Procedure for making abdominal radiographs

Patients should be encouraged to urinate and defecate prior to radiography. An empty colon and a small urinary bladder can lessen visceral crowding and reduce summation artifacts.

For elective radiographs, the GI tract should be empty. Many abdominal radiographs are rendered non‐diagnostic due to the presence of ingesta. A 12‐hour fast allows the stomach to empty and an 18‐ to 24‐hour fast allows the intestines to empty. Patients should have free access to water during fasting.

When practical, drugs that affect GI motility should be discontinued 24–48 hours prior to abdominal radiography, including sedatives and anesthetic agents that slow GI peristalsis (e.g., xylazine).

Cleansing enemas may be needed to empty the large intestine. Either a mixture of water and mild soap or a saline solution are acceptable enemas and should be administered 2–4 hours prior to radiography. Enema solutions that are below body temperature may help stimulate the expulsion of gas from the colon. Enemas are contraindicated in patients with acute abdominal pain, active vomiting, or a palpable abnormality in the abdomen. Enemas are not recommended prior to initial radiographs in patients being evaluated for GI disease. This is because an enema can alter the pattern of gas and fluid in the intestinal tract, which may mask or mimic pathology. Phosphate enemas are not recommended because they can be toxic, especially in cats and small dogs.

Clean and dry the patient’s hair coat and skin as needed prior to radiography. Move any monitoring equipment, such as ECG leads, out of the field‐of‐view (FOV). Note any cutaneous or subcutaneous growths that may create superimposition artifacts.

For accurate technical settings, measure the thickness of the abdomen at its widest part, which typically is over the liver at the level of the xyphoid or 12th rib.

Center the x‐ray beam in the middle of the abdomen, midway between the diaphragm and the pelvic inlet. This helps minimize distortion at either end of the abdomen. Landmarks for centering include the level of the umbilicus or just caudal to the last rib.

Collimate the FOV to include the entire abdomen from the diaphragm to the pelvic inlet. The landmark for the diaphragm is about the T8 thoracic vertebra and the landmark for the pelvic inlet is the greater trochanters of the femurs. Collimation should include the spine dorsally and the body walls laterally and ventrally (Figures 4.2 and 4.3)

The pelvic limbs should be moved caudally to eliminate superimposition. Avoid overstretching the patient as this will distort the shape of the abdomen and increase visceral crowding, especially in cats and small dogs. Align the spine and sternum in the same anatomical plane.

Use body markers to indicate Right or Left and whether horizontal beam radiography was performed.

Make the exposure at the end of expiration to minimize motion artifact as most patients pause breathing momentarily at the end of expiration. Expiration also allows the diaphragm to move cranially, which expands the abdominal cavity and lessens visceral crowding.

Standard views of the abdomen

Orthogonal radiographs are needed for complete evaluation of the abdomen. Standard radiographs for most abdominal studies include either a right or left lateral view (Figure 2.16) and a VD view (Figure 2.17). The DV view is not routinely made because serosal margins are less distinct due to visceral crowding.

The right lateral view is useful to move stomach gas into the fundus and to move stomach fluid into the pylorus (Figure 4.4). The radiographic appearance of fluid and gas in the stomach depends on the volume of each. In a right lateral view, the kidneys tend to be further separated than in a left lateral view and the margins of the liver and spleen may be easier to identify. The left lateral view allows stomach gas to rise into the pylorus and stomach fluid to flow into the fundus. This is helpful when differentiating a fluid‐filled pylorus from a cranioventral abdominal mass.

The VD view is useful to move stomach gas into the body of the stomach and to move stomach fluid into the fundus and pylorus (Figure 4.5). The DV view allows stomach gas to rise into the fundus and pylorus and stomach fluid to flow into the body of the stomach. It usually is easier to position a patient straight for a VD view than a DV view (Figure 2.18), however, some patients will not tolerate being in dorsal recumbency. When neither a VD nor DV view is possible, make both the right lateral and the left lateral views. Making both right and left lateral views can alter the positions of the moveable viscera and change the distribution of fluid and gas in the hollow organs, which may reveal lesions that were not initially evident.

Supplemental views of the abdomen

Supplemental views sometimes are needed to confirm or further evaluate a suspected lesion. For example, when the stomach or colon is not recognized in initial radiographs, gas may be added to help identify them. To add gas to the stomach, a small volume of carbonated beverage may be given per os or air administered through a gastric tube. To identify the colon, a small volume of room air can be infused per rectum.

Flexed VD view (“frog‐legged view”) flexing the pelvic limbs with the patient in dorsal recumbency may be preferred to an extended VD view to eliminate superimposition of the inguinal skin folds and to lessen visceral crowding (Figure 4.6).

Oblique views are used to eliminate superimposition of overlying structures. For example, rotating the patient 10° or 20° from a true VD or lateral view may reduce summation of the spine or intestines on an area of interest (Figure 2.12). The oblique view can be further modified by collimating the field of view to a specific area and centering the x‐ray beam on that area to make a tangential view.

Tangential views are used to better visualize a peripheral lesion, such as a mass in an abdominal wall. The patient is rotated or the x‐ray tube is angled so the x‐ray beam strikes the lesion in profile rather than en face (Figure 2.13). The FOV should be collimated to the size of the lesion to lessen scatter radiation and sharpen radiographic detail.

Horizontal beam radiography takes advantage of gravity to reposition fluid, gas, and viscera. It may be used to detect small volumes of free fluid or gas in the abdominal cavity or to better visualize an intra‐abdominal mass that was obscured by viscera or fluid in the initial radiographs. Fluid flows to the dependent (down) part of the abdomen and gas rises to the uppermost part. The patient may be standing, held vertically, or positioned in lateral, dorsal or ventral recumbency, depending on the purpose of the view (Figure 2.15). The image receptor is placed alongside the patient, and the x‐ray beam is directed across the table (horizontally) to make a VD or lateral view. The source‐to‐image distance should be the same as for vertical radiography (typically 100 cm or 40 inches) and you should follow the same guidelines for centering the x‐ray beam and collimation.

