An abdominal aortic aneurysm is a weak, ballooned section of the body’s main artery as it runs through the belly, and a CT scan is how doctors see it most completely. It shows how large the bulge is, what lines its inside, and whether it looks stable. The size measured on CT is the number that most shapes whether you are watched or offered repair.
What a CT Angiogram Shows That Other Scans Miss
A CT angiogram, or CTA, is a CT scan taken while iodine dye flows through your arteries, making them show up brightly. Because of that, it shows the tissue around the aorta and exactly how far the aneurysm extends, and the images can be rebuilt and viewed in any direction.1PubMed Central. Abdominal aortic aneurysm: pictorial review of common appearances and complications It has largely replaced the older test of threading a catheter into the artery and injecting dye directly.2PubMed Central. Abdominal aortic aneurysm: pictorial review of common appearances and complications
The appointment itself is simple. You lie on a table that slides through a ring-shaped scanner for a few minutes. The dye goes in through a vein in your arm and can cause a brief warm feeling. You will be asked to hold your breath for short periods so the images stay sharp. The results usually go to your vascular specialist rather than straight to you.
Ultrasound is the usual tool for screening and for regular size checks.3JOURNAL OF CLINICAL AND DIAGNOSTIC RESEARCH. Observational Study Comparing the Measurement of Abdominal Aortic Aneurysm (AAA) Diameter in Different Morphology with Ultrasound and CT Scan On average, though, it reads an aneurysm about 3 mm smaller than CT does.4JOURNAL OF CLINICAL AND DIAGNOSTIC RESEARCH. Observational Study Comparing the Measurement of Abdominal Aortic Aneurysm (AAA) Diameter in Different Morphology with Ultrasound and CT Scan In one screening comparison, ultrasound missed roughly a third to over 40% of aneurysms that CT, even without dye, picked up.5PubMed Central. Abdominal ultrasound-scanning versus non-contrast computed tomography as screening method for abdominal aortic aneurysm – a validation study from the randomized DANCAVAS study So if your ultrasound and CT numbers differ by a few millimetres, the scan type may explain it rather than real growth. Ask which scan type will be used for follow-up so your measurements stay comparable.
Part of the gap comes from shape. Aneurysms are often lopsided, so a measurement taken in one direction can miss the widest point, which CT can measure from any angle.
On your report, the aneurysm is measured on cross-sections, slices taken across the body, and radiologists usually pick the widest measurement from outer wall to outer wall. The report usually also gives the length of the aneurysm and where it starts relative to the kidney arteries, which tells the surgeon how much healthy artery is available. The scan also shows the iliac arteries, the branches into the legs. They sometimes balloon as well, and if so they are mentioned too.
Size on Your Report and When Repair Is Considered
- Under 30 mm: usually a normal belly aorta.6The Lancet. Abdominal aortic aneurysm
- 30 mm or more: conventionally diagnosed as an aneurysm.7The Lancet. Abdominal aortic aneurysm
- About 55 mm: the usual point at which repair is recommended.8PubMed. Rupture Risk of Large Abdominal Aortic Aneurysms: A Scoping Review
An aneurysm forms as the wall loses its stretchy elastic fibres and muscle cells and thins.9The Lancet. Abdominal aortic aneurysm10PubMed. Abdominal aortic aneurysms Aneurysms of any size can burst, but the risk climbs as the diameter grows.11PubMed. Abdominal aortic aneurysms The repair threshold balances that rupture risk against the risks of surgery, and when rupture risk is lower, the right size to repair is larger.12PubMed Central. Size thresholds for repair of abdominal aortic aneurysms warrant reconsideration Some experts argue the 55 mm line should be reconsidered as rupture estimates change.13PubMed Central. Size thresholds for repair of abdominal aortic aneurysms warrant reconsideration
Reports may give the size as 4.2 cm or 42 mm, which mean the same thing. The key figure is usually labelled maximum diameter. When you compare new and old numbers, use the same unit and, if you can, the same scan type.
Most aneurysms found below the repair size are not operated on. A diagnosis starts monitoring, not necessarily surgery, and growth over time is watched as closely as any single measurement.
Between 30 and 55 mm, watchful waiting means rechecking with scans at set intervals, and the gap between scans typically shortens as the aneurysm gets larger. Not smoking and controlling blood pressure are standard advice during this time.
About 55 mm is the usual line for most men, but doctors may suggest repair at a lower size in women, whose aortas are smaller and who tend to rupture at smaller sizes. Rapid growth, new pain, or an unusual saccular shape can prompt repair sooner than the usual line. Your age, heart and lung health, and other illnesses are weighed against the risk of surgery.
The Clot Lining the Aneurysm
Your report may mention intraluminal thrombus, a layer of clot stuck to the inside wall of the aneurysm, which this article calls the clot lining. It is present in about 70 to 80% of people with an aneurysm.14PubMed Central. The Abdominal Aortic Aneurysm and Intraluminal Thrombus: Current Concepts of Development and Treatment Blood usually still flows freely through the channel in the middle.15PubMed Central. The Abdominal Aortic Aneurysm and Intraluminal Thrombus: Current Concepts of Development and Treatment
On a dye-enhanced scan, that bright channel of flowing blood sits in the centre, and the clot appears as a duller grey layer between the channel and the outer wall. That is why the outer measurement is larger than the width of the bright channel, and why the report should be based on the outer wall.
