Ultrasound in Companion Animals

Introduction

The objective of this presentation is to present a very basic introduction and overview of small animal ultrasonography. Ultrasonography is rapidly becoming one of the most readily utilized imaging techniques in small animal medicine. The cost of the equipment has gone down, and the quality and ease of use of the equipment has gone up over the last decade. Where owning an ultrasound was once a specialty item, now, many small animal hospitals have ultrasound machines in their practice.

Ultrasound is a non-invasive modality to image soft tissues. It uses sound waves to generate images that are projected onto a screen which gives real time representation of the anatomy of the patient, in all three dimension, unlike radiography, which only generates a two dimensional image of a three dimensional object.

A radiograph allows the doctor to appreciate size, shape, position, contour and opacity of internal organs. An ultrasound allows the operator to evaluate size, shape, position, contour, echogenicity and composition of the tissue being evaluated. The most common imaging is called B-mode that produces a two dimensional cross sectional image.

Physics of Ultrasound

A piezoelectric crystal housed in a hand held transducer produces sound waves that are propagated through soft tissues of the body. The piezoelectric crystal spends 1% of its action transmitting sound waves, and 99% of the time receiving back the reflected waves. The frequencies produced by the crystals range from 1.0 MHz to greater than 20 MHz. The higher the MHz for the probe the greater the detail of the image (high resolution), but the shallower the depth of field. Conversely, the lower the MHz, the greater the depth of field, but, there is a lesser quality and a lack of detail (poor resolution) compared to the higher frequency transducers.

Examples of tissue types and frequency recommendations:

The sound waves are stopped by bone and gas. The sound wave, as it passes through different tissue densities (called acoustic impedance), is reflected back to the transducer, which then acts as a receiver. The echos are converted to electric impulses and displayed on the ultrasound screen. The more sound that reflects back from the tissue, the brighter the image on the screen.

When a sound wave crosses a tissue interface only a portion of that wave is reflected back to the receiver. The amount reflected depends on the amount of difference in the acoustic impedance between the two tissues. Acoustic impedance refers to the density of the tissue and the speed that the sound wave travels within that tissue. This determines the echogenicity of the selected tissue.

Echogenicity refers to the amount of sound returned (attenuation) to the transducer from an object or a tissue interface. Low echogenicity refers to a minimal amount of returned sound. High echogenicity refers to a maximum amount of returned sound. When referring to the images generated by the ultrasound wave, there are four main categories: Anechoic (no image – screen is black), hypoechoic (less reflection than other tissues), isoechoic (the same reflection as other tissues) and hyperechoic (more reflection than other tissues). These will be described in more detail later.

For B mode ultrasonography the standard convention is to have the screen black and the image various shades of grey.

When comparing ultrasound images to radiographic images, the following applies:

The echogenic scale of tissues on an ultrasonic image range as follows:

Mastering ultrasonography does take time. There is an initial “easy” phase where the operator can utilize an ultrasound and readily generate an image. Since most of what is needed in a clinical practice is generally concentrated around the “big five” structures – Liver, Kidneys, Spleen, Bladder, Intestines – this is a relatively easy task to master. However, there is a steep learning curve that is encountered when more detailed studies are pursued. For instance, when imaging adrenals, pancreas, lymph nodes and other smaller objects.

One of the biggest learning obstacles is understanding the concept of artifacts. To help with this, a few definitions may prove useful:

Reflection – Redirection of the ultrasound beam back to the source. This gives rise to the diagnostic ultrasound images.

Absorption – when the sound wave is absorbed in the target tissue it gets converted to heat, and not reflected back to the receiver.

Scatter – When the ultrasound beam encounters an interface that is irregular or tangential, or smaller than the sound beam, and the sound reflects off in different directions and not necessarily back to the receiver.

Shadow – This is a major source of artifacts. The pulse is unable to reach deep tissues due to highly reflective tissue interfaces, reflecting sound waves back to the receiver, thus casting a black shadow over the tissues beyond the interface. A classic example here is bone or a urinary calculi inside a fluid filled bladder.

Enhancement – This is the opposite of shadowing where the echoes from deeper tissues reflect louder (brighter) than superficial tissues (tissue on the backside of the urinary bladder or ball bladder are two good examples).

Refraction – This is where the sound wave is displaced alongside the walls of a fluid filled structure, and produces a negative artifact (black lines).

Reverberation – Sound waves resonating and reflecting back and forth between two tissue planes multiple times before it finally returns to the receiver. Air/fluid and soft tissue/gas interfaces are common sources of reverberation. On the ultrasound screen this appears as highly echoic parallel lines that recur at regular intervals. This is also seen when trying to image the lungs.

The Ultrasound Machine

The ultrasound machine has two different types of transducers (probes). The convex probe has a smaller curved face (referred to as the footprint) is preferred for smaller contact surfaces and smaller subjects, such as feline/canine abdomens, eyes, and hearts.

The linear probe is best for superficial structures such as cat intestines, tendons, etc.

Probes are regulated by the actual machine to which they are attached. There are multiple settings on the ultrasound machine. Generally, once a study has been started (e.g. abdomen, heart, eye, etc.) and the settings are determined, they don’t need to be changed frequently. A probe can be either a single frequency probe (e.g. 5 MHz) or a variable frequency probe (e.g. 3-5-8 MHz) that has multi-purpose usage.

“Power” refers to the intensity of the sound output that is emitted by the transducer.

“Gain” regulates the amplification of the returning echos, regardless of the depth or origin of the reflection. The higher the gain, the brighter the image. Time-Gain Compensation (TGC) refers to selective areas of amplification of the returning echos.

