Methods and systems are provided for treating a vascular structure having a defect, for example, a cerebral artery having a weakened wall that has formed an aneurysm. The methods include substantially entrapping a quantity of blood within a vascular defect, and introducing a quantity of a crosslinking agent, for example as a liquid solution, into the entrapped blood. The crosslinking agent is a compound in which each molecule of the compound has at least two nucleophilic-reactive functional groups. The crosslinking agent is allowed to combine with and react with the substantially entrapped blood, and a substantially solid mass made of crosslinked blood is formed within the defect.