Normal Abdominal Ultrasonography of the Horse
The small colon is located in the left paralumbar fossa medial or ventral to the spleen. With its small diameter, sacculations, and packed serpentine loops that suspend from the dorsal mesocolon, often only small sections of the surface of loops are visible ultrasonographically as short sharply curving hyperechoic lines. Like the left colon, the motility is slow and luminal gas typically prevents visualization of the contents and the distal walls.
The small intestine is hard to visualize in normal horses unless a peristaltic wave generates transient expansion of the lumen from movement of fluid contents. The medial location of the ileum precludes distinct identification. The jejunum most reliably is found in the left inguinal area, medial to spleen and the left ventral colon. The small intestine has the most visible motility of any part of the gastrointestinal tract, with peristaltic waves producing rhythms contractions. The fluid contents of the lumen enables distinction of the wall thickness (2 to 4 mm) and visualization of the distal wall in either its long or short axis. In the normal horse, luminal diameters rarely exceed 3 cm and should be seen contracting down to obscurity. Fasting and sedation with alpha-two agonists will individually and additively decrease motility of the small intestine.2,3
Ultrasonographic Anatomy of the Right Side of the Abdomen
Three anatomic structures that provide characteristic proximity that can be identified in the right rostral abdomen at the level of the shoulder are the liver, the descending duodenum, and the right dorsal colon. The liver can be located from the sixth to the fourteenth intercostal spaces between the diaphragm and the right dorsal colon. Air in the lungs dorsally often interferes with hepatic imaging. Only a small portion of the right side of the liver can be imaged and estimates of size rely on its expanse across intercostal spaces.5 It is unusual for the liver to be seen beyond the fifteenth intercostal space or in the same transverse plane as the right kidney, except at the most rostral aspect of the kidney.4 The ventral edges of the normal liver should be distinctly sharp. Like the spleen, the architecture of the liver is relatively homogenous, though more vessels are visible in the liver and the general echogenicity of the liver is less than the spleen. Portal veins have more connective tissue in their walls and thus have more echogenic walls than the hepatic veins.5 Often short segments of smaller portal veins appear as hyperechoic parallel lines. In some smaller horses, the portal vein can be seen entering the hilus deep on the medial side of the image. The common bile duct and its branches within the liver are not normally visible. 4,5
The position of the duodenum is fixed by its suspending mesoduodenum. It can reliably be found descending the right middle abdomen at about the level of the shoulder and is located between the liver and the right dorsal colon where it can be imaged transversely in short axis. Like the jejunum, patience must be exercised when looking for the descending duodenum as one must wait for a peristaltic contraction to deliver fluid through the lumen. Otherwise, the duodenum appears flattened. It would be unusual for the entire duodenal diameter to exceed approximately 4 cm in normal horses during peristaltic propulsion of ingesta.10 The duodenum contracts between 1 to 4 times per minute in fed horses,10 though contractions will be reduced in anorexic, starved or heavily sedated horses. The duodenum can be followed to the level of the ventral right kidney, wherein it crosses medially into the abdomen and is no longer distinguishable. The wall of the duodenum is less than 4 mm in thickness.
The right dorsal colon has no sacculations and consistently appears as a hyperechoic curved line adjacent to the liver. Once the right dorsal colon is located, if one slides the transducer ventrally, often the junction between the right dorsal and right ventral colons is identifiable. The right ventral colon has sacculations. Like the left colons, the right colons’ wall thickness should be less than 4 mm, motility is sluggish, and the contents and far walls are normally obscured by the presence of luminal gas.