A tour of the abdomen: Scanning in lateral recumbency – The right side (Proceedings)

This brief overview of abdominal ultrasound is meant to be just that – an overview. If you are serious about ultrasound, you will take it upon yourself to review on the ultrasound references in physics such as that by Kremkau. Ultrasound images can be 100% artifactual, when inappropriate use of the technology is applied. Ultrasound examinations require time and effort on the part of the sonographer. These types of introductory classes are great for getting an overview, but they are not the ends of the line for learning how to perform an ultrasound examination. Most abdominal ultrasounds can be performed in 20 to 40 minutes; however, complex studies may require additional time. Ultrasound requires patience and technical expertise that is only learned after mastering your equipment, the physics of ultrasound and ramifications in identifying abnormal sonographic anatomy. As you are starting to do ultrasound examinations, read and thoroughly digest Nyland and Mattoon's book on small animal ultrasound. Find a colleague with the same interest and start a journal club weekly to review this book in depth and other articles/references that come up. Additionally, start collecting a normal database and scan everything. Look at specifics (left kidney for 2 to 3 weeks) vs. trying you do a complete study and over 6 months you will start to recognize normal from abnormal. Be sure to take at least 2 weekend short courses with different instructors each year for 3 years. This is a minimum investment of 3 years on your part and in order to get "good" at it, you need to practice, practice and practice.

A Tour of the Abdomen

One of the most difficult aspects of abdominal imaging is establishing a set pattern of "exploration", especially compared with echocardiography, where well established imaging planes are preset. The goal then is to evaluate the heart from these imaging planes and take specific measurements. In abdominal imaging, conventional windows are not pre-established. Each structure or organ has a characteristic echogenicity, shape, pattern, location, margin and number. Each organ should be evaluated using multiple imaging planes. When abnormalities are identified, they should be evaluated in multiple imaging planes. The two most common methods used for evaluating the abdomen include dorsal or lateral recumbency. I currently use lateral recumbency as a starting point. HOWEVER, you should learn to use both techniques, as there are advantages and disadvantages of each. You should be comfortable in doing both. I will roll the dog or cat into dorsal recumbency only when required to evaluate a particular structure or a possible abnormality more completely. Aspirates and biopsies are done with the animal in the appropriate recumbency that puts the area of interest the closest to the surface with the least amount of possible structures between the area of interest and the skin. Abdominal radiographs are still the best way to "survey" the abdomen and should be used as the first line for evaluating abdominal disorders. Ultrasound can then be used to evaluate specific structures for specific reasons. The two imaging modalities are in fact complimentary as they provide different information.

Transducer placement and eye-hand coordination takes some practice and getting used to making adjustments while monitoring the screen. There are three basic movements of the ultrasound transducer that one needs to master. The first is distance motion. This means that the transducer moves in a cranial or caudal direction and thereby the structures in the image move also in a cranial or caudal direction. The other distance motions include dorsal and ventral directions and oblique directions. The second type of transducer motion is non-distance, angular motions. In this case, the transducer stays in the same position on the animal, but is angled in a cranial, caudal, dorsal or ventral direction. The third type of transducer motion is a rotational motion whereby the orientation of the beam is redirected from a cranial-caudal orientation to a dorsal-ventral orientation or an oblique orientation. This type of motion would be one used when moving from a right, parasternal long axis image to a right, parasternal, short-axis image of the heart. When moving the ultrasound transducer, the motions should be smooth. Usually you do not have to press very hard wit the probe. Tweaking the transducer can be just as much of an art form with technical expertise as playing with the ultrasound instrumentation. One should remember that more often than not, one is using an oblique imaging plane versus a true dorsal, sagittal or transverse imaging plane. On every transducer is a small point or marker that gives the orientation of the primary ultrasound beam. On the ultrasound screen, there is also a marker that identifies the mark on the transducer. Place one finger on this marker on the transducer so that you always know the appropriate image orientation and you do not get lost during the scan.

