Page 696 - Clinical Small Animal Internal Medicine

664 Section 7 Diseases of the Liver, Gallbladder, and Bile Ducts

“Acoustic shadowing” occurs when highly attenuating fine hyperechoic line of the hepatic capsule; however, in
VetBooks.ir structures (such as bone and calculi) leave the deeper tis- some patients the demarcation between fat and liver is
not as clear and the falciform fat may be misdiagnosed as
sues relatively devoid of echoes, appearing artificially
hypoechoic or anechoic. In the normal liver, acoustic
shadowing may be recognized deep to the ribs when hepatomegaly.
scanning through an intercostal space. “Reverberation” is Biliary System
recognized as a closely spaced series of parallel hypere- The normal gallbladder is seen just to the right of midline
choic bands running across the screen and is caused by as a rounded fluid‐filled structure with a thin well‐defined
sound waves reverberating between the transducer and a echogenic wall. Normal gallbladder size varies widely and
very reflective gas interface. When examining the liver, is larger in fasted or anorexic patients. The normal gall-
this artifact is most frequently identified deep to a gas‐ bladder wall measures up to 1 mm across in cats and up
filled stomach. to 2–3 mm in dogs. Gallbladder sludge is a common and
often incidental finding, recognized as echogenic, non-
shadowing material settled dependently within the
Ultrasound Evaluation
gallbladder lumen. The normal intrahepatic biliary tree
A consistent, logical approach to ultrasound evaluation cannot be identified, although in some patients, espe-
is recommended, with an initial assessment being made cially cats, it is possible to identify the normal common
of the position, size, and shape of the liver, followed by bile duct leaving the porta hepatis ventral to the hepatic
more detailed evaluation of the parenchymal structure, portal vein and entering the proximal duodenum at the
vasculature, biliary system, and lymph nodes. major duodenal papilla.
The position of the liver is most easily evaluated rela-
tive to the position of the surrounding organs. The dia- Vasculature
phragm should be seen as a curved hyperechoic line on Normal hepatic arteries are not identified on ultrasound.
the cranial aspect of the liver, demarcating the interface The hepatic portal vein enters the liver at the porta hepa-
between liver and air‐filled lung, with a mirror‐image tis before branching into left and right portal veins and
artifact sometimes seen beyond the diaphragm. Caudally, arborizing throughout the liver. The portal vasculature is
the normal liver is in contact with the spleen on the left, seen as branching, tapering anechoic tubes with charac-
the stomach centrally and the right kidney on the right. teristically echogenic walls. The caudal cava enters the
Changes in position are more commonly encountered liver dorsal to the portal vein, receiving venous drainage
due to changes in the adjacent organs or structures (e.g., via the hepatic veins as it crosses the right side of the
rupture of the diaphragm) than due to abnormalities of liver. Unlike the portal veins, the walls of the hepatic
the liver itself. veins cannot usually be seen.
Although radiography is usually more useful in evalu-
ating the overall size and shape of the liver, caudal exten- Hepatic Lymph Nodes
sion of the liver beyond the costal arch and rounding of The hepatic and splenic lymph nodes receive the major-
the normally sharply pointed caudoventral margin in ity of lymphatic drainage from the liver and are located
patients with generalized hepatomegaly may also be respectively at the porta hepatis and along the course of
identified on ultrasound. Localized hepatomegaly is usu- the splenic vein. The normal lymph nodes are small,
ally easier to identify with ultrasound than with radiog- ovoid, isoechoic to the surrounding mesentery and
raphy, and is recognized by distortion and/or bulging of usually not identified. Enlarged lymph nodes are typi-
the hepatic margins, often together with a localized cally hypoechoic and rounded or irregular.
change in the appearance of the hepatic parenchyma.

Parenchymal Structure Imaging Features of Hepatobiliary
The normal liver (see Figure 61.5) is of medium echo- Disease
genicity with a slightly coarse but uniform echotexture,
interrupted only by the hepatic vasculature and gallblad- Parenchymal Disease
der. Although the hepatic margins are generally well
defined, the liver lobes are not usually individually iden- Conditions classified as parenchymal disorders include
tified unless they are separated out by the presence of vacuolar diseases (e.g., steroid hepatopathy, hepatic lipi-
free abdominal fluid. The liver is bordered on the ventral dosis), amyloidosis, acute and chronic hepatitis, cirrho-
aspect by variable amounts of falciform fat. This fat is sis, hepatic abscessation, granulomatous disease, and
coarser in appearance than the normal hepatic paren- hepatic necrosis (e.g., due to toxic, ischemic or immune‐
chyma and is usually demarcated from the liver by the mediated disease).