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Aortic Aneurysms
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- Abdominal aneurysm size can be measured with ultrasound imaging in most patients; thoracic
aortic aneurysm size is best measured with CT
- Patients with thoracic aneurysm diameter >6 cm and abdominal aneuryms diameter > 5 cm are
referred for evaluation for aneurysm repair
- CT angiography with three dimensional reconstruction is performed for pre-operative assessment
prior to aneurysm repair and for follow-up imaging after endovascular repair
- One time ultrasound screening for abdominal aortic aneurysms is recommended for those who
are at high risk
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Abdominal
aortic aneurysms (AAAs) cause 1.3% of deaths among men aged 65-85 years
in developed countries, resulting in approximately 15,000 deaths per
year in the United States. The prevalence of abdominal aortic aneurysms
3-5 cm in diameter is 1.3% in men aged 45-54 and 12.5% in men aged
75-84. In comparison, the prevalence in women is 0% and 5.2%
respectively. Prevalence varies by race and AAAs are very rare in
Asians. In comparison, thoracic aortic aneurysms (TAAs) cause about
0.01% of deaths, which are equally distributed between women and men.
Risk factors that increase the likelihood of aneurysms include family
history, smoking, and hypertension.
Aortic aneurysms are
defined as dilatations of ≥3 cm diameter. Most are asymptomatic and are
found incidentally on routine physical examination or radiologic
examinations conducted for another purpose. Some are seen with plain
film radiography, which can detect AAAs if calcification is present,
whereas TAAs can appear as a widening of the mediastinal silhouette,
enlargement of the aortic knob, or tracheal deviation. However, plain
films do not detect all aneurysms and is not suitable for measuring the
size of an aneurysm. Other aortic aneurysms are incidentally detected
by ultrasound, CT, MRI, or nuclear medicine scans.
In addition to incidental findings, some AAAs are found by screening,
which has been recommended as one time ultrasound examinations of
patients at risk because of the prevalence of AAAs and the serious
consequences of their rupture.
| Indications for Imaging for Abdominal Aortic Aneurysm |
| 1. Screening |
| Men ≥65-75 years who have ever smoked |
| Men ≥60 years with sibling or parent with AAA |
| Women ≥ 65 years with cardiovascular risk factor |
| Both men and women >50 years with family history of aneurysmal disease |
| 2. Diagnostic |
| Palpable pulsatile abdominal mass |
| Unexplained lower back or abdominal pain |
| Known aneurysmal disease in extremities |
| Follow-up of previously demonstrated abdominal aortic aneurysm |
| Follow-up of patients with aortic or iliac endoluminal graft |
| Indications for Referral to a Vascular Specialist |
| AAA ≥ 5.0 cm |
| TAA ≥ 6.0 cm |
| Aneurysm expansion ≥0.5 cm in one year |
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Figure 1.
Abdominal aortic aneurysm. Intravenous contrast enhanced CT images of
the abdominal aorta demonstrates an aneurysm measuring 5.4 cm in
diameter. Axial images (A) demonstrate intraluminal thrombus (arrow).
The 3-D reconstruction images (B) demonstrates that the aneurysm
extends from the level of the renal arteries (solid arrow) to the iliac
bifurcation (dashed arrow).
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Diagnostic Imaging
If an aneurysm is incidentally detected by CT or MRI, no further
imaging is required for evaluation. Ultrasound is recommended for
measuring AAAs detected by standard radiography because ultrasound is
accurate, less expensive than other imaging modalities, and avoids
exposure to ionizing radiation. Accuracy is estimated to be within 3 mm
for longitudinal, anterior-posterior, and transverse
direction. However, sensitivity of ultrasound is limited by bowel
gas and body habitus and is estimated to be 92-99% with a specificity
of 100%.
CT without contrast is recommended for measuring the overall dimensions
of TAAs. Three-dimensional (3-D) reconstruction is necessary to
accurately measure the diameter of aneurysms because cross-sectional
imaging may exaggerate the size of an aneurysm in a tortuous aorta,
especially in the thorax. CT with intravenous contrast provides more
detail because it can be used to measure thickness of the aortic wall
outside of calcified deposits, detect bleeding within a thrombus, and
demonstrate the presence of para-aortic fibrosis. Therefore, unless
contra-indicated, contrast-enhanced CT is generally performed.
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| Figure 2.