Place the animal in right lateral recumbency with the limbs facing the sonographer and the head facing to the sonographer's left. Place the ultrasound machine to the left or right of the sonographer based on the dominant hand or the hand that will drive the transducer. I usually place the machine to the left and drive the transducer with my right hand (I am right handed). Start with imaging the left paralumbar fossa with the transducer oriented such that an oblique dorsal plane is produced. Find and longitudinally align the aorta (near field) and the caudal vena cava (far field). You may not be able to place the aorta or caudal vena cava in the same imaging plane. Trace these vessels caudally to the trifurcation of the caudal abdominal aorta into the sacral (midline) and the external iliac arteries. At this branching, evaluate for the presence of medial iliac lymphadenopathy. If an enlarged lymph node is present, freeze the image and measure it. This is going to be true for all abnormalities. If you stop moving the transducer at this point you can see the pulsatile nature of the aorta and its caudal branches. When evaluating this area from the right side, too much compression will result in the luminal collapse of the caudal vena cava. Move the transducer ventrally using distance motion to evaluate the urinary bladder. Once identified, evaluate from the dorsal and transverse imaging planes. To completely evaluate the caudal abdomen, one may have to shift the position of the ultrasound transducer from the paralumbar fossa position to a position between the legs, over the caudal and ventral abdomen. Lift the animal's left leg and evaluate in the caudal pelvic region for the prostate in the male and the uterus in the female. In male dogs that have been neutered, the prostate can be difficult to find. It may be easier to image the bladder transversely and move the transducer in a caudal direction through the trigone to the urethra. The first structure after the urinary bladder in line with the urethra will be the prostate. Once imaged in the transverse plane, you can turn the transducer into the dorsal or sagittal plane and longitudinally localize the prostate.

Prostatic Ultrasound: the prostate is a bilobed gland located in the caudal abdomen just caudal to the trigone of the urinary bladder. Prostatic parenchyma normally has a uniform echogenicity, being slightly hyperechoic to surrounding tissues. In older dogs that were neutered at an early age, the prostate can be very small and isoechoic to the surrounding tissue. The prostate may appear as a focal enlargement of the urethra when evaluating the urethra in the long axis or transverse planes. Abnormalities in size, shape, echogenicity and position can be detected sonographically. In a recent study, dogs that have been neutered early in life and have an enlarged prostate with mineralization have a 100% predictive value for prostatic adenocarcinoma. In dogs with the suspicion of prostatic cancer, thoracic radiographs should be obtained to evaluate for pulmonary metastasis. If medial iliac lymphadenopathy is noted, then abdominal radiographs should be obtained to evaluate for metastatic spread to L6 and L7 lumbar vertebrae. Hyperechoic changes most commonly are associated with benign prostatic hypertrophy and fibrosis within the gland. Hypoechoic changes are less common, but are often associated with prostatic adenocarcinoma. Other differential diagnostic considerations for hypoechoic lesions in the prostate include intraparenchymal cysts, hemorrhagic cysts, and glandular cystic hyperplasia and abscess formation. Anechoic or cavitary cysts can also be present in glands that are hyperplastic. Prostatic mineralization is feature of prostatic carcinoma although chronic inflammatory prostatic disease may also result in dystrophic mineralization of the prostate. Anechoic or hypoechoic cystic structures adjacent to the prostate gland are consistent with paraprostatic cyst formation.

Evaluation of the uterus can be difficult when the dog is not pregnant, not in estrus and not post-partem. Early diagnosis of pregnancy can be done evaluating for vesicles and/or heart beats between 16 to 20 days in dogs and 11 to 16 days in the cat post ovulation and breeding. Fetal viability late in gestation can also be assessed. Sonography provides a poor method for counting fetuses and typically a lateral and VD abdominal radiograph after ossification has occurred (day 42) should be used to determine actual fetus number and pelvic positioning if a dystocia is present. Uterine enlargement can be assessed via ultrasound. Pyometra (hydro and mucometra) can be assessed as fluid accumulations within the uterus. The uterine walls can become thickened in chronic metritis cases where open drainage to the vagina is present. Numerous tiny cysts can be seen within the uterine wall in dogs or cats with glandular cystic hyperplasia, endometriosis or endometritis.

