How to Check for a Brain Aneurysm: Tests & Screening

Brain aneurysms are checked with noninvasive imaging, most commonly CT angiography or MR angiography, both of which can detect outpouchings in brain arteries without surgery. Which test you get depends on whether doctors are investigating an emergency like a sudden severe headache or screening someone at higher risk who feels perfectly fine. These two scenarios follow very different paths and involve different levels of urgency, accuracy trade-offs, and follow-up.

The Emergency Path

When someone arrives at the emergency room with the classic “worst headache of my life,” the first concern is whether a brain aneurysm has ruptured and caused bleeding around the brain, a condition called subarachnoid hemorrhage. The initial test is almost always a plain (non-contrast) CT scan of the head. This scan is fast and widely available, and when performed within six hours of headache onset, it catches subarachnoid hemorrhage with extremely high reliability. A prospective study of nearly a thousand patients scanned within that window found that CT identified every single case of subarachnoid hemorrhage, with both sensitivity and specificity at 100%.¹ After six hours, though, sensitivity drops to about 86%, because the blood begins to break down and becomes harder to distinguish on the scan.¹ A systematic review across multiple studies confirmed an overall CT sensitivity above 98%, but that average includes scans done both early and late.²

If the initial CT is negative but the clinical suspicion is still high, the traditional next step is a lumbar puncture. A spinal tap draws a small amount of cerebrospinal fluid and checks for blood products, specifically a yellowish discoloration called xanthochromia that develops when red blood cells break down. Studies have shown that combining red blood cell counts with xanthochromia analysis reliably identifies ruptured aneurysms missed on CT, allowing timely treatment.³ Whether every patient with a negative CT truly needs a lumbar puncture remains debated, particularly when the scan was done within those first six hours, but in practice many emergency departments still perform one if a ruptured aneurysm cannot be ruled out clinically.⁴

Once bleeding is confirmed or strongly suspected, the next step is to find the aneurysm responsible. That typically means CT angiography, which involves injecting iodine-based contrast dye and scanning the brain’s blood vessels in detail, or in some cases proceeding directly to catheter angiography.

CT Angiography and How Well It Works

CT angiography is the workhorse test for visualizing brain aneurysms. It is quick, widely available, and reasonably accurate. A meta-analysis pooling data from studies using modern multi-detector scanners found an overall sensitivity above 95% on a per-aneurysm basis and above 97% on a per-patient basis.⁵ These numbers reflect how far the technology has come with 16- and 64-row scanners, which significantly outperform older single-detector machines, especially for small aneurysms under 4 mm.⁵

The catch is that CT angiography performs unevenly depending on aneurysm size and location. One study comparing CT angiography to the gold standard (catheter angiography) found sensitivity of about 81% for one radiologist and 90% for another, with the smallest aneurysm detected measuring about 2.2 mm.⁶ Another study focused on ruptured aneurysms found that sensitivity dropped to just 30% for aneurysms under 3 mm, compared to about 82% for those between 3 and 5 mm.⁷ Location matters too: aneurysms near the posterior communicating artery were detected less often than those in other common anterior circulation locations.⁷ So while CT angiography is excellent for most aneurysms, it has blind spots with tiny ones and certain tricky locations.

MR Angiography for Screening and Follow-Up

For people without an acute emergency, MR angiography is often the preferred imaging choice. It uses magnetic resonance imaging rather than X-rays, so there is no radiation exposure and usually no contrast dye is needed. A technique called time-of-flight (TOF) MR angiography generates images of blood flow through arteries without injecting anything. This makes it particularly suitable for screening healthy people who have risk factors and for monitoring known aneurysms over time.

Detection rates with MR angiography are strong. In a study comparing different MRI approaches, 1.5-Tesla time-of-flight imaging detected about 91% of aneurysms, while adding a gadolinium-based contrast agent pushed that up to about 97%. Moving to a stronger 3-Tesla magnet with contrast also reached about 97%.⁸ The contrast-enhanced version on 1.5-Tesla scanners was particularly better at catching small incremental growth in aneurysms already being monitored, which is one of the most important tasks in surveillance.⁸ Among neuroradiologists surveyed about how to follow small unruptured aneurysms, a majority favored noncontrast MR angiography for routine surveillance, and about 84% recommended imaging at least once a year.⁹

One limitation of MR angiography is that it can have trouble distinguishing a true aneurysm from an infundibular dilation, which is a normal funnel-shaped widening where certain small arteries branch off. These dilations are harmless, but they can look suspicious on a scan. Specialized fusion imaging combining different MRI sequences can help tell them apart.¹⁰ Higher-resolution imaging at 7 Tesla has also shown promise for this specific problem, potentially reducing the need for repeat scans or invasive follow-up when the diagnosis is uncertain.¹¹

Digital Subtraction Angiography

Digital subtraction angiography remains the gold standard for evaluating brain aneurysms.¹² It involves threading a thin catheter through an artery, typically starting in the groin, up into the blood vessels of the brain, and injecting contrast dye directly while taking rapid X-ray images. The technique produces the most detailed view of aneurysm anatomy and remains essential for treatment planning.

