Diagnostic imaging of the stomach

Standard radiographic examination of the stomach enables assessment of the shape, size, position and contents of this organ and, in the case that there is gas within its lumen, an initial evaluation of the gastric wall. In fact, the intragastric gas can be exploited as a natural contrast agent, useful also for visualising the different parts of the stomach in the various radiographic projections_¹.

 Gastrography, carried out with a single contrast agent or with double contrast (a radio-opaque dye and then air)_^1-4, can be used to study gastric motility and the characteristics of gastric emptying. The contrast agents used for the gastrointestinal system, their doses, the times to take the X-ray films and the organs opacified at the given times are reported in the following table.      

Radiographically visible alterations

Size: the state of gastric filling depends on the amount and type of food eaten by the animal as well as on the amount of gas. Stenoses secondary to phenomena of chronic infiltration or fibrosis, or spasms of the pylorus can delay normal gastric emptying.

Position:

Torsion (Fig. 1): gastric torsion means rotation of the stomach from 90° to 360° around its major axis. The most common amount of torsion is 180°, with a variable degree of gastric dilatation. Clockwise rotation is more common that anticlockwise rotation. The spleen normally follows the rotation of the stomach and, therefore, appears enlarged on X-rays. The torsion is often preceded by gastric dilatation, although torsion can occur without prior dilatation of the stomach. This fact suggests that gastric dilatation is not the only factor predisposing to gastric torsion. The characteristic X-ray signs of gastric torsion are reported in the table below.

Considering the greater visibility of the pylorus, it is advisable to take the right-lateral view first. Chest X-rays are also indicated, particularly in elderly patients, in order to exclude obvious, serious pathologies (masses, severe cardiomegaly) which could influence the therapeutic decisions.

Gastric dilatation_¹: gastric dilatation means severe distension of the stomach which contains a large amount of gas. Food-induced gastric dilatation is most common (Fig. 2) and is secondary to the intake of an abnormal amount of food. The key feature differentiating gastric dilatation from torsion is that in the former the position of the stomach and its relations to the other organs remain unchanged.

Content_¹: a radiographic examination can reveal the presence of metallic, radio-opaque mineral or osseous foreign bodies. Plastic objects and pieces of wood may also be visible if sufficiently large and surrounded by gas. The presence of food within the gastric lumen can create confounding images because the food may be confused for foreign bodies during the interpretation of an X-ray.

Changes in the gastric wall_¹: it is difficult to evaluate gastric wall thickening (caused by inflammation, hypertrophy, benign or malignant neoplasms) with plain X-rays because the presence of fluid in the stomach can mask, or sometimes simulate, an increase in wall thickness. In the absence of other more accurate diagnostic methods (ultrasonography, endoscopy, computed tomography, etc.) contrast media can be used (positive or double contrast) (Figs. 3 and 4. Courtesy of Professor S. Citi, University of Pisa). In cases of chronic renal failure, there may sometimes be an increase in the radio-opacity of the gastric submucosa due to mineralisation.

Hiatus hernia: this occurs when part of the stomach enters the thorax through the oesophageal hiatus; the condition may be congenital or traumatic. Two types of hiatus hernia have been recognized: a sliding hiatus hernia, in which the gastro-oesophageal junction and part of the gastric fundus are displaced cranially into the thorax, and a rolling (para-oesophageal) hernia, in which part of the stomach becomes located in the thorax parallel to the caudal part of the oesophagus. Radiographically, a mass can be seen in the caudal part of the thorax close to the left crus of the diaphragm; given the partial displacement of the stomach, its shape appears abnormal and the oesophagus is dilated_^6,7. Since the sliding hiatus hernia is a dynamic condition, it may not be visible with a single radiographic image or, if visible in a first image, may not be found in a subsequent radiographic control_¹ (Fig. 5, image at T0, and Fig. 6, image at T180). Positive contrast may be used to confirm the diagnosis (Fig. 7). Radiographic studies may also show a possible gastro-oesophageal intussusception_⁸.

ULTRASONOGRAPHY

Ultrasonography is a low-cost, practical and sensitive method for studying the stomach: it enables an assessment of the thickness of the gastric wall and its layers and evaluation of gastric motility. The examination can be repeated easily and so is suitable for following the evolution of a lesion. The animal to be assessed should always be kept fasted for at least 12 hours prior to the examination since the presence of gas and food can hamper a good view of the organ.

The fundus and body of the stomach are located on the left in both the dog and the cat. When the stomach is empty, the mucosal folds tend to collapse and in transverse section have a “flower-like” appearance, while in longitudinal scans they form parallel, hypo- and hyper-echoic striae.