The lining matters for growth. Aneurysms with it grew about twice as fast as those without: around 2 mm a year versus 1 mm.16PubMed Central. Intraluminal Thrombus Predicts Rapid Growth of Abdominal Aortic Aneurysms In aneurysms of 3 to 4 cm, those with clot grew nearly twice as fast.17PubMed Central. Intraluminal Thrombus Predicts Rapid Growth of Abdominal Aortic Aneurysms The clot starves the wall underneath of oxygen and feeds inflammation that weakens it.18PubMed Central. The Abdominal Aortic Aneurysm and Intraluminal Thrombus: Current Concepts of Development and Treatment It may also cushion the wall from pressure, though that depends on how firmly it is attached.19PubMed Central. The Abdominal Aortic Aneurysm and Intraluminal Thrombus: Current Concepts of Development and Treatment
This clot is not the same as a clot in the legs or lungs, and it is not something that typically breaks off and travels. It is a usual part of how aneurysms develop, and your doctor uses it mainly as a clue about growth speed. People with larger amounts of clot had about twice the rate of heart attacks, strokes and similar events over 4 years, roughly half versus a quarter.20PubMed Central. Thrombus volume is associated with cardiovascular events and aneurysm growth in patients who have abdominal aortic aneurysms
If your report mentions it, ask whether it changes how often you will be scanned, whether you should be on medicines that protect the heart and arteries, and whether it affects which repair would suit you later.
Signs on CT That an Aneurysm Is Unstable or Leaking
Most ruptures show up as a pool of blood behind the abdominal lining, next to the aneurysm.21PubMed. Spectrum of CT findings in rupture and impending rupture of abdominal aortic aneurysms A break in the wall, or a gap in the ring of calcium in the wall, can point to where it tore.22PubMed. Spectrum of CT findings in rupture and impending rupture of abdominal aortic aneurysms
Some signs appear before a rupture. A bright crescent inside the wall means fresh bleeding into it and is a sign rupture may be near.23PubMed. Spectrum of CT findings in rupture and impending rupture of abdominal aortic aneurysms24PubMed Central. Signs of impending rupture in abdominal aortic and iliac artery aneurysms by computed tomography: Outcomes in 41 patients Other warning signs include fast enlargement, bleeding into the clot lining, and small bulges or blisters on the wall.25PubMed. Spectrum of CT findings in rupture and impending rupture of abdominal aortic aneurysms26PubMed Central. Signs of impending rupture in abdominal aortic and iliac artery aneurysms by computed tomography: Outcomes in 41 patients An aneurysm draped over the backbone suggests a leak that is being held in place.27PubMed. Spectrum of CT findings in rupture and impending rupture of abdominal aortic aneurysms Any of these findings usually prompts urgent assessment for repair rather than routine follow-up.
A warning sign means the wall is under stress, not that it has torn, so there may be time for an urgent planned repair. Radiologists usually phone the treating doctor the same day if they see one, so you may be contacted quickly.
In an emergency, sudden severe belly or back pain, fainting or a racing heart can prompt an urgent CT. The scan takes minutes and shows surgeons where to act. Anyone with a known aneurysm and these symptoms should call emergency services.
Infected aneurysms are a rare type. Infection can cause a lumpy aneurysm with swollen tissue around it, sometimes with gas. These tend to grow and leak fast, often within weeks, so fever plus belly or back pain in someone with an aneurysm should be raised promptly.
How CT Is Used to Plan a Repair
Many aneurysms are repaired with EVAR (stent-graft repair), a fabric-covered stent passed up through the groin arteries to line the aneurysm. The graft must seal against a stretch of healthy artery just below the kidney arteries, called the neck.28PubMed Central. Patient Selection Criteria for Endovascular Aortic Aneurysm Repair: Optimizing Outcomes Standard device guidance asks for a neck at least 15 mm long and 18 to 32 mm wide, not sharply bent.29PubMed Central. Suitability of the Aortic Neck Anatomy for Endovascular Aneurysm Repair in Korean Patients with Abdominal Aortic Aneurysm An unsuitable neck, followed by groin and hip arteries that are too narrow, are the main reasons people cannot have this repair.30PubMed. Improvement of patient eligibility with the use of new generation endografts for the treatment of abdominal aortic aneurysms. A comparison study among currently used endografts and literature review
CT measures more than the neck. It gives the length of the aneurysm and the angle of the neck, since a neck that bends sharply can stop the graft sitting flat against the wall. It checks how much calcium and clot lie in the neck, because heavy amounts there can make a seal less reliable. It also maps the branches off the aorta, such as the kidney arteries, which the graft must not block.
