Abdominal ultrasound has the advantage over radiography in its ability to see small structures like adrenal glands and evaluate the abdomen in the presence of peritoneal effusion. Additionally, Doppler imaging allows us to evaluate blood flow, including direction and velocity.

Adrenal Glands

Due to their small size, a good anatomic understanding of where the adrenal glands are located is crucial for adrenal gland identification with ultrasound. The position of the adrenal glands in relation to regional vasculature is very consistent, making vascular landmarks quite helpful for identification of adrenal glands.

Left Adrenal Gland

The left adrenal gland is located caudal to the celiac and cranial mesenteric arteries (first two branches of the abdominal aorta; these vessels exit the ventral aspect of the aorta a short distance from each other) and cranial to the left renal artery and vein (these have a distinct curve just prior to entering the aorta and caudal vena cava). The left phrenicoabdominal vein crosses the left adrenal gland ventrally and is usually visualized with ultrasound. The left phrenicoabdominal artery crosses the left adrenal gland dorsally and is often too small to visualize with ultrasound. The left adrenal gland lives to the left of the aorta and medial to the cranial pole of the left kidney. The left adrenal gland in dogs is bilobed in shape (peanut or dumbbell shape) and more ovoid in cats.

Right Adrenal Gland

The right adrenal gland is more dorsal and cranial than the left adrenal gland, making visualization more difficult. Additionally, obesity, bowel gas and deep-chested conformation can hinder identification of the right adrenal gland. The medial surface of the right adrenal gland is in close proximity to the lateral wall of the caudal vena cava. The right adrenal gland is cranial to the right renal artery and vein and it is the same cranial-caudal level as the celiac and cranial mesenteric arteries. Similar to the left adrenal gland, the right phrenicoabdominal vein crosses the right adrenal gland ventrally and is usually well visualized and the right phrenicoabdominal artery crosses the adrenal gland dorsally and not usually visualized. In dogs, the shape of the right adrenal gland is variable (elongated, arrowhead or boomerang shaped). In cats, the right adrenal gland is oval in shape, similar to the feline left adrenal gland. In cats, the right adrenal gland is often located fairly cranial in respect to the cranial pole of the right kidney.

Normal Adrenal Gland Size

In dogs, the size of the adrenal gland varies with patient body weight. In a study that evaluated adrenal size in populations of dogs under 10 kg with and without hyperadrenocorticism, a cut off of 0.6 cm thickness of the caudal pole of the adrenal gland was 75% sensitive and 94% specific for differentiating normal adrenal glands from adrenal hyperplasia.¹ As there is a small population of canine patients with pituitary-dependent hyperadrenocorticism that do not have adrenal gland enlargement and larger dogs will normally have larger adrenal glands, interpretation of adrenal size should be made in conjunction with clinical and laboratory findings.

In cats, because they are more uniform in size there is a fairly narrow normal range of adrenal gland size which is 0.4 cm to 0.5 cm in thickness. Cats can also have incidental mineralization of the adrenal glands.

Alterations to Adrenal Gland Size Include:

• Bilaterally small adrenal glands: Addison’s disease (hypoadrenocorticism), chronic steroid therapy
• Bilaterally large adrenal glands: pituitary-dependent hyperadrenocorticism
• Focal adrenal nodules and masses (unilateral or bilateral): hyperplasia, adenoma. If lesion is greater than 2.0 cm in diameter, there is increased likelihood of neoplastic etiology (adenocarcinoma, pheochromocytoma) versus benign causes.² Metastasis can occur in the adrenal glands. If a functional tumor is present in an adrenal gland, the contralateral adrenal gland may be small in size. When an adrenal gland mass is identified, evaluate for vascular invasion (most often into the phrenicoabdominal vein and caudal vena cava).

Abdominal vasculature, lymph nodes and peritoneal and retroperitoneal spaces

In addition to vascular structures serving as landmarks for identification of other structures, ultrasound is utilized to evaluate the vessels themselves for size, echogenicity, thrombi, velocity and direction of blood flow. Particular vessels of interest during an abdominal ultrasound evaluation include evaluation of the aortic trifurcation. This is a common site for thrombi, especially in cats, and is a landmark for the sublumbar lymph nodes. A right intercostal window is helpful for evaluating the vasculature at the level of the hepatic hilus including the aorta, caudal vena cava and portal vein in patients with suspected portosystemic shunt, right heart disease or portal hypertension.