Abdominal compression is a useful technique to displace overlying viscera away from an area of interest (Figure 2.19). For example, the abdomen may be gently compressed using a low opacity paddle (e.g., a plastic or wooden spoon) to push the intestines away from the urinary bladder, the uterus, a kidney, or a mass. The x‐ray exposure is decreased to compensate for the reduced abdominal thickness; typically, the exposure time is reduced by half. Compression radiography is contraindicated in patients with a tense or painful abdomen, peritoneal effusion, or a mass that is at risk for rupture.

The urethral view is used to better visualize the urethra in male dogs by moving the pelvic limbs cranially while the patient is in lateral recumbency (Figure 2.20).

Immature animals

1. Liver and kidneys are relatively larger than in adults.
   
2. Serosal margins are less distinct due to a weaker opacity interface (Figure 4.7). The abdomen of a juvenile (less than six months old) typically contain less fat than in an adult. In addition, juvenile fat is mostly brown fat. The brown fat in immature animals is more opaque than adult fat due to its higher water content. To confirm the patient is immature, look for open growth plates.

Aged animals

1. Liver may normally extend caudal to the costal arch.
   
2. Spleen may be smaller than expected due to atrophy.
   
3. Abdomen may appear distended due to weak muscles.
   
4. Mineralized adrenal glands occasionally are visible in older cats as an incidental finding.

Obese animals

1. Abdomen commonly appears distended.
   
2. Large amounts of intra‐abdominal fat may displace viscera from their expected locations.
   
3. Small intestine may be crowded into the central abdomen. It often is displaced toward the right side in obese cats and ventrally in obese dogs.
   
4. Excessive fat in cats and small dogs can outline the serosal margins and may improve visualization of abdominal viscera. In large dogs, however, obesity increases scatter radiation which degrades radiographic contrast and detail.

Emaciated animals

1. Ventral abdomen may appear tucked or concave.
   
2. Serosal margins are less distinct due to loss of intra‐abdominal fat. Examine other parts of the patient to help confirm emaciation (e.g., lack of subcutaneous fat).

Effects of positioning

1. Spleen tends to be less visible in a left lateral view.
   
2. Liver sometimes appears larger in a right lateral view than in a left lateral.
   
3. Spleen and liver are more likely to be in contact with each other in a right lateral view which may result in a mistaken diagnosis of hepatomegaly.
   
4. Kidneys tend to overlap more in a left lateral view and to be further separated in a right lateral view.
   
5. Pulling the pelvic limbs caudally can cause visceral crowding in the abdomen and may produce superimposed skin folds in a VD view. Position the pelvic limbs away from the area of interest, but in a neutral position or flexed in a VD view (e.g., frog‐leg view).
   
6. Four views of the abdomen (right lateral, left lateral, VD, and DV) allows gravity to redistribute fluid and gas, which may aid in recognizing lesions, particularly when investigating GI disease.

Normal radiographic anatomy

The borders of the abdominal cavity include the diaphragm cranially, the sublumbar muscles dorsally, the pelvic inlet caudally, and muscular walls laterally and ventrally. The sublumbar muscles are also called hypaxial muscles or psoas muscles.

Parietal peritoneum lines the diaphragm and muscular walls. Visceral peritoneum covers most of the abdominal organs and forms the mesentery. Between the parietal and visceral peritoneum is the peritoneal space (Figure 4.8).

The peritoneal space is considered a “potential space” because it contains only a small volume of fluid. It is not visible in radiographs. The peritoneal space surrounds the abdominal organs that are covered by the visceral perito neum. The organs that usually are visible in survey radiographs include the liver, spleen, stomach, intestines, and urinary bladder. Peritoneal structures not generally seen include the gall bladder, pancreas, ovaries, uterus, lymph nodes, and blood vessels. The kidneys are located in the retroperitoneal space.

The retroperitoneal space is the area dorsal to the peritoneal space and ventral to the sublumbar muscles. It extends from the diaphragm to the pelvis. The retroperitoneal space communicates cranially with the mediastinum (via the aortic hiatus) and caudally with the pelvic canal. In addition to the kidneys, structures in the retroperitoneal space include the adrenal glands, ureters, aorta, caudal vena cava, cisterna chyli, and lymph nodes. The kidneys usually are visible in survey radiographs and the aorta sometimes is identified. The other retroperitoneal structures rarely are distinguished unless enlarged or mineralized. A notable exception are the paired deep circumflex iliac arteries which branch from the caudal aorta at nearly a 90° angle. In a lateral view, one or both of these vessels often is seen end‐on to appear as a focal mineral opacity structure ventral to the caudal lumbar spine, sometimes mistaken for a ureteral calculus (Figure 4.9). This finding is similar to seeing an end‐on pulmonary vessel and mistaking it for a mineralized lung nodule (Figures 3.145 and 3.146). Rarely will more than one of these vessels be visible in radiographs.

There is no direct communication between the peritoneal space and the retroperitoneal space. Disease in one compartment rarely involves the other. Fat, however, is stored in both compartments. Fat deposits tend to be largest in the falciform ligament, omentum, mesentery, and caudal retroperitoneal space. The radiographic appearance of peritoneal and retroperitoneal fat normally is similar in opacity and homogeneity. Large fat deposits may be mistaken for a mass or fluid, particularly in overweight cats (Figure 4.10). However, most masses and all biological fluids are soft tissue opacity and should not be mistaken for fat.

Normal intra‐abdominal lymph nodes are not visible in survey radiographs. The visceral nodes are located in the peritoneal space and the parietal nodes are in the retroperitoneal space. The visceral nodes receive lymphatics from the cranial abdomen and drain into the cisterna chyli. The parietal or sublumbar nodes receive lymphatics from the spine, adrenal glands, kidneys, pelvic canal, and pelvic limbs and also drain into the cisterna chyli. The cranial mesenteric nodes are the largest visceral lymph nodes. They are located near the root of the mesentery. The medial iliac nodes are the largest sublumbar nodes and they are located ventral to the L5–7 vertebrae. There is considerable variation in the number and location of the sublumbar lymph nodes among dogs and cats. Sometimes these nodes are absent.