From the caudal abdomen, reposition the transducer in the left paralumbar fossa and longitudinally localize the caudal abdominal aorta and caudal vena cava. Using the aorta, move the transducer cranially slowly, keeping the aorta in the middle of the screen and in long axis from your oblique dorsal plane. Occasionally sweep in a dorsal and ventral direction to image for the possibility of ureteral stones. Continue cranially until the position of the kidney has been reached.

In intact females, evaluation for the ovaries can be difficult in anestrus. The ovaries are normally small structures that are located caudal and somewhat lateral to the kidneys. When the dog is in lateral recumbency the left ovary may wind up just being caudal to the left kidney. Under optimal scanning conditions, the ovaries may be visualized, however, more often than not, will be only seen if there are multiple cysts or are abnormally enlarged. Ovarian tumors are rare and are usually heteroechoic. When one sees mineralization within an ovarian mass, a teratoma should be considered.

The next structure to be evaluated is the left adrenal gland. There are many anatomic landmarks that will help in the finding and evaluation of the left adrenal gland. The first is to ignore the renal position itself and find the left renal artery coming from the aorta. As you have the aorta in long axis, the left renal artery is the first major vessel that can be seen leaving the aorta and coming laterally into the near field. Just cranial to this vessel and within the left renal artery "hook" is the first place to start looking for the caudal pole of the left adrenal gland. The adrenal gland is typically a bilobed or peanut shaped structure in small dogs, ovoid in cats, long and narrow in medium sized dogs and sometimes thin or curvilinear in large dogs. The adrenal gland is hypoechoic relative the hyperechoic retroperitoneal fat. In certain cases one can differentiate the outer hypoechoic cortex of the adrenal gland with the inner hyperechoic corticomedullary interface. A thin hypoechoic line may be seen within this central echogenic portion of the gland. Once the adrenal is visualized, fine-tuning the image will consist of two motions. The first is a non-distance motion, most probably angling the transducer cranially. The second motion is a rotational motion of the transducer so that the transducer's marker is in an oblique dorsal plane with the reference marker of the transducer pointing in a dorsal and cranial direction. If the adrenal could not be identified with the retroperitoneal fat just cranial to the left renal artery, continue moving the transducer slightly cranial and sweep in a non-distance dorsal and ventral motion. These motions are not big adjustments, but we are talking several cm of directional changes in the dorsal and ventral direction keeping the transducer in a dorsal imaging plane. From the ventral surface of the aorta, you will visualize two circular arteries that originate just cranial to the left kidney position. These vessels are the celiac artery and the cranial mesenteric artery in a cranial to caudal direction respectively. The left adrenal can be triangulated between these two vessels as the cranial markers and the left renal artery as the caudal marker. It has gotten to the point that if I can not find the left adrenal gland in this area on my initial sweeps, then I will find these two markers and start with non-distance rotational and dorsal ventral angular motions (slight) to find the left adrenal gland. In dogs with Addison's disease the left adrenal can be difficult to identify. If I still have not found the left adrenal gland, I back up and find the long axis image of the mid abdominal aorta and start again. The easiest animals to start identifying the adrenal gland are the Minature Schnauzer breed or small dogs such as the Poodle. Practicing to identify the left adrenal gland will help you as a sonographer develop your eye-hand coordination for optimizing the image. Remember to continuously adjust the image as needed. When evaluating the medial iliac lymph nodes, the image depth may have only been 3 to 4 cm for most animals. When evaluating for the left adrenal gland, you may have increased the depth now to 6 to 7 cm.

