Vessel wall MRI detection of an angiographically occult A3 blister aneurysm presenting with subarachnoid hemorrhage: illustrative case.

backgroundBlister aneurysms (BAs) constitute an uncommon and distinct subtype of intracranial aneurysm. Owing to their inconspicuous morphology and nonbranching location, they are often not apparent on conventional angiographic studies. The pathophysiology of BAs remains incompletely elucidated, with arterial dissection considered a principal pathogenic mechanism. OBSERVATIONS: A 49-year-old woman presented with a spontaneous subarachnoid hemorrhage of unknown etiology. Initial lumen-based imaging was nondiagnostic. Vessel wall MRI (VW-MRI) discerned a small hemispheric protrusion without a definable neck, arising from the left A3 segment. Postcontrast imaging demonstrated aneurysmal wall enhancement, a feature correlated with recent rupture in the context of a subarachnoid hemorrhage, implicating this lesion as the presumed hemorrhagic source. Concomitantly, eccentric mural thickening with intrinsic T1 and T2 hyperintensity was evident along the parent artery, supportive of intramural hematoma and underlying arterial dissection. LESSONS: This case emphasizes the adjunctive value of VW-MRI in the diagnostic workup of suspected BAs. By targeting the arterial wall beyond the luminal contour, VW-MRI enhances lesion visibility and delineates associated parent artery abnormalities. https://thejns.org/doi/10.3171/CASE26248.