The Liver part 1: Normal Appearance

The Liver part 1: Normal Appearance

This is the first in a series of descriptive articles looking at organ specific ultrasonographic appearance in canine and feline patients. This feature will focus on liver parenchymal evaluation excluding the gall bladder and biliary system, which will be described separately.

Indications

Ultrasonography is essential in the investigation of suspected liver pathology.

Indications include:

  • Icterus
  • Investigating biochemical changes associated with liver disease
  • Ascites
  • Pyrexia of unknown origin
  • Hepatomegaly
  • Cranial abdominal mass
  • Cranial abdominal pain
  • Trauma
  • Potential metastases
  • Facilitation of fine needle aspirate and/or biopsy sampling
  • Use of doppler ultrasonography to assess vascular patterns and blood flow

Ultrasound Vs. Radiography

Both techniques should be considered complementary, though ultrasound allows a detailed examination of the internal structure of the liver. Radiography can be used to assess the size of the liver relative to surrounding structures such as the costal arch and stomach. Radiographs can also give information as to whether pathology is focal or more diffuse, including extension beyond the liver.

lateral x-ray

Image 1 - A left lateral radiograph of the abdomen of a normal dog. The liver can be seen entirely within the costal arch. The red arrowheads mark the cranial and ventral margins of the liver.

Patient preparation

The liver can be evaluated with the patient positioned in dorsal, left lateral and/or right lateral recumbency. Hair clipping should encompass the entirety of the cranial abdomen as well as the two most caudal intercostal spaces, especially in large or deep chested breeds of dog and/or where microhepatica is suspected.

Gas in the stomach can cause acoustic shadowing and reverberation artefacts which may prevent good visualisation of the liver during an ultrasound exam. Therefore, fasting the patient prior to the procedure is important; 12 hours is usually adequate. Consideration should be given to sedation and analgesia to improve patient comfort and to reduce stress and panting which may lead to aerophagia or motion artefact and consequently reduced image quality.

A curvilinear or sector probe is preferred for liver imaging. The smaller foot print and wedge-shaped field of view allows a large area of the liver to be visualised simultaneously and facilitates examination through the small intercostal spaces. A 7.5MHz or higher frequency probe can be used for imaging cats and small to medium sized dogs. A 5MHz or lower probe is preferred to facilitate imaging in large or giant breed dogs.


Normal Appearance

Liver diagram

Image 2 – A schematic illustration of the liver and surrounding anatomy.

The liver is located in the cranial abdomen within the ribcage. The liver is positioned cranially to the stomach with the cranial margins of the liver being adjacent to the diaphragm. Ultrasonographically, the region of the diaphragm appears as a hyperechoic line which represents the interface between the liver and diaphragm tissue and the air-filled lung. The curved and highly reflective nature of this interface can result in the formation of a mirror image artefact in which soft tissue with an appearance similar to that of the liver can be visualised cranial to the diaphragm. Care should be taken when interpreting the image and this should not be mistaken for a diaphragmatic hernia.

Mirror image

Image 3 - Mirror image artefact – In this image the highly reflective curved shape of the diaphragm interface (marked by the red arrow) is causing the formation of a mirror image artefact. This has led to parts of the liver and the gall bladder (containing a large amount of hyperechoic sludge) to appear visible distal to the diaphragm (marked with a red arrowhead).


The caudal border of the liver extends to the level of the spleen on the left, the stomach centrally and with the right kidney positioned in the renal fossa of the caudate lobe.

Spleen

Image 4 - Liver and spleen – This ultrasound image shows the liver (red arrow) bordering the spleen (arrowhead). This highlights the more hypoechoic appearance of the liver relative to the spleen and the coarser echotexture.

RK

Image 5 - This image shows a portion of the caudate lobe of the liver (red arrow) with the right kidney (red arrowhead) positioned adjacent to it within the renal fossa. Note the isoechoic appearance of the liver parenchyma and renal cortex.

