Ultrasonography of the Pancreas
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Assessment of the pancreas should form part of a routine abdominal ultrasound examination.
Specific indications for pancreatic ultrasonography include:
- Weight loss
- Anorexia
- Vomiting
- Diarrhoea
- Abdominal pain
- Palpable abdominal mass
- Diabetic patients who are refractory to standard therapy
- Icterus
- Hypoglycaemia
Normal Anatomy
The pancreas is a thin, elongated organ consisting of lobules of glandular tissue surrounded by connective tissue. It can be subdivided into 3 parts: the left lobe, body, and right lobe.
An illustration of the pancreatic position and surrounding anatomy in the dog and cat.
- The left lobe of the pancreas lies adjacent to the greater curvature of the stomach and extends towards the cranial pole of the left kidney. The lobe passes cranioventrally to the splenic vein and the transverse colon lies caudally to the left lobe.
- The body is situated at the level of the pyloroduodenal junction in the right cranial abdomen. In cats, the body lies more centrally. The portal vein passes dorsally to the body of the pancreas.
- The right lobe lies in the mesoduodenum, medial and dorsal to the descending duodenum and ventral to the right kidney. In cats, the caudal right lobe can curve in a cranial direction, appearing more “hook-like”.
The anatomy of the ductal system varies between species. In dogs, the pancreatic duct joins the duodenum at the major duodenal papilla, adjacent to the common bile duct. With most, an accessory pancreatic duct joins the duodenum at the minor papilla though anatomic variation exists. In cats, the common bile duct fuses with the pancreatic duct before entering the major papilla. Only 20% of cats are estimated to have an accessory pancreatic duct1.
Examination Technique
Due to the proximity of gastrointestinal structures, intraluminal gas will create reverberation artefacts which can obscure the adjacent pancreas. The presence of gas can be reduced through fasting the patient; where possible, a minimum of 12 hours fasting is recommended prior to an ultrasound examination of the pancreas.
Because the right lobe of the pancreas sits adjacent to the descending duodenum in the right cranial abdomen, it is important to include the last 3-4 intercostal spaces in the clipped area. It may be necessary to use the intercostal windows to view parts of the pancreas in this area, especially in deep chested dogs.
The highest frequency setting that gives adequate depth of penetration to view the area of the pancreas being examined should be used in order to maximise axial resolution. In most patients, a transducer with a frequency range of 8-15Mhz is ideal. However, in larger dogs, transducers with a lower frequency range (5-8Mhz) may be necessary to ensure adequate ultrasound penetration. Either a micro-convex or linear transducer can be used for the examination. Linear transducers are capable of higher frequency settings and are ideal for smaller patients and superficial areas of the abdomen. However, their larger footprint makes them harder to manoeuvre around the body. In comparison, micro-convex transducers produce a lower frequency range though their smaller, curved surface make them ideal in larger patients and for imaging using the intercostal spaces.
The normal pancreas has an inconspicuous, poorly marginated appearance. Therefore, the use of anatomic landmarks to guide an examination of the pancreas is essential.
The patient is positioned in right or left lateral recumbency depending on the area of the pancreas to be examined.
Left Lobe of Pancreas
With the transducer in sagittal orientation, the probe can be slid up the costal arch to roughly mid abdomen level. Identify the spleen and the splenic vein which is visible in the far field, deep to the body of the spleen.
The left lobe of the pancreas lies immediately caudal to the greater curvature of the stomach and follows the course of the splenic vein as it runs to join the cranial mesenteric vein forming the portal vein. By sliding the probe, you can follow the course of the splenic vein towards the portal vein. The area of the left lobe of the pancreas is between the splenic vein and portal vein at the level where both are visible.
Bordering the area of the left lobe of the pancreas the greater curvature of the stomach lies immediately cranially, the transverse colon is caudal, and the spleen lies laterally. These structures can be identified as boundaries to mark the area where the left pancreatic lobe can be seen.
