What are the known risks of a brain angiogram?

Possible allergy. While generally safe, this procedure can be complicated by allergy to the contrast embolus causing a stroke dissection of the artery causing a stroke perforation of a vessel causing bleeding at the site of the insertion of the catheter or more distally damage to the vessel at site of insertion of the catheter requiring surgical repair.
Brain angio. Depending on the health of the pt there can be risks of bleeding, stroke, kidney damage, allergic reaction, death etc. Usually the condition requiring an angiogram is more concerning.