Abnormal size of the abdomen

A small or contracted abdomen may be due to loss of body fat or displacement of the abdominal viscera, the latter resulting from a hernia or rupture of the diaphragm or body wall. In emaciated patients, the abdomen may appear “tucked up” with a concave ventral border (Figure 4.11). The serosal margins often are indistinct because there is insufficient intra‐abdominal fat to create a visible opacity interface. Both the peritoneal and retroperitoneal spaces tend to be small.

Abdominal distention may involve the peritoneal space, the retroperitoneal space, or both. Disease in one space usually does not involve the other, which means the other space generally appears normal in radiographs. Conditions that can enlarge both spaces include excess fat in overweight animals and hemorrhage, the latter when caused by coagulopathy or severe trauma. Leakage of urine can also enlarge one or both spaces.

An abdomen may appear distended due to weakening of the supporting ligaments and muscles. Weakening may be associated with old age or Cushing’s Syndrome (hypercortisolism). The abdomen may appear to sag or to be pendulous, but serosal margins remain distinct, unless there is concurrent intra‐abdominal fluid or inflammation.

Abnormal abdominal wall

Thickening (widening) of a body wall may be localized or diffuse. A thickened wall may or may not be accompanied by altered opacity. Localized thickening typically is due to a mass.

A thickened abdominal wall that contains gas opacity may result from a penetrating wound, herniation of a gas‐filled structure, or migration of emphysema. Penetrating wounds (e.g., bite wounds, injections, surgery) generally produce a multifocal gas pattern, whereas a herniated bowel segment tends to be localized and confined. Migration of emphysema typically originates in the thorax (e.g., pneumomediastinum) and can become quite diffuse (Figure 3.63).

Fat opacity in a diffusely thickened abdominal wall may be due to obesity or a dissecting lipoma. Obesity is bilateral, whereas lipomas tend to be unilateral. Localized fat opacity may represent an isolated lipoma or a hernia containing omental or falciform fat. Fat in the body wall may displace and outline muscle layers, providing an opacity interface with the soft tissue opacity of the muscles.

A thickened abdominal wall that is soft tissue opacity may be due to subcutaneous fluid, a mass, or displaced viscera. Soft tissue opacity swelling may obscure fascial planes and muscle layers.

Mineral or metal opacity in the abdominal wall usually is readily identified and may represent dystrophic mineralization or a foreign object. Superimposition artifacts can mimic abnormal opacity.

Abdominal hernia or rupture

A hernia is a protrusion through a natural opening; the peritoneum remains intact. A rupture is an acquired loss of body wall integrity; the peritoneum is damaged. The two rarely are distinguished in radiographs. The term hernia, therefore, commonly is used as a catch‐all phrase to describe the displacement of abdominal viscera outside the abdominal cavity.

Displaced viscera create variable degrees of extra‐abdominal swelling. Swelling caused by a hernia may be difficult to distinguish from swelling caused by a tumor, cyst, or other mass. The opacity of the swollen area may provide clues to its contents.

Fatty tissues tend to enter a hernia first (e.g., omentum, mesentery, falciform ligament). The most commonly displaced soft tissue structures are the intestines, stomach, liver, spleen, urinary bladder, and uterus. The identity of a displaced organ may be determined by recognizing its shape or its pattern or by noticing that it is absent from its normal location. GI structures often can be identified by their intraluminal gas. Displaced intestines commonly create curvilinear gas opacity tubes that extend beyond the limits of the abdominal cavity. A strangulated or incarcerated GI structure can dilate greatly to produce a very large cyst‐like mass (Figure 3.48).

The abdominal cavity may appear “empty” when a significant amount of viscera is displaced. The serosal margins of the remaining organs usually are well‐visualized unless surrounded by fluid or there is insufficient body fat.

Abdominal hernias vary in etiology and location (Figure 4.12). Diaphragmatic hernias, including traumatic, hiatal, and peritoneopericardial, are discussed in Chapter 3: Thorax. An umbilical, inguinal, or scrotal hernia may be congenital or acquired and may appear as either a soft tissue, fat, gas, or mixed opacity swelling in its respective area. Perineal hernias result from a weakened or ruptured pelvic floor and most often occur in older, intact males. They typically produce a swelling under the tail and may displace the rectum. Contents in a perineal hernia may include fluid, fat, urinary bladder, prostate gland, uterus, or/and intestine. Intra‐abdominal hernias occur when an organ protrudes through a tear in the mesentery. These may be visible in radiographs as a static bowel segment or dilated bowel if the segment becomes incarcerated (i.e., mesenteric hernia).

Abnormal opacity in the abdominal cavity

Abdominal radiographs that are less opaque than expected may be due to poor radiographic technique, excess intra‐abdominal fat, or free gas in the peritoneal or retroperitoneal space (see differential diagnoses). Excess fat is due to obesity or a large intra‐abdominal lipoma (Figure 4.13). Free gas may result from a penetrating wound, ruptured GI structure, recent laparotomy, or infection with gas‐producing bacteria.

Abdominal structures or areas that appear more opaque than expected in radiographs may be due to poor radiographic technique, increased intra‐abdominal fluid, or visceral crowding (see differential diagnoses). Increased fluid and visceral crowding both lead to less distinct serosal margins. Fluid may result from inflammation or effusion. The type of fluid cannot be determined from radiographs because all biological fluids are soft tissue opacity.

Gas in the abdominal cavity

Gas is common in the GI tracts of dogs and cats. The volume of gas can vary greatly between patients and may or may not be clinically significant. Abnormal GI gas usually is accompanied by dilation or a persistently abnormal pattern.

