Abdominal aortic aneurysm (AAA) is defined as a dilatation of the infrarenal aorta exceeding 50% of its normal diameter (2.0 cm), typically >3 cm. Atherosclerosis is the most common etiological factor; male sex, advanced age, smoking, and family history are the primary risk factors. Most patients are asymptomatic and diagnosed incidentally. Rupture risk correlates with aneurysm diameter: <1% per year at <4 cm, 3-15% at 5-5.9 cm, and 20-40% at >7 cm. CTA is the gold standard for diagnosis and preoperative planning, providing detailed assessment of aortic morphology, branching pattern, mural thrombus extent, and iliac arteries. Screening ultrasound is recommended for men aged 65-75. Treatment is endovascular (EVAR) or open surgical repair; 5.5 cm is the accepted threshold.
Age Range
55-85
Peak Age
70
Gender
Male predominant
Prevalence
Common
The pathogenesis of AAA is based on progressive loss of structural integrity of the aortic wall. The atherosclerotic process begins in the intima and extends to the media, causing proteolytic degradation of elastin and collagen fibers (matrix metalloproteinases — MMP-2 and MMP-9). Apoptosis of smooth muscle cells in the media and elastin fragmentation reduce wall resistance; according to Laplace's law (wall tension = pressure x radius / wall thickness), increasing radius increases wall tension, creating a vicious cycle of expansion. Mural thrombus forms in the dilated lumen due to turbulent flow causing platelet activation and fibrin deposition — appearing on CTA as a hypodense crescent or ring between the lumen and aneurysm wall. The thrombus impedes oxygen diffusion, increasing wall ischemia and accelerating proteolytic activity. In inflammatory AAA (5-10%), marked periaortic fibrosis and inflammation cause wall thickening and adhesion to surrounding tissues — a thick wall enhancing in the delayed phase is characteristic. The predilection for the infrarenal segment is attributed to ischemic wall weakening from decreased vasa vasorum density in this region.
Posterior aortic wall draping over the vertebral body like a curtain — a critical finding indicating loss of posterior wall integrity and rupture/impending rupture. Confirms rupture diagnosis when accompanied by retroperitoneal hematoma.
Infrarenal aortic diameter >3 cm with crescent or ring-shaped hypodense mural thrombus surrounding the lumen. The thrombus shows low density (30-50 HU) while calcified plaques along the wall are seen at high density (>150 HU). The outer diameter (adventitia to adventitia) must be measured including the mural thrombus. Acute thrombus may be hyperattenuating (60-70 HU), chronic thrombus hypoattenuating (30-40 HU). Crescent sign — a crescent-shaped high-density area within mural thrombus — indicates acute hemorrhage and impending rupture.
Report Sentence
Fusiform aneurysmal dilatation of the infrarenal aorta measuring __x__ mm is noted with mural thrombus surrounding the lumen.
In the arterial phase, the contrast-filled true lumen appears brightly hyperdense (300-400 HU) while mural thrombus remains non-enhancing and hypodense (30-50 HU). This phase best demonstrates the difference between true lumen diameter and outer aneurysm diameter. Iliac artery extension, accessory renal arteries, and visceral branching pattern are assessed in this phase for EVAR planning. Proximal neck length (at least 15 mm required), diameter, and angulation are measured.
Report Sentence
On arterial phase CTA, the infrarenal aortic aneurysm demonstrates a contrast-enhanced true lumen diameter of __ mm with surrounding mural thrombus yielding a total aneurysm diameter of __ mm.
In inflammatory AAA, the delayed phase (3-5 min) shows thick (>5 mm), homogeneous wall enhancement. Periaortic soft tissue thickening causes retroperitoneal adhesions and may encase the ureters, duodenum, and IVC. Normal AAA shows minimal or no wall enhancement. The inflammatory component may be associated with IgG4-related disease or retroperitoneal fibrosis.
Report Sentence
Delayed phase demonstrates marked aneurysm wall thickening (__ mm) with enhancement and periaortic soft tissue reaction, consistent with inflammatory AAA.
B-mode US demonstrates infrarenal aortic anteroposterior diameter >3 cm. Mural thrombus appears as an echogenic-hypoechoic layered structure around the lumen — fresh thrombus is hypoechoic, organized thrombus is echogenic. Aortic wall calcifications appear as hyperechoic foci with posterior acoustic shadowing. Outer-to-outer wall measurement is the standard for US screening. Obesity and bowel gas may limit visualization. US sensitivity for AAA detection is 95-100%.
