In another study,109
a negative CTA result reliably excluded aneurysms when
head CT showed the classic perimesencephalic SAH pattern
or no blood. Digital subtraction angiography (DSA) was
indicated if there was a diffuse aneurysmal pattern of aSAH,
and repeat delayed DSA was required if the initial DSA
findings were negative, which led to the detection of a small
aneurysm in 14% of cases. When the blood is located in the
sulci, CTA should be scrutinized for vasculitis, and DSA is
recommended for confirmation