 

Ventral to the liver is the falciform ligament. It can protrude between the left and right lobes of the liver and contains a variable amount of fat. The falciform ligament appears iso- or hyperechoic to the liver parenchyma and has a coarser echotexture. In cats, the falciform ligament can be mistaken for hepatomegaly, therefore careful interpretation is necessary. Obese cats may exhibit liver tissue which appears hyperechoic compared with the falciform fat1.

Falciform 

Image 6 - Falciform Fat – In this sagittal oblique image of the liver the falciform fat is visible in the near field and demarcated by the red arrowheads.

The liver can be described as having 4 lobes – the left, quadrate, right and caudate. The left and right lobes can be further divided into medial and lateral parts2. Divisions between the lobes are not visible ultrasonographically unless a peritoneal effusion is present. This is due to the close spatial proximity of the lobes and the minimally visible liver capsule. The margins of the liver taper to a point.

Normal liver parenchyma has a uniform, medium echogenicity. It is important to remember that ultrasound machine settings will influence the appearance of liver tissue. Using consistent settings, the echogenicity of the liver can be evaluated relative to that of surrounding organs. The parenchyma appears hypoechoic relative to splenic tissue and isoechoic relative to the cortex of the right kidney. When evaluating the liver in this way consideration should be given to the possibility of pathology affecting other organs which may change their appearance.

The liver parenchyma is interrupted by the hepatic and portal veins. The portal veins can be distinguished from the hepatic vessels by their comparatively hyperechoic walls. Doppler ultrasonography can be used to distinguish between hepatic and portal veins based on their relative flow direction. The intrahepatic biliary tree is not visible in normal patients.

Portal Vessels

Image 7 - Portal Vessels – In this image of the liver the hyperechoic walls of the portal vasculature are visible (red arrow). A hepatic vein is marked by the red arrowhead.

The gall bladder sits between the quadrate and right lobes of the liver. The normally fluid-filled lumen can cause acoustic enhancement of the liver tissue visualised deep to the gall bladder. The liver parenchyma in the far field, below the gall bladder, will appear more echogenic.

Acoustic enhancement

Image 8 - Acoustic Enhancement – In this image the central fluid filled gall bladder is causing acoustic enhancement of the tissue distal to it. This has caused the liver parenchyma marked by red arrowheads to appear more hyperechoic.

Evaluating liver size utilising ultrasonography is subjective and can vary depending on breed, body condition score and conformation. Like radiography, the surrounding anatomy can be used as markers, such as the position relative to the costal arch and diaphragm to stomach distance.

References

1 Nicoll R.G., O’Brien R., Jackson M.W. (1998) Quantitative ultrasonography of the liver in obese cats. Veterinary Radiology and Ultrasound 39: 47-50.

2 Dyce K.M., Sack W.O. and Wensing C.J.G. (1996) Textbook of Veterinary Anatomy, W.B. Saunders, London, pp 136-140, 429-431.

Further reading

d’Anjou M.A., Penninck D. (2015) Liver, In: Atlas of Small Animal Ultrasonography 2nd edn., Eds: Penninck D. and d’Anjou M.A., John Wiley & Sons, Chichester, pp 183-238.

Gaschen L. (2009) Update on hepatobiliary imaging. Veterinary Clinics of North America: Small Animal Practice 39: 439-467.

Griffin S. (2018) Feline abdominal ultrasonography: what’s normal? what’s abnormal? The liver. Journal of Feline Medicine and Surgery 21: 12–24.

Larson M. (2018) Liver and Spleen, In: Textbook of Veterinary Diagnostic Radiology 7th edn., Ed: Thrall D., Elsevier, St. Louis, pp 792-822.

Liffman R., Courtman N. (2017) Fine needle aspiration of abdominal organs: a review of current recommendations for achieving a diagnostic sample. Journal of Small Animal Practice 58: 599–609

Nyland T.G., Larson M. and Mattoon J.S. (2015) Liver, In: Small Animal Diagnostic Ultrasound 3rd edn., Eds: Mattoon J.S. and Nyland T.G., Elsevier, St. Louis, pp 332-399.

Rademacher N. (2011) Liver, In: BSAVA Manual of Canine and Feline Ultrasonography., Eds: Barr F. and Gaschen L., BSAVA publications, Gloucester, pp 85-99.

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