The Gastrointestinal Tract
Since gas is a barrier to ultrasound, ultrasound examinations of the gastrointestinal tracts have been considered as limited. However, in recent years, more and more ultrasonographic abnormalities have been reported in the literature. These abnormalities include gastroenteritis, gastrointestinal neoplasia, intussusception, linear foreign bodies, hypertrophic gastropathy, etc. Appropriate patients preparation such as overnight fasting should avoid excessive gas in the lumen and thus allow better visualization of the gastrointestinal tract. In addition, use of a high frequency transducer such as 7.5 or 10 MHz should improve resolution needed for detection of mural lesions. A real-time evaluation of the gastrointestinal movement is an additional benefit of ultrasound examinations.
Normal Ultrasonographic Appearance
The gastrointestinal tract should be evaluated on: (1) thickness, (2) layered appearance, and (3) contents. The normal gastric wall should measure no more than 6 mm, when it is moderately distended. The measurement is performed at the "valley" portion of the rugal folds in the gastric fundus. The normal intestinal wall should measure no more than 5 mm in duodenum, 4 mm in jejunum and ileum, and 2 mm in colon. In normal gastrointestinal walls, 5 layers should be visualized:
(1) lumen (hyperechoic), (2) mucosa (hypoechoic), (3) submucosa (hyperechoic), (4) muscularis (hypoechoic), and (5) serosa(hyperechoic). With lower frequency transducers, three layers may be seen: lumen (hyperechoic), wall (hypoechoic), and serosa (hyperechoic). However, the lower frequency transducers do not provide enough resolution to identify subtle lesions. Thus, without the higher frequency transducers, gastrointestinal ultrasound examinations are most likely unrewarding.
Gastrointestinal motility should be evaluated. The motility may be hyper or absent. Stay in one area for about 20 sec for evaluation of the motility. The pyloric motility should be 5 to6 peristalsis/min. Intestinal motility should be less than that. Increased motility may suggest acute obstruction or irritation due to enteritis.
Gastric contents are difficult to judge. Food materials in the stomach may appear abnormal like a foreign body. You need to exercise caution on hasty diagnosis of gastric foreign body.
Gastrointestinal Foreign Bodies
Gastrointestinal foreign bodies are difficult to diagnose. Radiographic examinations are helpful to confirm a dilated segment of intestines or actual foreign bodies. Although the cost may become a limiting factor, ultrasound and radiographic studies complement to each other. There are few case reports on ultrasonographic diagnosis of gastric foreign bodies including linear foreign bodies. You may indeed find these foreign bodies, but you may also miss these or even misdiagnose as present or positive. Thus, you need to be very careful with the diagnosis. In my experience, gastrointestinal foreign bodies are not reliable diagnosis by ultrasonography.
This may be a rare situation that you can make a definitive diagnosis on ultrasound. A bowel loop within another loop creates a characteristic ultrasound finding. That is, multicentric layers of walls are seen on transverse scans, and mass effects are noted on longitudinal scans. On the transverse scans, "target" appearances may be seen. You should still seek underlying causes. Since recurrence is common, you may need to perform post-operative examinations prior to discharging patients. In addition, prior to surgery, you may repeat the examination or at least palpate the mass. I have seen spontaneous resolution under general anesthesia, most likely due to decreased intestinal movement.
Ultrasonography is most helpful in this condition. The most common ultrasound findings with gastrointestinal neoplasia are abnormal hypoechoic thickening of the stomach or bowel walls, loss of its normal layered appearance, and changes in the contour of the mucosal and/or serosal surfaces. It is relatively easy to detect this lesion.
Distribution of lesions may be diffuse or focal. It is difficult to determine a cell type from ultrasound appearance. This is very important to remember. Common neoplasia includes gastrointestinal adenocarcinoma in dogs and lymphoma in cats. Although some common ultrasonographic findings associated with these neoplasias have been described, cytological or histological diagnosis should be achieved. Fine needle aspiration techniques can be used under ultrasound guidance. Since the wall is thickened, usually you will see the needle in the thickened wall. Percutaneous aspiration of neoplasia always carries a risk of spreading or seeding of the neoplasia. This should be well warned to clients prior to the procedure. Although it is rare to see severe hemorrhage associated with this procedure, it is always wise to check clotting parameters prior to the fine needle aspirate.
Occasionally with a moderate amount of fluid in the gastric lumen, a focal loss of gastric mucosa and submucosa may be apparent ultrasonographically. The inflammation and edema associated with ulceration may cause a loss of the layered appearance and thickening of the gastric wall. Endoscopic examination is still the best procedure to detect the ulcer and obtain histological samples. In my experience, a true peptic ulcer is rare. Gastric ulcers are most likely secondary to neoplasia. Cytologic examination is extremely important.
The ultrasonographic appearance of uremic gastropathy has been described in four dogs. Ultrasonographic characteristics included gastric wall thickening and the presence of an echogenic line in the superficial gastric mucosa, representing gastric mucosal mineralization. The normal layered appearance of the gastric wall was lost in three dogs but was preserved in one dog, reflecting variation in the depth of gastric wall inflammation, edema, and necrosis. All dogs did not survive. A layered mineralization of the gastric mucosa should be considered as an unfavorable sign in uremic dogs.
Chronic Hypertrophic Pyloric Gastropathy
Ultrasonographic findings characteristic of chronic hypertrophic pyloric gastropathy (CHPG) include gastric distention and thickening of the pyloric wall. Examination of the pylorus in a transverse plane shows an evenly thickened hypoechoic ring (representing the muscularis) surrounding the pyloric lumen. In six dogs with CHPG that were examined ultrasonographically the thickness of the pyloric wall was greater than 9 mm and the thickness of the muscular layer was greater than 4 mm.
References are available upon request.