An aneurysm is a localized bulging or ballooning in the wall of a blood vessel, which can occur in any artery throughout the body. A computed tomography (CT) scan can visualize an aneurysm, but visibility depends on the specific CT method used and whether the aneurysm has ruptured. A standard CT scan is often the first tool used in an emergency to assess for bleeding around the brain, a sign of rupture. However, specialized CT imaging is required to clearly map the blood vessels and definitively diagnose the location, size, and shape of an unruptured aneurysm.
The Difference Between Standard CT and CT Angiography
The technical difference between a standard CT scan, known as a non-contrast CT (NCCT), and a CT Angiography (CTA) lies in the use of an intravenous contrast agent. NCCT does not involve injecting a dye and is primarily used in emergencies to assess for hemorrhage. If an aneurysm has ruptured, the escaped blood appears as a bright white area, typically indicating a subarachnoid hemorrhage (SAH). However, an unruptured aneurysm is generally not clearly visible on an NCCT because the blood density inside the sac is similar to the surrounding brain tissue.
The CTA procedure overcomes this limitation by requiring the rapid injection of an iodine-based contrast material into a vein. As the contrast travels through the bloodstream, it illuminates the arteries, making them appear bright white on the CT images. This allows the radiologist to visualize the blood vessel structure in detail, including the size, shape, and exact location of the aneurysm sac. CTA is considered the definitive CT method for diagnosing unruptured aneurysms and is highly accurate for lesions larger than 3 millimeters.
Aneurysm Visualization Based on Location
The visibility and the preferred CT protocol depend heavily on the aneurysm’s location in the body, particularly differentiating between those in the brain and those in the body’s major arteries. For cerebral (brain) aneurysms, the immediate concern is rupture, which causes bleeding into the space surrounding the brain, known as subarachnoid hemorrhage. When a patient presents with sudden severe headache, a non-contrast CT is performed first to quickly confirm or rule out this hemorrhage.
If a subarachnoid hemorrhage is detected, a CTA is performed immediately afterward to locate the source, as the contrast highlights the ruptured vessel. In patients who have no bleeding but are considered high-risk due to family history or other factors, CTA is a common non-invasive screening tool to look for unruptured aneurysms. The high sensitivity of modern CTA technology for aneurysms larger than 3 to 5 millimeters makes it a reliable method for evaluating these intracranial vessels.
Aortic and peripheral aneurysms, such as an abdominal aortic aneurysm (AAA) or a thoracic aortic aneurysm (TAA), are typically assessed using CTA as the primary imaging modality. These aneurysms involve the body’s largest artery, and the CTA is used to accurately measure the vessel’s diameter, monitor its growth over time, and look for signs of dissection. The contrast injection provides clear visualization of the vessel’s lumen and wall, which is essential for planning surgical or endovascular repair.
When CT Results Are Inconclusive or Negative
Despite the advanced capabilities of CT Angiography, limitations can lead to inconclusive or negative results even when an aneurysm is present. A major challenge is the size of the lesion, as the sensitivity of CTA decreases for very small aneurysms, often referred to as microaneurysms, which are typically less than 3 millimeters in diameter. Modern CTA techniques have improved detection rates for these tiny lesions, but a small percentage can still be missed.
Aneurysms located near dense bone structures, such as those at the base of the skull or in the internal carotid artery, can also be difficult to visualize clearly due to artifacts caused by the bone. Technical factors, including involuntary patient movement during the scan or poor timing of the contrast injection, can degrade the image quality and obscure a small aneurysm. Furthermore, patients with compromised kidney function may not be able to receive the necessary high dose of contrast material, limiting the effectiveness of the CTA.
If there is a strong clinical suspicion of an aneurysm despite a negative or inconclusive CT or CTA result, a physician may recommend specialized testing. These follow-up imaging studies include Magnetic Resonance Angiography (MRA), which uses magnetic fields instead of X-rays, or Digital Subtraction Angiography (DSA). DSA is an invasive procedure that involves threading a catheter through the blood vessels but remains the gold standard for providing the most detailed images of the vascular system.