Important Notes on Veterinary Ultrasound Examination of Dogs

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The most common ultrasound examinations of dogs and small animals performed by veterinary ultrasound are focused ultrasound assessment of the abdomen and chest (often called AFAST and TFAST, respectively) and lung ultrasound. Each of these examinations has its own goals, but they are complementary when used together in patients, and it is usually recommended to complete all three examinations in the examination.

Generally, veterinary ultrasound machines with curved probes (slightly convex) are used for ultrasound examinations of small animals such as dogs. The commonly used frequency settings range from 5 MHz for large dogs (> 20 kg) to 7 MHz for small dogs (dogs and cats ≤ 20 kg).

Preparation for examination with veterinary ultrasound machine:

It is usually not necessary to shave the hair during the examination. The fur is separated and the coupling agent is applied to the desired area. However, sometimes the thick hair in the dog causes poor image quality and more details are required. Shaving can improve the image quality.

For ultrasound examination, the patient can be placed in the left or right lateral position, or the sternum can be scanned. This figure shows the left lateral position. The 4 sites to be evaluated include the subxiphoid or diaphragmatic (DH) site (1), the right paralumbar or hepatorenal (HR) site (2), the supra-midline bladder or colic (CC) site (3), and the left paralumbar or splenorenal (SR) site (4). A variation of the technique whereby a “flash” scan (dog in left lateral decubitus in position 4 or dog in right lateral decubitus in position 3) can be substituted for the gravity-dependent view if the goal is to identify free fluid and renal assessment is not critical. At each site, the ultrasound probe is initially placed longitudinally over the underlying organ and fanned out at a 45° angle and moved 2.5 cm in cranial, caudal, left, and right directions.

1. The subxiphoid or hepatorenal (DH) view is obtained by placing the probe at an approximately 45-degree angle just posterior to the xiphoid process (approximately where the ribs first come together ventrally). The caudal end of the probe is directed toward the caudal end of the patient. The subxiphoid view allows assessment of specific target structures, including the liver lobes, gallbladder (hypoechoic), hepatomembranous interface (visualized as a white hyperechoic curved line that separates the abdominal and thoracic cavities), heart and pericardial cavity, and pleural cavity. This transverse membrane is used to assess the patient's fluid status.

Important considerations for dogs with veterinary ultrasound

Patients with negative veterinary ultrasound scans, stable symptoms, or persistent clinical signs often benefit from serial examinations.

A negative veterinary ultrasound scan does not rule out internal injury or pathology. Ultrasound is unlikely to reveal pathology that is located more than a few millimeters within the lung and does not extend to the periphery. Veterinary ultrasound is also a focused examination for specific structures and pathology may be missed in areas that are not assessed within the scanned area.

Patients with panting or shallow breathing may be difficult to assess for signs of gliding if B-lines are not present.

Gliding signs are only visible during the dynamic phases of inspiration and expiration and disappear between breaths (static phase of breathing and during apnea). Single-lung intubation (i.e., left or right mainstem bronchus intubation) will result in a lack of gliding signs in the unintubated lung.

 Movement of your hand, probe, or patient may result in a false-positive gliding sign; keep your hand, patient, and ultrasound probe still while looking for gliding signs. Abdominal scans may produce false-positive results when normal hypoechoic abdominal structures are interpreted as free fluid. Structures commonly mistaken for free fluid include the gallbladder, common bile duct, hepatic veins, caudal vena cava, and occasionally the GI wall and/or GI contents. Using both transverse and longitudinal views helps avoid misinterpretation of normal abdominal structures.