Gas within the wall or parenchyma of an organ (pneumatosis) is abnormal. Pneumatosis has been reported in the stomach, intestines, spleen, liver, uterus, gall bladder, and urinary bladder. It most often is caused by a gas‐producing bacterial infection (e.g., E. coli, pseudomonas), particularly in animals with diabetes mellitus. Pneumatosis also can result from damage to the mucosa and leakage of gas into the wall of the organ.

Pneumoperitoneum is free gas in the peritoneal space. The etiology of pneumoperitoneum may not be evident from radiographs alone. Patient history and results of physical examination often provide valuable diagnostic information.

Penetrating wounds frequently are accompanied by subcutaneous emphysema. Note: superimposition of subcutaneous gas can mask or mimic a pneumoperitoneum. Post‐operative gas generally is resorbed within a week, but gas can persist in the peritoneal space for as long as three to four weeks. In patients with pneumoperitoneum caused by a ruptured stomach or bowel segment, the damaged organ may be abnormally distended.

Small volumes of peritoneal gas appear as irregular‐shaped bubbles or pockets of gas. Bubbles usually are unevenly distributed and frequently become trapped among the segments of small intestine. They appear as sharply‐marginated pockets of gas that are more linear or triangular in shape than the normal rounded, flowing intestinal gas pattern (Figure 4.14).

Peritoneal gas rises to the highest part of the abdomen. Larger volumes of gas tend to collect between the diaphragm and the liver (Figure 4.14). Gas adjacent to the diaphragm, liver and stomach enhance their visualization due to the gas:soft tissue opacity interface. As gas continues to fill the peritoneal space, more and more serosal margins may become more distinct. However, pneumoperitoneum often is accompanied by peritoneal inflammation or effusion, which may obscure serosal margins.

A large volume of free gas in the peritoneal cavity can cause an overall decrease in abdominal opacity which may be mistaken for an overexposed radiograph. Examine the extra‐abdominal structures to determine whether the exposure was correct.

Peritoneal gas tends to be freely moveable. Its distribution is affected by gravity and the position of the patient at the time of radiography. Small volumes of free gas may be easier to detect with horizontal beam views (Figure 2.15). In lateral recumbency, gas rises to the uppermost part of the peritoneal space and tends to collect under the ribs (Figure 4.15). Left lateral recumbency is preferred over right lateral recumbency because the gas that normally is present in the gastric fundus and colon may be mistaken for peritoneal gas. In dorsal recumbency, free gas is likely to collect between the ventral body wall and the liver (Figure 4.16). Keep in mind that it takes some time for peritoneal gas to redistribute, so allow the patient to remain recumbent for about 5–10 minutes prior to making the radiograph. For patients in distress, a standing lateral view may be preferred, in which case free gas rises dorsally to collect in the sublumbar region.

Pneumoretroperitoneum is free gas in the retroperitoneal space. The margins of the kidneys, aorta, and sublumbar muscles often become more visible due to the gas outlining their margins (Figure 3.63). Causes of a pneumoretroperitoneum include a penetrating wound, migration of gas from a pneumomediastinum, and infection with gas‐producing bacteria.

Fluid in the abdominal cavity

An increase in intra‐abdominal fluid may result from inflammation or effusion. The type of fluid cannot be determined from radiographs.

Peritoneal inflammation (peritonitis) creates more fluid due to increased blood flow and capillary leakage. The inflammation may be localized or diffuse. Serosal margins in the inflamed area appear hazy and indistinct due to fluid effacement. Peritonitis can progress to effusion and complete loss of visualization of serosal margins. Effusion may or may not cause abdominal distention. The retroperitoneal space is normal as long as the inflammation is confined to the peritoneal space.

Peritonitis can alter peristalsis in the nearby intestines and cause stasis in one or more bowel segments. A static bowel segment is called a sentinel loop. Sentinel loops are segments of intestine that persist relatively unchanged in size, shape, content, and position in serial radiographs. Often it is this static gas pattern that is first detected in patients with peritonitis. Pancreatitis is a common cause of localized peritonitis. It may lead to sentinel loops in the duodenum and transverse colon (Figures 4.17 and 4.48).

Peritoneal effusion (or ascites) is free fluid in the peritoneal space. There are numerous possible etiologies (see differential diagnoses). Small volumes of fluid may be visible as streaks of soft tissue opacity mixed with intra‐abdominal fat. The fat provides the opacity interface to see the free fluid as thin, faint, curvy or “wispy” lines (Figure 4.18). “Fluid‐streaking of fat” tends to be easiest to see at the periphery of the abdominal cavity, where there are fewer superimposed structures. Sometimes a wet hair coat can mimic fluid‐streaking of fat, but wet hair extends beyond the limits of the abdominal cavity.

Larger volumes of peritoneal fluid lead to an overall increase in peritoneal opacity and greater effacement of serosal margins (Figure 4.19). The radiograph may appear underexposed. Examine the retroperitoneal space and the skeletal structures to determine whether the exposure was correct. In patients with peritoneal effusion, the small intestine often is suspended in the fluid and evenly distributed (unless displaced by a mass). Gas‐filled bowel loops rise to the top of the fluid and commonly collect in the mid‐abdomen. Severe peritoneal effusion can distend the abdomen and push the diaphragm cranially. In these cases the diaphragm may appear stationary in serial radiographs.

The retroperitoneal space remains normal as long as fluid is confined to the peritoneal space. The kidneys frequently are visible through the peritoneal effusion because they are surrounded by retroperitoneal fat.

Retroperitoneal fluid typically is due to leakage of urine or blood, but can be caused by inflammation (see differential diagnoses). Fluid partially or completely obscures the margins of the kidneys and sublumbar muscles. Large volumes of fluid may distend the retroperitoneal space and displace the colon and small intestine ventrally (Figure 4.20). Peritoneal serosal margins remain distinct as long as the disease is confined to the retroperitoneal space.

Mineral opacity in the abdominal cavity

Mineral opacity material is a frequent finding in the GI tracts of dogs and cats and may or may not be clinically significant. It may be associated with food, bones, or foreign objects.

