Doctors diagnose brain aneurysms using specialized imaging that maps the blood vessels inside the skull. The specific test depends on the situation: a ruptured aneurysm causing sudden, severe symptoms calls for emergency CT scanning, while an unruptured aneurysm is typically found through CT angiography or MR angiography, sometimes during a scan done for a completely unrelated reason. About 2 in 100 people have an undetected brain aneurysm, and the path from suspicion to confirmed diagnosis usually involves two or three steps.
Emergency Diagnosis After a Rupture
When an aneurysm ruptures, blood leaks into the space surrounding the brain, a condition called subarachnoid hemorrhage. The hallmark symptom is a sudden, explosive headache often described as the worst of your life. In the emergency room, the first step is a non-contrast CT scan of the head. This scan is fast and highly reliable at detecting fresh bleeding, especially within the first 6 to 12 hours.
If the CT looks normal but doctors still suspect bleeding based on your symptoms, the next step is a lumbar puncture (spinal tap). A small sample of spinal fluid is collected and examined for signs of blood breakdown products, specifically a yellowish discoloration called xanthochromia. This test needs to be performed at least 12 hours after symptoms started, because the body needs that long to break down any blood in the spinal fluid into detectable compounds. If the fluid comes back clear with no discoloration, subarachnoid hemorrhage is effectively ruled out.
Once bleeding is confirmed, doctors move quickly to pinpoint the aneurysm’s location and shape using one of the angiography techniques below, because treatment decisions hinge on those details.
CT Angiography: The Most Common First Test
CT angiography (CTA) is the workhorse of aneurysm detection. It involves injecting a contrast dye into a vein, then taking rapid CT images as the dye flows through the brain’s blood vessels. The entire scan takes only a few minutes, making it practical for both emergencies and planned evaluations.
CTA’s accuracy depends heavily on the size of the aneurysm. For aneurysms larger than 10 mm, sensitivity is essentially 100%. For those between 7 and 10 mm, it’s 99%, and for 4 to 6 mm, it’s 90%. The weakness is with very small aneurysms: for those 3 mm or smaller, CTA catches only about 45% of them. Overall, the test has an 83% sensitivity and 93% specificity, meaning it’s good at confirming aneurysms that are present while rarely flagging something that isn’t there.
Those numbers matter in practice. If your CTA shows a clear aneurysm of 4 mm or larger, the diagnosis is usually reliable enough to start planning next steps. If the CTA is negative but clinical suspicion remains high, doctors will often follow up with a more detailed test.
MR Angiography: No Radiation, No Contrast
MR angiography (MRA) uses magnetic resonance imaging to visualize blood vessels without radiation or injected dye, making it a good option for screening and follow-up monitoring. It’s often the test of choice when someone with risk factors, like a strong family history, needs periodic surveillance.
MRA’s limitations mirror CTA’s: small aneurysms are hard to see. In a large systematic review, 90% of the aneurysms MRA missed were smaller than 5 mm, and 45% of those were under 3 mm. On the flip side, 82% of false positives (spots that looked like aneurysms but weren’t) were also under 3 mm. This means that at very small sizes, MRA can both miss real aneurysms and flag harmless structures.
The strength of the MRI magnet plays a role. Scanners operating at 3 Tesla show a trend toward better accuracy compared to the older 1.5-Tesla machines that are still common in many facilities. If you’re being screened electively, a 3-Tesla scanner may offer a meaningful edge in detection.
Digital Subtraction Angiography: The Gold Standard
Digital subtraction angiography (DSA) remains the definitive test for brain aneurysms. It’s more invasive than CTA or MRA, but it produces the most detailed images of the brain’s blood vessels, down to branches that other scans can’t resolve clearly.
During the procedure, a thin catheter is inserted into an artery in the leg and guided up to the blood vessels in the brain. Contrast dye is injected through the catheter while X-ray images are captured. The “subtraction” part refers to the software removing bone and soft tissue from the image, leaving only the blood vessels visible in sharp detail. The procedure typically takes 1 to 2 hours, and you’ll need to lie flat for several hours afterward while the puncture site in your leg heals.
Doctors turn to DSA when CTA or MRA results are inconclusive, when they need precise measurements before planning surgery or coiling, or when they need to distinguish a true aneurysm from a look-alike structure. It carries a small risk of stroke (roughly 0.5 to 1%), so it isn’t used as a first-line screening tool.
When Aneurysms Are Found by Accident
Many brain aneurysms are discovered incidentally, during scans ordered for something else entirely, like investigating headaches, head injuries, or sinus problems. Prevalence estimates from large reviews suggest roughly 2.3% of the general adult population harbors an unruptured aneurysm. That number climbs for certain groups, including people with atherosclerosis or pituitary tumors being evaluated with angiography.
An incidental finding can be unsettling, but most small, unruptured aneurysms never cause problems. When one is discovered, doctors assess its rupture risk based on several factors before recommending treatment or monitoring. The PHASES scoring system, developed from pooled data across six large studies, weighs six variables: your age, whether you have high blood pressure, whether you’ve had a previous subarachnoid hemorrhage, the aneurysm’s size, its location in the brain, and your geographic background (rupture rates differ across populations). A higher combined score points toward more aggressive management, while a lower score often supports a watch-and-wait approach with periodic imaging.
How Doctors Tell Aneurysms From Look-Alikes
Not every bulge on a brain scan is an aneurysm. One of the most common mimics is an infundibulum, a small funnel-shaped widening where a branch vessel meets a larger artery. Infundibula are harmless, but on a scan they can look remarkably similar to a tiny aneurysm.
Radiologists use a specific set of criteria to tell them apart. Infundibula are almost always smaller than 3 mm, have a cone-like shape that’s widest where the vessel originates and tapers toward the tip, and lack the defined “neck” that protrudes from the parent artery in a true aneurysm. One of the most reliable measurements is the outflow angle, the angle between the bulge and the parent vessel. In studies comparing the two, over 98% of infundibula had an outflow angle less than 90 degrees, while over 92% of true aneurysms measured 90 degrees or more.
True aneurysms also come in two basic shapes. Saccular aneurysms, the most common type, balloon out from one side of the vessel like a berry on a stem. Fusiform aneurysms involve the entire vessel wall bulging outward in all directions. The shape matters because it influences both rupture risk and which treatment options are feasible.
Who Should Be Screened
Routine screening for brain aneurysms isn’t recommended for the general population because most aneurysms never rupture, and the anxiety and follow-up testing from false positives can cause real harm. Screening is typically reserved for people at elevated risk. This includes those with two or more first-degree relatives (parent, sibling, child) who have had a brain aneurysm or subarachnoid hemorrhage, people with autosomal dominant polycystic kidney disease, and those with certain connective tissue disorders.
If you fall into one of these groups, screening usually starts with an MRA, repeated every 5 to 7 years if the initial scan is clear. If an aneurysm is found, follow-up imaging is scheduled more frequently, often every 6 to 12 months initially, to check whether the aneurysm is stable or growing. Growth over time is one of the strongest signals that intervention may be needed.