Normal Abdominal Ultrasonography of the Horse
Ultrasonographic Anatomy of the Left Side of the Abdomen
If imaging is started on the left rostral side of the abdomen, the stomach should be located deep to the spleen between the ninth to the thirteenth intercostal spaces at approximately the level of the shoulder. In this location, the only part of the stomach that normally can be seen is the wall of the greater curvature, which can be reliably identified as a curved line with proximity to the adjacent spleen and the gastrosplenic vein.7 If the stomach extends beyond the 14th intercostal space in a horse that has not recently eaten, it would be an indication of gastric distension or displacement by other viscera. The stomach has the thickest wall of the gastrointestinal tract, measuring roughly 7 mm from the serosal to the mucosal/lumen interface. When the stomach is empty, the wall may be up to 1 cm thick. Since only the dorsal portion of the greater curvature can be seen and the lumen generally contains gas in this location, often the contents of the stomach are not visible and the curved wall appears hyperechoic. If gastric fluid is present ventrally, a distinct gas/fluid interface may be apparent in the lumen.
The size and location of the spleen is highly variable, though it should be identifiable immediately adjacent to the body wall, from the left ventral eight intercostal space to the paralumbar fossa. It may remain to the left of the midline, or extend slightly beyond the right of the ventral midline. The only measurement of the spleen that can be reliably obtained is its central thickness or depth, which usually is less than 15 cm.5 In some horses, in the rostral ventral left abdomen, the most rostral aspect of the spleen can be seen either lateral or medial to the liver. 5 Normally the spleen's ultrasonographic architecture is homogenous with vessels rarely visible. The general echogenicity of the spleen should be greater than that of the liver and kidney.
The left kidney can be found between the sixteenth to seventeenth intercostal space and the first to third lumbar vertebra, medial or deep to the spleen, between the level of the tuber coxae and the tuber ischii.8,9 Rarely, the left kidney may directly appose the left body wall.5 Gas in the small colon or left colon or lung may preclude transabdominal viewing of the left kidney. The left kidney is 15 to 18 cm long, though this measurement is difficult to obtain in its long axis (i.e. dorsal plane that is parallel to the spine) because of interference from the ribs.9 The height (11 to 15 cm in slightly oblique transverse plane) and thickness or depth (5 to 6 cm) measurements are more reliably obtained. The corticomedullary junction should be distinct, with the cortex approximately 1 cm thick. The renal cortex is more echogenic than the adjacent medulla, except in areas of the medulla where interlobar vessels course centrally to form the renal pyramids, which are most readily visible in the middle regions of the kidney, as compared to the poles.9 Adjacent to the renal pyramids, centrally located at the rostal and caudad regions of the kidney are the terminal recesses wherein urine is collected and carried to the pelvis. In these areas, the renal pyramids appear as distinct hypochoic "circles," converging on the hyperchoic and indistinctly parallel lines of the terminal recesses. The walls of the renal pelvis are best imaged in the hilus and also appear as parallel to diverging hyperechoic lines that are often accentuated by the presence of fat in the renal pelvis.9 The renal artery and vein can sometimes be identified medial to the kidney at the hilus in transverse planes. The normal left ureter cannot be imaged.
The left colon is located ventromedial to the spleen. The left ventral colon is sacculated. and has “sluggish” motility. The wall of the colon should measure less than 4 mm. The left dorsal colon is not sacculated and may be located dorsal, lateral, medial, or even ventral to the left ventral colon. Gas in the left ventral colon may preclude identification of the left dorsal colon when it lies medial or dorsal to the left ventral colon. Gas in the colons typically generates a hyperechoic appearing wall with an indistinct luminal border and intraluminal acoustic shadowing that precludes identification of the contents and the medial walls.