The pyloric antrum is located perpendicularly to the midline plane and in transverse section has a classical “bull’s eye” appearance. The pylorus can be seen on the righ t of the midline plane in the dog and closer to the midline in the cat. The contractions of thewall occur approximately four to five times a minute in a fasting state; some anaesthetics, such as ketamine, acepromazine and xylazine, and prokinetic drugs can alter normal intestinal motility_^9-11. Measured by ultrasound, the thickness of the wall of the stomach ranges from 2 to 5 mm in the dog_^12-14 and from 1.7 to 3.6 mm in the cat_^12-16, depending, in part, on the degree of distension of the organ. The gastric wall has five layers_^12-15 (Fig. 8).

Abnormal ultrasound findings

Pyloric stenosis: ultrasonographically, pyloric stenosis is characterized by symmetrical thickening of the pylorus, which can reach a thickness of up to 1 cm; the increase particularly involves the muscle layer. The stomach appears greatly dilated by fluids, gas and food_^17,18. There has been one published report of a case of pyloro-gastric intussusception in a dog_¹⁹.

Gastric foreign bodies: ultrasound examination may be able to identify gastric foreign bodies, although the presence of food and gas can hamper their visualisation or create artefacts (for example, residues of food) mimicking the presence of foreign bodies. Stones, metal objects, plastic and wood create strongly reflective, hyperechoic interfaces and tend to produce a posterior acoustic shadow_²⁰ (Fig. 9); besides a posterior acoustic shadow, some hollow objects can also give rise to reverberation because of the gas within them. When the foreign body causes a complete obstruction, there may initially be an increase in peristalsis and gastric dilatation.

Gastritis: in acute forms there may be diffuse thickening (up to 6-7 mm) of the wall of the stomach, with the normal structure of the layers maintained, while in chronic forms of gastritis there may also be a partial or total loss of the normal organization of the layers of the gastric wall_²¹ (Fig. 10) making the condition difficult to differentiate by ultrasound from a neoplasm_²²; a  biopsy is, therefore, indicated in these cases. Gastritis may be complicated by ulcers, which appear as focal thickenings of the wall, with loss of the layers of the wall and a central area that is often hyperechoic because of the presence of gas and that tends to create an interruption in the gastric wall_²³.

Uraemic gastropathy: this condition can be found in patients with chronic renal failure. Areas of mineralisation of the mucosa and submucosa can be seen, sometimes accompanied by considerable changes in the normal architecture of the gastric wall layers_^12,13,24.

Neoplasms: The benign neoplasms most frequently found in the stomach are leiomyomata and adenomatous polyps_²⁵, whereas the most common malignant neoplasms in this site are adenocarcinoma_²⁶ (Fig.11), leiomyosarcoma_²⁷ and lymphosarcoma (this last particularly in the cat)_²⁸. The ultrasound signs most frequently observed, alone or in combination, in the case of gastric neoplasm are presented in the following table.

HIGHER LEVEL IMAGING TECHNIQUES

Computed tomography(CT) is now a technique widely used also in veterinary medicine, not only for the direct study of the stomach, but also for an accurate and complete staging of the patient with a gastric neoplasm.

The gas physiologically present within the lumen of the stomach acts as a natural contrast agent, which is useful when carrying out CT studies. A specific study (hydro-CT) can be useful in order to improve the visualisation of the wall. Hydro-CT is performed by introducing tepid water (the optimal amount is 30 ml/kg) into the stomach through a gavage tube and repeating the CT carrying out a plain scan and then another after administration of the contrast agent. This method provides better definition of the presence, extent and characteristics of a lesion of the gastric wall_³¹ (Fig.12).                    

There are also studies in which scintigraphy has been used to investigate the stomach of animals_^32,33, but the technique is currently not used in veterinary practice in Italy.

