Background The ability to diagnose sidewall cerebral aneurysms (SCAs) on an angle measurement basis may be useful in clinical practice. A study was undertaken to evaluate the effect of an outflow angle (OA)-assisted approach.

Methods MR angiography (MRA) images of 438 patients with suspected SCAs and other cerebrovascular diseases were separately evaluated using the subjective approach and the OA approach. The approaches were then exchanged for confirmation of unclear cases. An OA of ≥90° was considered to represent SCA positivity. The accuracy, sensitivity, and specificity of the OA-assisted approach were determined using patient-based, aneurysm-based, and size-based evaluations.

Results Digital subtraction angiography (DSA) detected 301 SCAs in 267 patients and no SCAs in 171. An OA of ≥90° was observed for 271 aneurysms in 244 patients (true positives); the OA approach misinterpreted OA as <90° for 29 aneurysms in 29 patients (false negatives) and missed one aneurysm. The subjective approach detected 309 SCAs in 273 patients. This approach misdiagnosed 10 patients (false positives) and missed two aneurysms in two patients (false negatives). The OA-assisted approach detected 300 SCAs in 267 patients and no SCAs in 171, overlooking one aneurysm. Patient-based evaluation yielded high accuracy, sensitivity, specificity, and positive and negative predictive values for the OA-assisted approach.

Conclusions The OA-assisted approach for SCA diagnosis effectively reduced the false-positive rate obtained with the subjective approach with high accuracy, sensitivity, and specificity, suggesting that MRA based on this approach can be a reliable alternative to DSA in SCA screening and diagnosis.