I can understand your confusion. Both
MRI and CT
scan can be used to screen for an aneurysm/dilatation.
MRI is a safer option as it does not expose a patient to radiation, but the quality of image is poor as compared to CT scan (which exposes a patient to radiation). In case of doubt, CT scan imaging is done. So, yes, it is possible that a MRI is unable to catch a small aneurysm or gives a false image of an aneurysm/dilatation.