Giant cell arteritis (GCA), also known as Horton's disease, is an inflammatory arteritis of the large and medium-sized arteries, with an estimated incidence of 17.8/100,000 in people over 50.

The disease presents potential ophthalmological, neurological, cardiac and aortic vascular complications, making diagnosis an emergency in cases of suspected Horton's disease.

only corticosteroid therapy started as early as possible can prevent these complications.

Diagnosis has historically relied on temporal artery biopsy, but the recent ACR/EULAR 2022 classification criteria propose alternatives to this invasive examination, in particular imaging tests such as temporal artery ultrasound and PET scans. Although not included in these latest recommendations, high-definition wall MRI can also provide arguments in favor of this diagnosis, and avoid the need for a temporal artery biopsy, the sensitivity of which is only 75%. The investigators recently demonstrated in a prospective cohort that wall MRI, possibly coupled with temporal artery ultrasound or retinal angiography, was far superior to temporal artery biopsy in diagnostic performance.

The main limitation of these imaging tests is the lack of data in the literature on the evolution of abnormalities over time, and in particular after initiation of oral corticosteroid therapy. This uncertainty makes it difficult to use these examinations to monitor disease activity, particularly in cases of suspected relapse, a frequent situation in which the clinician is regularly put at fault due to an often frustrating symptomatology and the possible absence of a frank biological inflammatory syndrome.

The investigators propose to conduct a study aimed at describing the evolution of cranial vessel wall abnormalities on wall MRI and ultrasound by systematically repeating these examinations at 1 month, 3 months from the initial MRI performed at diagnosis, in addition to the follow-up performed as part of care at 6 and 12 months from diagnosis. In the event of a relapse in the intervening period, a new MRI scan can be performed and compared with the most recent MRI scan, to look for evidence of disease activity.

Inclusion Criteria:

• Patient aged 50 and over
• Having received informed consent to participate in the study
• Affiliated or beneficiary of a social insurance scheme
• Patients with giant cell arteritis according to ACR/EULAR 2022 criteria
• Diagnosed with MRI and ultrasound.

Exclusion Criteria:

• Absolute or relative contraindication to MRI (incompatible implantable device, claustrophobia, etc.)
• Hypersensitivity to gadobutrol
• Patient under legal protection
• Pregnant or breast-feeding women

Patients treated at the Fondation A de Rothschild Hospital for Horton's disease.