Aneurysm morphology describes the shape, size, and geometric characteristics of a brain aneurysm. Since not only where the aneurysm is located but also what form it has directly affects the rupture risk and the choice of treatment method, morphology is one of the fundamental components of clinical evaluation.

Aneurysms are basically divided into two groups according to their shape. Saccular (sac-like) aneurysms are the most common type; they protrude outward from the vessel wall like a pouch and are usually connected to the main vessel by a narrow neck. These are typically seen at vascular branching points. Fusiform aneurysms, on the other hand, widen the entire circumference of the vessel in a spindle shape and do not form a distinct neck; they are more frequently associated with atherosclerosis or vessel wall diseases.

The principal criteria used in morphological evaluation are as follows. Size (maximum diameter) is one of the best-known determinants of rupture risk; in general, as the aneurysm grows, the bleeding risk increases. Neck width is critical for treatment planning; wide-necked aneurysms may require stent or balloon support during endovascular coil treatment. The aspect ratio and the dome-to-neck ratio express the ratio of the sac height to the neck width; when this ratio is high, the rupture risk is considered to increase. In addition, irregularities in the aneurysm wall, lobulation (a multi-lobed structure), and small protrusions emerging from the wall (blebs) are also evaluated as signs of instability and high rupture risk.

The importance of morphology is directly reflected in practical decisions. While a smooth, small, narrow-necked aneurysm may carry a lower risk, a large, irregular, lobulated, and wide-necked aneurysm both carries a higher bleeding risk and requires a more complex treatment approach. For this reason, aneurysm morphology, when evaluated together with location and patient characteristics, constitutes one of the most important foundations of the treatment decision.