in patients with active giant cell arteritis (GCA) started on prednisone to understand if DOTATATE can identify more areas of active blood vessel inflammation than FDG.

The aim of this study is to prospectively evaluate the potential use of 68-Ga HA-DOTATATE PET/CT for detecting medium-large vessel inflammation in a small number of GCA patients with active disease, as compared to FDG PET/CT, and to understand if DOTATATE uptake correlates with disease activity. Fifteen patients with active GCA (either newly-diagnosed or recently relapsed disease) who are receiving glucocorticoids will be enrolled. Participants will undergo baseline FDG PET/CT and baseline DOTATATE PET/CT scan. Tracer uptake will be described in 8 major vascular territories. DOTATATE PET/CT will be repeated again in 6 months, and tracer uptake will be correlated to clinical disease activity, and medication use.

Inclusion Criteria:

Participants must:

1. Meet the revised GCA diagnosis criteria (modified from ref 1)
2. Have either newly-diagnosed or relapsing disease
3. Have active disease (modified from ref 1, to remove ESR/CRP requirements)
4. Have been empirically started on glucocorticoid treatment (or had baseline glucocorticoid dose increased, in the case of relapsers) ≤ 2 weeks of enrolment
5. Participant must be have had (or will be willing to undergo) an urgent FDG-PET/CT as part of routine clinical care.

GCA diagnosis criteria (modified from ref 1, to remove ESR requirement).

1. Age ≥ 50 years

2. And at least 1 of a. or b. :

   1. Unequivocal cranial symptoms of GCA (new headache, scalp or temporal artery tenderness, ischemia-related vision loss, jaw/mouth claudication
   2. Unequivocal symptoms of polymyalgia rheumatica (PMR), defined as shoulder and/or hip girdle pain associated with inflammatory stiffness

3. And at least 1 of a. or b.:

   1. Temporal artery biopsy revealing features of GCA
   2. Evidence of large-vessel vasculitis by angiography or cross-sectional imaging study such as magnetic resonance angiography (MRA), computed tomography angiography (CTA), or positron emission tomography-computed tomography (PET-CT)

Exclusion Criteria:

• Patients not meeting the above criteria or who are unable to provide informed consent will be excluded.