Liver
Diagnostic Imaging
Basic Information 
Overview and Goal(s)
• Trans-abdominal ultrasonography is the most commonly used imaging modality to evaluate the equine liver. It allows determination of hepatic size and contour, as well as detection of alterations in the hepatic parenchyma (abscesses, neoplastic masses, diffuse hepatitis, hepatic lipidosis) or evidence of obstruction of the biliary tree (dilated bile ducts, choleliths).
• Radionucleotide imaging of the liver and mesenteric contrast portography are occasionally used to provide additional information regarding aspects of hepatic function and blood flow but require specialized equipment and technical expertise. Ultrasonographic evaluation of the liver is discussed in detail in the following sections. As the use of radionucleotide imaging and contrast portography is typically confined to referral institutions, these modalities are covered briefly in the “Alternatives and Their Relative Merits” section in this chapter.
Equipment, Anesthesia
Preparation: Important Checkpoints
• Ultrasonographic evaluation of the liver may be performed in a standing, unsedated adult horse and in a standing or recumbent foal.
• The patient may be sedated as necessary to ensure patient compliance for a comprehensive examination or if an ultrasound-guided liver biopsy is anticipated.
• The right and left flanks should be clipped as follows:
Right: From the sixth to sixteenth intercostal spaces, ventral to a line extending from the right olecranon to the right tuber coxae
• After clipping, the skin should be wiped with alcohol and coated with a thin layer of ultrasound contact gel.
Possible Complications and Common Errors to be Avoided
• No specific complications are associated with the ultrasound procedure itself.
• It is important to note that the entire liver is not visible with trans-abdominal ultrasound in the adult horse and in most foals, due to the position of the liver relative to the lung fields, diaphragm, stomach, and large colon. In addition, some atrophy of the right liver lobe is common in older horses and can limit visualization of the liver in these cases. Thus difficulty imaging the liver does not always imply hepatic pathology or diminished hepatic mass.
• Ultrasonography provides no information regarding hepatic function, and ultrasonographic abnormalities are not always found even with severe hepatic disease or hepatic failure. Thus a normal hepatic ultrasonographic appearance does not exclude the possibility of hepatic disease.
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