Imaging of the liver and pancreas

Introduction

The various imaging modalities now available in small animal practice permit exceptional opportunities for the diagnosis of many conditions involving the liver and the pancreas. This paper briefly reviews the anatomy of both organs and discuss the pros and cons for each technique.

The liver

The liver is the largest organ within the abdomen, occupying most of its cranial aspect. Located between the diaphragm and the stomach, the liver is divided in several lobes: the right lateral and medial lobes, the left lateral and medial lobes, the quadrate lobe, and the caudate lobe, which has a papillar process and a caudate process. The liver has two venous systems, portal and systemic.

The liver is closely associated to the gallbladder, which is located between the right medial and quadrate lobes, and the right kidney is in contact with the caudate process of the liver.

Radiography

Radiographs allow evaluation of liver size and contours, but do not allow evaluation of parenchymal changes unless associated with gas or mineralization. Two orthogonal views of the patient’s abdomen are necessary (lateral and ventrodorsal) and should be taken on expiration. It is important to include the cranial margin of the diaphragm on abdominal radiographs to entirely evaluate the liver.

The hepatic silhouette cannot be differentiated from the diaphragm and is delineated caudally by the stomach. The caudoventral aspect of the normal liver has angular margins (Figure 1a) (Figure 1b). The gastric axis may help to evaluate liver size as it should be visible on a lateral radiographic view between a line drawn at 90 degrees to the spine and a line parallel to the last pair of ribs. If the gastric axis is displaced beyond the last pair of ribs, or if the caudoventral margin of the liver extends caudal to the ventral aspect of the stomach, it indicates hepatomegaly. If the stomach is displaced cranially then the liver is probably small, although this can be normal in deep-chested dogs such as the Boxer, Doberman or Great Dane. Hepatic diseases are often associated with ascites which may preclude evaluation of the liver, but the presence of peritoneal fluid, characterized by loss of serosal detail and, in severe cases, a pendulous abdomen, should be easily recognized on abdominal radiographs. 

A liver mass may deform the hepatic contour and may induce various mass effects (i.e., the secondary pathological effects caused by a mass pushing on or displacing surrounding tissue). A liver tumor will displace the pylorus caudomedially if coming from the right side, or will distort the cranial contour of the gastric fundus if originating from the center or left side of the liver.

The gallbladder cannot normally be seen on radiography. However, in some cats, the ventral aspect of the gallbladder can project beyond the ventral margin of the hepatic silhouette on the falciform fat, and should not be mistaken for a mass. Cholelithiasis is a common cause of mineralization of the liver, and if involving the intrahepatic bile ducts will have a branching appearance on radiography (Figure 2).

Ultrasound

Ultrasound is a very useful modality to complete evaluation of the liver as it is cost effective and non-invasive, and in most cases can be accomplished without sedation. Ultrasound is particularly recommended if hepatomegaly or peritoneal fluid is detected on radiography, but its usefulness may be limited in large dogs or if the stomach is markedly distended with gas. The liver can be imaged with the probe positioned just below the xyphoid process, scanning from the left to the right in a sagittal plane, or cranial to caudal in the transverse plane will allow evaluation of the whole organ. Depth has to be set appropriately to include the entire liver. The diaphragm cannot be differentiated from the liver parenchyma, and the diaphragmatic margin will delimit the cranial aspect of the liver, as with radiography. This diaphragmatic margin is characterized by a curved line of reverberation artifacts produced by air within the lungs. A mirror image is a common artifact found on hepatic ultrasound and is characterized by the projection of the liver image cranial to the diaphragm; this should be recognized and not be confused with a diaphragmatic hernia or thoracic mass.

The liver has a homogenous parenchyma that has a mildly granular echotexture and is hypoechoic to the falciform fat and the spleen (Figure 3a) (Figure 3b) (Figure 3c), and iso- to hypoechoic to the right kidney 1. The liver contours should be smooth and regular, delineated by a thin hyperechoic capsule. Separation of the liver lobes should be apparent, unless there is free fluid within the peritoneal cavity. The hepatic veins are easily visualized within the liver parenchyma as tubular anechoic structures, and the portal veins are differentiated from the systemic veins by their hyperechoic walls. Additionally, the degree of visibility of the portal vessels may also be used to assess hepatic echogenicity. 

