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The respiratory system may be involved in all systemic vasculitides, although with a variable frequency. The aim of our review is to describe radiographic and high-resolution CT (HRCT) findings of pulmonary vasculitides and to correlate radiological findings with pathological results. Lung disease is a common feature of antineutrophil cytoplasmic autoantibody-associated small-vessel vasculitides, including granulomatosis with polyangiitis (Wegener’s), eosinophilic granulomatosis with polyangiitis (Churg–Strauss) and microscopic polyangiitis. Pulmonary involvement is less frequent in immune-complex-mediated small-vessel vasculitides, such as Behçet’s disease and Goodpasture’s syndrome. Pulmonary involvement associated to large-vessel (gigantocellular arteritis and Takayasu’s disease) or medium-vessel (nodose polyarteritis and Kawasaki’s disease) vasculitides is extremely rare. The present review describes the main clinical and radiological features of pulmonary vasculitides with major purpose to correlate HRCT findings (solitary or multiple nodules, cavitary lesions, micronodules with centrilobular or peribronchial distribution, airspace consolidations, “crazy paving”, tracheobronchial involvement, interstitial disease) with pathological results paying particular attention to the description of acute life-threatening manifestations. A thorough medical history, careful clinical examination and the knowledge of radiological patterns are mandatory for a correct and early diagnosis. INTRODUCTION The systemic vasculitides are a heterogeneous group of disorders characterized by an inflammatory destructive process affecting blood vessels. The lung is frequently involved with various clinical presentations. Pulmonary vasculitides may be primary and, in most cases, idiopathic disorder or secondary to other conditions such as infectious diseases, connective tissue diseases, malignancies and hypersensitivity disorders. Primary pulmonary vasculitides are rare disorders, with an incidence of 20–100 cases per million and a prevalence of 150–450 per million. The most widely used approach to classifying primary vasculitides is based on the size of the affected vessels (large, medium, small). Primary vasculitides most commonly associated with thoracic involvement are small-vessel antineutrophil cytoplasmic autoantibody (ANCA)–associated vasculitides represented by granulomatosis with polyangiitis (Wegener’s) (GPA), eosinophilic granulomatosis with polyangiitis (EGPA) (Churg–Strauss) and microscopic polyangiitis. Pulmonary involvement is less frequently associated to small-vessel immune-complex-mediated vasculitides as Goodpasture’s syndrome and Behçet’s disease. Pulmonary involvement related to large-vessel vasculitides (Takayasu arteritis, giant-cell arteritis) and medium-sized-vessel vasculitides (polyarteritis nodosa and Kawasaki disease) is rare. Radiological manifestations of primary pulmonary vasculitides are extremely variable and can be characterized by several findings including small-vessel wall thickening, nodular lesions, cavitary lesions, micronodules with centrilobular and peribronchial distribution, “ground-glass” opacities and/or consolidations, “crazy-paving” pattern, tracheobronchial stenoses and aneurysmatic dilatation of pulmonary arteries. The knowledge of these signs and a careful correlation of specific clinical, laboratory, radiological and pathological features are mandatory for early diagnosis of vasculitides also in biopsyproven cases. Chest radiography is often non-specific and does not show the right pattern and extent of thoracic involvement. Highresolution CT (HRCT) is the most effective method to evaluate the characteristics, distribution and evolution of lung disorders. As mentioned above, the primitive pulmonary vasculitides are rare disorders and represent one of the most complex diagnosis in medicine characterized by non-specific symptoms and signs common to other diseases such as infectious, connective tissue and neoplastic disease; in addition, the diagnosis of vasculitis is based on the identification of specific patterns of clinical, radiological, laboratory and histopathological abnormalities. The association of serological testing for ANCA with other immunopathological markers such as immunoglobulin A, serum cryoglobulins and anti-glomerular basement membrane is equally important for the diagnosis. CLASSIFICATION OF THE VASCULITIDES The size of the predominantly involved vessels strongly influences the clinical and radiological features of the different forms of vasculitides and is therefore one major criterion for the classification of vasculitides. Primary vasculitides are classified into large-vessel vasculitides, medium-sized-vessel vasculitides and small-vessel vasculitides. Large-vessel vasculitides affect the aorta and its largest branches; the medium-sized-vessel vasculitides affect the main visceral arteries and their branches; and the small-vessel vasculitides affect the capillaries, venules and arterioles. Note how all three categories affect arteries, but only small-vessel vasculitis affects also capillaries and venules. The 2012 Revised International Chapel Hill Consensus Conference Nomenclature of Vasculitides elaborated a new classification system (Table 1). In this review, we describe the radiological, pathological and clinical features of primary vasculitides that most frequently involve the lung, represented by primary small-vessel ANCAassociated vasculitides [EGPA (Churg–Strauss), granulomatosis with polyangiitis (Wegener’s) and microscopic polyangiitis] and primary immune-complex-mediated vasculitides (Behcet’s disease and Goodpasture’s syndrome). We also discuss the radiological findings and the underlying causes of diffuse alveolar haemorrhage (DAH) which is Table 1. Classification of vasculitides Large-vessel vasculitis Giant-cell arteritis