Because it is invasive, it carries real if small risks. In the general population undergoing catheter angiography (which includes people already predisposed to stroke), complication rates of about 1-2% have been cited. But among patients being evaluated specifically for aneurysms or vascular malformations without active bleeding, the combined risk of permanent and temporary neurological complications was about 0.3%, with permanent damage essentially at 0%.¹³ Still, because CT and MR angiography have improved so dramatically, catheter angiography is now reserved mainly for cases where noninvasive imaging is inconclusive or when treatment is being planned in the same session.

It is worth knowing that even catheter angiography is not perfect. Some aneurysms hide from initial imaging, particularly when they are very small, are located behind blood clots after a rupture, or go into temporary spasm. Reports suggest that somewhere between 2% and 20% of initial angiograms can be falsely negative, and a proportion of those missed aneurysms are eventually found on repeat imaging.¹⁴ This is why repeat angiography is sometimes recommended when clinical suspicion remains high despite a negative first study.

Who Should Be Screened

Most brain aneurysms never rupture and never cause symptoms. They are found in roughly 2-4% of the general adult population, with the rate climbing with age: one study using 3-Tesla MR angiography found aneurysms in about 1.3% of people under 40 but around 6.7% in those over 70.¹⁵ A large population-based study in Rotterdam detected aneurysms in about 2.3% of participants.¹⁶

Because most aneurysms are small and low-risk, screening the entire population would generate a lot of anxiety, follow-up imaging, and in some cases unnecessary procedures for what amounts to an incidental finding that will never cause harm. Screening makes the strongest case for people at elevated risk of both harboring an aneurysm and having it rupture. Modeling studies support screening for people with two or more first-degree relatives (parent, sibling, child) who have had aneurysmal subarachnoid hemorrhage, and for people with autosomal dominant polycystic kidney disease. Evidence also supports screening those with even a single affected first-degree relative.¹⁷

Smoking and high blood pressure also raise the odds. A meta-analysis found that smokers had about a third higher odds of having an unruptured aneurysm detected on screening compared to non-smokers.¹⁸ A study screening people aged 35 and older with hypertension, atherosclerosis, and smoking history found that the estimated risk of finding an aneurysm ranged from about 1.6% in younger male non-smokers up to about 13% in older women who currently smoked.¹⁹ These numbers are useful for clinicians weighing whether to order screening in people who do not have a family history but who stack up other risk factors.

When to Screen and How Often

If screening is appropriate, the question becomes when and how often. Cost-effectiveness analyses point to a practical strategy: screen at ages 40 and 55 for people with a family history of subarachnoid hemorrhage. This two-scan approach yielded the best balance of health gains per dollar spent.²⁰ More frequent screening from age 20 to 80 every seven years still produced additional health benefits at acceptable costs, but the returns diminished with each added scan.²⁰ A European analysis reached similar conclusions, finding that screening at 40 and 55 offered the highest net health benefit for people with one affected first-degree relative.²¹

Whether screening the general population without family history is worthwhile is more contentious. An older cost-effectiveness model found that when the annual rupture rate of detected aneurysms was very low (around 0.5% per year, consistent with some large international data), screening actually produced net harm because the risks and costs of treatment outweighed the benefits.²² At a higher rupture rate of about 2% per year, screening became cost-effective. This highlights a real tension in the field: the value of screening depends heavily on what you assume the natural rupture risk is, and different studies have produced different estimates.

What Happens After an Aneurysm Is Found

Finding an unruptured aneurysm does not automatically mean it needs treatment. Many small aneurysms are simply watched over time. To help decide between treatment and observation, doctors often use the PHASES score, which estimates five-year rupture risk based on factors including age, blood pressure history, aneurysm size and location, history of prior subarachnoid hemorrhage, and geographic region.²³ A study of how the score performs in practice found that aneurysms with PHASES scores above 3 were more likely to be treated, while those scoring 3 or below were more likely to be observed, with reasonable accuracy in predicting which aneurysms stayed stable.²⁴

For aneurysms placed on surveillance, most neuroradiologists recommend repeat imaging at least annually. Roughly 59% of surveyed specialists favored lifelong follow-up for small unruptured aneurysms, recognizing that some aneurysms grow slowly over years before becoming dangerous.⁹ This ongoing monitoring typically uses noncontrast MR angiography, avoiding the radiation and contrast exposure that would come with repeated CT angiograms.

