LIVER ULTRASOUND

Preparation:

• Starved patient

• Sedated where possible

• Full abdominal clip

• Lots of ultrasound gel

In these articles the approach to abdominal ultrasound will be via a right lateral recumbency and then a left lateral recumbency approach. It is equally valid to image in dorsal recumbency

Approach

Cats and smaller breeds of dog:

• Abdominal approach starting at the xiphisternum

Medium to larger breeds of dog:

• Often require imaging via intercostal approach as well as via the abdomen

• Intercostal approach is especially important for larger deeper chested dogs - especially to image the right liver lobes

In both cases the liver is scanned in longitudinal and transverse plains

The different lobes of the liver cannot be distinguished via ultrasound unless there is an abdominal effusion

• LONGITUDINAL IMAGE OF THE LIVER - FAN UP TO ASSESS THE LEFT LIVER

  • With the probe marker towards you place the probe on the midline at the xiphisternum and angle cranially

  • In cats and smaller dogs drop the tail of the probe to fan upwards to assess the left liver lobes

  • In larger dogs it is often also necessary to move the probe up the body wall/ image via an intercostal space

  • Compare the echogenicity of the liver to the spleen

LONGITUDINAL IMAGE OF THE GALL BLADDER AND RIGHT LIVER

• Return the probe to the xiphisternum/midline and then fan down, lifting the tail of the probe to bring in the gall bladder

• Further fanning down towards the table brings in the right side of the liver

TRANSVERSE VIEWS OF THE LIVER

• Next move the probe back to the xiphisternum/midline position and rotate the probe 90 degrees so that the marker is pointing to the right side of the patient (down towards the table when in right lateral recumbency)

• Fan the probe in cranial and caudal directions to image the liver in cross section at the midline, left and right side of the liver

Note

• In larger dogs it will not be possible to adequately assess the right liver fully with the patient in right lateral recumbency

• The dog will need to be moved into left lateral recumbency and the liver scanned sub-costally from the right and through the ribs in large deep chested dogs

Assess the liver size, shape, echogenicity and echotexture - NOTE - some conditions may have characteristic ultrasonographic features but these changes are not always present and could reflect multiple disease processes. process. For this reason it is important to take into account the whole clinical picture - history, exam, bloodwork changes, ultrasound findings and liver cytology/histology. See the approach to liver disease section - canine & feline

NORMAL ULTRASONOGRAPHIC APPEARANCE OF THE LIVER

The normal hepatic parenchyma is homogenous with a relatively coarse echotexture. It is normally hypoechoic when compared to the spleen. Hepatic and portal veins can be seen throughout. Portal veins have hyperechoic walls unlike the hepatic veins. The gallbladder is just to the right of the midline. It’s size is variable - larger in fasted animals. GB wall is thin, hyperechoic and normally around or under 1mm. Some sludge may be normal in dogs. Normal intra-hepatic bile ducts are not visible in the liver

Normal GB in a fasted dog. Note the thin GB wall.

An acoustic enhancement artefact can be seen distal to the GB making the liver appear more hyperechoic here. This is an artefact of more sound waves being present here as they have not been as attenuated passing through the fluid filled GB. This artifact can be useful in helping to differentiate cysts (which will have distal acoustic enhancement) with low echogenicity solid nodules/masses.

Edge shadowing can also be seen - a refraction artefacts created when ultrasound waves are bent as they encounter a curved surface tangentially. This occurs commonly when the kidneys, urinary bladder, jejunum in the short axis plane or gallbladder is imaged.

ASSESSING LIVER SIZE

Enlargement is relatively subjective but often rounded liver margins can be seen:

HEPATIC NODULES

Hypoechoic liver nodule differentials:

• Benign nodular hyperplasia (very common)

• Metastasis

• Primary hepatic neoplasia

• Lymphoma

• Hematoma

• Abscess

• Necrosis

Fine needle aspirates or biopsy is needed to differentiate between them

With the exception of target lesions, benign vs malignant nodules cannot be differentiated on imaging. FNA or biopsy is required. Nodular hyperplasia is seen commonly in older dogs and are most often hypoechoic but can vary in echogenicity in size.

EXAMPLES OF PRIMARY HEPATIC NEOPLASIA

Hepatoma’s and well differentiated carcinomas are the most common large primary liver tumour in dogs and cannot be differentiated on ultrasound or fine needle aspirate. Biopsy is required. With massive lesions CT can be useful to assess feasibility of surgery

DIFFUSE CHANGES IN ECHOGENICITY AND ARCHITECTURE