Aortic aneurysm imaging
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An abdominal aortic aneurysm is a dilatation of the abdominal aorta that is 50% greater than the normal diameter or greater than 3 cm. They are usually asymptomatic but can cause pain or rupture. Ultrasound is used to monitor AAA size and growth rate, with more frequent scans needed as the aneurysm enlarges. For aneurysms between 5-5.5 cm in men or 5 cm in women, the risk of rupture increases and surgical repair is considered. The goal is to repair before the aneurysm reaches 6 cm in diameter to reduce the risk of fatal rupture.
Aortic aneurysm imaging
- 1. Aortic aneurysm- Imaging Dr. M Sanal kumar Guided by, Dr. G Sudha Dr. T R Saravanan
- 2. Abdominal aortic aneurysms (AAA) are focal dilatations of the abdominal aorta that are 50% greater than the proximal normal segment or that is greater than 3 cm in maximum diameter. Epidemiology Its prevalence increases with age. Males much more commonly affected than females (with a male:female ratio of 4:1).
- 3. Clinical presentation Most AAAs are asymptomatic unless they leak or rupture. Unruptured aneurysms may uncommonly cause abdominal or back pain, or a pulsatile mass, if large. Ruptured aneurysms present with severe abdominal or back pain, hypotension and shock.
- 5. Anatomy The aorta passes through the diaphragm at the level of the T12 vertebral body. It lies slightly to the left of the midline and bifurcates at the level of L4 vertebral body. The surface anatomy landmarks corresponding to these two points are the xiphoid process and the umbilicus. The length of the abdominal aorta is about 13 cm (6 inches). Most scanning of the aorta will therefore take place in the short distance between the sternum and the umbilicus. Immediately below the diaphragm, the celiac trunk is the first major vessel to arise from the aorta in the midline anteriorly. This short (usually less than 1 cm) vessel can often be seen sonographically in the transverse plane, dividing in a “wide Y”. The fork on the patient’s right is the common hepatic artery, heading to the porta hepatis; the fork on the patient’s left, is the splenic artery. This sonographic view is known as the “seagull sign”.
- 6. About 1 cm inferior to the celiac trunk, arises the superior mesenteric artery (SMA). Measurements of the proximal aorta to use as a comparison with distal measurements are made at this level. One centimeter below the SMA, the renal arteries arise on either side.. Thus, these three major vessels occur within about 3 centimeters of the diaphragm. 90% of all AAA’s will occur distal to this point.
- 7. Most AAAs begin below the renal arteries and end above the iliac arteries. The size, shape, and extent of AAAs vary considerably. Like aneurysms of the thoracic aorta, AAAs may be broadly described as either fusiform (circumferential) or saccular (more localized).
- 9. Causes Atherosclerosis (most common) Inflammatory abdominal aortic aneurysm Chronic aortic dissection Vasculitis, e.g. Takayasu arteritis Connective tissue disorders, e.g. Marfan syndrome Ehlers-Danlos syndrome Mycotic aneurysm Traumatic pseudoaneurysm Anastomotic pseudoaneurysm
- 10. The natural history of abdominal aortic aneurysms (AAA) is that of slow expansion and rupture with devastating consequences. The risk of rupture is proportional to the size of the aneurysm and the rate of growth. Differing rates of rupture for a given aneurysm size have been reported in the literature but the general consensus is that aneurysms greater than 5.0 cm in women and 5.5 to 6.0 cm in men carry a significantly increased risk of rupture and should be treated. Furthermore, aneurysms that expand greater than 10 mm per year are also at significant risk of rupture and are considered for treatment even when less than 5.0 cm.
- 11. Ultrasound Ultrasound assessment is simple, safe and inexpensive. It has a reported sensitivity of 95% and specificity close to 100%. It is usually the preferred choice for monitoring of small aneurysms.
- 12. Technique for ultrasound scanning of the aorta 1) Orientation. Start in the transverse plane (pointer to “9 o’clock”), high in the epigastrium, using the liver as a sonic “window”. Identify the vertebral body (a dark, rounded shape, with dense shadow). 2) Identify the aorta on the patient’s left, and the IVC (patient’s right) “above” the vertebral body on the ultrasound image. 3) In real time obtain transverse images of the aorta from the celiac to the bifurcation. 4) Obtain views of the iliacs if possible. 5) Rotate the probe’s pointer clockwise from the "9 o' clock" to the “12 o’clock” position for sagittal views from the celiac to the bifurcation. 6) Attempt to obtain: 1) at least 3 transverse views, labeled, “high”, “middle”, “low”, with calipers. One view should show the maximal aortic diameter. 2) Sagittal view(s) from the celiac to the bifurcation
- 17. Aorta is visualized first in short axis and then in long axis. Large (>7 cm) abdominal aortic aneurysm with mural thrombus and hypoechoic areas is noted outside aorta, which may represent rupture. Colorflow Doppler illustrates turbulent flow within lumen.
- 18. Using Sonography to Monitor the Growth of an Abdominal Aortic Aneurysm The accepted method of monitoring abdominal aortic aneurysm is as follows: If aneurysm measures < 4cm in diameter, the patient is scanned once each year. If aneurysm measures between 4cm- 5cm in diameter, the patient is scanned every 6 months. If aneurysm measures between 5cm - 5.5cm in diameter, the patient is scanned every 3 months. Once the aneurysm reaches 5.5cm in diameter, the patient is scheduled for surgical repair. It is important to repair the aneurysm before the diameter reaches 6cm because of increased risk of rupture. If an aneurysm is growing rapidly, repair be scheduled sooner to avoid rupture.
- 19. The mortality rate from a ruptured AAA is high (59-83%) of patients succumb to death before they make it to hospital or undergo surgery. The operative mortality rate for those who make it to surgery tends to be around 40%.
- 20. Pearls and Pitfalls •Obtain measurements of the aorta from outer wall to outer wall. Since aneurysms will often contain a thrombus, one may accidentally mistake the inner rim of the thrombus for the aortic wall. Doing this will lead a falsely decreased measurement of the true aortic diameter. •Avoid oblique or angled cuts if possible, especially with a tortuous aorta, which will exaggerate the true aortic diameter. •Transverse views are needed because many AAAs have larger transverse than AP diameter. •A small aneurysm does not preclude rupture: Any symptoms consistent with rupture in a patient with an aortic diameter greater than 3.0 cm should have this diagnosis (or alternative vascular catastrophes) ruled out. •Scanning should be systematically performed in real-time from the diaphragmatic hiatus to the bifurcation in order to avoid missing small, localized saccular aneurysms.