An aneurysm is a localised, irreversible dilatation or "ballooning of an artery. The walls of the artery, which are normally quite resilient, weaken and stretch to the point where the artery can be more than twice its normal size. When this happens the wall becomes extremely thin and fragile and the artery is prone to leaking and/or bursting (rupture). Aneurysms may develop in any artery of the body and depending on its location and size, can have serious repercussions. Most commonly aneurysms affect the largest artery in the body (abdominal aorta) or the arteries behind the knee (popliteal). Aneurysms are classified by size and shape and whether they are intact, leaking or ruptured. Ultrasound is used to visualise the arteries and measure their size and assess them for the presence of blood clots.