References

1. Mahaffey MB, Frank PM. The stomach. In Thrall DE. Textbook of Veterinary Diagnostic Radiology. Saunders, 2007.
2. Miybayashi T, Morgan JP. Gastric emptyng in the normal dog: a contrast radiograph technique. Vet Radiol, 25:187; 1984.
3. Brawnwe WR, Bartels JE. Contrast radiography of the digestive tract. Indications, technique and complication. Vet Clin North Am, 13:500; 1983.
4. Evans SM, Lauffer I. Double contrast gastrography in the cat: technique and normal radiographic appearance. Vet Radiol, 22:2; 1981.
5. Kneller SK. Radiographic interpretation of the gastric dilatation-volvolus complex in the dog. J Am Anim Hosp Assoc 12:154; 1976.
6. Iawasaki M, DeMartin BW, DeAlvarenga J et al. Congenital hiatal hernia in a dog. Med Vet Pract 58:1018; 1977.
7. Peterson SL. Esophageal hiatal ernia in a cat. J Am Vet Med Assoc 183:325; 1983.
8. Van Camp S, Love NE, kumaresan S. Radiographic diagnosis-gastroesophageal intussusception in a cat. Vet Radiol Ultrasound, 39:190-192, 1998.
9. BargaiU.The  effect of xylazine hydrochloride on the radiographic appearance of the stomach and intestine in the dog. Vet radiol 23:60; 1982.
10. Steyn PF, Twedt D, Toombs W. The effect of intravenous diazepam on solid phase gastric emptyng in normal cats. Vet Radiol Ultrasound, 38: 469; 1997.
11. Scrivani PV, Bednarsky RM, Meyer CW. Effect of acepromazine and butorphanol on positive- contrast upper gastrointestinal tract examination in dogs. AM J Vet Res 59: 1227; 1998.
12. Penninck DG. Gastrointestinal tract. In Penninck DG. Atlas of Small Animal Ultrasonography. Blackwell, 2008.
13. Chetboul V, Tessier-Vetzel D. Esame ecografico del tratto digerente. In Chetboul V: esame ecografico di addome occhio e sistema nervoso del cane e del gatto. Masson, Paris, 2001.
14. Penninck DG, Nyland TG, Fisher PE, et al. Ultrasonography of normal canine gastrointestinal tract. Vet Radiol, 30, 6, 272-275; 1989.
15. Newell SM, Graham JP, Sonography of the normal feline gastrointestinal tract. Vet Radiol Ultrasound, 40, 1, 40-43; 1999.
16. Gogging JM, Biller DS, Debey BM, et al. Ultrasonographic measurement of gastrointestinal wall thickness and the ultrasonographic appearance of the ileocolic region in healthy cats. J Am Anim Hosp Assoc, 36: 224-228; 2000.
17. RhodesWH, Brodey RS. The differential diagnosis of pyloric obstruction in the dog. J Am Vet Radiol Soc 6:65; 1965.
18. Biller DS, Partington BP, Myabashi T, Leveille R. Ultrasonographic appearance of chronic hypertrophic pyloric gastrophaty in the dog. Vet Radiol Ultrasound, 35, 1, 30-33; 1994.
19. Lee H, Yeon S, Lee H, Chang D, et al. Ultrasonographic diagnosis­-pylorogastric intussusception in a dog. Vet Radiol Ultrasound,46: 317–318; 2005.
20. Tidwell AS, Penninck DG. Ultrasonographic of gastrointestinal foreign bodies. Vet Radiol Ultrasound 33: 160-169; 1992.
21. Baez Jb, Hendrick MJ, Walker LM, et al. Radiographic, ultrasonographic, and endoscopic findings in cats with inflammatory bowel disease of the stomach and small intestine: 33 cases (1990-1997). J Am Vet Med Assoc 215: 349-354; 1999.
22. Penninck DG, Smyers B, Webster CRL, et al. Diagnostic value of ultrasonography in differentiating canine enteritis from intestinal neoplasia. Vet Radiol Ultrasound 44: 570-575; 2003.
23. Penninck DG, Matz ME, Tidwell AS. Ultrasonographic detection of gastric ulceration. Vet Radiol Ultrasound, 38:308-312; 1997.
24. Grooters AM, Miyabayashy T, Biller DS, et al. Sonographic appearance of uremic gastropathy in four dogs. Vet Radiol Ultrasound, 35: 35-40; 1994.
25. DianaA,PenninckDG, KeatingJH.Ultrasonographic appearance of canine gastric polyps. Vet Radiol Ultrasound, 50:201–204; 2009.
26. Penninck DG, Moore AS, Fliatto J. Ultrasonography  of canine gastric epithelial neoplasia. Vet Radiol Ultrasound, 39: 342-348; 1998.
27. Myers NC, Penninck DG. Ultrasonographic diagnosis of gastrointestinal smooth muscle tumors in the dog. Vet Radiol Ultrasound, 35: 391-397, 1994.
28. Penninck DG, Moore AS, Tidwell As, et al. Ultrasonography of alimentary lymphosarcoma in the cat. Vet Radiol Ultrasound 35: 299-304; 1994.
29. Penninck DG. Characterization of gastrointestinal tumors. Vet Clin North Am, Small Anim Pract, 28, 4: 777-797, 1998.
30. Kaser-Hotz B, HauserB, Arnold P.Ultrasonographic findings in canine gastric neoplasia in 13 patients.Vet Radiol Ultrasound, 37: 51-56; 1996.
31. Terragni R, Vignoli M, Rossi F. Helical Hydro-TC (HHCT) in the study of stomach wall: normal tecnique and clinical application in dogs. 18_^th Annual ECVIM Congress-Ghent, Belgium, September 4-6; 2008.
32. Goggin JM, Hoskinson M,Kirk CA, et al.Comparison of gastric emptying times in healthy cats
33. simultaneously evaluated with radiopaque markers and nuclear scintigraphy.Vet Radiol Ultrasound, 40: 89-95; 1999.
34. Lester NV, Roberts DG, Newell SM, et al. Assessment of barium impregnated polyethylene spheres (bips®) as a measure of solid-phase gastric emptying in normal dogs-comparison to scintigraphy. Vet Radiol Ultrasound. 40: 465-471; 1999.