Brain aneurysms are diagnosed primarily through specialized imaging tests that map the blood vessels inside your skull. About 3% of adults have an unruptured brain aneurysm without knowing it, and the diagnostic path depends entirely on whether doctors suspect one has ruptured or are screening for one that hasn’t.
Two Very Different Diagnostic Paths
The process looks completely different depending on the situation. In an emergency, when someone arrives with a sudden, severe headache and doctors suspect bleeding in the brain, the priority is speed. A CT scan of the head is typically the first test, because it can detect blood pooling around the brain within minutes. If that scan confirms bleeding, doctors follow up with a vessel-specific imaging test to locate the aneurysm responsible.
In a non-emergency, the goal is screening. Someone with risk factors (like a strong family history) might undergo imaging purely as a precaution, looking for a bulge in a blood vessel that hasn’t caused any symptoms yet. These two paths use many of the same tools, but the urgency and sequence differ.
CT Angiography: The Fast First Look
CT angiography (CTA) is often the first imaging test used when an aneurysm is suspected. It involves injecting a contrast dye into a vein, then taking rapid CT images that highlight the blood vessels. The whole process takes only a few minutes, making it ideal in emergencies.
CTA is highly accurate for larger aneurysms. For aneurysms bigger than 10 mm, detection rates reach 100%. For those between 7 and 10 mm, sensitivity is about 99%, and for those between 4 and 6 mm, it’s around 90%. The limitation is with very small aneurysms: for those 3 mm or smaller, CTA catches only about 45% of them. False positives also become more common below 4 mm, meaning the scan may flag something that isn’t actually an aneurysm.
MR Angiography: No Radiation, No Dye
MR angiography (MRA) uses magnetic fields and radio waves instead of X-rays, so there’s no radiation exposure. It can produce detailed 2D or 3D images of brain arteries and identify an aneurysm’s size, shape, and location. MRA is commonly used for non-emergency screening because it doesn’t require contrast dye injection in many cases, making it lower risk for the patient.
Like CTA, MRA has trouble with very small aneurysms. False positives increase significantly for anything smaller than 3.5 mm. For this reason, when either CTA or MRA flags a small or uncertain finding, doctors often recommend a more definitive test.
Cerebral Angiogram: The Most Detailed Test
A cerebral angiogram (also called digital subtraction angiography) provides the most detailed view of the brain’s blood vessels. It’s used when CTA or MRA results are unclear, or when doctors need precise measurements before planning treatment.
During the procedure, a thin catheter is inserted into an artery in the groin or wrist and threaded up to the brain’s blood vessels. Contrast dye is injected through the catheter, and X-ray images capture the flow of blood in real time. This allows doctors to see even small aneurysms that noninvasive scans might miss or misidentify. Because it’s an invasive procedure with a small risk of complications, it’s reserved for cases where the less invasive scans haven’t provided enough information.
When Imaging Comes Back Negative but Suspicion Remains
Sometimes a person has the classic symptoms of a ruptured aneurysm, particularly a sudden thunderclap headache, but the CT scan looks normal. This doesn’t rule out bleeding. A small amount of blood can be missed on CT, especially if the scan is done more than a few hours after symptoms started.
In these cases, the next step is a lumbar puncture (spinal tap). A sample of spinal fluid is collected and analyzed for signs of blood breakdown products, specifically bilirubin. This test needs to be performed at least 12 hours after symptoms begin, because bilirubin takes time to accumulate in the spinal fluid. Samples taken too early can produce a false negative. The test also becomes unreliable after about 10 days, as the pigments clear naturally.
When performed within that window and analyzed with a light-based measurement technique called spectrophotometry, the test has a sensitivity of 100% and specificity of 95%, making it extremely reliable. Visual inspection of the fluid for yellow coloring, by contrast, is not considered dependable.
Physical Signs Doctors Look For
Before ordering imaging, a neurological exam can reveal clues. The specific signs depend on where the aneurysm sits and how large it is.
A classic red flag is a dilated pupil on one side, often accompanied by a drooping eyelid and difficulty moving that eye. This pattern points to an aneurysm pressing on the nerve that controls eye movement, most commonly at the posterior communicating artery, one of the most common aneurysm locations (about 23% of all brain aneurysms). Aneurysms in other locations can cause different problems: vision loss or visual field changes, weakness on one side of the body, difficulty speaking, or loss of smell. Very large aneurysms can compress surrounding brain tissue enough to mimic the symptoms of a tumor.
If a rupture has occurred, the exam typically reveals a stiff neck (from blood irritating the lining of the brain), reduced consciousness, and other neurological deficits depending on where the bleeding is concentrated.
Who Should Get Screened
Most people don’t need aneurysm screening. The guidelines from the American Heart Association and American Stroke Association identify one group with a clear benefit: people who have two or more first-degree relatives (parents, siblings, or children) with known brain aneurysms. In this group, about 12% will have an aneurysm themselves, four times the rate in the general population.
For these individuals, screening with MRA every 5 to 7 years, starting at age 20 and continuing through age 80, has been shown to be cost-effective. Certain genetic conditions that affect connective tissue, such as autosomal dominant polycystic kidney disease, also raise risk enough that doctors may recommend screening even without a family history of aneurysms.
What Happens After an Aneurysm Is Found
Finding an unruptured aneurysm doesn’t automatically mean surgery. Doctors assess rupture risk using several factors: your age, blood pressure history, the aneurysm’s size and location, whether you’ve had a previous brain bleed, and even your geographic background (rupture rates vary by population). These factors are sometimes formalized into a scoring system to guide decision-making.
Small aneurysms in low-risk locations may simply be monitored with repeat imaging every 6 to 12 months initially, then less frequently if the aneurysm stays stable. Larger aneurysms, those in higher-risk locations, or those that grow on follow-up imaging are more likely to be treated. The two main treatment options are a surgical clip placed at the base of the aneurysm or a less invasive approach where coils are threaded through a catheter to fill the aneurysm from the inside. Which approach is recommended depends on the aneurysm’s characteristics and your overall health.