Evaluation of the left kidney is next in line. Longitudinally localize the left kidney and optimize the image so that you can see the entire kidney. Sweep in a dorsal and ventral direction to evaluate the entire kidney from cortex to cortex. Renal size varies with patient size, however, most cat kidneys are between 3.2 and 3.8 cm in length. Sonographically, the kidney can be divided into three different zones. The outer hyperechoic renal cortex, the middle hypoechoic medulla and the inner renal sinus and pelvic area (hyperechoic due to fat in the renal sinus). The renal cortex is usually slightly hypoechoic (or isoechoic) to the liver. The normal cortical to medullary ratio is around 1.5:1. The size of the medulla can increase physiologically as a result of iatrogenic diuresis as well as slight dilation of the renal pelvis. Differential diagnostic considerations for increased echogenicity of the renal cortex include glomerulonephritis, interstitial nephritis, acute tubular necrosis, ethylene glycol toxicity (really bright cortex) nephrocalcinosis, nephrosclerosis, lymphoma, amyloidosis and chronic pyelonephritis. Definitive diagnosis requires cytological evaluation. Focal renal changes can include cyst formation, previous renal infarct, renal metastasis, granulomas and abscessation. Severe hydronephrosis can be caused by renal obstructions secondary to a number of causes. Ascending recurrent infections secondary to an ectopic ureter may also cause unilateral hydronephrosis. Sub capsular fluid accumulation can be seen in trauma (lymphoma cells, hemorrhage or urine accumulation) or perinephric cyst formation in cats. Neoplastic renal disorders usually have complex sonographic patterns with internal derangement, septation and a heteroechoic appearance. Cystic adenomas can also have multiple cysts within the renal parenchyma. Congenital renal disorders consist of small hyperechoic kidneys with lack of normal cortical:medullary development or multiple renal and hepatic cystic disorders as seen in some breeds of cat (Himalayan, Persian).

From the kidney, the focus turns to the area just cranial to the kidney. The most obvious structure in this region is the spleen. The spleen will be located in the near field, will be triangular in shape when imaged in the dorsal plane, and will have a homogeneous parenchyma with the exception of the splenic portal vessels that collect centrally along the mesenteric border of the spleen (far field). The spleen should be evaluated for changes in echogenicity. Common changes seen in the spleen include hyperechoic fibrotic or fat nodules noted along the mesenteric border of the spleen at the hilus where the vessels enter and exit the spleen. The field of view should be decreased to evaluate the spleen optimally. The spleen should be traced from a dorsal (head of the spleen) to a ventral location (tail of the spleen) using distance motion of the transducer. Once the spleen has been evaluated, reposition the transducer along the left paralumbar fossa just caudal to the last rib. The head of the spleen should be in the near field, the stomach is in the cranial aspect of the mid to far field and the colon is in the caudal aspect of the dorsal plane of the mid to far field. Once in this position, evaluate the fat in the central portion of the image for the pancreas. If you rotate the transducer so that the splenic portal vein is longitudinally localize, the pancreas will be located just above or below this vessel position depending upon the exact plane the transducer is located. More often than not the pancreas is located above the portal vessel. One way to identify the pancreas, which is normally isoechoic with the surrounding peritoneal and mesenteric fat is to look for two parallel hyperechoic lines that are normally only 1 to 2 mm apart. This structure is the pancreatic duct and appears within the center of the left limb of the pancreas. When evaluating structures deep within this region, it is sometimes possible to see right adrenal masses and/or caudate lobe of the liver, liver masses or perihepatic lymphadenopathy. Before assuming the mass to be pancreatic in origin, be sure to evaluate the right side of the patient's abdomen. Also, if evaluating a dog for a portosystemic shunt, the most common extrahepatic shunt vessel is a left gastric. Sometimes from this position, the shunt vessel can be identified as it goes deeper toward the caudal vena cava.