Uses of Ultrasonography

As mentioned, ultrasonography allows visualization of the soft tissues in real time, in multidimensions. It also allows both distinction between tissue types as well as the ability for virtual objective assessment of internal structures, such as measurements of intestinal wall thickness.

Positioning

Whereas patient positioning is operator preference, most ultrasonograhers prefer to have the patient in dorsal recumbency for abdominal scans. Some prefer to have the patient in either right or left lateral recumbency. Regardless, the patient’s abdomen should be shaved to bare skin from just cranial to the xiphoid caudally to the pubis and laterally to at least the costo-chondral junctions.

For cardiac examinations, most basic studies can be accomplished using a window between the ribs with the patient in RIGHT lateral recumbency, although left lateral and ventral, sub xiphoid approaches are also used.

For the purpose of this lecture, the major structures that general veterinary practitioners should be readily able to evaluate are considered to be the “big five.” That is, the Liver, Kidneys, Spleen, Bladder and Intestines. In addition, we will make a few brief remarks regarding evaluating the heart in dogs and cats.

Abdominal cavity

The ultrasound is ideal for detecting abdominal fluid, soft tissue masses, urinary calculi and more. Ultrasound is ideal for detecting even small amounts of peritoneal fluid. In addition, the presence of free fluid acts as an acoustic window, thus enhancing visualization of other abdominal structures. Where it can be difficult to ascertain distinct abdominal masses using conventional radiography, an ultrasound can readily differentiate between the different acoustic densities between liver, spleen and masses.

The liver is the larges organ in the abdomen, and is generally the place where the ultrasound study should begin. It is evaluated for architecture and contour and general echogenicity. Focal lesions, metastasis, nodular hyperplasia, abscesses, cysts, neoplasia, vascular congestion, shunts, fistulas and cirrhosis can all be evaluated.

With practice ultrasound guided fine needle aspirates and Tru-cut biopsies are easily accomplished with accuracy and safety.

Between the right medial and lateral lobes of the liver can be found the hepatobiliary system – specifically the Gall Bladder and Bile Duct. Cholelithiasis, biliary obstruction, bile duct dilation, sludging and Gall Bladder mucocoeles are all identified. Secondary obstruction of the biliary duct due to pancreatic disease or duodenal pathology, or foreign body presence, is also possible.

The spleen is a dynamic organ and readily moves around in the peritoneal cavity. It sized can be influenced by disease and or certain drug administration. For instance, the phenothiazine tranquilizer acepromazine causes splenomegaly, and generally should not be used prior to abdominal ultrasound evaluations.

Splenic neoplasia is probably the most common finding using the ultrasound. Since approximately 50% of all splenic masses are benign, the finding of splenic disease with ultrasound certainly makes splenectomy a viable next step.

Splenic torsions, infections, infarctions, cysts, hematomas and nodular hyperplasia are all readily imaged with the ultrasound. With practice differentiation between the different pathologies will be more apparent.

Kidneys and Bladder are readily seen with the ultrasounds. Evaluation of the kidneys, measurements of their size, renal blood flow and overall appearance is accomplished with ultrasound. For kidneys, neoplasia (e.g. lymphoma), cysts, abscesses, hydronephrosis, glomerulonephrosis/nephritis, mineral deposition, fibrosis, hypoplasia, dysplasia and toxicosis (e.g. ethylene glycol) can all be determines.

Investigation into the urinary bladder is commonplace, with evaluation of wall thickness, presence of masses, urinary sediment or calculi and number of stones, dilation of ureters, and position of the ureteral openings (an advanced procedure) can all be performed. In addition, using ultrasoundguided cystocentesis allows

In intact males the prostate and testicles can be scanned for presence of pathology such as neoplasia or abscessation.

Ovarian activity, uterine pathology, pyometra can all be assessed.

The intestines are readily moveable, but, the duodenum and large colon are fairly stationary. The ileum and jejunum will move about with positioning and probe pressure, but, once learned, can be readily identified. This is important as it allows intestinal wall thickness to be measured. This is imperative when evaluating intestinal lymphoma or inflammatory bowel disease. In addition, ultrasonography is useful in determining the presence of foreign bodies such as string and plastic, items that may be radiolucent on x-ray studies.

Imaging the pancreas takes some practice, but once learned, evaluation for the presence of cysts, masses, neoplasia or pancreatitis is readily accomplished.

Measurements and evaluations of the abdominal lymph nodes are done with the ultrasound where these structures may not be visualized using standard radiography.

Thorax

While evaluating the thorax is beyond the scope of this lecture, learning how to look for fluid lines in the pleural space is relatively easy to master. In addition, one does not need to be a cardiologist to evaluate cardiac wall thickness to determine the predominant pathology in a cat with cardiac signs. If the ultrasound equipment has the ability, Doppler technology allows visualization of valvular regurgitation such as is seen with myxomatous degeneration of the A-V valves. Also, identifying adult heartworms in the right ventricle and pulmonary artery of a dog is generally not difficult once a person gains some experience.

Summary 

Ultrasound diagnostics is no longer something confined to universities and specialty hospitals. The technology is getting better and the cost of the equipment has been steadily decreasing over the past decade. The machines are user friendly and affordable. The learning curve is initially quick and the diagnostic benefits are great.

References

There are many good ultrasound references available, both in book form and on line. In addition, there are many telemedicine services that offer help with evaluating both radiographs and ultrasound images. Two textbooks that I find useful:

• Mattoon J, Nyland T. Small Animal Diagnostic Ultrasound (3rd edition). Elsevier, St. Louis. 2015.
• Chetboul V, Bussabori C, de Madron E. Clinical Echocardiography of the Dog and Cat. Elsevier, St. Louis. 2015