Introduction

Vomiting is a common presenting complaint in cats and may be caused by a variety of gastrointestinal and non-gastrointestinal disorders. Diagnostic imaging plays an important role in the work-up of the vomiting cat. While advanced imaging modalities (CT, MRI) are gaining popularity in the evaluation of the small animal abdomen, radiographs and ultrasound remain the imaging techniques of choice. Contrast procedures may also be indicated in certain cases.

Abdominal Radiographs¹

Routine abdominal radiographs should include right lateral and ventrodorsal views. Due to re-distribution of gas and fluid with gravity an additional left lateral view is helpful in the evaluation of pylorus and proximal duodenum and is strongly recommended in vomiting animals. Additionally, a horizontal beam view may be helpful in cases of suspected pneumoperitoneum. Evaluation of abdominal radiographs includes assessment of serosal detail (increased in cases of pneumoperitoneum; decreased in cases of abdominal effusion, visceral crowding and lack of intra-abdominal fat), extra-abdominal structures, and abdominal organs which are systematically evaluated using Roentgen signs (number, size, shape, opacity, margination, location). In normal cats the liver, spleen, kidneys, urinary bladder, and gastrointestinal tract are visible. Parenchymal organs are of homogenous soft tissue opacity. The liver is sharply marginated and contained within the costal arch. The spleen is only visible on the VD view as a small triangle caudal to the gastric fundus. The kidneys are bean shaped and fairly symmetric in size. On a VD view they measure 2.4–3.0x length of L2 (intact cats) and 1.9–2.5x length of L2 (spayed/neutered cats), respectively. The urinary bladder is variable in size. The appearance of the gastrointestinal tract (GIT) is variable. The stomach may contain ingesta, gas and/or a small amount of fluid. The small intestine usually contains a mixture of gas and fluid, and the colon contains gas and formed faecal material. Solid material should only be present in the stomach and the colon. Gastric and intestinal wall thickness is difficult to evaluate on survey radiographs due to luminal contents. Overall small intestinal diameter should not exceed 12 mm or 2x the height of L2. Especially in fat cats, the extremity of the left lobe of the pancreas may be visible on the VD view. Adrenal glands may be visible if mineralized. The only reproductive organ seen is the uterus in pregnant cats which is of soft tissue opacity until mineralized foetal structures appear. Normal abdominal lymph nodes are not visible.

Abdominal Ultrasound^2,3

The ultrasonographic examination complements abdominal radiographs. Ultrasound is inferior to radiographs; e.g., in the assessment of liver size, in the detect ion of mineralized or metallic peritonea l foreign bodies, and in the detection of a small volume peritoneal or organ associated gas. Ultrasound is superior to radiographs, e.g., in the assessment of internal organ structure and the detection of a small volume effusion. Also, ultrasound allows guided tissue sampling (fine-needle aspiration, biopsy) for further evaluation. With the exception of critically ill/unstable patients the ultrasound examination of a vomiting cat should include evaluation of all abdominal organs. The normal liver is hypoechoic to the spleen and similar in echogenicity as adjacent falciform fat. The gall bladder is thin walled (<1 mm) and contains anechoic bile. The spleen is small (<1 cm in thickness) and relatively hyperechoic. The kidneys are smoothly marginated and have a distinct cortico medullary definition. The renal pelvis is usually empty, although a renal pelvis diameter of 1–2 mm may be normal secondary to diuresis or back pressure from a distended urinary bladder. Normal ureters are not visible. The urinary bladder is thin walled and contains urine which may contain echogenic particles in suspension. The adrenal glands are more rounded than in dogs and measure less than 5 mm in thickness. The stomach, duodenum, jejunum, ileum and colon can be distinguished based on differences in position, wall thickness, appearance of the wall layers, and contents. The gastric wall measures less than 4mm in thickness and has prominent submucosal and muscularis layers. Gastric contents are highly variable; however, normal cats usually have little to no fluid within the stomach. The duodenum and jejunum have distinct layers (thick inner mucosal layer; thin submucosa, muscularis, serosa) and measure up to 2.5 mm in wall thickness. These intestinal segments typically contain a small amount of gas and fluid. The ileum is comparatively thick (up to 3.2 mm) and characterized by a prominent hyperechoic submucosal layer. The colonic wall thickness depends on contents and ranges from less than 1mm when filled with faeces to more than 2 mm when empty. The body and left lobe of the pancreas are readily visualized caudal to the stomach, especially in older cats. The normal pancreas measures less than 1 cm in thickness and contains a central duct. Some abdominal lymph nodes (gastric, medial iliac, ileocolic) are occasionally visualized in normal cats, are of similar echogenicity as the spleen, and measure less than 5 mm in thickness. Female reproductive organs which may be visible include the ovaries and a pregnant uterus; internal male reproductive organs are not seen.