Focal mineral opacity structures outside the GI tract may represent calculi or dystrophic mineralization. These are seen more often in the hepatobiliary and urinary tracts. Some focal areas of mineral opacity appear to be free in the peritoneal cavity. These sometimes are Bates bodies, which appear as discrete, round, or oval‐shaped structures with smooth borders and less opaque centers (Figure 4.21). Bates bodies can vary in size and may be fixed in position or freely moveable, changing location in serial radiographs. They generally are considered incidental findings, possibly representing nodules of fat necrosis from a previous episode of pancreatitis or peritonitis. They are reported most often in older cats.

Vascular mineralization is uncommon in dogs and cats. It typically appears linear and heterogenous (Figure 4.22) and may be associated with atherosclerosis or hypercalcemia. Mineralization usually involves the arteries (e.g., aorta, iliac, celiac, mesenteric).

Metal opacity in the abdominal cavity may represent a foreign object (e.g., projectile, ingested foreign object), surgical implant (e.g., sutures, vascular band), or contrast medium.

Normal radiographic anatomy

The liver is located in the cranial abdomen. It is connected by ligaments to the diaphragm, caudal vena cava, stomach, duodenum, and right kidney. The falciform ligament attaches the liver to the ventral body wall and serves as a storage reservoir for fat.

The liver consists of six lobes: right lateral, left lateral, right medial, left medial, caudate, and quadrate (Figure 4.27). The gall bladder lies between the quadrate lobe and the right medial lobe. It is not visible in radiographs unless enlarged, emphysematous, mineralized, or filled with contrast medium. The right kidney lies in the renal fossa of the caudate liver lobe. Because the kidney and the liver are the same opacity, the cranial pole of the right kidney may not be visible unless surrounded by fat or gas.

Liver in the lateral view

The triangular‐shaped liver is located between the diaphragm and the ventral body wall (Figures 4.2 and 4.3). Its margins normally are even and well‐defined. The cranial border of the liver blends with the caudal border of the diaphragm (Both are soft tissue opacity with no other opacity between them). The dorsal liver border is formed by the caudate lobe, but it is not well visualized because it blends with the adjacent soft tissues. The caudal border of the liver is adjacent to the stomach and also poorly visualized. The ventral liver border usually is visible because it is adjacent to falciform fat.

The caudoventral border of the liver forms an acute angle at about the level of the costal arch. The costal arch is the curved line formed by the caudal costal cartilages (Figure 4.28). The caudoventral edge of the liver is a frequently used landmark to evaluate liver size. When the edge extends caudal to the costal arch, the liver commonly is considered to be enlarged. However, this interpretation of liver size is based on palpation of the liver in a clinical setting. It is important to realize that palpable findings do not always correlate with radiographic findings. The costal arch is typically mineral opacity. It is located cranial to the palpable arch because muscle, skin, and subcutaneous tissue are between the true arch and the fingers during palpation. In radiographs, of normal dogs and cats, the caudoventral edge of the liver frequently extends caudal to the visible costal arch. This is usually evident during inspiration and in right lateral recumbency. The liver may also extend further caudal than expected in animals with weak supporting ligaments (e.g., old age, endocrinopathy). Do not rely on the position of the caudoventral edge of the liver to diagnose hepatomegaly. The key radiographic finding with hepatomegaly is rounding of the liver margins.

Liver size is also assessed in a lateral view by evaluating the position of the gastric axis. The gastric axis is an imaginary line drawn between the fundus and pylorus of the stomach (Figure 4.28). In a lateral view, the normal gastric axis is somewhere between perpendicular to the spine and parallel with the ribs, depending on the patient’s body conformation and the size of the stomach. The gastric axis tends to be more perpendicular to the spine in deep‐chested dogs (e.g., Afghan Hound, Collie) and in patients with a full stomach. It often is more parallel with the ribs in shallow‐chested breeds (e.g., Bulldog, Pug) and in patients with an empty stomach.

The gall bladder in a lateral view is located in the cranioventral part of the liver. Mineral or gas opacity in this area may be associated with the gall bladder. However, artifacts created by superimposition of the ribs, costal cartilages, and the caudal lung lobes must be differentiated from a hepatobiliary lesion.

A large gall bladder can create a convex, soft tissue opacity bulge along the ventral border of the liver, particularly in cats (Figure 4.29).

Abnormal liver position

The liver normally lies within the costal arch. Its position can vary somewhat depending on the patient’s body conformation and phase of respiration. Again, the liver tends to be positioned further cranially in deep‐chested patients and more caudal in patients with a wide, shallow thorax. Displacement of the liver may be due to a mass effect or loss of abdominal integrity.

Cranial displacement of the liver most often results from a hernia or rupture of the diaphragm. This is because the cranial position of the liver is limited by an intact diaphragm. A large intra‐abdominal mass can push both the liver and diaphragm cranially (e.g., distended stomach, advanced pregnancy, severe splenomegaly).

Caudal displacement usually is due to an expanded thoracic cavity (e.g., severe pleural effusion, severe pneumothorax). Weakening of the supportive ligaments along the ventral body wall can lead to a pendulous abdomen that allows the liver to move caudally. Ligaments frequently weaken and stretch in older animals and in patients with hypercortisolism.

Lateral or ventral displacement of the liver usually is due to loss of body wall integrity, which may result from trauma or a congenital abnormality.

Dorsal displacement is caused by a ventral mass effect. In patients with a large amount of falciform fat, the liver may appear to be dorsally displaced (Figure 4.30).

Abnormal liver size

As mentioned earlier, liver size is assessed by the shape and position of its caudoventral edge and the position of the gastric axis. Describing a liver as small often is a subjective assessment. Enlargement of the liver is recognized by the rounding of the hepatic margins, caudal extension of the liver edge and caudal displacement of the stomach. Unless the change is severe, the diagnosis of a small liver or a large liver can be difficult from radiographs. The causes and radiographic signs of each are discussed in more detail on the following pages.

