Ultrasound for Vets, by Dr.Kishor Mahind---Liver

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The Liver:

The liver was imaged using either a 3.5 MHz or 5 MHz mechanical sector transducer. Transducer was placed immediately behind the xiphisternum, in the midline and angled craniodorsally to image a transverse section of the liver. The transducer was then moved gradually caudally, remaining in the middle; until the caudal extremity of the ventral liver lobes was revealed. The angulation of beam was then adjusted in a dorsoventral direction until the distance from the skin surface to the diaphragm line. If the beam was angled too far dorsally, a clear diaphragmatic line was lost. If the beam was angled too far ventrally, the depth of liver was reduced. Once the optimal plane of section had been determined, the image was frozen at the full expiration to minimize the distance between the skin surface and diaphragm. The distance from skin surface to midpoint of diaphragm is measured to give indicator of liver size. The head of the transducer is then rotated through 90° to image a longitudinal section of liver in midline. The dorso-ventral angulation of beam was adjusted until the caudal liver surface was as near as possible, while retaining a clear image of the diaphragm line and liver parenchyma. The image is frozen at maximum expiration and a vertical line taken from the skin surface to diaphragm. Mid-hepatic line is relatively close to the midline in dog. Measurements are therefore taken in midline. By using sector scanners it is rarely possible to image entire liver in longitudinal section, so the length of liver cannot be measured accurately. It is found that reproducibility of measurement taken from transverse plane of section is good.

Site for scanning liver Anatomy

The liver is not mobile and so its location always remains the same. It lies immediately behind the diaphragm, which appears on the ultrasound image as a curved, hyperechoic line, which will move in time with the animal's respiration. There should be nothing between the liver and the diaphragm.

The gall bladder is a pear shaped anechoic structure, which is located to the right of midline within the liver, and its visualization confirms identification of the liver.

Hepatic arteries are too small to be seen on ultrasound examination but we can identify both the hepatic portal veins and the hepatic veins. The walls of the hepatic veins are not visible on ultrasound so these vessels appear as anechoic tubular structures. They arise in the periphery of the liver and increase in diameter to converge at the porta hepatis where they communicate with the caudal vena cava.

The walls of the portal veins have a larger amount of fat and fibrous tissue within them, which produces a hyperechoic appearance on ultrasound. These vessels therefore appear as tubular anechoic structures branching out towards the periphery of the liver with white walls. The walls appear as converging lines or circles depending whether the beam transects the vessel in long or short axis and this gives an appearance similar to the 'tramline and donut' effect seen with thoracic radiographs. These vessels are also widest centrally and decrease in diameter towards the periphery of the liver.

The presence of blood vessels throughout the liver produces a very coarse appearance to the parenchyma with hyperechoic speckling produced by the tapering portal veins and this is the characteristic appearance of the liver on ultrasound.

Gall Bladder Blood Vessels

Abnormal findings:

Focal parenchymal lesions may be hypoechoic or hyperechoic. In small animals such foci are usually neoplastic, but other possibilities include abscesses, infarcts, hematoma and hyperplasia.
Diffuse parenchymal disease is more difficult to detect. A diffuse heterogeneity of the parenchyma may be associated with diffuse neoplasia or advanced cirrhosis.
Gallstones may be seen as echogenic foci, which cast acoustic shadows, lying in the dependent part of the bladder. Abnormal thickening of gall bladder wall can be due to inflammatory change or edema. The major advantage of USG has been that it allows an easy detection of both radiolucent and radiopaque choleliths, because choleliths create an ultrasonographic artifact referred to as acoustic shadowing. Important factors in creation of acoustic shadowing are the chemical composition of the choleliths and the size and location of choleliths in the bladder. On an empirical basis, acoustic shadowing forms one of the cornerstones in the diagnosis of cholelithiasis.

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