Small vessel vasculitides mainly affect small intraparenchymal arteries, arterioles, capillaries, and venules. They are divided into two categories; ANCA-associated small vessel vasculitis and immune complex-associated small vessel vasculitis.

”Fibromuscular dysplasia (FMD) is a vascular disease that may result in stenosis, dissection, or aneurysm of nearly all arterial distributions, with the renal and extracranial carotid and vertebral arteries most commonly affected (1). Medium-sized arteries are typically affected, and the disease often involves multiple vascular territories within an individual patient. The etiology of this disease is unclear. It is believed to be neither an inflammatory nor an atherosclerotic process, but genetic and environmental risk factors may play a role in the condition.”
Screening CT Angiography of the Aorta, Visceral Branch Vessels, and Pelvic Arteries in Fibromuscular Dysplasia.
Bolen M et al
JACC Cardiovasc Imaging. 2017 May;10(5):554-561

“The clinical presentation is driven by the vascular beds affected. Renal involvement usually presents with hypertension, and carotid artery involvement is often heralded by pulsatile tinnitus, headache (generally the migraine type), and transient ischemic attack or stroke (often due to carotid or vertebral artery dissection). FMD may also be discovered incidentally in an asymptomatic patient who is imaged for another clinical indication.”
Screening CT Angiography of the Aorta, Visceral Branch Vessels, and Pelvic Arteries in Fibromuscular Dysplasia.
Bolen M et al
JACC Cardiovasc Imaging. 2017 May;10(5):554-561

“Abnormalities including beading, aneurysm, dissection, and stenosis/occlusion were noted in aortic, renal, mesenteric, and iliac distributions. The most commonly affected vessels were the renal arteries (n = 76 [67%]), followed by the lower extremity/iliac arteries (n = 37 [32%]). Aortic abnormalities were less frequently encountered (n = 3 [3%]), including 1 case with mild dilation (4.2 cm) of the ascending aorta and 2 cases of dissection involving the descending aorta, 1 with mild dilation (4.4 cm). Reformatted images were crucial, affecting final assessment in 56% of cases evaluated by reader 1 and 36% evaluated by reader 2.”
Screening CT Angiography of the Aorta, Visceral Branch Vessels, and Pelvic Arteries in Fibromuscular Dysplasia.
Bolen M et al
JACC Cardiovasc Imaging. 2017 May;10(5):554-561

(Medicine 2018:97;49)

Mayo Clinic study of 64 patients

• Aorta involved in 56%
  • Ascending Aorta 35%
  • Aortic arch 38%
  • Descending Aorta 41%
  • Supra renal AA 40%
  • Infra renal AA 46%
• Aortic branches involved in 26%
  • Iliacs 24%. Renal 25%, SMA 14%, Celiac 16%, Subclavian 10%, Carotid 10%, Coronaries 14%
• Perinephric fibrosis strongly associated with aortic disease (OR 5-7)

Vasculitis is a systemic disease characterized by non-infectious inflammation of the blood vessels. The cause can be primary (idiopathic) or secondary to an underlying disease. The precise pathophysiology of idiopathic vasculitides is unknown. Therefore, classification is based on vessel size—large, medium, small, and variable. Large vessel vasculitis (LVV) predominantly involves the large vessels (aorta and its main branches) but can also affect medium- or small-sized vessels. In variable-vessel vasculitis (VVV), any vessel can be affected, and no type of vessel predominates. 
Multimodality Imaging of Large Vessel Vasculitis
Ayaz Aghayevhttps://doi.org/10.2214/AJR.21.26150

Multidetector CT is a useful noninvasive imaging modality for the evaluation of vasculitis and vasculitis mimics because CT can provide the information of the vessel wall change and other accompanied findings. Although CT features of various vasculitis are often overlapping, CT features via consideration of the involved vessel type, location, morphology, and associated systemic disease, can be useful in narrowing down the differential diagnosis .Getting familiar with CT features will help radiologists to establish appropriate diagnosis for vasculitis.