What features on CTA/MRA are most helpful for differentiating large vessel vasculitis from atherosclerosis?

While calcified plaque is not typically mistaken for large vessel vasculitis, distinguishing non-calcified atherosclerotic lesions from large vessel vasculitis is challenging. An experienced vascular radiologist is an invaluable resource in such situations if available. 

The first step is to assess pre-test probability of large vessel vasculitis by determining if there are other clinical features suggestive of vasculitis and what risk factors for atherosclerosis are present in the patient being evaluated.

With respect to imaging, the most important feature I look for that is suggestive of vasculitis is whether the wall thickening is smooth and concentric, typically involving long segments of the vessel. In contrast, non-calcified plaque is often eccentric and has a more focal or "bumpy" appearance. 

Fat stranding surrounding the vessel, if present can also be helpful. However, it is important to make sure fat stranding is not due to a dissection which can also sometimes mimic vasculitis or atherosclerosis.

FDG-PET and MRA can also be helpful to assess large vessel vasculitis, but these tests have their limitations in differentiating LVV from athero because atherosclerosis can be FDG avid and can have vessel wall enhancement on MRA, particularly with early lesions. However, the FDG avidity with athero is also typically more focal than the smooth and concentric uptake seen in LVV. Some studies have reported that the intensity of FDG uptake is higher in LVV compared to atherosclerosis, however, because generally FDG PET avidity is not assessed with standardized instruments in current clinical practice, this differentiation can be hard to make.