Small liver (microhepatia)

In general, a liver that occupies fewer than 2 intercostal spaces is considered small (Figure 4.31). In a lateral view, the caudoventral edge of the liver does not extend to the costal arch. The gastric pylorus may be located cranial to the fundus, making the gastric axis slope from caudodorsal‐to‐cranioventral. The most common causes of microhepatia are a portosystemic vascular shunt and hepatic cirrhosis.

A portosystemic vascular shunt is an abnormal communication between the portal vein and the systemic circulation. The shunt may be congenital or acquired, partial or complete, intra‐hepatic or extra‐hepatic. In all of these, blood from the portal system is allowed to bypass the liver. In radiographs, the liver is small, but the hepatic margins usually remain even and well‐defined. The kidneys may be large to compensate for the underperforming liver.

Hepatic cirrhosis is scarring and fibrosis of the liver due to chronic inflammation. In radiographs, the liver is small with irregular margins, the latter due to nodular regeneration. Hepatic cirrhosis frequently progresses to liver failure and peritoneal effusion.

Radiographic diagnosis of microhepatia is easier when the entire liver is small and less obvious when only a portion of the liver is affected. Venous portography can be used to investigate a suspected portosystemic shunt (Figure 4.32). Hepatic vessels will be poorly opacified if there is severe shunting of blood. The procedure for venous portography, as well as its indications and contraindications, is described in the Contrast Radiography section of Chapter 2.

Large liver (hepatomegaly)

Hepatomegaly may involve part or all of the liver. Partial liver enlargement usually is due to a mass. Generalized enlargement may be due to endocrinopathy, venous congestion, inflammation, or neoplasia (see differential diagnoses).

In a lateral radiograph of a patient with generalized hepatomegaly, the caudoventral edge of the liver is rounded and extends a significant distance caudal to the costal arch (Figures 4.33 and 4.34). The stomach is displaced caudally and dorsally, producing a more craniodorsal‐to‐caudoventral slope to the gastric axis. The severity of stomach displacement depends on the degree of liver enlargement. Severe hepatomegaly can also push the proximal duodenum, right kidney, and transverse colon further caudally and may limit movement of the diaphragm.

Generalized hepatomegaly often is accompanied by a distended abdomen, either due to the large size of the liver, a weak body wall, enlargement of other viscera, or peritoneal effusion. Venous congestion and infiltrative diseases such as lymphoma frequently lead to enlargement of multiple organs and varying amounts of peritoneal effusion.

Liver mass

Masses growing in or on the liver generally lead to asymmetric hepatomegaly and an abnormal liver shape. The adjacent viscera may be unevenly displaced, depending on the size and location of the mass (Figure 4.35). A left liver mass can displace the gastric fundus caudally and medially, sometimes pushing the fundus caudal to the pylorus. The head of the spleen and the intestines may be pushed caudally. Right liver enlargement can displace the gastric pylorus, proximal duodenum, and right kidney caudally and medially.

A centrally located liver mass may displace the gastric pylorus caudally and dorsally (Figure 4.36). A central hepatic mass can extend caudal to the stomach, particularly if it is pedunculated, and may appear separate from the liver. In these cases, the liver mass may be misdiagnosed as part of the spleen or other nearby structure.

In addition to mistaking a hepatic mass for part of the spleen, the spleen may be mistaken for part of the liver. In a lateral view the tail of the spleen sometimes blends with the caudoventral edge of the liver, mimicking a large liver. In these cases the opposite lateral view often allows the liver and spleen to be distinguished as separate structures.

A large gall bladder may produce an intra‐hepatic mass effect. As mentioned earlier, gall bladder distention may produce a swelling or bulge along the ventral liver border in a lateral view (Figure 4.29).

Abnormal liver opacity

The normal liver is uniform soft tissue opacity. Mineral or gas opacity in the liver is abnormal and frequently involves the biliary system. Occasionally, dystrophic mineralization or a gas‐producing bacterial infection occurs in a long‐standing hepatic tumor or abscess, or in an area of liver necrosis (e.g., liver lobe torsion). Gas also can enter the liver parenchyma from the GI tract via the portal system, which may occur due to a necrotic stomach (e.g., gastric dilatation and volvulus). Abnormal opacity in the liver parenchyma typically appears as an irregular or mottled pattern localized in part of the liver.

Abnormal opacity in the gall bladder is seen in the right cranioventral part of the liver. An emphysematous gall bladder most often is associated with diabetes mellitus. Gas initially forms in the bladder wall and eventually fills the lumen to produce multiple tiny bubbles that conform to the shape of the gall bladder (Figure 4.37). Emphysema can extend into the bile ducts to create a tree‐like pattern of gas opacity, similar in appearance to air bronchograms in consolidated lungs.

Mineral opacity in the gall bladder or bile ducts may represent cholelithiasis or dystrophic mineralization. Choleliths (gallstones) can vary greatly in size, number, and opacity (Figure 4.38). They may or may not be clinically significant. Not all gallstones are mineral opacity. When in the common bile duct, mineral opacity calculi are called choledocoliths and may be visible between the gall bladder and the duodenum (Figure 4.38). Mineralization of the gall bladder wall typically appears as a thin, well‐defined, shell‐like structure (Figure 4.39). Mineralization in the bile ducts appears as a linear or branching pattern in either an isolated part of the liver or scattered throughout the liver (Figure 4.40).

Normal radiographic anatomy

The spleen is a relatively flat and elongated organ with even, well‐defined margins. The edges of the spleen form acute angles (less than 90°). Because it is thin, the spleen often is difficult to identify in radiographs except where it overlaps or “folds” on itself. In these areas the spleen is essentially doubled in thickness and opacity. The two layers of spleen summate to create a triangular or wedge‐shaped appearance (Figure 4.41). Seeing a splenic triangle in a radiograph does not mean you have identified the entire spleen; rather, you have found the starting point to begin examining the spleen.