Endovascular stent. Axial (A) and 3-D reconstruction (B) CT images with
intravenous contrast demonstrates a endoluminal stent originating at
the level of the renal arteries (solid arrow), bifurcating into two
limbs that terminate in the internal iliac arteries (dashed arrow) |
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Surveillance
The annual risk of AAA rupture for aneurysms <4 cm in diameter is
zero, while the risk increases to 3-15% for those that are 5.0-5.9 cm
in diameter. The risk of rupture in women with AAAs of 4.0-5.5 cm is
four times greater than that for men, perhaps because they have smaller
aortas than men. In the thorax, the annual risk of rupture is 3% at
5.0-5.9% diameter, increasing to 7% for TAAs ≥6 cm.
Evidence from clinical trials indicates that surveillance of AAAs that
are less than 5.5 cm is both safe and cost-effective. The intervals
between imaging examinations are shorter for larger aneurysms because
growth rates tend to be greater as the diameter of AAAs increase. In
addition, smoking increases growth rate of AAAs by 15-20% and growth
rates are higher in patients with low body mass index (BMI), whereas
diabetes decreases growth by >30%. Based on large surveillance
studies of growth rates, optimal surveillance intervals between
ultrasound examinations for AAAs have been calculated. If these
intervals are followed, no more than 1% of patients are predicted to
have an aneurysm exceeding 5.5 cm at the time of examination, the size
at which repair is recommended. Surveillance imaging for known TAAs is
recommended at 6-12 month intervals.
| Optimal Surveillance Intervals for Abdominal Aortic Aneurysm* |
| Aneurysm diameter |
Screening Interval |
| 3.5 cm |
36 months |
| 4.0 cm |
24 months |
| 4.5 cm |
12 months |
| 5.0 cm |
3 months |
| *From Brady et al., 2004. Recommendations restrict probability of breaching the 5.5 cm limit to <1%. |
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CTA and MRA
If an AAA is ≥5cm or a TAA is ≥6 cm, CT angiography (CTA) is indicated
to demonstrate the three dimensional anatomy of the aneurysm and its
relationship to branch vessels, which is needed to help determine
whether endovascular or open surgical repair is preferable. Catheter
based angiography is seldom used in surgical planning because it is
more invasive than CTA and is not suitable for 3-D reconstruction. If
nephrotoxicity of CT contrast agents is a concern, MR angiography (MRA)
is an alternate imaging modality; but the images are of lower
resolution and the relatively large quantities of gadolinium contrast
used doses not completely eliminate the concern for nephrotoxicity.
Reconstruction of CTA or MRA images into 3-D images allows examination
of the precise anatomy of the aneurysm, which can be examined from all
angles. These images aid selection of the most suitable stent graft
device and can be used to simulate insertion of a device.
Stent graft devices are used at the MGH in about 70% of AAAs and 40-50%
of TAAs. The success rate for intra-luminal stent graft placement for
aortic aneurysms is high at MGH, with <1% requiring conversion to
surgical repair. CT is recommended for patients following intervention
at 6 months, 1 year, and annually thereafter. CT follow up is required
by the FDA for some devices.
Scheduling
MRI, CT or US for diagnosis and evaluation of aortic aneurysms may be
ordered through ROE (http://mghroe
) for appointments at the MGH Main Campus, Mass
General West Imaging Waltham, and MGH Chelsea Health Center, or by
telephone 617-724-XRAY (9729) for all locations.
Further Information
For further questions, please contact Alan Greenfield, M.D.
, Cardiovascular Radiologist at 617-726-8788.
We would like to thank Dr. Greenfield and Richard P. Cambria, M.D., Chief, Division of Vascular and
Endovascular Surgery, for their assistance and advice for this issue.
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This article provided useful information about the appropriate use of imaging studies:
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References
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Brady AR, Thompson SG, Fowkes FG, Greenhalgh RM, Powell JT. (2004) Abdominal aortic aneurysm expansion: risk factors and time intervals for surveillance. Circulation 110:16-21
Hirsch AT, Haskal ZJ, Hertzer NR, et al. ACC/AHA 2005
Practice Guidelines for the management of patients with peripheral
arterial disease (lower extremity, renal, mesenteric, and abdominal
aortic): Circulation 113:e463-654.
Isselbacher EM. (2005) Thoracic and abdominal aortic aneurysms. Circulation 111:816-828.
Svensjo S, Bengtsson H, Bergqvist D. (1996) Thoracic and thoracoabdominal aortic aneurysm and dissection: an investigation based on autopsy. Br J Surg 83:68-71.
U.S. Preventive Services Task Force (2005) Screening for Abdominal Aortic Aneurysm: Recommendation Statement. Ann Intern Med. 142:198-202.
The United Kingdom Small Aneurysm Trial Participants (2002) Long-term outcomes of immediate repair compared with surveillance of small abdominal aortic aneurysms. N Engl J Med. 346:1445-1452.
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