A registrar happens to do bedside ultrasound on a 55 years old male with abdominal pain and obtains following images.

Answers:

1. Above is transvers view of distal aorta , and curvilinear probe has been used. Curvilinear probe is low frequency probe and gives higher depth so ideal for looking at deeper structures hence used in AAA/ AFAST scans.
2. Hypoechoic ( black)  half circle at the bottom of image is vertebral body shadow which is the main landmark for identifying abdominal aorta. Remember ultrasound waves do not pass through the bones and you get a  black shadow as above . Other example is rib shadow when doing FAST scan.

The round structure on right of image ( left of patient ) is Aorta , while the elliptical structure on left of image ( right of patient ) is IVC. Following points will help further to differentiate between two.

a) Aorta lies on left of  while IVC lies on right of vertebral body ( patient ).

b) Aorta appears round in shape, while IVC appears elliptical mostly like a tear drop.

c) Both structures can be pulsatile so do not get trapped.

Below is coloured demonstration of above structures.