Ultrasound is as accurate as computed tomography (CT) and more accurate than angiography in measuring aneurysmal diameter. Specific indications for ultrasound evaluation of the aorta include abdominal pain in hypotensive patients and elderly patients with unexplained back, flank, or abdominal pain.
SONOGRAPHIC CONSIDERATIONS The aorta is located in the midline of the abdomen, to the left of the inferior vena cava (IVC), just anterior to the spine and posterior to all other abdominal contents (Fig 2.95-4:.). The aorta normally tapers as it progresses distally, and any diameter greater than 3 cm is abnormal. An ultrasound examination that images the aorta from the diaphragm to its distal bifurcation is extremely accurate at confirming or refuting the diagnosis of abdominal aortic aneurysm (AAA). Although pressure on the transducer usually displaces intervening bowel gas, complete visualization is sometimes impossible. Such examinations are considerated indeterminate.
FIG. 295-4. Transverse view of a normal abdominal aorta and inferior vena cava (IVC). Note vertebral body is represented by the large hypoechoic area in the far field.
With experience, it is generally not difficult to differentiate the aorta from the IVC/Fog. The IVC, usually to the right of the aorta, has thinner walls and changes remarkably in size with probe pressure and the Valsalva maneuver. Both the aorta and the IVC are pulsatile, although the gentle, undulating pulsation of the IVC can, with experience, be differentiated from the forceful, centripetal aortic pulsation.
The primary sonographic finding of AAA is an aortic diameter greater than 3 cm. When AAA is suspected, the aorta should be imaged in the transverse and sagittal planes from the diaphragm to its bifurcation at the level of the umbilicus. Transverse images measured horizontally from outside wall to outside wall are the most reliable in accurately determining true aortic size. Echogenic thrombus within the outer margins of AAAs is very common, and care must be taken to identify the outer limits of the aortic wall (Fig..295-5). While ultrasound is very accurate in measuring AAA size, it is often impossible to determine sonographically whether the AAA has ruptured, since rupture most often occurs into the retroperitoneal space. A hypoechoic retroperitoneal hematoma may be seen but is often very difficult to identify. AAA rupture into the peritoneal space is usually fatal.
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