A b figure 58 distal bifurcation of the aorta into

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(A) (B) Figure 5.8 Distal bifurcation of the aorta into the iliac arteries (arrows in A , calipers in B ) above the vertebral body shadow (V). *, inferior vena cava). Diagnostic ultrasound 121 Abdominal aorta ultrasound
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Aorta versus vena cava Differentiating between the IVC and the aorta ( Figure 5.12 ) may seem straightforward, but a few points are worth remembering. Of course, holding the probe marker to the patient’s right will generally reveal the aorta on the right of the screen (the side without the screen marker, thus indicating the patient’s left). In addition, the aorta is a thicker-walled structure than the vena cava and often develops calcifications as sequelae of atherosclerotic plaque. Thus, it may appear to have brightly Figure 5.9 Probe positioning for longitudinal imaging. The probe marker (green dot) faces cephalad. Figure 5.10 Longitudinal aorta (A) with SMA branch point (*) visible. 122 Diagnostic ultrasound Abdominal aorta ultrasound
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echogenic walls. The aorta is generally round, while the IVC is often elliptical or teardrop-shaped. The SMA will generally be visible proximally immediately above the aorta, not the IVC. The aorta is actively pulsatile; however, as mentioned, transmission of pulsations to the IVC from the aorta and the right ventricle can make this distinction difficult. The aorta is not compressible with probe pressure, whereas the vena cava is. The Figure 5.12 Inferior vena cava (IVC) versus aorta (A). True Diameter False Diameter Beam placement Screen image Figure 5.11 Beam placement determines apparent diameter through a cylinder. Diagnostic ultrasound 123 Abdominal aorta ultrasound
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normal aorta tapers as it progresses distally, whereas the vena cava gets somewhat larger as it approaches the renal vessels. Finally, with deep inspirations (sniff test) the IVC will change caliber, whereas the aorta will not. Adequate visualization of the entire length of the aorta is required to exclude AAA. If the diameter of the aorta (from outer wall to outer wall) appears normal over this length, then this excludes a ruptured AAA with an essentially 100% negative predictive value [ 6 –9], although false-positive screening exams have been reported [ 10 ]. Several studies of bedside AAA screening evaluations performed by emergency physicians have reported high rates of technically limited studies ranging from 8% to 33% [ 11 –13], with that number likely influenced by operator experience and difficult body habitus [ 13 ]. Again, remember that an aortic diameter > 3 cm and an iliac artery diam- eter > 1.5 cm are considered abnormal. Do not forget to evaluate the iliac arteries – aneurysmal dilatation and rupture of the iliacs can carry significant morbidity and mortality. Scanning tips Trouble with aorta scanning Bowel gas in the way? ² Apply pressure to minimize artifact caused by bowel gas interposed between the probe and the aorta. Occasionally, it will be necessary to hold constant pressure to force peristalsis of the overlying bowel out of the field of view. If obesity and/or bowel gas still degrade the quality
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