How to choose imaging modalities in large vessel vasculitis

Different types of imaging in large vessel vasculitis have specific characteristics which should be known to make an informed decision on which modality to use.

Regular monitoring of disease activity in large vessel vasculitis is recommended by American and European guidelines [1]. Monitoring can be performed with inflammatory markers and imaging. It is now known that new lesions may form despite sustained clinical and laboratory remission, as shown by autopsy findings that demonstrate ongoing vascular inflammation even in patients in clinical remission. Thus, information about the vessel wall is needed to identify inflammation before the damage has occurred.

Ultrasound, MRI/magnetic resonance angiography (MRA) and fluorodeoxyglucose (FDG) PET/CT are modalities that can be used to monitor disease activity. Ultrasound has the advantages of being non-invasive and inexpensive. New stenosis or wall thickening (suggesting active disease) and the response to treatment can be detected with ultrasound. The halo sign disappears between 2 days and several weeks, especially in cranial vessels, but wall swelling can persist in larger extracranial arteries for months to years. Ultrasound has many advantages: it can be done at the bedside, is inexpensive, non-invasive, and can visualise axillary arteries. However, it cannot assess vessels behind bone, and it is limited by sonographer expertise and resolution of the machine.

MRI/MRA provides a look into the lumen and the vessel wall, having the potential to identify vascular stenosis, occlusions, aneurysms, ectasia, and mural thickening. It provides information on disease extent, damage, and activity, and does not involve radiation or iodinated contrast. Nonetheless, in a study, it did not correlate with clinical disease activity and >50% of patients in clinical remission had active disease by MRA. MRI visualises the aorta and is cheaper and more accessible than FDG PET/CT. On the other hand, cranial and large vessels cannot be imaged sufficiently in one examination. Besides, there are the known contraindications of MRI (e.g. pacemakers) and the higher costs compared with ultrasound.

FDG PET/CT monitoring evaluates cellular metabolic changes in the inflamed vessel wall as opposed to morphologic changes caused by inflammation. It is more sensitive than MRA for disease activity and uptake may predict increased risk for relapse. Also, PET uptake may reflect response to treatment changes. The disadvantages of FDG PET/CT include the fact that it may not adequately show the lumen. Furthermore, its availability is limited, it is expensive, and there is a lack of consensus of cut-off criteria for inflammation. Radiation is emitted to the patient when using CT, and it cannot be used in patients with elevated blood glucose. Finally, it is unclear whether to escalate treatment based on PET alone. Dr Anisha Dua concluded by saying that imaging can add information outside of clinical assessment in large vessel vasculitis. Imaging modalities, new methods and tracers need to be incorporated into clinical trials to generate evidence and to better understand the significance of specific findings.

1. Dua AB. Role of Imaging in Treatment and Follow-Up for Patients with Large-Vessel Vasculitis. 2F053, ACR Convergence 2020, 5-9 Nov.

Posted on 18 January 2021