Transverse ultrasound image of the liver. The aorta (A), caudal vena cava (B), and portal vein (C) are visible in cross-section, with the white dashed lines measuring the cross-sectional area of the aorta and portal vein. The area of the portal vein was obtained by acquiring a transverse image of the vessel (perpendicular to its long axis) and traced its outline. From this tracing, the area was calculated by the ultrasound machine software. The same measurement was acquired for the aorta at the same level and the area calculated in the same manner.