LIVER ULTRASOUND HEPATIC VASCULATURE

liver ultrasound - hepatic vasculature
The portal veins radiate from the porta hepatis, where the main portal vein (MPV) enters the liver. They are encased by the hyperechoic, fibrous walls of the portal tracts, which make them stand out from the rest of the parenchyma. Also contained in the portal tracts are a branch of the hepatic artery and a biliary duct radical. These latter vessels are too small to detect by ultrasound in the peripheral parts of the liver, but can readily be demonstrated in the larger, proximal branches. At the porta, the hepatic artery generally crosses the anterior aspect of the portal vein, with the common duct anterior to this. In a common variation the artery lies anterior to the duct. Peripherally, the relationship between the vessels in the portal tracts is variable. The three main hepatic veins, left, middle and right, can be traced into the inferior vena cava (IVC) at the superior margin of the liver. Their course runs, therefore, approximately perpendicular to the portal vessels, so a section of liver with a longitudinal image of a hepatic vein is likely to contain a transverse section through a portal vein, and vice versa. Unlike the portal tracts, the hepatic veins do not have a fibrous sheath and their walls are therefore less reflective. Maximum reflectivity of the vessel walls occurs with the beam perpendicular. The anatomy of the hepatic venous confluence varies. In most cases the single, main right hepatic vein (RHV) flows directly into the IVC, and the middle and left have a common trunk. In 15–35% of patients the left hepatic vein (LHV) and middle hepatic vein (MHV) are separate. This usually has no significance to the operator. However, it may be a significant factor in planning and performing hepatic surgery, especially tumour resection, as the surgeon attempts to retain as much viable hepatic tissue as possible with intact venous outflow.

Haemodynamics of the liver
Pulsed and colour Doppler to investigate the hepatic vasculature are now established aids to diagnosis in the upper abdomen. Doppler should always be used in conjunction with the real-time image and in the context of the patient’s presenting symptoms. Used in isolation it can be highly misleading. Familiarity with the normal Doppler spectra is an integral part of the upper-abdominal ultrasound scan. Doppler of the portal venous and hepatic vascular systems gives information on the patency, velocity and direction of flow. The appearance of the various spectral waveforms relates to the downstream resistance of the vascular bed.

The portal venous system
Colour Doppler is used to identify blood flow in the splenic and portal veins. The direction of flow is normally hepatopetal, that is towards the liver. The main, right and left portal branches can best be imaged by using a right oblique approach through the ribs, so that the course of the vessel is roughly towards the transducer, maintaining a low (< 60°) angle with the beam for the best Doppler signal. The normal portal vein diameter is highly variable but does not usually exceed 16 mm in a resting state on quiet respiration. The diameter increases with deep inspiration and also in response to food and to posture changes. An increased diameter may also be associated with portal hypertension in chronic liver disease. An absence of postprandial increase in diameter is also a sign of portal hypertension. The normal portal vein (PV) waveform is monophasic with gentle undulations which are due to respiratory modulation and cardiac activity. This characteristic is a sign of the normal, flexible nature of the liver and may be lost in some fibrotic diseases. The mean PV velocity is normally between 12 and 20 cm per second but the normal range is wide. (A low velocity is associated with portal hypertension. High velocities are unusual, but can be due to anastomotic stenoses in transplant patients.)

The hepatic veins
The hepatic veins drain the liver into the IVC, which leads into the right atrium. Two factors shape the hepatic venous spectrum: the flexible nature of the normal liver, which can easily expand to accommodate blood flow, and the close proximity of the right atrium, which causes a brief ‘kick’ of blood back into the liver during atrial systole. This causes the spectrum to be triphasic. The veins can be seen on colour Doppler to be predominantly blue with a brief red flash during atrial contraction. Various factors cause alterations to this waveform: heart conditions, liver diseases and extrahepatic conditions which compress the liver, such as ascites. Abnormalities of the hepatic vein waveform are therefore highly unspecific and should be taken in context with the clinical picture. As you might expect, the pulsatile nature of the spectrum decreases towards the periphery of the liver, remote from the IVC.

The hepatic artery
The main hepatic artery arises from the coeliac axis and carries oxygenated blood to the liver from the aorta. Its origin makes it a pulsatile vessel and the relatively low resistance of the hepatic vascular bed means that there is continuous forward flow throughout the cardiac cycle. In a normal subject the hepatic artery may be elusive on colour Doppler due to its small diameter and tortuous course. Use the MPV as a marker, scanning from the right intercostal space to maintain a low angle with the vessel. The hepatic artery is just anterior to this and of a higher velocity (that is, it has a paler colour of red on the colour map).