Aneurysms are most often diagnosed through imaging tests that create detailed pictures of your blood vessels. The specific test depends on where the suspected aneurysm is located and whether it’s causing symptoms or was found by accident. Many aneurysms produce no symptoms at all and are discovered during scans ordered for completely unrelated reasons.
How Brain Aneurysms Are Found
When a brain aneurysm is suspected, a CT scan is typically the first test ordered. This specialized X-ray can quickly reveal bleeding in or around the brain, which is the most urgent concern. If the CT scan suggests an aneurysm or if doctors need more detail, a CT angiogram follows. This involves injecting a contrast dye into a vein so that blood flow through the brain’s arteries becomes clearly visible on the scan. The dye highlights the shape and location of any aneurysm present.
MRI and MR angiography offer another approach. MR angiography uses magnetic fields and radio waves instead of radiation to capture detailed images of the brain’s arteries. It can detect the size, shape, and precise location of an aneurysm. MRI tends to take longer than CT but avoids radiation exposure, making it a common choice for monitoring known aneurysms over time.
In some cases, neither CT nor MRI provides enough information. That’s when doctors turn to a cerebral angiogram, also called a cerebral arteriogram. This is a more invasive procedure: a thin catheter is threaded through a blood vessel, usually starting in the groin, up to the arteries in the brain. Contrast dye is injected directly, producing the most detailed images available. Digital subtraction angiography (DSA) remains the gold standard for confirming aneurysm details and planning treatment, but because it requires arterial access, it’s reserved for situations where non-invasive scans aren’t conclusive enough.
There’s one more test that comes into play in a specific scenario. If you have symptoms of a ruptured aneurysm (a sudden, severe headache being the hallmark) but the CT scan doesn’t show bleeding, a lumbar puncture can help. This procedure draws a small sample of cerebrospinal fluid from the lower back. Blood in that fluid suggests a rupture that the CT missed.
Brain Aneurysm Size Categories
Once a brain aneurysm is detected, it’s classified by size, which directly influences decisions about monitoring versus treatment. The National Institute of Neurological Disorders and Stroke defines three categories:
- Small: less than 11 millimeters in diameter, roughly the size of a large pencil eraser
- Large: 11 to 25 millimeters, about the width of a dime
- Giant: greater than 25 millimeters, wider than a quarter
Smaller aneurysms are far more common and often monitored with periodic imaging rather than treated immediately. Larger ones carry higher rupture risk and are more likely to require intervention.
How Aortic Aneurysms Are Detected
Aortic aneurysms, which occur in the body’s largest artery, use a different diagnostic path than brain aneurysms. The aorta runs from the heart down through the chest and abdomen, and aneurysms can develop in either segment.
For abdominal aortic aneurysms, ultrasound is the primary screening and diagnostic tool. It’s painless, uses no radiation, and can accurately measure the diameter of the aorta. The normal infrarenal aorta (the section below where the kidney arteries branch off) averages about 2.0 cm in diameter. Ninety-five percent of adults have an aorta measuring 3.0 cm or less. An aorta wider than 3.0 cm is generally considered aneurysmal, defined as a dilation at least 50 percent greater than the normal diameter. This threshold applies to both men and women, though women tend to have slightly smaller aortas on average.
When an abdominal ultrasound finds an aneurysm or when more precise measurements are needed, CT angiography provides highly detailed cross-sectional images. This is especially important if an aneurysm is approaching the size where repair might be recommended, because surgeons need exact dimensions and information about the aneurysm’s relationship to nearby blood vessels.
For thoracic aortic aneurysms (those in the chest), CT angiography and MR angiography are the primary imaging tools. Transesophageal echocardiography, where an ultrasound probe is passed down the throat to get close to the heart and aorta, can also detect thoracic aneurysms. However, it has slightly lower sensitivity than CT or MR angiography, partly because it has a blind spot in portions of the ascending aorta and upper arch. It also requires sedation and can’t visualize the abdominal aorta, so it’s typically used as a complement to other imaging rather than a standalone test.
Screening Before Symptoms Appear
Most aneurysms cause no symptoms until they become large or rupture, which makes screening critical for people at higher risk. The U.S. Preventive Services Task Force recommends a one-time abdominal aortic aneurysm screening with ultrasound for men aged 65 to 75 who have ever smoked. “Ever smoked” is defined as having smoked 100 or more cigarettes in a lifetime, so even people who quit decades ago qualify.
For men in that same age range who have never smoked, screening is offered selectively based on other risk factors like family history. The Task Force has not established a routine screening recommendation for women, though women with risk factors (particularly smoking history or a family history of aortic aneurysm) may benefit from discussing screening with their doctor.
There is no equivalent population-wide screening program for brain aneurysms. However, people with a strong family history of brain aneurysms or certain genetic conditions (like polycystic kidney disease or connective tissue disorders) may be offered MR angiography to check for unruptured aneurysms.
Aneurysms Found by Accident
A significant number of aneurysms are discovered incidentally, during imaging ordered for something else entirely. You might get a head CT after a car accident, an MRI for chronic headaches, or an abdominal scan for kidney stones, and the radiologist spots an aneurysm that wasn’t causing any problems. Studies looking at large numbers of cerebral angiograms have found asymptomatic, unruptured brain aneurysms at a rate of roughly 0.6 to 1.5 percent across all adult age groups from 35 to 84.
An incidental finding can be unsettling, but it’s actually an advantage. Unruptured aneurysms found early can be monitored with periodic imaging, and your doctor can assess whether the size, shape, and location warrant preventive treatment or watchful waiting. Many small, incidentally discovered aneurysms never require intervention.
What Happens After Diagnosis
The diagnostic process doesn’t end with finding the aneurysm. Doctors assess several factors to determine next steps: the aneurysm’s size, its location, its shape (irregular shapes carry more risk), how fast it’s growing, and your overall health. For brain aneurysms, this often means repeat imaging every 6 to 12 months initially. For aortic aneurysms, ultrasound or CT scans are repeated at intervals that depend on the current diameter.
If treatment is being considered, more detailed imaging may be needed for surgical planning. This is where catheter-based angiography often comes in. CT angiography is less invasive and works well for many planning scenarios, but if the images aren’t detailed enough to guide a procedure, digital subtraction angiography provides the highest-resolution view of the blood vessels and any residual or recurrent aneurysm after previous treatment.
The key takeaway is that aneurysm diagnosis is rarely a single test. It’s typically a sequence, starting with the fastest and least invasive option and escalating to more detailed imaging only when needed.