Thomas Spillmann, Dipl. vet. med., Dr. med. vet.
Professor of Small Animal Internal Medicine, Department of Equine and Small Animal Medicine, Faculty of Veterinary Medicine, University of Helsinki
Imaging techniques for the diagnosis of pancreatic diseases have following four aims (1):
To diagnose a pancreatic disease
To assess its spatial extension
To reveal differential diagnosis and etiology of the disease
To assess indication and viability of surgical interventions
Imaging techniques that have been used in humans and also animals are regarding their invasiveness: plain and contrast radiography, abdominal ultrasound, contrast enhanced computed tomography, endosonography, endoscopic retrograde pancreatography and laparoscopy with pancreatic biopsy. Radiography has proven to be the least sensitive and specific technique to diagnose pancreatic diseases. Therefore the following review will not cover X-ray but the diagnostic advantages and limitations of all the other mentioned techniques.
2. Abdominal Ultrasound
The advantage of abdominal ultrasound is the low invasiveness, relatively easy accessibility in veterinary practice, and good visualization of the normal and diseased canine pancreas when high quality equipment is used by an experienced ultrasonographer (2). Combined with fine needle aspiration for cytology, ultrasound can contribute to the diagnosis of inflammation (necrosis), pseudocyst or tumor (3). A special advantage is the possibility of arbitrary repetition of the ultrasonographic examination (1).
Limitations are caused by the close association of the pancreas with the stomach and duodenum. Intragastric and intraduodenal gas can disturb or even prevent the imaging of the whole pancreas. Other negatively influencing factors are abdominal pain, difficulties in sonographic differentiation between necrosis and tumor or acute and chronic pancreatitis (2). Therefore abdominal ultrasound has to be seen as a basic imaging technique in the diagnostic work up of pancreatic diseases. It's limitations in assessing the full extend of morphologic changes, in differentiating pancreatic necrosis or chronic pancreatitis from pancreatic carcinoma and in taking tissue samples indicates the necessity to use more advanced imaging techniques. A future improvement is expected by the introduction of contrast enhanced ultrasound (CEUS) into veterinary medicine. This technique allows a better differentiation between inflammation and neoplasia and to assess the extent of morphological changes (4).
3. Computed Tomography
Contrast enhanced computed tomography (CT) has become a corner stone in the diagnosis of pancreatic diseases in people. It is used to differentiate acute from chronic pancreatitis, to assess severity and extent of inflammatory and neoplastic processes and for planning the surgical approach when indicated. CT has also been used successfully in dogs to describe the appearance of the normal pancreas and to diagnose acute and chronic pancreatic diseases with high accuracy (5-9).
However, due to difficulties in accessibility, necessity for anesthesia and contrast application as well as high examination costs, CT has not yet achieved the same significance in veterinary gastroenterology as it has for people.
Endosonography of the pancreas is performed with equipment that combines flexible endoscopy with ultrasound. Main indication for endosonography in people is the search for very small tumors and metastasis in liver and pancreas that can not be visualized by abdominal ultrasound or CT. Due to high sensitivity and the possibility for exact sampling of biopsies out of sonographically abnormal areas endosonography helps to assess the actual size and localization of tumors and to assess the necessary extent of a surgical intervention (10).
A drawback for veterinary medicine is the high costs of the equipment. The technique has been used in dogs for a limited number of studies that proved its usefulness for imaging the normal and diseases pancreas (11-13).
5. Endoscopic Retrograde Pancreatography
Endoscopic retrograde pancreatography (ERP) is an imaging technique that combines endoscopy and fluoroscopy. It has been used for decades in people for diagnosing and staging chronic pancreatitis and pancreatic carcinoma by assessing contrast images of the pancreatic duct system and by taking brush cytology samples (14).
Current attempts to introduce this imaging technique also into veterinary medicine revealed in healthy dogs that ERP is technically possible and easier to perform than endoscopic retrograde cholangiography. It was possible to establish reference images and values for the length of both parts of the accessory pancreatic duct for healthy beagles (15). Using ERP in dogs with chronic gastrointestinal problems, the procedure was successful in 21 of 30 cases (70% success rate). It revealed abnormal findings of pancreatic ducts in 2 dogs with proven pancreatic acinar atrophy but showed normal images in the other patients (16). The low prevalence of abnormal duct findings can be explained by the fact that the patients for the procedure were not specifically selected for the suspicion of a pancreatic disease. ERP did not cause any mild or severe side effects such as acute pancreatitis neither in healthy dogs nor in dogs with chronic gastrointestinal problems (15,17).
The use of this promising technique is limited by the size of available duodenoscopes with a 90 degree side view optic. Duodenoscopes used in adult people have a tip diameter of 11-13 mm and can not be used in patients of < 10 kg. Pediatric endoscopes could be used but unfortunate they are currently very expensive.
Laparoscopy has been used for minimal invasive direct imaging of the canine pancreas and the withdrawal of tissue biopsies for histology. It was shown that it is an excellent tool to diagnose and differentiate pancreatic diseases such as chronic pancreatitis, pancreatic adhesions to adjacent organs, and partial pancreatic acinar atrophy. Side effects are minimal and pancreatic biopsy has not been reported to cause severe or even lethal complications. Therefore this technique is highly recommended especially to diagnose chronic pancreatitis (18-23).
The only disadvantage is the relative high initial investment for the necessary equipment. The positive experience with diagnostic laparoscopy also for hepatic diseases has made this technique to a valuable diagnostic for the veterinary gastroenterologist (20,21).
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