As with radiography, liver size will be evaluated subjectively, as the caudoventral aspect should not extend beyond the gastric fundus. The right lobes project more dorsally and are often better evaluated using a caudal inter-costal window, between the right 10^th or 11^th intercostal space. This window is also useful to evaluate the gallbladder, which is normally visible with an anechoic content and a very thin hyperechoic wall. In most middle-aged dogs, mobile echoic material is noted within the gallbladder. This is rather unusual in the cat and if seen may prompt further investigation of the biliary function. Intrahepatic bile ducts are not normally visible.

The main changes that can be appreciated on ultrasound are alterations in liver size (most often hepatomegaly) and echogenicity, or the presence of a nodule or mass. Ultrasound is very sensitive to detect parenchymal changes but not specific, and any change should be interpreted with regard to the clinical signs. For example, hyperechoic hepatomegaly may lead more towards a diagnosis of lipidosis in an icteric cat and will be a common finding in a diabetic dog. In these two particular diseases, the liver parenchyma will also be hyperattenuating (Figure 4). Acute hepatitis can be characterized by a hypoechoic hepatomegaly, whilst in chronic cases the liver may become heterogeneous with irregular margins. 

A liver mass may vary in its appearance, but will generally be seen as heterogeneous and can deform the hepatic margin. The portion of the liver involved can be identified, but it may be difficult to correctly target the exact hepatic lobe affected. On the other hand ultrasound can be useful to identify the liver when investigating the origin of a large abdominal mass.

It is important not to interpret gallbladder changes as a hepatic mass, and this is especially true in cases of mucocoele where the gallbladder is filled with heterogeneous, organized and static material. 

Ultrasound is also very sensitive for the detection of liver nodules but again is not specific to identify their nature, and many hepatic nodules are benign. A hepatic cyst will appear as an anechoic, rounded structure in orthogonal planes that will induce an acoustic enhancement. Ultrasound-guided fine needle aspirates or biopsies may be performed if necessary.

Decreased liver size may be a consequence of chronic hepatitis and cirrhosis, and in this case irregular margination may be seen on ultrasound; ascites is often also present. However, a small liver can also be indicative of a congenital portosystemic shunt in younger patients, especially where there is an extrahepatic shunt. An intrahepatic shunt can usually be easily seen as an abnormally large, curved, tortuous vessel within the liver parenchyma that connects the portal flow to the hepatic portion of the caudal vena cava.

Where there is chronic biliary obstruction, intrahepatic bile ducts can be seen on ultrasound as mildly tortuous, tubular anechoic structures; color Doppler technique may help differentiate the ducts from blood vessels.

Computed tomography (CT) 

Imaging via CT will require the patient to be under general anesthesia followed by intravenous injection of iodinated contrast medium. It is very helpful to image the entire liver, especially in large dogs in which ultrasound may be limited. Dogs may be placed in either dorsal or ventral recumbency and images acquired in the transverse plane may be reformatted in different planes. The liver will normally have a homogeneous soft tissue attenuation. CT is especially recommended to evaluate the exact location and possible dissemination of a hepatic mass if surgery is to be considered (Figure 5). It is also very useful to evaluate vascular anomalies, and especially portosystemic shunts (intra or extrahepatic); in this case three post-contrast medium injection acquisition times are required, during the arterial, portal and venous phases 2.

Magnetic resonance imaging (MRI)

MRI of the liver is not frequently used in veterinary medicine and applications are still limited. However its superior contrast resolution may help differentiate benign from malignant lesions in the liver. It requires general anesthesia of the patient, gadolinum contrast injection and a high field MRI (at least 1.5 Tesla) to avoid respiratory-induced motion artifacts.

The pancreas

The pancreas is a small organ which can be divided into three anatomic sections; the right lobe is located along the mesenteric border of the duodenum, the body of the pancreas runs along the caudal aspect of the stomach, and the left lobe sits alongside the descending colon. The contours of the organ are typically irregular.