For descriptive purposes, the spleen is divided into three parts; head, body, and tail. The head is the proximal part of the spleen that is anchored to the fundus of the stomach by the gastrosplenic ligament. The body of the spleen is the middle part. The tail of the spleen is the distal part. Both the body and tail of the spleen are freely moveable. In dogs, the body and tail can vary considerably in size and position. In cats, they are more constant and proportionately smaller than in dogs.

Normal radiographic anatomy

The pancreas is divided into three parts; right limb, left limb, and body. The body is the part between the two limbs and it is located in the right cranial abdomen near the gastroduodenal angle (Figure 4.46). The gastroduodenal angle refers to the junction between the stomach and the duodenum.

The right limb of the pancreas extends caudally from the body of the pancreas to about the level of the L3‐4 vertebrae. It is positioned between the descending duodenum and the ascending colon. The left pancreatic limb extends transversely from the body, across midline, and toward the left kidney. It is positioned between the greater curvature of the stomach and the transverse colon.

The normal pancreas seldom is seen in radiographs due to its small size and similar opacity to the adjacent tissues. When surrounded by fat, the left pancreatic limb may be visible in a VD/DV view, particularly in cats (Figure 4.47). In these cases it appears as a thin, linear, soft tissue opacity structure within a triangle formed by the stomach, spleen, and left kidney.

Pancreatic disease

Diseases of the pancreas may or may not be evident in radiographs. Characteristic findings of pancreatic disease include increased opacity, indistinct serosal margins, and a mass effect in the area of the pancreas (Figure 4.48). These findings may be present near the gastroduodenal angle or along the descending duodenum or caudal to the stomach. Note: when examining these areas, use caution when interpreting subtle increases in opacity and mildly indistinct margins. In a VD view of many patients there is less fat and more visceral crowding in the right cranial abdomen than in the left cranial abdomen.

A mass effect in the area of the pancreas may be caused by pancreatic swelling or a pancreatic mass (e.g., tumor, cyst, abscess). Both pancreatitis and pancreatic neoplasia produce similar radiographic signs and cannot reliably be differentiated. In a VD view, a pancreatic mass effect may displace the duodenum to the right and widen the gastroduodenal angle. The transverse colon may be displaced caudally, resulting in increased distance from the stomach. In a lateral view, the duodenum may be displaced either ventrally or dorsally.

Pancreatic diseases can cause a regional peritonitis that diminishes peristalsis in the nearby intestines. Regional inflammation near the descending duodenum or transverse colon may lead to a localized functional ileus known as a sentinel loop. A sentinel loop is a static segment of bowel, often recognized in radiographs by its static gas pattern that remains relatively unchanged in serial radiographs. The segment appears similar in size, shape, and content between images. Sentinel loops do not only occur with pancreatitis; any site of peritoneal inflammation can lead to a functional ileus.

Pancreatic disease can progress to a more diffuse peritonitis and peritoneal effusion, further obscuring serosal margins. Pancreatic adenocarcinoma can metastasize throughout the peritoneal space to produce a diffuse, mottled, hazy pattern (i.e., carcinomatosis).

Severe pancreatic disease may cause a gastric outflow problem and persistent distention of the stomach. It can also lead to hepatitis and hepatic lipidosis. Pancreatic mineralization is rare but may result from chronic inflammation or fat necrosis.

Normal radiographs do not rule out pancreatic disease. Follow‐up radiographs or other imaging modalities may be needed in patients that do not respond to appropriate therapy.

Signs of pancreatic disease sometimes are evident in an Upper GI study. Radiographic signs may include a duodenal sentinel loop, delayed gastric emptying, delayed transit through the intestines, or thickening of the duodenal or gastric wall.

Normal radiographic anatomy

The stomach is located caudal to the diaphragm and liver. Its caudal border is convex and called the greater curvature. Its cranial border is concave and called the lesser curvature (Figure 4.49). For descriptive purposes, the stomach is divided into four parts: cardia, fundus, body, and pylorus.

The cardia is the small area of the stomach that joins the esophagus. It is not radiographically distinct from the fundus.

The fundus is the dome‐shaped pouch that comprises the dorsal and left lateral portions of the stomach. It stores undigested food and gas.

The body of the stomach is the large distensible part located between the fundus and the pylorus.

The pylorus forms the distal third of the stomach. It consists of the pyloric antrum (the area before the gastric outlet), the pyloric canal (the passage that connects the stomach to the duodenum), and the pyloric sphincter (a thick muscular ring that helps regulate outflow from the stomach). In most adult dogs, the pylorus is positioned ventral to the fundus and to the right of midline. In cats, puppies, and barrel‐chested dogs, the pylorus tends to be closer to midline. The closer the pylorus is to midline, the more U‐shaped or J‐shaped the stomach appears in a VD/DV view (Figure 4.50).

The size, shape, position, and opacity of the stomach can vary significantly depending on its contents. An empty stomach generally lies within the costal arch, rarely extending caudal to the last pair of ribs. As the stomach fills, the fundus and body expand and the greater curvature becomes more convex. Filling increases the length and diameter of the pylorus and moves it further to the right. In a lateral view, a full stomach may touch the ventral body wall and extend caudally to the level of the umbilicus.

The stomach is able to expand because the gastric mucosa is formed into folds called rugae. The rugal folds generally are larger and more numerous in dogs than in cats and often more visible in the fundic region (Figure 4.49). Rugal folds can vary significantly in appearance and their radiographic interpretation requires experience and excellent visualization of the gastric mucosa. In general, each fold should be similar in height and width (thickness) to the width of the spaces between folds.

The mucosal margin of the stomach rarely is identified with certainty in survey radiographs, because fluid and most ingesta are the same opacity as the mucosa, contrast radiography is usually needed. Prior to performing gastrography, however, always make four view survey radiographs of the stomach: right lateral, left lateral, VD, and DV. In many cases, these four views will redistribute gas and fluid in the stomach lumen and will help clarify questionable findings. Four views are also important during gastrography to allow different parts of the stomach to fill with contrast medium. This is particularly useful when searching for mural lesions and identifying the different parts of the stomach.

