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Background
Aneurysmal dilation of the intercostal arteries is a rare phenomenon that may mimic intrathoracic pathology such as pulmonary nodules, pleural lesions, or soft tissue masses. This entity has been described to be associated with conditions such as Kawasaki's disease, neurofibromatosis type 1, and aortic coarctation; however, this finding may also be idiopathic as well. For chest radiographers, it is important to reinforce vascular anomalies, such as intercostal artery aneurysm as a potential differential for an enhancing lesion of the chest wall on contrast-enhanced imaging. Although rare, this differential diagnosis should also be considered for an appropriately positioned mass in the intercostal space on a noncontrast examination. This exhibit seeks to educate radiologists and trainees regarding the etiology, pathophysiology, and cross-sectional imaging appearance of intercostal artery aneurysms.

Educational Goals / Teaching Points
Review the imaging appearance of intercostal artery aneurysms on computed tomography (CT), magnetic resonance imaging (MRI), and catheter directed angiography. Discuss the described clinical associations of intercostal artery aneurysms found in the literature. Identify potential imaging mimics of intercostal artery aneurysms. Discuss treatment options for intercostal artery aneurysms.

Key Anatomic/Physiologic Issues and Imaging Findings/Techniques
Identification of intercostal artery aneurysms can be difficult. On noncontrast enhanced CT, these may appear as soft tissue masses adjacent to the pleural, giving the appearance of a pleural lesion or pulmonary nodule depending on the size of the aneurysm. Routine contrast enhanced chest CT may falsely identify intercostal artery aneurysms as an enhancing soft tissue mass. As the intercostal nerve runs alongside the vascular bundle, the appearance on CT and MRI may mimic a nerve sheath tumor. CT angiography is the modality of choice for diagnosis, which may clearly delineate communication of the aneurysm with the intercostal artery with the average intraluminal Hounsfield unit within the aneurysm matching that of the aorta. On MRI, these entities present as hypointense on T1 sequences, hyperintense on T2 sequences, and avidly enhancing on postcontrast images.

Conclusion
Intercostal artery aneurysms are a rare clinical entity that can mimic a variety of intrathoracic pathologies. Imaging findings might be ambiguous across multiple modalities and imaging protocols; however, CT angiography and catheter directed angiography remain the imaging modalities of choice for identifying this vascular anomaly.