The next major organ is the liver. Using distance and angular motions of the transducer, increase the depth and appropriate multi-frequency transducer shifts/focal zone shifts to image deeper within the abdomen. Angling cranial, one should start to image normal hepatic parenchyma. Most of the liver can be imaged with the transducer placed caudal and ventral to the xyphoid process or along the ventral caudal margins of the costal cartilages. In cases of microhepatia, an intercostal approach may be necessary. For evaluating the left side of the liver, the ventral intercostal approach provides the access for evaluation of most of the liver without lung interference. The liver should be evaluated for its size, margins, overall echogenicity and major intrahepatic structures. The liver lobes are not easily differentiated unless there is a peritoneal effusion that separates the different hepatic lobes. The liver echogenicity should be slightly hyperechoic or isoechoic to the renal cortex and hypoechoic relative to the spleen. The major intrahepatic structures include the medium and large sized portal veins and hepatic veins, the porta hepatis (hepatic hilus) and the gall bladder. Portal veins can be distinguished from the surrounding hepatic veins by their origin from the central portal vessel at the porta hepatis and the brighter hyperechoic walls when compared with the walls of the hepatic veins. The hepatic veins also can be connected to the caudal vena cava deep and dorsal within the liver. Changes in the size, shape, margin and echogenicity of the liver are indicators of hepatic parenchymal disease. Diffuse changes in hepatic echogenicity may be difficult to recognize and are subjective in nature. A diffuse increase in echogenicity can be caused by lipidosis, fibrosis or cirrhosis, lymphoma and steroid hepatopathy. Decreases in echogenicity can be seen with lymphoma, leukemias, and passive congestion secondary to right heart failure, chronic-active hepatitis and steroid hepatopathy. In the early stages of hepatic lipidosis, a decrease in overall hepatic echogenicity may also result. Ultrasound has not realized the original intent of tissue characterization for specific hepatic disease and due to overlap in disease appearances within the liver; the final diagnosis is dependent upon hepatic biopsy. However, ultrasound is good for evaluating parenchymal changes and monitoring response of a disease process to treatment.

The gall bladder is easily identified within the right hepatic parenchyma as an anechoic ovoid structure with a neck that extends caudally. Distal acoustic enhancement is a common feature of the liver parenchyma deep to the gall bladder. The bile duct and intrahepatic biliary ducts are normally not seen in the dog. The bile duct in the cat can be traced and is usually ≤ 3 mm in diameter. It is common to identify echogenic debris within the gall bladder that is gravity dependent. The overall size of the gall bladder can be difficult to assess and can be enlarged after fasting. Feeding a fatty meal or administering cholecystokinin and monitoring the size of the gall bladder can be used as a method for determining the presence of an extra hepatic obstruction. The most common cause of an extra hepatic obstruction is secondary to a pancreatitis with inflammatory obstruction of the bile duct and pancreatic duct as they course through the pancreas.

Now it is time to flip the patient over to a left lateral recumbent position. Starting in the right paralumbar fossa, we are going to repeat the same procedure for the caudal abdomen evaluating the caudal abdominal great vessels and the urogenital tract. Once this has been accomplished place the transducer in the right paralumbar fossa and longitudinally localize the caudal vena cava. Excessive pressure can collapse this vessel so be careful that you do not mistake the aorta for the caudal vena cava. Using a cranial distance motion, move the transducer cranial along the paralumbar fossa to a position just caudal to the 13th rib. Angle the transducer cranially and identify the right kidney and the caudal lobe of the liver. By sweeping the transducer in a dorsal to ventral plane evaluate for the caudal vena cava. Just beside or cranial to the level of the kidney (but not worrying about keeping the kidney in the imaging plane, just like on the left side), gently sweep the transducer dorsally and ventrally and back again.