In some animals, especially overweight cats, a submucosal layer of fat may be visible in the stomach wall. Submucosal fat appears as a thin line of fat opacity that follows the curvature of the stomach (Figure 4.51). The fat layer may be mistaken for gas in the stomach wall.

Gas and fluid in the stomach

In most patients, the stomach contains some gas or food, making its identification relatively simple. The distribution of material in the stomach varies with its volume and the position of the patient at the time of radiography. Gas generally rises to the highest parts of the stomach and fluid flows with gravity into the dependent parts. Deliberately changing the position of the patient is useful to redistribute gas, fluid, and contrast media in the stomach. In situations where there is only a small volume of material in the stomach, the patient should be rotated a full 180° for a few seconds and then back to the position desired to ensure the gas or fluid moves to a different part of the stomach.

Figures 4.52–4.55 illustrate the distribution of fluid and gas in the stomach based on the position of the patient. Each figure includes both a radiograph and a cross‐sectional CT image to help describe the appearance of the stomach in right lateral, left lateral, dorsal and ventral recumbencies. Table 4.1 summarizes the distribution of gas and fluid in different parts of the stomach.

Patient position	Fundus	Body	Pylorus
Right lateral	Gas	Gas	Fluid
Left lateral	Fluid	Fluid	Gas
Dorsal (VD view)	Fluid	Gas	Gas/Fluid
Ventral (DV view)	Gas	Fluid	Gas

Abnormal stomach position

The normal position of the stomach is influenced by the volume of its contents as well as the patient’s body conformation and the phase of respiration. For example, in deep‐chested dogs the stomach tends to be more cranial in position than in barrel‐chested dogs. The stomach naturally moves caudally during inspiration and cranially during expiration.

Abnormal position of the stomach may be due to a nearby mass effect or a hernia (see differential diagnoses). The cardia is the least mobile part of the stomach because it is relatively fixed in position by its attachment to the esophagus.

Abnormal stomach size

A largely distended stomach may be physiologic or pathologic. Follow‐up radiographs sometimes are needed to distinguish the two. Recent ingestion of a large amount of food or water can lead to marked gastric distention (Figure 4.56). Ingestion of a large amount of foreign material can produce a similar appearance (Figure 4.57). Because normal and abnormal gastric contents can appear similar in radiographs, a follow up study often is useful. A stomach that can empty normally will naturally be smaller in follow‐up radiographs, provided the patient does not eat, drink, or vomit between studies.

Disorders that prevent normal gastric emptying may be physical or functional, congenital or acquired, acute or chronic, partial or complete. Physical obstructions can result from foreign material, thickening of the stomach wall, or gastric malpositioning. Functional outflow problems usually are due to pyloric disease. The degree of gastric distention depends on the severity and duration of the disorder and the patient’s ability to vomit.

A stomach that is largely distended has been described as both dilated and dilatated. The terms often are used interchangeably. Specifically, dilation refers to the passive act of enlarging whereas dilatation describes the state of abnormal enlargement. Dilatation is dilation beyond normal dimensions. Gastric dilatation sometimes is called gastric bloat.

A largely distended stomach creates a mass effect in the cranial abdomen. The intestines, spleen, and sometimes the kidneys often are displaced caudally. An extremely distended, mostly fluid‐filled stomach may be difficult to recognize in radiographs because it is homogenous in opacity and its margins extend much further caudally than expected (Figure 4.58). A gas pocket floating on top of the gastric fluid may be mistaken for the actual size of the stomach if the fluid‐filled portion is not identified.

In patients with a chronic gastric outflow problem, multiple, tiny, mineral opacity objects may be visible in the dependent portion of the stomach (Figure 4.58). This finding is called the gravel sign. The gravel sign can occur anywhere in the GI tract. It results from the sand‐like sedimentation of heavier particles in an area of chronic obstruction. In the stomach, the gravel sign often is easiest to see in a lateral view and typically in the pylorus.

Gastric dilatation and volvulus (GDV)

GDV can occur when the stomach rotates around its axis. It is more common in large, deep‐chested dogs, but can occur in any breed of dog and at any age. It is rare in cats but has been reported in a cat with a diaphragmatic hernia. Patients with GDV frequently present as an emergency, in a life‐threatening situation. Patients in severe distress must be stabilized prior to imaging.

During gastric volvulus, the pylorus rotates from its normal position in the ventral, right side of the abdomen to the dorsal, left side (Figures 4.59 and 4.60). Volvulus also twists the esophagus at the cardia of the stomach. The duodenum follows the pylorus and moves dorsally and to the left, often wrapping around the esophagus. GDV results in an inability to eructate and an obstructed pyloric outflow.

The radiographic appearance of GDV is summarized in Table 4.2. Note: radiographic findings depend on the stomach contents, the degree of gastric distention, and the extent of rotation. The key to diagnosis of GDV is to locate the pylorus (Figures 4.60–4.63). Locating the pylorus is essential to differentiate GDV from gastric dilatation only. A displaced pylorus is easiest to identify when it contains gas. Because the pylorus typically rotates dorsally and to the left, the right lateral and DV views tend to be the most diagnostic. Remember, the dependent (down) parts of the stomach fill with fluid (and ingesta) while the non‐dependent (up) parts fill with gas. In right lateral and ventral recumbencies, a pylorus that is displaced to the left and dorsally will be “up” and more likely to fill with gas.

Pylorus is abnormally positioned, usually dorsally and to the left. Compartmentalization of the stomach. Abnormally positioned spleen with variable splenomegaly. Reflex ileus in small intestine. Dilated esophagus. Small caudal vena cava and small cardiac silhouette.

The classic appearance of GDV in the right lateral view is commonly called compartmentalization of the stomach (Figures 4.60 and 4.61). Compartmentalization appears as a gas‐filled pylorus positioned craniodorsal to a gas‐and‐fluid‐filled fundus, and with a band of soft tissue opacity between them. Compartmentalization is considered by many to be pathognomonic for GDV but, like most radiographic signs, it is not 100% diagnostic.