Radiography

The normal pancreas cannot usually be identified on radiography as it is too small. However, in some overweight cats with a large amount of peritoneal fat, the left lobe of the pancreas can be seen alongside the medial aspect of the spleen adjacent to the cranial pole of the right kidney, and should not be mistaken for an abnormality. Radiographs may, however, still be useful if a pancreatic disease is suspected, as pancreatitis may induce indirect changes such as loss of serosal detail in the cranial abdomen, enlargement of the pyloro-duodenal angle, and gaseous dilation of the duodenum due to secondary induction of a functional ileus. Additionally, some pancreatic tumors can be mineralized, and a cranial abdominal mass with foci of mineralization may be of pancreatic origin.

Ultrasound

Ultrasound is quite useful to image the pancreas although thorough evaluation requires some experience. The normal pancreas is discretely heterogenous and generally slightly hypoechoic to the surrounding fat, with ill-defined margins 3. It may be iso-echoic to the surrounding fat in cats and hyperechoic in the Yorkshire Terrier 4. Assessment of the pancreas relies mainly on identification of specific landmarks. To evaluate the right lobe it is important to image the duodenum from the right kidney caudally to the pylorus cranially. In the dog, the pancreatico-duodenal vein can be easily identified as a tortuous, tubular anechoic structure parallel to the medial aspect of the duodenum (Figure 6a); the tissue around this vessel is the pancreas. Color Doppler can be used to better characterize this vessel. In the cat, it is the pancreatic duct that will be visualized in this location, and this will help localize the right lobe of the pancreas. The pancreatic duct is physiologically dilated in the cat (and especially in older cats, up to 3 mm in diameter); it joins the common bile duct at the level of the major duodenal papilla in this species. The body of the pancreas is located caudal to the pylorus and ventral to the portal vein between the stomach and the transverse colon. The left pancreatic lobe can be visualized at the lateral aspect of the descending colon caudal to the gastric fundus, medial to the spleen and cranial to the cranial pole of the left kidney. The pancreatic duct may also help to localize this lobe in the cat (Figure 6b). The thickness of the pancreas can be measured in cats and should not exceed 1 cm 5. 

Acute pancreatitis is generally characterized by hypoechoic and heterogenous thickening of the pancreas, surrounded by hyperechoic and hyperattenuating fat. Fluid is also frequently present in the vicinity of the pancreas. The adjacent duodenal wall is often thickened and plicated, with loss of definition of its layers (Figure 7). Abdominal pain can often limit examination of the pancreas and necessitate analgesia or sedation. Concomitant abscessation or cyst formation may occur, and will be identified as a rounded structure filled with hypoechoic to anechoic fluid. Such lesions can be drained via ultrasound guidance. Chronic pancreatitis can be more difficult to recognize and may be characterized by heterogeneous areas within the pancreatic parenchyma and foci of hyperechoic fat adjacent to it. 

Nodular hyperplasia is commonly seen in older cats and is characterized by hypoechoic, well-defined nodules measuring less than 1 cm in diameter 6. Pancreatic tumors are often hypoechoic and slightly heterogeneous, and will distort the pancreatic contours (Figure 8). Pancreatic carcinoma are often associated with carcinomatosis, which is characterized by peritoneal fluid accumulation and hypoechoic nodules dispersed within the mesentery and onto the peritoneum.

Insulinomas usually appear as small hypoechoic nodules and may be difficult to visualize. Metastases to the liver or adjacent lymph nodes are often detected before the primary tumor is identified.

Computed tomography

CT is useful to fully evaluate the pancreas, which will appear as an irregularly marginated, soft tissue attenuating organ along the landmarks describes in the section above. The technique can be particularly useful to search for insulinomas as it will not be limited by gas in the gastrointestinal tract, and it is more sensitive for the detection of small lesions, although this will usually require dual‐phase computed tomography. An insulinoma is characterized on imaging by a hypo-attenuating nodule that will have a strong enhancement during the arterial phase of the study but not during the other phases 7.

Imaging can be invaluable when investigating diseases of the liver and pancreas, but it is essential that the clinician is familiar with the normal anatomy and appearance on the chosen imaging modality. It is also necessary to be aware of the limitations when using such diagnostic techniques, but appropriate care and a standardized approach should enable beneficial results in most cases.