1/ Anatomy of the stomach.
Below is a dorso-ventral diagram of the stomach.
The stomach is found cranial to the last pair of ribs, dorsal and caudal to the liver.
The distal 1/3 rd
is the pyloric portion. Since the stomach is in the transverse plane, the pyloris is found right to the midline and the rest of the stomach on the left of the midline.
The greater curvature of the stomach is along the surface of the stomach and originates from the cardia and extends caudo-ventrally around the pyloris. (see below).
2/ Animal preparation for examination
Preparation involves several steps.
1.
Firstly starve the animal for 12 hours prior to scanning for elimination of all abdominal contents.
2.
Perform cleansing enema for 2hours before the stomach is to be scanned.
3.
Remove excess gas using an orogastric tube.
4.
Restrict water intake.
5.
Then scan the abdominal area along transverse and oblique planes using a 7.5mhz transducer.
6.
The stomach can then be filled with water for a second scan to improve imaging.
7.
Remember to take advantage of the effect of gravity on the luminal contents when using ultrasound. When evaluating the right side, make the animal lie on its right side so the fluid sinks and gas floats to the left.
8.
If the whole stomach is to be scanned, tip the animal so it lies in dorsal recumbancy. The gas rises to the chest but can be easily displaced when the transducer is pressed along the abdomen.
After these steps are followed the animal should be ready for an ultrasound examination.
Next the area that is to be scanned should be clipped to remove any hairs that may interfere with the ultrasound beam. The area to be clipped should be a little larger than the area to be scanned. Then use alcohol to soften the skin and remove any dirt etc. Then coat the ultrasound transducer with a gel that is hypoallergenic and a great ultrasound beam transmitter.

3/ Selection of the transducer.
The transducer used to scan the stomach is usually around 7.5mhz. A 7.5mhz transducer is a high-resolution transducer. This is important when imaging the walls of the stomach.
Increasing the frequency decreases penetration but great penetration through the stomach isn’t usually necessary as the opposite wall is usually obstructed from view by the luminal contents of the stomach.
For visualization of the peristaltic movement; greater penetration of the stomach is needed. We need to visual the whole stomach for this situation but since the resolution is decreased, it is not good for assessing the separate wall layers. Greater penetration requires a decreased frequency so a 5.0mhz transducer is used.
Small footprint transducers are the best choice to avoid shadowing caused by ribs. The stomach can be imaged between the ribs using a sector scanner with a small footprint.
4/ Normal expected sonographic findings.
The normal appearance of the stomach involves hyper echoic gastric fluids surrounded by rugal folds giving a satellite appearance. If real time imaging is used using a scanner with a good frame rate, peristalsis may be visualized.
There are 5 layers distinct to the stomach. The appearance alternates from hypo to hyper echoic. There is:

An inner hypoechoic layer – mucosal surface

Central hyperechoic layer – submucosa

Outer hyperechoic layer – subserosa and serosa

Inner hypoechoic layer – mucosa

Outer hypoechoic layer – muscularis propria
The mucosal layer is the thickest layer but the thickness of each layer doesn’t equal the corresponding histological layer. Total wall thickness is from 3 – 5 mm but it depends on distension of the stomach.
Normal luminal contents of the stomach are:
1.
Mucus – echogenic without shadowing
2.
Gas – hyperechoic with distal shadowing
3.
Fluid – Anechoic
The normal peristaltic contraction rate of the stomach and proximal duodenum in normal dogs is around 4-5 contractions per minute.

5/ Common abnormalities.
1.
Hypertrophic pyloric stenosis – Symptoms of this disease include chronic intermittent vomiting after eating. The pyloris when visualized under ultrasound is about 9mm thick and the muscularis layer is up to 4mm thick. Area is usually distended with fluid.
2.
Gastric foreign bodies – Gastric foreign bodies are identified by their acoustic shadows. Any structure under the foreign body wont be able to be seen due to the body’s highly reflective/attenuating surface. There is also some gastric dilatation and hyper peristalsis may be evident.
3.
Gastritis
– There stomach wall thickening. If the wall is more than 6-7 mm thick then pathogenesis should be suspected.
4.
Gastric ulcers – Focal area thickening in the stomach wall with a central area of echogenic focus.
5.
Uremic gastroenteropathy – Uremic gastroenteropathy is indicated by hypertrophy of the gastric wall and rugal folds. There is also mineralisation evident in the submucosa and vasculature. This is within the superficial layers.
The thickening usually passess 9mm.
6.
Gastric neoplasia – There are several types.
 polyps – Seen in a fluid filled stomach.

Leiomyomas – circular hypoechoic mass. It is continuous with the muscular layer.

Adenocarcinoma – Localised diffuse wall thickening.

Lymphosarcoma – thickened hypoechoic mucosa.

Leiomyosarcoma – Large solitary masses of intramural origin.
6/ References
Dubbins PA. The Gastrointestinal tract.
Clinics in Diagnostic Ultrasound:
Gastrointestinal Ultrasonography. 1988:195
Hamco LD. Gastrointestinal tract.
Small Animal Ultrasound. 8:149-173, 1996.
Hudson JA, Mahaffey MB. The Gastrointestinal Tract. Practical Vet Ultrasound. 9:107-
133
Pennique DG. Ultrasonography of the gastrointestinal tract.
Thrall DE. Textbook of Veterinary Diagnostic Radiology.
Rantanen NW. Diseases of the Abdomen.
Diagnostic Ultrasound. Veterinary Clinic North
American Equine Practise. 2:67, 1986
Wilson SR. The Gastrointestinal tract, in Diagnostic Ultrasound . Volume 1. Mosby
Yearbook 1991.