Reading the word aneurysm on a brain imaging report is an unsettling experience, and the word infundibulum sitting next to it rarely makes things clearer. Yet these two findings appear together on reports all the time, because distinguishing between them is one of the routine judgments a radiologist makes when reviewing images of the arteries at the base of the brain.
The short version is reassuring: an infundibulum is a normal anatomical variation, not an aneurysm. It is a small, funnel-shaped widening where one artery branches off another, and it is present in a meaningful share of healthy adults. Understanding what separates the two, and why the distinction takes care to make, turns an alarming line on a report into a manageable piece of information.
An infundibulum is a cone-shaped or triangular flare at the origin of a small branch artery, most classically where the posterior communicating artery comes off the internal carotid artery. The word itself simply means funnel. Rather than the branch vessel leaving the parent artery at a clean right angle, the junction widens gently before narrowing into the branch, producing a small bulge that can superficially resemble an aneurysm on imaging.
The defining feature is that the branch artery emerges from the apex of the bulge, the narrow tip of the funnel. Blood flow through it remains smooth and laminar. Nothing about the vessel wall has weakened or thinned. It is a variation in how the plumbing is shaped, not a defect in the plumbing.
A saccular aneurysm, sometimes called a berry aneurysm, is a genuine outpouching of a weakened arterial wall. Instead of a funnel that tapers into a branch, it forms a rounded sac attached to the side of the artery by a neck, with no vessel leaving from its tip. Blood swirls inside it rather than flowing straight through, and the sac itself has the potential to enlarge over time.
Unruptured intracranial aneurysms are more common than most people expect. Classic estimates place the prevalence at roughly 3% of adults, and studies using modern high-resolution imaging report higher detection rates, largely because smaller aneurysms are now visible that previously were not. More than nine in ten are discovered incidentally on scans performed for entirely unrelated reasons. The overwhelming majority never cause a problem, and management for smaller ones is frequently observation with periodic imaging rather than intervention.
Three features do most of the work. The first is size: an infundibulum is conventionally under 3 millimeters at its widest point. Anything larger tends to be treated as an aneurysm until proven otherwise. The second is shape: an infundibulum is conical or triangular, tapering smoothly, while an aneurysm is rounded or lobulated with a distinguishable neck. The third, and the most decisive, is whether a branch artery arises from the apex. If a vessel comes off the very tip of the bulge, that supports an infundibulum. If the bulge is a dead end, that supports an aneurysm.
MR angiography and CT angiography resolve most cases confidently. One dedicated study comparing posterior communicating artery infundibula against small aneurysms found CT angiography distinguished them with roughly 95% sensitivity and 100% specificity. Even so, very small outpouchings and their relationship to tiny perforating branches can be genuinely ambiguous on any noninvasive study, and catheter angiography remains the reference standard when a definitive answer is needed. This is why reports sometimes describe a finding as favored to represent an infundibulum rather than declaring it outright, and why a short-interval follow-up scan is occasionally recommended.
Radiologists also draw on how a lesion appears across multiple sequences rather than a single image, in much the same way that specialized sequences like diffusion-weighted imaging resolve questions elsewhere in the brain that standard images leave open.
The consensus is that infundibula carry a very low risk, meaningfully lower than a true aneurysm of comparable size. They are widely regarded as a normal variant. That said, the medical literature does contain isolated case reports of infundibular dilations that ruptured or later developed into aneurysms, which is why the honest answer is very low rather than zero, and why some clinicians favor a single follow-up study to confirm stability.
For a true unruptured aneurysm, guidance from the American Heart Association and American Stroke Association emphasizes that decisions weigh estimated rupture risk against the risk of treatment. Size, location, shape, growth over time, and personal and family history all feed into that calculation. People with two or more first-degree relatives who have had an aneurysmal hemorrhage, or with polycystic kidney disease, fall into a higher-risk group where screening and closer surveillance are more readily recommended.
Because most brain aneurysms produce no symptoms until something goes wrong, nearly all of them are found by accident. That is the same reason the infundibulum question arises so often: when you image the brain arteries of people who feel entirely well, you find variations in a lot of them.
A comprehensive scan is not a promise that nothing will ever be found. It is a way of building visibility into what your own anatomy looks like now, so that a future scan has something to be compared against. Some people find that clarity genuinely reassuring, while others find that any finding, however benign, takes some processing. Both reactions are normal, and we have written before about how patients experience anxiety around scanning and what tends to help.
If you want the broader picture of how brain aneurysms are evaluated and what a scan can and cannot tell you, our earlier article on brain aneurysms and full-body MRI covers that ground, and our health library entry on aneurysm offers a shorter reference definition.
A finding on a brain scan is information, not a verdict. Whether it turns out to be an infundibulum, a small stable aneurysm, or nothing at all, the value lies in knowing rather than wondering, and in having a baseline your future imaging can be measured against.
If you are considering comprehensive imaging or want to understand what a scan of the brain and body actually covers, our team is glad to talk it through. Take the next step toward proactive healthcare by reviewing what a whole-body scan includes.
Frequently asked questions
Is an infundibulum a type of aneurysm?
No. An infundibulum is a normal funnel-shaped widening at the point where a small artery branches off a larger one, with the branch vessel emerging from its tip. An aneurysm is an outpouching of a weakened arterial wall with no vessel leaving from it. They are separate findings, though they can look similar on imaging.
How large can an infundibulum be?
The conventional threshold is under 3 millimeters. A conical bulge smaller than that, with a branch artery arising from the apex, is generally read as an infundibulum. A rounded bulge larger than 3 millimeters is typically evaluated as a possible aneurysm.
Does an infundibulum need treatment or follow-up?
Treatment is not standard. Some clinicians recommend a single follow-up scan to confirm the finding has not changed, since rare cases of enlargement have been reported. Beyond that, an infundibulum is usually documented and left alone. Your neurologist or neurosurgeon makes that call.
How common are unruptured brain aneurysms?
Roughly 3% of adults are estimated to have one, with newer high-resolution imaging studies reporting higher detection rates. More than 90% are found incidentally during imaging done for another reason, and most never cause symptoms.
Can MRI detect a brain aneurysm without contrast dye?
Yes. MR angiography uses the motion of flowing blood itself to generate images of the arteries, so it can visualize the vessels at the base of the brain without injected contrast. Very small aneurysms remain more difficult to characterize on any noninvasive study, which is why catheter angiography is still used when certainty is required.
Sources
- Infundibular dilation and aneurysm at the origin of the posterior communicating artery: differential diagnosis by CT angiography
- Guidelines for the Management of Patients With Unruptured Intracranial Aneurysms
- Unruptured intracranial aneurysms: Why should we focus on small aneurysms? A comprehensive update of recent findings
- An infundibulum of thalamoperforator arteries: Importance of angiographic images for appropriate diagnosis
- Endovascular Embolization of Ruptured Infundibular Dilation of Posterior Communicating Artery: A Case Report
