Objective To evaluate whether the aneurysm outflow angle (OA) at MR angiography (MRA) might serve as discriminant for accurate diagnosis of, and differentiation between, small sidewall cerebral aneurysms (SCAs) and infundibula.

Methods Between June 2007 and July 2015, 426 consecutive patients with SCAs completed both an MRA and DSA examination. Of these, 156 patients with small SCAs and 52 patients with infundibula were included in this study. A patient with an OA ≥90° was defined as having a SCA, while those with OA <90° were defined as having an infundibulum.

Results DSA identified 172 SCAs in 156 patients and 55 infundibula in 52 patients. The average OA on MRA was 102.96°±13.36° (range 60°–151°) in 172 SCAs of 156 patients. An OA of ≥90° was seen for 159 (92.4%) small SCAs in 147 patients, while an OA of <90° was observed for 13 SCAs. The average OA on MRA was 69.05°±14.26° (range 35–107°) in 55 infundibula of 52 patients. An OA of ≥90° was seen in one patient with one infundibulum; while an OA of <90° was observed for 54 infundibula (98.2%) in 51 patients. The average OA in SCAs (n=172) was greater than the average OA in infundibula (n=55; 102.96° vs 69.05°, p<0.001).

Conclusions The OA at MRA could serve as discriminant for accurate diagnosis of, and differentiation between, small SCAs and infundibula.