Veterinary Clinical Sciences, School of Veterinary Medicine, Louisiana State University, Baton Rouge, LA, USA
Most clinics adhere to a standard protocol for completing radiographic studies depending on the anatomical region of interest. On some occasions, important diagnostic information can be obtained with an additional view. Typical views of the abdomen include a ventro-dorsal (VD) and right or left lateral view. Often times, the right lateral recumbent view is routinely chosen because the spleen is better identified, and the kidneys are better separated from each other in this view.1 Simple additional techniques include positional radiography (left lateral abdominal projection, horizontal beam radiography, humanoid thoracic view), compression radiography, and simple negative contrast radiography (pneumocolon and pneumogastrogram) used in combination with the above.
Personal preference largely determines which standard views to obtain for the abdomen. Gravity, mobility of organs, and the changes in distribution of gas and fluid with recumbency can be used to the clinician’s advantage to optimize diagnostic capability of radiographic studies in many common clinical scenarios.
Left Lateral Abdominal View
Luminal contents within stomach and small intestine often contain some degree of fluid and gas. Because patients are positioned in recumbency (rather than standing) and views are obtained with a vertical beam (rather than horizontal beam), the classic appearance of a combination of contents in the gastrointestinal tract is a summation of opacities created by both gas and fluid. When a combination of soft tissue and gas material is present within the stomach or small intestines, the difference in opacity between these two can sometimes be used as a “natural” radiographic contrast to highlight margins where gas acts as a negative contrast agent. Fluid and mobile soft tissue material will be present in the gravity dependent part of the stomach or small intestines while gas rises to the non-gravity dependent part. By altering recumbency, the redistribution of gas within the gastrointestinal tract (without administration of additional contrast agents) has been used to diagnose pyloric outflow obstructions, locate obstructions within small intestines, highlight intussusceptions, distinguish colon from dilated small intestine, rule out gastric malpositioning, and definitively locate the pylorus, among other applications.2-5
Specifically, in the left lateral recumbent view, gas rises to the right-sided pylorus and antrum, while fluid and soft tissue that are mobile fall to the region of the fundus on the left side. This is useful in cases of suspected pyloric outflow obstruction, where material that is non-mobile at the level of the antrum can be highlighted by gas limiting the effect of border effacement of soft tissue foreign objects or masses by fluid contained within the stomach.
Horizontal Beam Radiography
The usefulness of horizontal beam radiography is also based on the principle of gravity dependency of fluid versus gas. In the abdomen, the main purpose for obtaining a horizontal beam view is to reliably diagnose free peritoneal gas. Small volumes of free gas can be missed on survey radiographs, most commonly because gas bubbles are assumed to be associated with the gastrointestinal tract. Larger gas accumulations can also be overlooked, as the only finding may be a cranial or generalized decrease in abdominal opacity, which can be missed or mistaken for gas within the fundus of the stomach.
Horizontal beam views of the abdomen can be obtained in one of two ways. The basis of these techniques is to force gas to accumulate in a predictable location (i.e., non-gravity dependent) and radiograph this location. One method is to position the animal in left-sided recumbency. The tube head and detector are positioned horizontally such that a VD view of the abdomen centered at the cranial abdomen is obtained. In this view, free peritoneal gas will accumulate caudal to the diaphragm on the right side. Left-sided recumbency is preferred over right-sided recumbency due to positioning of gas within the stomach. Gas that is free or contained in the stomach will both rise to similar, non-gravity dependent locations. In left recumbency, the gastric gas will rise to the pyloric antrum, which is more desirable because it is smaller than the fundus and gas contained within the stomach is less likely to be confused for free gas.5 A second method is to position the animal in dorsal recumbency with the cranial half of the animal slightly elevated. The tube head and detector are positioned horizontally such that a lateral view of the animal centered at the cranial abdomen is obtained. Free peritoneal gas will accumulate caudal to the diaphragm ventrally in the non-gravity dependent part of the peritoneal space.
The purpose of compression radiography is to isolate a specific region of interest or remove superimposition by use of a radiolucent compression device.6 The most typically used device is a radiolucent wooden spoon or spatula; however, any rigid radiolucent device with a handle can be used. The indications for this study include isolation of specific organs of interest that are otherwise obscured by superimposition or to better delineate margins of a normal or abnormal structure seen on survey radiographs. This technique has been used to better delineate palpable or radiographically visible abdominal masses, isolate the urinary bladder to prove presence of absence of calculi, remove superimposition of the gastrointestinal tract on kidneys, separate small intestines from each other, and confirm presence of uterine enlargement, among other potential applications.
Negative Contrast Radiography
The inherent contrast between soft tissue/fluid and gas, as already discussed, is the basis for negative contrast radiography. Pneumogastrography and pneumocolonography are the two main procedures discussed that use room air as the contrast agent. These procedures are often used in combination with positional or compression radiography. For example, in a dog with clinical signs suspicious of mechanical pyloric outflow obstruction, the stomach may contain very little gas to contrast with material in the pylorus in performing a left lateral abdominal view. In such a case, the gas needed to create contrast may be added by simply administering room air into the stomach. Additionally, in some circumstances, it is beneficial to determine the precise location of the colon so that small intestinal foreign material or dilation can be confirmed or ruled out. Negative contrast is beneficial in that it is free of cost and will not obscure luminal material or the mucosal surface, unlike positive contrast agents.7
A pneumogastrogram can be used to determine size, shape, and position of the stomach. It can also be used on some occasions to highlight foreign objects, especially those located in the pylorus8 and makes the left lateral recumbent view more effective. It is more commonly used in animal suspected of pyloric outflow obstruction or in cases of a cranial abdominal mass where the stomach is not well identified. Mechanical obstructions originating in the proximal duodenum often show absence of radiographically visible small intestinal dilation (especially where vomiting is effective at removing luminal contents) and can be highlighted by a pneumogastrogram performed in left lateral recumbency.
A pneumocolonogram can be used to determine location of the colon and to highlight mural, luminal (foreign material), or extramural narrowing of the colon. It is commonly used to differentiate small from large intestine in cases suspect for mechanical obstruction or when the origin of dilated intestinal loops cannot be positively determined.7 Pneumocolonograms are useful in determining the origin of inhomogeneous or foreign material as being in small or large intestine, as material in the colon is typically nonsurgical, unless it is linear in nature and plication is seen. Once the location of the colon is determined, the remaining loops can be assumed to be small intestine.
Rather than adhering to a standardized protocol for obtaining an abdominal or thoracic set of radiographic views, consider the use of an additional view in another recumbency, using compression, or administering a contrast agent as available as room air to provide useful clinical information.
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