The structure indicated by the arrow is the middle hepatic vein. This is confirmed by both its anatomical location within the liver and its Doppler waveform characteristics.
Key ultrasound and Doppler features:
* The middle hepatic vein runs between the right and left hepatic lobes and drains into the inferior vena cava (IVC). On grayscale imaging, it appears as a tubular anechoic structure extending toward the IVC.
* On spectral Doppler, hepatic veins (including the middle hepatic vein) demonstrate a characteristic triphasic waveform due to pressure changes in the right atrium. This triphasic pattern is clearly visible in the Doppler tracing below the image.
* This differs significantly from the monophasic low-resistance flow of the hepatic artery or the continuous hepatopetal flow of the portal vein.
Differentiation from other options:
* A. Left portal vein: Would show continuous, hepatopetal flow (toward the liver) and lies more anterior and medial within the liver.
* C. Proper hepatic artery: Small-caliber vessel with low-resistance pulsatile waveform (not triphasic).
* D. Inferior vena cava: Lies posterior to the liver and demonstrates phasic flow with respiration, but this vessel is more centrally located and not shown in this field of view.
References:
Rumack CM, Wilson SR, Charboneau JW, Levine D. Diagnostic Ultrasound. 5th Edition. Elsevier, 2018.
Chapter: Hepatic Vasculature, pp. 90-95.
AIUM Practice Parameter for the Performance of Hepatic Doppler Ultrasound Examinations, 2020.
Radiopaedia.org. Hepatic vein Doppler waveform: https://radiopaedia.org/articles/hepatic-vein-doppler- waveform