Body of Pancreas
Place the transducer caudal to the xiphisternum with the probe marker orientated cranially. Identify the stomach caudal to the liver and fan the transducer towards the patient’s right side to the point where the stomach just disappears from the image. This is the level of the pylorus. Fan the probe back towards the patient’s left until the image of the pyloric area of the stomach just appears again and then rock the probe to angle slightly caudally. Fanning the probe dorsally and ventrally from this point, the portal vein should be identified. The portal vein is the largest ventral vessel and should be visible on the right-hand side of the screen as a tubular, anechoic structure. The area of the body of the pancreas is visible between the portal vein and pyloric area of the stomach at this level.
Right Lobe of Pancreas
With the probe in a sagittal orientation, identify the right kidney. From this point slide the probe ventrally. The duodenum is positioned laterally, immediately deep to the right body wall and is usually the first loop of small intestine visualised in the ultrasound image. In some patients the ascending colon will be visible between the kidney and the duodenum. The duodenum is identifiable by its lateral position and the straight course it travels in the right abdomen. It has larger wall thickness compared to other parts of the small intestine due to a prominent mucosal layer. With the duodenum visualised in a sagittal plane make small fanning movements dorsally to identify the area of the right lobe of the pancreas, which is dorsomedial to the descending duodenum. In canine patients, the pancreaticoduodenal vein can help to identify the area of the right pancreatic lobe. It is visualised as an anechoic tubular structure within the pancreas. Rotating the probe through 90 degrees to achieve a transverse image of the duodenum, the pancreas can sometimes be visualised as an irregular triangular area adjacent to the dorsomedial border of the duodenum.
Normal appearance
Normal pancreatic tissue is thin and appears isoechoic to the surrounding mesenteric tissue and fat, making it indistinct. It is hypoechoic relative to the spleen and iso- to hyperechoic relative to the liver.
In dogs, the body and left lobe are harder to visualise in comparison to the right lobe. Whereas in cats, the right lobe is harder to visualise in comparison to the body and left lobe.
Right lobe of the pancreas in a dog. This image shows a transverse plane image of the right lobe adjacent to the dorsomedial border of the descending duodenum. The borders of the pancreas are marked with the dotted yellow line and the pancreaticoduodenal vein can be seen as a central round anechoic (black) area within the pancreatic tissue.
Body of the pancreas in a cat. In this sagittal plane image, the body of the pancreas is visible as a roughly triangular area of tissue (red arrow) caudal to the pylorus (blue arrow) and ventral to the portal vein (green arrow).
Left lobe of the pancreas in a dog. This sagittal plane image of the left cranial abdomen shows the left lobe of the pancreas as a roughly triangular area of tissue. The digital callipers are placed on the image to measure the pancreas width. The stomach, filled with intraluminal gas, is visible cranially (blue arrow), the transverse colon caudally (red arrow) and the spleen occupies the near field (green arrow). The splenic vein is visible as a circular anechoic area between the spleen and pancreas.
Normal ranges are available for measuring the thickness of the pancreas in cats and dogs:
Dog2: Right lobe – Normal width: 0.39-1.6cm, mean= 0.81cm
Left lobe – Normal width: 0.36-1.4cm, mean= 0.65cm
Body – Normal width: 0.35-1.1cm, mean= 0.63cm
Cat3: Right lobe – Normal width: 0.3-0.57cm, mean 0.43cm
Left lobe – Normal width: 0.46-1.03cm, mean= 0.65cm
Body – Normal width: 0.46-0.9cm, mean= 0.64cm
The pancreatic duct can be differentiated from any vessels using Doppler ultrasound. The duct is not readily identifiable in dogs but is more frequently identifiable in cats as a central anechoic area within the pancreas, most easily identifiable within the left lobe. In cats the duct diameter increases with age: mean duct diameter in cats ≥10 years 1.3mm (range 0.6-2.4mm)3.
Ultrasonographic appearance of pancreatic pathology
It is important to recognise that the pancreas can appear ultrasonographically normal despite the presence of pathology.
Pancreatitis
The diagnosis of pancreatitis requires a multimodal approach, and it is important to interpret ultrasound findings along with history, physical exam findings and the results of serum assays such as the specific canine pancreatic lipase assay4.