Report Sentence
An aneurysm of the infrarenal aorta measuring __ mm in anteroposterior diameter is noted with mural thrombus surrounding the lumen.
Color Doppler demonstrates turbulent flow pattern within the aneurysm sac — described as yin-yang sign or swirling pattern. Antegrade flow is preserved in the true lumen while mural thrombus appears as an avascular (flow-free) zone. Spectral Doppler shows a low-resistance biphasic waveform. Tardus-parvus pattern in distal iliac arteries suggests occlusive disease.
Report Sentence
Color Doppler demonstrates turbulent flow within the aneurysm sac with mural thrombus appearing as an avascular zone.
On MRA, mural thrombus shows heterogeneous T1 signal: acute thrombus is T1 hyperintense due to methemoglobin content, chronic organized thrombus is T1 iso- to hypointense due to fibrosis. On gadolinium MRA, the true lumen enhances brightly while thrombus does not enhance. On bright blood techniques (TOF, CE-MRA), patent lumen is hyperintense, thrombus is hypointense. On dark blood techniques (black blood TSE), wall morphology and thrombus layering are better assessed. MRA is an alternative to CTA in patients who cannot receive nephrotoxic contrast.
Report Sentence
MRA demonstrates infrarenal aortic aneurysm with heterogeneous signal mural thrombus surrounding the lumen; the true lumen is patent and enhancing.
In ruptured or impending rupture AAA, the posterior aortic wall drapes over the vertebral body like a curtain — draped aorta sign. It indicates loss or weakening of posterior wall integrity. The fat plane between the posterior aortic wall and vertebra is obliterated. When seen with retroperitoneal hematoma, it confirms rupture diagnosis, but even without hematoma it may indicate impending rupture.
Report Sentence
The posterior wall of the aneurysm drapes over the vertebral body (draped aorta sign), suggesting impending/active rupture requiring urgent evaluation.
Criteria
Symmetric dilatation involving the entire circumference; most common type (90%)
Distinct Features
Symmetric diameter increase, usually surrounded by mural thrombus, infrarenal location, long segment involvement
Criteria
Eccentric dilatation on one side of the aortic wall; focal outpouching
Distinct Features
Asymmetric, focal ballooning, wide or narrow neck, important when mycotic aneurysm suspected, rupture risk may be higher than fusiform
Criteria
AAA + periaortic wall thickening (>5 mm) + delayed wall enhancement + surrounding tissue adhesion
Distinct Features
5-10% of all AAAs, may cause ureteral obstruction, abdominal/back pain more common, elevated CRP/ESR, may be IgG4-related, surgery challenging (adhesions)
Criteria
Aneurysm neck adjacent to renal arteries (juxtarenal) or involving renal arteries (pararenal)
Distinct Features
EVAR technically challenging, fenestrated/branched stent may be needed, high renal ischemia risk, open surgery more often required
Distinguishing Feature
Rupture shows retroperitoneal hematoma, active contrast extravasation, draped aorta sign, hemoperitoneum — absent in intact AAA
Distinguishing Feature
Dissection shows intimal flap and dual lumen (true+false); AAA has no flap, dilatation is symmetric/asymmetric with mural thrombus
Distinguishing Feature
Mycotic aneurysm is usually saccular, eccentric, rapidly growing, with periaortic gas/fluid collection; AAA develops slowly without infectious findings
Distinguishing Feature
Penetrating ulcer shows focal ulcer crater and subintimal hematoma; AAA shows diffuse fusiform widening and circumferential mural thrombus
Urgency
urgentManagement
surgicalBiopsy
Not NeededFollow-up
6-monthAAA management depends on diameter: <4 cm surveillance (annual US), 4-5.4 cm close follow-up (6-monthly US/CT), >=5.5 cm or >0.5 cm/6-month growth rate indicates EVAR or open surgical repair. Symptomatic AAA (abdominal/back pain, pulsatile mass) requires urgent intervention regardless of size. Rupture mortality is 80-90% (including those not reaching hospital), 40-50% in operated patients. US screening programs (65+ men) have reduced rupture-related mortality by 40%.
AAA >5.5 cm or those growing >1 cm/year are indications for surgical repair (open or endovascular — EVAR). Small aneurysms (<5.5 cm) are followed with ultrasound surveillance. Rupture mortality is 80-90%. Screening: one-time US recommended for men aged 65-75 who have ever smoked.