Basic abdominal ultrasound (Proceedings)

Small animal patients are commonly presented to the veterinarian because of signs referable to the abdominal cavity due to congenital anomalies, dietary indiscretion, parasitic infestation and infectious or inflammatory disease. Abdominal ultrasound provides valuable clinical information about the peritoneal cavity, great vessels, abdominal viscera and lymph nodes obtained in a non invasive fashion, with no confirmed adverse biologic effects, and usually not necessitating sedation or anesthesia. This paper reviews the techniques for performing the pediatric abdominal ultrasound scan.

The use of ultrasound during evaluation of the pediatric patient with signs referable to the abdominal cavity provides valuable information obtained in a non invasive fashion with no confirmed adverse biologic effects. Additionally, minimal or no sedation is generally required to complete an abdominal scan in the pediatric patient. Abdominal ultrasound provides useful data in a short period of time. The normal paucity of intra abdominal fat in pediatric patients results in less informative abdominal radiography, but actually improves ultrasonographic imaging. (Abdominal fat attenuates the ultrasound beam.) Image quality is improved with small patient size as a higher frequency scanhead can be employed. Acquisition of special equipment for pediatric ultrasonography is usually not necessary as scanheads selected for small animal (especially feline) clinical use are appropriate for most pediatric cases.

Equipment

Small animal patients are best evaluated using an ultrasound machine equipped with a curvilinear variable frequency scanhead (6.0-8.0 MHz). Many portable machines now have available a high frequency linear scanhead (8.0 -10.0 MHz) which will improve quality and also allow evaluation of smaller regional anatomy (thyroid, parathyroid, cryptorchid testes).

Preparation

The small animal patient should be placed in dorsal recumbency within a padded V-trough, and gently restrained by assistant(s) holding the forelimbs and hindlimbs. Sedation is rarely required for the basic abdominal scan unless marked pain or apprehension is present. Allowing the patient to become accustomed to this restraint before initiating clipping or scanning usually minimizes struggling and resultant aerophagia.

Clipping the cranioventral abdominal hair using a No. 40 blade and wetting the skin with water, tincture of zephiran or 70% isopropyl alcohol, followed by a liberal amount of ultrasound gel permits the best acoustic coupling of the scanhead to the patient, improving the image obtained. Some pediatric patients have scant ventral hair coats and will not require clipping. Care should be taken to avoid excessive chilling of pediatric patients secondary to the application of room temperature liquids followed by evaporation. Electric warming devices (warm water blankets) may cause electronic interference with the ultrasound equipment; warm water bottles or their equivalent are superior.

Fasting as much as is safely possible in the small animal patient minimizes gastric ingesta obscuring imaging of the liver and gastrointestinal gas accumulation interfering with visualization of other abdominal viscera. Preventing urination immediately prior to the examination permits better evaluation of the urinary bladder.

Serial evaluations can provide useful information when the clinical status of the small animal patient has changed; clinicopathologic deterioration, progressive lethargy or obtundation, acute pain, changes in abdominal palpation findings or refractory vomiting or diarrhea warrant repeat evaluation for ultrasonographic signs indicating intussusception, perforation and/or peritonitis have evolved.

The Normal Abdomen

Regardless of the clinical history, the abdomen should be evaluated methodically with the animal in dorsal decumbency. Place the scanhead under the xyphoid with the beam in sagittal plane. Visualization of the liver is achieved by fanning the beam from right to left. The gall bladder is seen on the right; the left liver lobes are seen ventral and sometimes caudal to the stomach. Turning the beam to transverse allows for visualization of the liver between stomach and gall bladder. This view is good for evaluation of the hepatic border, echogenicity of hepatic parenchyma and portal architecture. The portal vessels have very echogenic walls.

Resuming the sagittal plane, scan to the left of the dog past the stomach to the spleen. The spleen will be visualized ventrally in the near field. Splenic border, parenchyma and shape should be evaluated. Following the spleen transversely down the left body wall, you will see the left kidney.

Once visualization of the kidney is achieved, turn to the sagittal plane and evaluate the renal border, cortical echogenicity and pelvic architecture. Dilatation of the renal pelvis is best seen in the transverse plane. The adrenal is located medial to the cranial pole of the kidney. In sagittal, maintaining strong hand pressure, scan medially to visualize the linear aorta and the renal artery. The left adrenal is located cranial to the left renal artery and caudal to the left cranial mesenteric artery. The left adrenal is visualized as a bi-lobed structure with the phrenicoabdominal vein at its waist.

With a transverse beam back in the middle of the abdomen, scan caudally to a large hypoechoic structure, the urinary bladder. Evaluate bladder wall and lumen contents, and, dorsal to the bladder, the major vessels (caudal vena cava and aorta). Sub lumbar lymph nodes will be seen at the aortic bifurcation into the iliac arteries, adjacent to the bladder wall. Sagittal scanning of the urinary bladder caudally will allow visualization of the urethra (and prostate in the male).

At the edge of the right ribcage at the renal fossa of the liver the right kidney will be found. The right kidney should be evaluated as was the left (renal border, cortical echogenicity and pelvic architecture). By scanning sagittally between the right kidney and the caudal vena cava with a fanning technique, the right adrenal is visualized just lateral to the caudal vena cava. In transverse, find the right kidney, and lateral to the kidney, the duodenum.

At the cranial end of the kidney medial to the duodenum will be the right limb of the pancreas. The right pancreatic limb is identified by visualizing the caudal pancreaticoduodenal vein within the structure. Turning to the sagittal plane, follow the pancreas, scanning medially to the angle of the body and left limb, or sagittally scan the caudal border of the stomach. The pancreatic body is seen caudal to the stomach, cranial to the splenic vein. The left limb is found caudal to the splenic vein and midline to the cranial pole of the left kidney.

Returning to the transverse plane in mid abdomen at the mesenteric root, scan for mesenteric lymph nodes and small bowel wall changes. It may take 2-3 passes to evaluate the entire abdomen scanning in a uniform serpentine fashion.