Ultrasonographic changes seen in acute pancreatitis include:
- Enlargement of the pancreas
- Hypoechoic areas within the pancreas
- Hyperechogenicity of the surrounding mesentery
- Dilation of the pancreatic and/or common bile duct
- Focal free abdominal fluid
- Thickened gastric and/or duodenal wall
- Ileus and corrugation of local intestine
- Formation of pancreatic cysts or abscesses
Because the pancreas can appear unremarkable on ultrasound examination despite a high clinical suspicion of pancreatitis, repeat ultrasound examinations can be useful to detect changes over time and monitor response to treatment5.
Pancreatitis affecting the left pancreatic lobe in a dog. This sagittal plane image of the left cranial abdomen shows some of the ultrasonographic changes seen in pancreatitis; the left lobe is visible as a hypoechoic area in the centre of the image (the digital callipers are placed to measure pancreatic thickness). The surrounding mesentery has become relatively hyperechoic. Note that the normal anatomic landmarks are still visible; the stomach cranially (blue arrow), spleen in the near field (green arrow,) and transverse colon caudally (red arrow).
Pancreatitis in the right pancreatic lobe of a dog. In this transverse plane image, the duodenum can be seen (red arrow). The right lobe of the pancreas is visible on the dorsomedial aspect of the duodenum and has become diffusely hypoechoic and enlarged (blue arrow). The neighbouring mesentery has become hyperechoic (asterisk) and there is thickening of the duodenal wall adjacent to the pancreas.
Pancreatic oedema can be seen in cases of pancreatitis, though hypoproteinaemia and portal hypertension can also cause oedema. The pancreas may appear thickened with hypoechoic stripes.
Chronic pancreatitis can be challenging to diagnose and the ultrasonographic appearance of the pancreas can be variable. There is an overlap between the changes seen in cases of acute pancreatitis and those observed in some cases of chronic pancreatitis; the pancreas can appear hypoechoic with hyperechoic surrounding mesentery. However, other signs such as pancreatic thickening and a hyperechoic or irregular pancreas can be seen in chronic cases6,7.
Nodular Hyperplasia
Pancreatic nodular hyperplasia is an incidental finding seen in older dogs and cats8. It can appear as multiple small, hypo- to isoechoic nodules within the pancreas. There can be an overlap in the appearance of malignant pathology and nodular hyperplasia, therefore cytology and histopathology are necessary to distinguish between the two8.
Pancreatic Cysts
Cysts can be incidental findings (true cysts) or can be formed following pancreatitis (pseudocysts). These appear as round, anechoic areas within the pancreatic tissue which show distal acoustic enhancement.
Pancreatic Abscess
Pancreatic abscesses can form as a sequalae to pancreatitis. These appear as areas with thick, irregular walls and a hypoechoic centre. Flocculent fluid can be seen within the abscessed area, as can reverberation from gas particles if gas-producing bacteria are present. Differentiating abscesses from cystic lesions, pancreatic necrosis or tumours is difficult and sampling for cytology or histopathology is necessary for diagnosis7.
Pancreatic Neoplasia
It can be difficult to differentiate pancreatic neoplasia from other pancreatic conditions using ultrasound alone. It is important to consider findings alongside the patient’s physical examination and the results any bloodwork. Cytology and/or histopathology samples can be essential for diagnosis, as can CT or MRI evaluation. Serial ultrasound examinations can be used as pancreatitis can improve with treatment, whereas neoplastic conditions will not.
Neoplasia can appear as nodules or masses within the region of the pancreas. However, diffuse forms of pancreatic neoplasia are also possible. Neoplastic changes can vary in echogenicity and may cause local fluid accumulation.
Exocrine tumours that arise from the ductal system or acinar cells are the most common form of pancreatic neoplasia in small animal patients. However, the incidence of pancreatic exocrine carcinoma in dogs is less than 0.5%9. A recent study showed that metastatic spread was detected in 78% of cases at the time of diagnosis9. Therefore, it is important to assess neighbouring structures such as the liver and local abdominal lymph nodes in addition to the pancreas. Similarly, pancreatic neoplasia and/or inflammation can cause obstruction of the biliary system, leading to common bile duct and hepatic duct dilation.
