Before pathology can be discussed, the normal appearance of the liver, biliary system, and pancreas will be reviewed. Determination
of liver size via US is not accurate and is best done on radiographs. Ultrasound is best performed with the animal in dorsal
recumbency (on their back) and the area must be clipped free of hair. Generous amounts of isopropyl alcohol and US gel should
be applied. In deep chested breeds, such as boxers and greyhounds, you may need to scan via the intercostal spaces to completely
image the liver. The liver consists of 6 lobes: right lateral, right middle, left lateral, left middle, caudate and quadrate.
Normally, you cannot distinguish between lobes with US. The liver has a medium level homogeneous echotexture (is slightly
more echogenic than renal cortex, but less than that of spleen at the same imaging depth). The liver is moderately coarse
compared to the spleen and has smooth serosal margins.
The intrahepatic vasculature appears as anechoic, tubular structures in the liver parenchyma that taper towards the periphery.
There are three types of intrahepatic vessels. The portal veins have hyperechoic walls. The hepatic veins have walls that
are not normally visible. And lastly, the hepatic arteries, are very small and not usually seen. The extrahepatic vasculature
consists of the caudal venal cava, the portal vein and the aorta. The caudal vena cava is anechoic, tubular and the walls
are not seen. The caudal vena cava is in the dorsal abdomen and is located to the right of midline. It can be traced into
and through the diaphragm. The portal vein is also anechoic and tubular but has hyperechoic walls. It is located ventral
to the caudal vena cava and is located more on midline. The aorta is located to the left of midline and is the most dorsally
located vessel. Doppler evaluation of the aorta shows arterial flow profile, while both the caudal vena cava and the portal
vein have venous flow profiles. The gall bladder and bile ducts are located within the hepatic parenchyma. They are anechoic,
smoothly marginated, variously sized (depends on time of most recent meal), and thin walled. The bile ducts are differentiated
from intrahepatic vessels with Doppler, as they will have no flow. The gall bladder is located to the right of midline and
there is distal acoustic enhancement. The normal common bile duct can be difficult to identify due to small size. A normal
common bile duct in a cat is 4mm or smaller and in a dog 3mm or smaller.
The pancreas is considered normal if you cannot see it on US. The normal pancreas has a body and right and left lobes. The
body of the pancreas is arbitrarily defined, and is in the middle of the right and left lobes. The right lobe is just dorso-medial
to the descending duodenum. The left lobe is located just caudal to the body of the stomach, just cranial to the transverse
colon. The right lobe is usually easier to find than the left. To definitively identify the pancreasy, you initially will
need to find the descending duodenum. The descending duodenum is usually the lateral most bowel loop in the right cranial
abdominal quadrant. Search medial to the descending duodenum and cranial to the right kidney. The left lobe is usually difficult
to see due to overlying gas in the stomach and colon. Scan the area just caudal to the greater curvature of the stomach and
cranial to the colon, medial to the spleen.
Now that we are familiar with the normal appearance of the liver, biliary system, and pancreas, pathology of these will be
discussed. It is important to realize that most sonographic signs of liver disease are non-specific. Clinically, lesions
of the hepatobiliary system can be divided into those that involve the liver parenchyma, vascular lesions, and lesions that
involve the gall bladder or biliary system alone. Lesions should also be classified as to their echogenicity, distribution,
presence of any artifacts, and effect on surrounding structures. Diseases that affect an organ in a diffuse manner are very
hard to diagnose, and careful inspection and comparison to surrounding organs is needed.