The right adrenal gland is a hypoechoic structure that is more oval in smaller dogs and cats and elongated in medium and large dogs just adjacent to the caudal vena cava (anechoic). Once part of the right adrenal gland is visualized, rotate the transducer 5 to 15 degrees so as to longitudinalize it. The right adrenal gland may parallel the caudal vena cava or may extend in a craniodorsal to caudoventral direction adjacent to the caudal vena cava. Adrenal gland tumors (adenomas, adenocarcinomas and pheochromocytomas) usually distort the right or left adrenal gland anatomy and form a circular mass lesion. These lesions usually are hypoechoic. Adrenal cortical tumors may mineralize (can NOT be used to differentiate an adenoma vs. adenocarcinoma), while pheochromacytomas tend to be locally invasive with tumor thrombus invading the caudal vena cava via the phrenicoabdominal vein. If the right adrenal gland is not visualized angle the transducer caudally, realign with the caudal vena cava and move cranial again. You may have to use a dorsal intercostal approach to visualize this area. Another possible angle that can be used to identify the right adrenal is to place your transducer along the caudal aspect of the costal cartilages around the middle of the abdomen and pointing the transducer dorsally, again looking to longitudinalize the caudal vena cava. Gently moving the transducer in a medial and lateral direction will place the right adrenal gland lateral to the caudal vena cava. The right dorsal intercostal approach is a great window for evaluating the aorta, caudal vena cava and the portal vein. This window can also be used for identifying the presence of single, extrahepatic portosystemic shunts that may be entering this location. The porta hepatis can also be evaluated from this window. Lymphadenopathy of the peri-gastric or peri-hepatic lymph nodes can be seen.

Once the adrenal gland has been identified and measured, the right kidney and the right side of the liver should be evaluated in a similar manner as described for the left side. Once this is accomplished, place the transducer in the right paralumbar fossa and get the major vessels in the long axis again. This time move the transducer in a distance motion using a ventral direction until there is a small intestinal loop that is located in the near field that extends from the cranial aspect of the image to the caudal aspect of the image. This small intestinal loop most likely is the duodenum. Trace this bowel loop cranially and connect it with the stomach. Move back to the gastroduodenal angle and look for any mass lesions in the region of the body of the pancreas and then down along the duodenum evaluating for the right limb of the pancreas. Similar features noted for the left limb of the pancreas can identify the pancreas. Although a vessel paralleling the pancreatic duct can also be identified. This vessel is the pancreaticoduodenal vein. Features of pancreatitis include: hypoechoic pancreas, duodenal hypomotility or ileus, peritoneal focal effusion, pseudocyst or abscess formation, hyperechoic mesenteric fat consistent with saponification.

The final area to evaluate is the gastrointestinal tract, and the mid abdomen. We have already evaluated the stomach, colon and duodenum. Sweeping in a cranial and caudal distance motion as well as in a dorsal and ventral distance motion is used to evaluate the entire middle abdomen. Evaluation of the mid abdomen for lymphadenopathy and focal peritoneal effusion should be the final stages of the abdominal evaluation.

Summary

Abdominal ultrasound is a complex tool for survey evaluation of the abdomen. The examination is just as complimentary to abdominal radiographs as thoracic radiographs are a critical part of the cardiac patient work up along with an echocardiogram. Abdominal ultrasonography requires time and patience and proper understanding of the equipment and the physics. One should us this hand out only as introductory material. The dedicated sonographer should have several textbooks of small animal abdominal ultrasound that have plenty of pictures. Differential lists can be obtained from these textbooks as can descriptions for the various types of disease processes, recognizing that the final diagnosis for tissue characterization will require cytology and/or histology.

References

Nyland TG, Mattoon JS. Small Animal Diagnostic Ultrasound, second edition. Philadelphia: WB Saunders, 2002.

Kremkau FW. Diagnostic Ultrasound. Principles, Instruments and Exercises, 6th ed. Philadelphia: WB Saunders, 2002.

Burk RL, Feeney D. Small Animal Radiology and Ultrasonography: A diagnostic atlas and text, third edition. Philadelphia: WB Saunders, 2002.

Penninck D and d'Anjou, M. Atlas of Small Animal Abdominal Ultrasound. Blackwell Publishing, 2008.