The Gastrointestinal Tract
Since gas is a barrier to ultrasound, ultrasound examinations of the gastrointestinal tract have been considered limited. However, in recent years, more and more ultrasonographic abnormalities have been reported in the literature. These abnormalities include gastroenteritis, GI 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 GI movement is an additional benefit of ultrasound examinations.
Normal Ultrasonographic Appearance
The gastrointestinal tract should be evaluated for: 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 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, five layers should be visualized: lumen (hyperechoic), mucosa (hypoechoic), submucosa (hyperechoic), muscularis (hypoechoic), and serosa (hyperechoic). With the lower frequency transducer, three layers may be seen: lumen (hyperechoic), wall (hypoechoic), and serosa (hyperechoic).
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 to 6 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. Foreign bodies may be observed. However, luminal gas may mimic a foreign body—extreme caution is needed to diagnose a 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 each other.
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.
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. Distribution of lesions may be diffuse or focal. It is difficult to determine a cell type from ultrasound appearance. 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 aspirations of neoplasia always carry a risk of spreading or seeding of the neoplasia. Clients should be informed of this risk.
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.
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. None of the dogs survived. A layered mineralization of the gastric mucosa should be considered 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.
This is another new area where ultrasound examinations delineate morphological abnormalities. With an improved resolution of recent ultrasound machines, the pancreas is readily visible in normal dogs. To maximize resolution, a high-frequency transducer (7.5 or 10 MHz) should be used. In addition, overnight fasting should provide a good acoustic window to evaluate the area of the pancreas. Most animals show clinical signs of vomiting. Often, the stomach contains a relatively large amount of fluid. Rolling the animals in slightly right-sided recumbency from dorsal recumbency moves the luminal gas to the fundus and improves the visualization of body and right limb of the pancreas. Dogs with pancreatic abnormalities may resist the pressure of the transducer. This information is also beneficial to suspect pancreatic abnormalities such as pancreatitis.
Normal Ultrasonographic Appearance
The normal pancreas is readily visible caudal to the gastric fundus and body, and medial to the duodenum. However, gas accumulation in the GI lumen obscures the pancreas. The normal pancreas is a triangular shaped, slightly hypoechoic structure with small hyperechoic foci (ducts). The surrounding fat is nearly isoechoic to the pancreas; this is a most important finding.
Severe and frequent vomiting is often associated with pancreatitis. Serum chemistry abnormalities may not be seen, depending on the phase and severity of the disease. Radiographic abnormalities (focal loss of detail in right cranial quadrant, dilated and fixed duodenum, etc.) have been described, but they may not be present in some cases.
When the area of the pancreas is seen as a hypoechoic irregular mass, pancreatic abnormalities should be suspected. The surrounding fat becomes drastically hyperechoic in a case of acute severe necrotizing pancreatitis due to saponification. The edges of the pancreas are indistinct. Often, gastrointestinal motility is reduced, and duodenal or gastric wall thickening is observed adjacent to the pancreatic mass. In chronic cases, the surrounding fat may be slightly hyperechoic. The edges of the pancreas may be better visualized. Pseudocyst formation has been reported in dogs as is seen in humans. These findings are noted in both dogs and cats.
It should be emphasized that pancreatitis and pancreatic neoplasia are difficult to differentiate on ultrasound examinations. Pancreatic aspirates may be beneficial in these cases. In addition to cytology, bacterial culture of the pancreatic aspirates can be performed.
Ultrasonographic identification of pancreatic neoplasia is difficult. Exocrine pancreatic neoplasia resembles pancreatitis ultrasonographically (as well as clinically), making differentiation more difficult. In most cases, the diagnosis of exocrine pancreatic neoplasia is made based on histopathology following surgical biopsy or necropsy in an animal in which signs of pancreatitis were unresponsive to medical therapy. Insulinomas have been detected on ultrasound, but it is usually not big enough to detect routinely. Carcinoma can be aggressive. Presence of hypoechoic or target lesions in the liver may suggest metastastic disease. In addition, the adjacent GI walls may be thickened due to direct extension of neoplasia. Cytological evaluation may be beneficial in diagnosis of neoplasia, when a mass effect is recognized.
The Adrenal Glands
When hyperadrenocorticism (Cushing's disease) is suspected, an ultrasound examination is useful to measure the adrenal glands. Visualization of the adrenal glands relies on: 1) high-frequency transducer (7.5 or 10 MHz); and 2) appropriate patient preparation with overnight fasting.
Left adrenal gland. The left adrenal gland can be best detected on the longitudinal plane. First, place the cranial pole of the left kidney in the middle of a monitor screen. Then, with a moderate amount of pressure on the transducer, move the transducer medially until you see the longitudinal anechoic structure of the aorta. The left adrenal gland should be located between the renal and cranial mesenteric arteries. The phrenicoabdominal vessel may be seen ventral to the adrenal body. The left adrenal gland is a peanut-shaped hypoechoic structure. Occasionally, you may see a hyperechoic zone within the gland, that represents the cortical-medulla demarcation. The normal thickness is no more than 7 mm. In cats, the adrenal gland is oval-shaped. Since it is mobile, it may be difficult to find. The normal thickness appears to be less than 5 mm.
Right adrenal gland. The right adrenal gland is more difficult to detect than the left mainly due to overlying gas shadows of intestinal tracts in dogs. First, place a longitudinal view of the right kidney in the middle of a monitor screen. The placement of the transducer should be subcostal (behind the right 13th rib). When you find the kidney, angle the transducer to the dorsal midline. Stay where you are or move the transducer medially. Apply pressure during this maneuver. This will displace a gas filled duodenum and other small intestinal loops medially so that they are not interfering with the view. You should see a large tubular vessel. This is the aorta, caudal vena cava, or portal vein. When you see the vessel, release the pressure on the transducer slightly. If the luminal diameter changes, it is the caudal vena cava. Then, move cranial and caudal to find the right adrenal gland. The right adrenal gland is located dorsal to the caudal vena cava and just caudal to a border to the liver. It is heart-shaped or arrowhead-like. The normal thickness is no more than 7 mm.
In pituitary dependent hyperadrenocorticism, both adrenal glands are enlarged. However, a few do not show this enlargement. That is, normal size of the adrenal glands does not rule out Cushing's disease. Most of the cases have diffuse hyperechoic liver due to steroid hepatopathy.
Occasionally, an adrenal mass is seen. Calcification of the adrenal gland is not a pathognomonic finding for neoplasia. Within various neoplasias, pheochromocytoma appears to invade the caudal vena cava. In addition, metastases to the adrenal glands can occur.
Adrenal atrophy is seen with use of o,p-DDD. Unless it is a functional tumor, the contralateral adrenal gland does not appear to atrophy.
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