If the anatomy doesn’t fit, open surgery or a custom device may be offered instead. Newer devices have widened the range of people who qualify.31PubMed. Improvement of patient eligibility with the use of new generation endografts for the treatment of abdominal aortic aneurysms. A comparison study among currently used endografts and literature review
The two approaches differ in practice. A stent graft uses small groin cuts and usually means a shorter hospital stay. Open surgery replaces the section through a belly incision. Stent grafts need lifelong scan checks, while open repair needs fewer. A decision aid comparing both can help you weigh them.
At the planning visit, ask whether your neck and leg arteries suit a stent graft, and what the alternative is if they don’t. Ask too how your clot lining or kidney function affects the plan.
Checking the Stent Graft With CT Afterward
The main thing follow-up scans look for is an endoleak, blood still flowing into the old aneurysm sac outside the graft.32PubMed Central. CT angiography for the assessment of EVAR complications: a pictorial review It happens in about 15 to 30% of people in the first month.33PubMed Central. CT angiography for the assessment of EVAR complications: a pictorial review Scans are taken right after dye goes in and again a little later, because some leaks only show up on the later images.34PubMed. Clinical significance of endoleak detected on follow-up CT after endovascular repair of abdominal aortic aneurysm
Leaks happen for several reasons. Small arteries branching off the sac can still feed it with blood from the back. Leaks at the top or bottom of the graft happen when the seal against the artery wall is incomplete or loosens over time, including when the aorta widens over the years, which is why checks continue long term.
The most common kind comes from small side arteries filling the sac backward, and up to 40% close on their own.35PubMed Central. CT angiography for the assessment of EVAR complications: a pictorial review These are usually watched, with treatment if they last over 6 months and the sac grows more than 5 mm.36PubMed Central. CT angiography for the assessment of EVAR complications: a pictorial review Leaks at the graft ends or from a gap in the graft are high-pressure and usually need prompt repair, often through another catheter procedure.37PubMed Central. CT angiography for the assessment of EVAR complications: a pictorial review Sometimes the sac grows with no leak visible, and other scan types are used to look for a hidden one.38PubMed Central. CT angiography for the assessment of EVAR complications: a pictorial review
The main comparison on each scan is the sac’s size against earlier scans, since a sac that is stable or shrinking suggests the repair is working. The report also looks at whether the graft has moved or kinked, and whether its limbs are still open. Because comparison matters, having scans done at the same centre or with images shared makes these checks easier.
Missing scheduled scans can let a silent leak go unnoticed. Ask whether some checks can be done by ultrasound to limit dye.
Contrast Dye and Your Kidneys
The iodine dye can narrow kidney blood vessels and injure kidney cells.39PubMed Central. Renal function-adjusted contrast medium volume is a major risk factor in the occurrence of acute kidney injury after endovascular aneurysm repair Kidney injury after a contrast CT scan has been estimated at up to 12% in the general population.40PubMed Central. Use of ultra-low contrast dose CT aortography for the management of aortic aneurysmal disease The risk is higher with conditions common in aneurysm patients, and more dye means more risk.41PubMed Central. Use of ultra-low contrast dose CT aortography for the management of aortic aneurysmal disease
People with existing kidney disease, diabetes, dehydration, or older age are more likely to have kidney injury after dye. Certain medicines, such as anti-inflammatory painkillers and some water pills, can add to the strain, so the team may ask you to hold them. Most people with healthy kidneys have no lasting problem.
Before your scan, a blood test of kidney function is often done first, and drinking fluids as advised can help protect the kidneys. Ask whether metformin or other medicines should be paused. Tell the scanning team about kidney disease, diabetes or past dye reactions beforehand.
There are alternatives. MRI without dye can give measurements as reliable as CT for repair planning.42PubMed. Preoperative planning for endovascular aortic repair of abdominal aortic aneurysms: feasibility of nonenhanced MR angiography versus contrast-enhanced CT angiography43PubMed. Prospective intraindividual comparison of unenhanced magnetic resonance imaging vs contrast-enhanced computed tomography for the planning of endovascular abdominal aortic aneurysm repair It is an option for people whose kidneys can’t handle dye.44PubMed. Prospective intraindividual comparison of unenhanced magnetic resonance imaging vs contrast-enhanced computed tomography for the planning of endovascular abdominal aortic aneurysm repair
Unexpected Findings Elsewhere on the Scan
Aneurysm CT scans often spot findings outside the blood vessels, and about half of those people needed further tests.45PubMed. Extravascular incidental findings at multislice CT angiography of the abdominal aorta and lower extremity arteries: a retrospective review study In one large group, about 5% had a cancer found, mostly one not known before.46PubMed. Extravascular incidental findings at multislice CT angiography of the abdominal aorta and lower extremity arteries: a retrospective review study In people scanned repeatedly after stent repair, about 12% had a possible cancer flagged over their follow-up.47CVIR Endovascular. Incidental extravascular findings in CT angiograms in patients post endovascular abdominal aortic aneurysm repair: clinical relevance and frequency So follow up on any extra finding your report lists, even if your aneurysm is stable. Ask who is responsible for arranging the next test, and keep a copy of the report to share with your family doctor.
The scan picks up so much because it covers the kidneys, liver, pancreas, bowel, adrenal glands, spine and lower lungs, so any abnormality in these areas can appear on the report. Many findings are variations or harmless.