ABDOMINAL AORTA AORTIC ANEURYSM

THE ABDOMINAL AORTA
The abdominal aorta can be visualized proximally in the midline, posterior to the left lobe of the liver. The coeliac axis and superior mesenteric artery (SMA) are easily demonstrated in longitudinal section (LS), arising from its anterior aspect. In transverse section (TS) the coeliac axis branches, the main hepatic and splenic arteries, may be better appreciated. Just below this level, the origin of the renal arteries is seen. The distal abdominal aorta, which runs more anteriorly, and bifurcation are frequently obscured by bowel gas in sagittal section. A coronal approach from the patient’s left side often overcomes this problem and is also useful in displaying the origin of the renal arteries. The aorta often becomes ectatic and tortuous with age, and it is not unusual to detect considerable calcification of the walls.

Aortic aneurysm
The most frequent referral for aortic scanning is to establish or monitor the presence of an aneurysm. The incidence of aortic aneurysm increases with For this reason, aortic aneurysms are monitored, and a graft placed within the vessel in aneurysms over 5 cm which are increasing in size. Postoperative complications of grafts, such as infection or pseudoaneurysm, are usually monitored with CT or MRI.

Discussions of the benefits of screening programmes for selected populations are ongoing. However, there is some evidence that, despite thereduction of mortality due to aneurysm rupture, overall mortality in men over 65 remains unaffected by screening,3 and it has not been widely adopted into patient management.

Most aneurysms are associated with atherosclerosis, which weakens the media of the wall, causing the vessel to dilate and eventually rupture. The aneurysm may be fusiform or saccular. Blood flow within it is turbulent, and the slow-flowing blood at the edges of the vessel tends to thrombose.

Surgery is always complicated by the involvement of the renal arteries. Fortunately, the vast majority of aneurysms are infrarenal, but it can be difficult to determine the relationship of the aneurysm to the renal artery origins on ultrasound, and CT is helpful in such cases. The use of angiography can be beneficial in this respect; however its disadvantage is that, unlike ultrasound, it displays only the lumen of the vessel and can underestimate the pathology present. Occasionally the aneurysm affects the bifurcation and common iliac arteries, which should be examined during the scan as far as possible. The true maximum diameter of the aneurysm should be ascertained in TS and LS. A true anteroposterior diameter is most accurately measured in LS, by ensuring the calipers lie in a plane perpendicular to the vessel axis at its widest part. To measure the lateral diameter in TS, care must be taken to keep the angle of the transducer perpendicular to the vessel axis to ensure an accurate and reproducible measurement.

The ability of ultrasound to locate the correct plane, regardless of vessel tortuosity, is a distinct advantage over CT, which may over- or underestimate the size of the aneurysm in an axial plane. Complications of aortic aneurysm Dissection of the aneurysm, in which the intima becomes detached, is uncommon in the abdomen. Ultrasound may visualize the intimal flap and the false lumen created between the media and intima often contains slower, more turbulent or even reversed flow. Layers of thrombus may mimic a dissection, and colour flow Doppler is particularly useful in such cases.

Leakage of an aneurysm may cause retroperitoneal haematoma, but CT is usually more reliable in detecting leaks than ultrasound. Rupture of an aortic aneurysm is not unknown in the ultrasound department or emergency department, and is accompanied by abdominal pain and severe hypotension. It is associated with a high mortality rate and is a surgical emergency. Involvement of the renal arteries may cause renal artery thrombosis and subsequently small kidney(s). Always check the kidneys at the time of scanning to ensure they are of normal size and appearance.