Ultrasonography in Animals

Ultrasonography is the probably the second most commonly used imaging modality in veterinary practice. It uses ultrasonic sound waves most commonly in the frequency range of 1.5–18 megahertz (MHz) to create images of body structures based on the pattern of echoes reflected from the tissues and organs being imaged.

Several types of image formats can be displayed. The most familiar one (and the one that creates the actual image of anatomy) is B-mode grayscale scanning.

The sound beam is produced by a transducer placed in contact with and acoustically coupled by means of a transmission gel to the patient. An ultrashort pulse of sound is directed into the animal, after which the transducer switches to the receive mode. Echoes occur as the sound beam changes velocity while passing from a tissue of one density to one of another density, even when the change occurs at nearly microscopic levels.

The greater the change in velocity, the greater the strength of the echo. A small percentage of these echoes are reflected back to the transducer, which then reconverts the energy of the echoes into electrical impulses recorded by the computer in the ultrasound machine. The strength of the echo, the time required for the echo to return after the pulse, and the direction that the sound beam was sent are all recorded. Using information from multiple echoes, the machine creates an image that represents the appearance of the tissues when cut in the same plane on an anatomical specimen.

In modern scanning systems, the sound beam is swept through the body many times per second, producing a dynamic, real-time image that changes as the transducer is moved across the body. This real-time image is easier to interpret and allows the examiner to scan continuously until a satisfactory image is obtained. The image can then be frozen and recorded in a digital format, which also allows for recording of short segments of the real-time scan. As for radiography and all other medical imaging systems, the accepted, legal format for digital ultrasound images is the DICOM standard.

Ultrasonography is most commonly used to evaluate soft tissue structures in the abdomen, musculoskeletal system, and thorax. The sound beam is totally reflected at soft tissue/gas interfaces and absorbed at soft tissue/bone interfaces; gas and bone also “shadow” any other organs beyond them. Despite these limitations, ultrasonography is still a useful tool to evaluate osseous, articular, and periarticular margins, and the pleural surface, particularly in large animals. Bowel gas can inhibit imaging of adjacent abdominal organs, and the heart must be imaged from locations that do not require the sound beam to pass through the lungs.

Ultrasonographic imaging is also limited in regard to the depth of tissue that can be examined. Many scanners are capable of displaying tissues to a depth of approximately 30 cm; however, the image can be quite noisy at that depth. This is because most tissue echoes do not return directly to the transducer but are reflected in some other direction. By a depth of 30 cm, the loss of energy from the sound beam results in echoes so weak the scanner cannot separate the returning echoes from the background electronic noise. In addition, some echoes not directly reflected can return to the transducer by reflecting off a tissue outside the beam path. Such echoes require longer to return to the transducer and are depicted at a spurious location, adding noise to the image.

Low-frequency transducers can scan deeper than high-frequency transducers, but resolution is decreased. There is much less loss of beam intensity in fluid media such as urine in the bladder, so if the beam passes through fluid, the maximum scanning depth can be increased at the expense of temporal resolution.

In equids, ultrasonography is most often used to detect and evaluate the presence of tears in the tendons and ligaments of the legs. Transrectal ultrasonography is also commonplace in large animals for assessment of the reproductive system. Examination of joints and the margins of bones around the joints in both large and small animals is also widely performed and yields information not available from standard radiographic evaluation. In small animals, soft tissue lesions of the ligaments, tendons, joint capsule, and articular cartilage of the shoulder and stifle joints are readily detectable by an experienced examiner. Most joints and muscles can be evaluated by ultrasonography if the operator is familiar with the normal anatomy and the manner in which pathological changes of those structures is manifested on the image.

Changes in the size and shape of organs, tissues, and structures are evident in most cases; however, evaluation of the echo pattern is based on comparison with that of organs and tissues the examiner has scanned in other animals. The person evaluating the scan must have a firm idea, developed from experience and comparison with known normals, of the normal echo pattern for each organ scanned with each transducer. The echo pattern will change between transducers because of changes in axial and transaxial resolution as well as transducer design. Comparison of the echogenicity of several tissues must be made because any organ can have increases or decreases in the echogenicity of its parenchyma.

Diseased organs may be uniformly altered in echogenicity or exhibit focal or multifocal changes. Focal changes are usually easier to detect than uniform changes. Ultrasonographic lesions are sometimes quite characteristic of a given disease process, but more often the changes are nonspecific. Although ultrasonography can be quite sensitive to detection of disease, the changes are not specific for a given disease in most cases unless a characteristic change in anatomical presentation is detected along with changes in echogenicity.

Advancements in ultrasound technology have improved the ability to detect diseases previously not able to be well characterized by ultrasonographic evaluation. Pancreatitis is an example. Historically, the pancreas was not considered an organ that could be evaluated with ultrasonography; however, ultrasonography has become a mainstay of assessing animals with suspected pancreatic disease. Yet, such assessments do not always agree with clinicopathological findings or physical examination. In some cases, the physical examination and clinicopathological data will suggest pancreatitis, but it is not detected on ultrasonographic examination. This is probably due to the difficulty of interrogating the entire pancreas using ultrasonography. In other cases, chronic pancreatitis can be indicated by ultrasonographic examination but poorly characterized by clinicopathological data because of the chronic status of the disease.

Hyperadrenocorticism is also frequently difficult to interpret on the basis of ultrasonographic examination or any other imaging modality because of the problems of benign adenomas that have no clinical importance in the adrenal glands and because in true cases of hyperadrenocorticism, the adrenal glands are being overdriven by a pituitary adenoma and are not themselves structurally abnormal.

Other instances of increased usage of ultrasonography are in the evaluation of other soft tissues, such as muscle and tendon injuries, as well as cartilage injuries in various joints, such as the shoulder and stifle.

Ultrasonography can also be used to direct biopsy instruments to acquire tissue for a specific pathological diagnosis. This method is much safer and more diagnostic than blind biopsy; further, in many cases, it obviates the need for an open surgical exploration. Lesions buried within large organs such as the liver and kidneys that might not be detectable at surgery may be detected and biopsied with ultrasonographic guidance.

Presurgical diagnosis permits more thorough and specific planning of surgical procedures and presurgical treatment of lesions. These procedures can frequently be safely performed with the patient under heavy sedation and analgesia. Ultrasound-guided biopsy and aspiration of lesions can also be performed in large animals without the need for general anesthesia.