Doppler imaging is helpful for evaluation of abdominal vasculature. Types of Doppler include color, power, pulse wave and continuous wave.

Color Doppler identifies the direction and relative velocity of blood flow.

Power Doppler is sensitive to slow flowing blood but does not identify direction of blood flow.

Pulse wave Doppler allows for evaluation of velocity and direction of flow in a specific vessel.

Continuous wave Doppler is used in echocardiography and allows for evaluation of high velocity blood flow.

Keep in mind that Doppler signal is dependent on the angle of the ultrasound probe in relation to blood flow. Best Doppler results are obtained when the probe angle is small (less than 60 degrees) in comparison to the direction of blood flow.

Lymph Nodes

Knowledge of normal lymph node anatomy and drainage patterns is important for identification and interpretation of abnormal lymph node findings. Normal lymph nodes are generally hyperechoic compared with vasculature and slightly hypoechoic compared to surrounding mesentery. The jejunal and medial iliac lymph nodes are normally the largest lymph nodes in the canine abdomen and thus are generally visible even in normal patients. Other abdominal lymph nodes are not often visualized unless lymphadenopathy is present. Normal lymph nodes size is dependent on patient size and age. Normal lymph nodes are elongated in shape and are much longer than they are thick.

Jejunal lymph nodes: Paired, one on either side of the cranial mesenteric artery. Usually on midline at the level of the umbilicus and just medial to the ileocecocolic junction.

Medial iliac lymph nodes: Paired, located lateral to their respective external iliac arteries at the level of the aortic trifurcation.

When lymph nodes are enlarged, the big question is are they reactive or metastatic. Cytologic or histopathologic sampling is required for definitive diagnosis; however, there are some trends that may prioritize one differential over the other. Metastatic lymph nodes tend to get rounded in shape and have a short axis to long axis ratio greater than 0.5, whereas reactive lymph nodes will be enlarged but generally retain an elongated shape. Metastatic lymph nodes tend to be hypoechoic versus reactive lymph nodes that either retain normal echogenicity or may have a hypoechoic outer rim. Reactive lymph nodes tend to retain a normal hilar blood flow pattern, whereas metastatic lymph nodes may exhibit peripheral blood flow.

Peritoneal and Retroperitoneal Spaces

The mesentery and abdominal fat provides the background echoes between abdominal organs. Inflamed mesentery and fat will be hyperechoic and hyperattenuating. When mesenteric inflammation is focal it can act like a neon sign pointing to the area of pathology. This can be quite helpful for identification of lesions. For instance, when surrounding the pancreas in acute pancreatitis, adjacent to a ruptured gallbladder or focal intestinal mass. The mesentery can also be a site of metastasis as is the case with carcinomatosis.

Peritoneal effusion can occur for a variety of reasons. Although the echogenicity of peritoneal effusion can imply the cellular/protein content (ex. transudates, urine likely anechoic; hemorrhage, exudates likely echogenic), cytology is necessary for diagnosis. Be aware that machine settings (ex: gain) can alter the echogenicity of fluids.

Ultrasound is more sensitive than radiology for identification of small volumes of free peritoneal fluid or gas. When small volumes of abdominal effusion are present, it tends to collect in typical locations. These include between the splenic head and left body wall, between loops of small intestine, cranial to the urinary bladder and surrounding the gallbladder. Small volumes of free fluid can be identified by their triangular or angular shape. Pneumoperitoneum can be identified by reverberation artifact caused by gas which most often collects in the most non-gravity dependent portions of the abdomen. If large volumes of gas are present within the abdominal cavity (such as in a postoperative abdomen), positional imaging can be utilized to help visualize abdominal structures as the reverberation artifact associated with gas can inhibit visualization of structures.

References

1.  Choi J, Kim H, Yoon J. Ultrasonographic adrenal gland measurements in clinically normal small breed dogs and comparison with pituitary-dependent hyperadrenocorticism. J Vet Med Sci. 2011;73(8):985–989.

2.  Pagani E, Massimiliano T, Chiara L, et al. Ultrasonographic features of adrenal gland lesions in dogs can aid in diagnosis. BMC Vet Res. 2016; 12:267.