**4. IgG4-related diseases: Chronic periaortits**

IgG4 -related disease is a newly recognized syndrome of unknown etiology characterized by fibroinflammatory condition, in which tumefactive lesions, a dense lymphoplasmacytic in‐ filtrate rich in IgG4-positive plasma cells. Various symptoms are observed according to the lesions involved, although patients feel well at the time of diagnosis without fever. Seventy percent of patients have elevated serum IgG4 concentrations [17]. CT imaging features of ar‐ terial lesions are characterized by homogeneous wall thickening and enhancement in the late phases after contrast infusion accompanying the increase of connective tissue indicating sclerosing inflammation [21]. This indicates chronic periaortitis, which resembles the infect‐ ed aneurysm with periaortic abscess (Figure 3). Therefore, the diseases should be differenti‐ ated from infected aneurysm by not only the imaging features but also clinical symptoms or negative blood cultures. Glucocorticoids typically the first line of therapy.