Pancreatic endocrine tumours are rare, with insulinoma being the most common form seen. Diagnosis can be challenging, and contrast enhanced CT is reported to have a better sensitivity for detection than ultrasound10.
Sampling Considerations
Ultrasound guided fine needle aspiration can be used to sample pancreatic lesions along with draining cavitary lesions. A 20- to 22-gauge spinal needle can be used and Doppler ultrasound can be useful in highlighting any local blood vessels that should be avoided. The technique is shown to be safe in cats11 and has a low rate of complication in dogs12.
References
- Otte C.M.A., Penning L.C., Rothuizen J. (2017) Feline biliary tree and gallbladder disease: aetiology, diagnosis and treatment. Journal of Feline Medicine and Surgery 19: 514-528.
- Penninck D.G., Zeyen U., Taeymans O.N., Webster C.R. (2013) Ultrasonographic measurement of the pancreas and pancreatic duct in normal dogs. American Journal of Veterinary Research 74: 433-437.
- Hecht S., Penninck, D.G., Mahony, O., King, R., Rand, W. (2006) Relationship of pancreatic duct dilation to age and clinical findings in cats. Veterinary Radiology & Ultrasound, 47: 287–294.
- Cridge H., Sullivant A.M., Wills R.W., Lee A.M. (2020) Association between abdominal ultrasound findings, the specific canine pancreatic lipase assay, clinical severity indices, and clinical diagnosis in dogs with pancreatitis. Journal of Veterinary Internal Medicine 34: 636-643.
- Hecht S. Henry G. (2007) Sonographic evaluation of the normal and abnormal pancreas. Clinical techniques in small animal practice. 22 : 115-121.
- Forman M.A., Steiner J.M., Armstrong P.J., Camus M.S., Gaschen L., Hill S.L., Mansfield C.S., Steiger K. (2021) ACVIM consensus statement on pancreatitis in cats. Journal of Veterinary Internal Medicine 35: 703– 723.
- Nyland T.G., Mattoon J.S. (2015) Pancreas. In: Small Animal Diagnostic Ultrasound 3rd edn., Eds. Mattoon J.S., Nyland T.G., Elsevier, St. Louis, pp 438-465.
- Hecht S., Penninck D.G., Keating J.H. (2007) Imaging Findings in Pancreatic Neoplasia and Nodular Hyperplasia in 19 cats. Veterinary Radiology & Ultrasound 48: 45-50.
- Pinard C.J., Hocker S.E., Weishaar K.M. (2021) Clinical outcome in 23 dogs with exocrine pancreatic carcinoma. Veterinary and Comparative Oncology 19: 109-114.
- Ryan D., Pérez‐Accino J., Gonçalves R., Czopowic, M., Bertolani C., Tabar M.D., Puig J., Ros C. Suñol, A. (2021) Clinical findings, neurological manifestations and survival of dogs with insulinoma: 116 cases (2009‐2020). Journal of Small Animal Practice doi:10.1111/jsap.13318.
- Crain S.K., Sharkey L.C., Cordner A.P., Knudson C., Armstrong P.J. (2015) Safety of ultrasound-guided fine-needle aspiration of the feline pancreas: a case-control study. Journal of Feline Medicine and Surgery 17: 858-863.
- Cordner A.P., Sharkey L.C., Armstrong P.J., McAteer K.D. (2015) Cytologic findings and diagnostic yield in 92 dogs undergoing fine-needle aspiration of the pancreas. Journal of Veterinary Diagnostic Investigation 27: 236-40.
Further Reading
Nyland T.G., Mattoon J.S. (2015) Pancreas. In: Small Animal Diagnostic Ultrasound 3rd edn., Eds. Mattoon J.S., Nyland T.G., Elsevier, St. Louis, pp 438-465.
d’Anjou M.A., Penninck D. (2015) Pancreas. In: Atlas of Small Animal Ultrasonography 2nd edn., Eds: Penninck D. and d’Anjou M.A., John Wiley & Sons, Chichester, pp 309-330.
Griffin S. (2020) Feline Abdominal Ultrasonography: What’s Normal? What’s Abnormal? The Pancreas. Journal of Feline Medicine and Surgery 22: 241-259.
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