Contrast Dye and Safety Considerations

Both CT angiography and some MR angiography protocols use contrast agents, and these carry a small but real set of risks you should know about. CT angiography uses iodine-based contrast, which can rarely trigger allergic-type reactions ranging from mild hives to serious anaphylaxis. There is also concern about contrast-induced kidney injury, though the actual risk is debated. The concern is greatest for people with significantly impaired kidney function.²⁵ For MRI, gadolinium-based contrast agents appear to have a wide safety margin in most patients, though a rare condition called nephrogenic systemic fibrosis can occur in people with very poor kidney function.²⁶ For screening purposes, the preference for noncontrast time-of-flight MR angiography sidesteps these contrast-related concerns entirely.

Aneurysms in Children

Brain aneurysms in children are rare and behave quite differently from those in adults. Pediatric aneurysms more often arise from identifiable causes like trauma, infection, or blood vessel abnormalities, and they tend to occur in different locations, with the posterior circulation affected more often than in adults.²⁷ There is also a male predominance in children, which flips from the female predominance seen in adult aneurysms.²⁸

A review of nearly 1,500 pediatric brain MR angiograms found aneurysms in about 3.3% of patients, but this was a population already undergoing imaging for clinical reasons, not a random sample. The vast majority of those children had an underlying condition, with sickle cell disease being the most common.²⁹ Interestingly, none of the 36 children with a first-degree family history of aneurysms in that study actually had one themselves, though the numbers are too small to draw strong conclusions about familial screening in kids.²⁹ Routine screening of children is not standard practice; imaging is pursued when there is a specific clinical reason.

Mycotic Aneurysms and Infective Endocarditis

Not all brain aneurysms are the typical berry-shaped saccular type. Mycotic aneurysms form when an infection, usually from bacterial endocarditis (an infection of the heart valves), seeds small clumps of bacteria into the brain’s blood vessels, weakening the artery wall. Detecting these is harder with standard noninvasive imaging. A meta-analysis of CT and MRI accuracy for cerebral mycotic aneurysms found a pooled sensitivity of about 71% and specificity of 90%.³⁰ In one study of endocarditis patients who underwent catheter angiography, CT angiography detected mycotic aneurysms in fewer than half of confirmed cases, and MR angiography caught about a third.³¹ Catheter angiography remains the more reliable test in this specific context, particularly when a patient with endocarditis presents with brain hemorrhage.³²

Artificial Intelligence in Aneurysm Detection

AI-assisted detection is an active area of development that is starting to reach clinical use. Roughly half of the research studies in a systematic review used deep learning methods, particularly convolutional neural networks, to flag potential aneurysms on brain imaging.³³ An external validation study of one commercially available AI platform tested on time-of-flight MR angiography found sensitivities ranging from about 88% for very small aneurysms (under 3 mm) up to 100% for larger ones (7 mm and above).³⁴ AI tools are also being developed specifically to help with the infundibular dilation problem, using scores from the neural network’s final layer to distinguish true aneurysms from benign widened vessel origins.³⁵

These tools are designed to act as a second reader rather than a replacement for the radiologist. Their strength lies in reducing the chance that a small or subtly located aneurysm is overlooked during the initial read. Given the volume of brain MRIs performed every day, even a modest improvement in catch rates could matter at the population level.

Transcranial Doppler

Transcranial Doppler ultrasound deserves a mention because it occasionally comes up in discussions about brain aneurysm monitoring, though its role is narrow. It is not used for detecting aneurysms themselves. Instead, it measures blood flow velocity through the brain’s major arteries and is primarily used after a subarachnoid hemorrhage to watch for vasospasm, a dangerous narrowing of arteries that can happen in the days following a bleed.

Compared to CT angiography for detecting vasospasm, transcranial Doppler has high specificity (about 97%) but lower sensitivity (about 72%), meaning it is good at confirming vasospasm when it flags it but misses some cases.³⁶ A systematic review found it performs best for the middle cerebral artery and is less reliable for other vessels.³⁷ Its main advantage is that it can be done at the bedside without moving a critically ill patient to a scanner, so it is used as a daily monitoring tool in neurointensive care rather than as a diagnostic test for the aneurysm itself.