Contrast Procedures

An oesophagram is the imaging study of choice to rule out disorders of the oesophagus, especially with a presenting complaint of regurgitation rather than vomiting. Other gastrointestinal tract contrast procedures (barium series, colonogram) have largely been replaced by ultrasound and endoscopic procedures. Similarly, urinary tract contrast studies have largely been replaced by ultrasound. However, an excretory urogram remains the technique of choice to evaluate ureteral integrity, and a cystourethrogram is helpful in identifying a bladder rupture or an intrapelvic urethral lesion inaccessible for ultrasonographic evaluation.

Imaging Findings in Cats With Primary Gastrointestinal Disease

Common gastrointestinal diseases in cats include foreign bodies, inflammatory disorders, neoplasia and ileus. Foreign bodies may be radio-opaque and easily detectable on radiographs. Non-opaque foreign bodies are a diagnostic challenge, and a diagnosis is usually made based on secondary signs (obstruction or plication of intestine in case of a linear foreign body) or using ultrasound. Inflammatory and neoplastic disorders are more commonly diagnosed using ultrasound, although advanced disease usually also results in radiographic changes. Inflammatory disorders result in mild to moderate generalized/multifocal intestinal wall thickening. Intestinal wall layers are usually maintained but are frequently altered (echogenicity changes of the mucosal layer, thickening of the muscularis layer). Mild to moderate mesenteric lymphadenopathy is common. Lymphoma is by far the most common gastrointestinal neoplasm in cats; other neoplasms include mast cell tumour, adenocarcinoma and smooth muscle tumours. A focal mass (or masses) with loss of wall layers is most consistent with an intestinal neoplasm, and enlargement of local lymph nodes is common. Unfortunately, overlap exists between imaging finding in cats with inflammatory and neoplastic disorders, and some intestinal neoplasms (lymphoma, mast cell tumours) behave the same way as inflammatory disease. lleus may be diagnosed either using radiographs or ultrasound. Obstructive ileus manifests as dilation of some intestinal segments while others retain their normal diameter; therefore, 2 different populations of intestinal segments are present. Paralytic ileus results in generalized intestinal dilation and decreased to absent intestinal motility noted on ultrasound.

Imaging Findings in Cats with Non-gastrointestinal Disease

Radiographic and ultrasonographic abnormalities may be detected in one or more organs of a vomiting cat. As imaging changes may or may not be clinically significant they have to be correlated with findings on clinical examination and laboratory analyses. Common liver diseases associated with vomitus in cats include cholangiohepatitis and lymphoma. Both result in increased hepatic size with variable echogenicity changes observed on ultrasound. Sludge associated with the gall bladder is suggestive of cholecystitis in cats. Renal disease is very common in cats and frequently results in vomiting. Inflammatory (e.g., FIP, acute nephritis) and neoplastic diseases (e.g., lymphoma) result in enlarged kidneys with variable echogenicity changes. Hydronephrosis also results in renomegaly on radiographs and is easily recognized by an enlarged renal pelvis on ultrasound. Visible ureters are concerning for obstruction and should be traced to identify an obstructive lesion. Chronic degenerative renal disease results in small and irregularly shaped kidneys. Urolithiasis is readily diagnosed using either radiographs or ultrasound. Peritonitis and diffuse spread of abdominal neoplasia (carcinomatosis) result in lack of serosal margin detail (often with a granular appearance) on radiographs. Ultrasound findings include abdominal effusion, hyperechoic mesenteric fat and mesenteric/omental nodules. Pancreatitis is common in cats and may be difficult to identify with imaging especially in chronic cases. A hypoechoic, irregular pancreas may be observed on ultrasound. Other possible findings include hyperechoic mesenteric fat, abdominal effusion and dilation of the pancreatic duct and common bile duct. Concurrent findings suggesting inflammatory bowel disease and/or cholangiohepatitis are common. Splenomegaly and enlarged abdominal lymph nodes may be noted on radiographs or ultrasound and suggest infiltrative inflammatory or neoplastic disease.

References

1.  Thrall DE. Textbook of Veterinary Diagnostic Radiology. 6^th ed. St. Louis, MO: Elsevier Saunders; 2013.

2.  Bafr F, Gaschen L. BSAVA Manual of Canine and Feline Ultrasonography. Gloucester: BSAVA; 2011.

3.  Pennick D, d’Anjou MA. Arias of Small Animal Ultrasonography. Ames, IA: Wiley Blackwell; 2015.