Ultrasonography is the second most commonly used imaging format in veterinary practice. It uses ultrasonic sound waves in the frequency range of 1.5–15 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 animal. 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 the sound beam that 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 anatomic 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 may 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 III standard.
Ultrasonography cannot be used to scan gas-filled or bony tissues. 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. 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.
Sonographic imaging is also limited in regard to the depth of tissue that can be examined. Most scanners will display tissues to a depth of ~24 cm, but the image is often 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 24 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 may 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 the urinary bladder, so if the beam passes through such a fluid media, the maximum scanning depth may be increased at the expense of temporal resolution.
Although ultrasound can be used to evaluate most soft tissues, including muscles, tendons, and ligaments, the heart and abdominal organs still constitute the majority of examinations performed in small animals. In scanning of the abdomen, the abdominal structures should be systematically evaluated. Each sonographer develops his or her own system of completely evaluating the abdomen. Systematic evaluation ensures that all structures are scanned. In the past, organs such as the adrenal glands and pancreas were seen only if diseased and enlarged, but modern ultrasound machines operated by an experienced sonographer produce images of such quality that the normal adrenal glands, pancreas, and lymph nodes are routinely imaged even in large dogs.
Ultrasonography also is widely used to evaluate the soft tissues of the musculoskeletal system. In equids, ultrasound is used to detect and evaluate the presence of tears in the tendons and ligaments of the legs. 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. Of course, ultrasound cannot be used to evaluate the bones themselves, so the two imaging methods are complementary. 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 pathology of those structures is manifest on the image.
Changes in the size and shape of organs, tissues, and structures are evident in most cases, but evaluation of the echo pattern is based on comparison with that of other 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 may have increases or decreases in the echogenicity of its parenchyma.
Diseased organs may be either uniformly altered in echogenicity or exhibit focal or multifocal changes. Focal changes are usually easier to detect than uniform changes. Sonographic 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 anatomic presentation is detected along with changes in echogenicity.
Ultrasound technology has improved the ability of sonographic examinations to detect diseases previously not well characterized by sonographic evaluation. Pancreatitis is a primary example. For many years, the pancreas was not considered an organ that could be evaluated with ultrasound, but evaluating it with ultrasound has become a mainstay of assessing animals with suspected pancreatic disease. However, it does not always agree with clinical pathologic evaluation or physical examination. In some cases, the physical examination and pathologic data will suggest pancreatitis, but it is not detected on sonographic examination. This is probably due to the great difficulty of interrogating the entire pancreas using ultrasound. In other cases, chronic pancreatitis may be indicated by the sonographic examination but poorly characterized by clinical pathologic data because of the chronic status of the disease.
Cushing disease is also frequently difficult to interpret on the basis of sonographic examination or any other imaging modality because of the problems of benign adenomas that have no clinical significance in the adrenal glands and because in true Cushing disease the adrenal glands are being overdriven by a pituitary adenoma and are not themselves structurally abnormal. Other instances of increased usage of ultrasound 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 pathologic diagnosis, and is much safer and diagnostic than blind biopsy. This obviates the need for an open surgical exploration in many cases. 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 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.
Echocardiography is ultrasonic evaluation of the heart. In the past, it was done using the M-mode format of displaying ultrasound information. A narrow beam of sound is projected into the heart, and the echo pattern and strength are displayed onto a persistence screen, with the x-axis of the display representing time (y-axis is depth), similar to the familiar format of an ECG. The pattern and amplitude of movement of the walls of the chambers of the heart and valves can be evaluated, as well as the size of the respective structures along the path of the sound beam. The M-mode format has very high temporal resolution and thus is especially suited to evaluation of rapidly moving structures such as heart valve leaflets. Considerable experience is required to obtain and interpret diagnostic studies. The M-mode examination has been coupled with real-time B-mode studies to improve the accuracy of beam placement and add additional information, such as shape of the chamber.
Ultrasonographic images are also used to acquire quantitative information about cardiac function. Measurement of specified parameters may be made on either the M-mode scan or on the two-dimensional B-mode image. Some advanced systems have the ability to produce a three-dimensional image of heart structures. Mathematical formulas are then applied to determine values for cardiac output, ventricular contractility, ejection fraction, ventricular wall stiffness, and other cardiac functions.
Doppler ultrasound makes use of the familiar phenomenon that sound emitted from a moving object such as a train has a different apparent frequency to someone standing still relative to the moving object. If the object is moving away from the observer, the frequency of the sound is lower; conversely, if the emitter is moving toward the observer, the frequency of the sound is higher. The same is true of diagnostic ultrasound. Echoes from moving RBCs change the frequency of the sound reflected back to the transducer. The amount by which the frequency is shifted is proportional to the velocity of the RBCs; whether it is a positive or negative frequency shift is used to determine blood flow direction. This is used to identify valvular regurgitation (insufficiency), increased flow velocity (as in stenosis), or abnormal movement of the blood in the heart or vessels elsewhere in the body.
Doppler signals may be displayed in two formats. In the first, spectral Doppler, a sound beam is used to evaluate a specific small volume within the vessel of interest. This display resembles the M-mode display, except that the frequency shift, or velocity, is substituted on the y-axis. It also shares high temporal resolution (millisecond) capabilities of the M-mode format. The second way to display Doppler frequency shifts is to select a larger area of the scan on a real time B-mode image, encoding the velocities and direction as a color spectrum. The color (usually red or blue) depicts blood flow direction, and the hue depicts mean flow velocity. This allows evaluation of larger areas, but at the price of lower temporal resolution. For this reason, color-encoded B-mode flow studies are used to guide placement of spectral sample volumes to acquire more accurate and complete information.
Thus, Doppler studies complement and improve the accuracy and specificity of echocardiograms. Quantitative evaluation of spectral Doppler studies also allows the examiner to determine values such as pressure gradients across valves and stenotic areas or resistance to flow of blood entering an organ. In some cases, abnormal blood flow patterns can be detected before obvious anatomic lesions are present.
Doppler evaluation of blood flow is not limited to the heart. It is often used to assess blood flow of vessels in the abdomen and other locations. It is the most specific way to do this and can be useful in detection of arterial or venous thrombosis or malformation.
Ultrasound contrast agents increase the reflectivity of blood and any tissue through which blood flows. Enhancement of blood reflectivity is usually accomplished by injection or formation of transient microscopic bubbles in the plasma. The increase in echogenicity is related to the amount of blood flowing through the tissue. The bubbles are quickly absorbed into the plasma and therefore do not constitute an embolism hazard. The ability to evaluate the vascularity of a tissue provides additional information about the type of lesion present. For instance, granulomas generally have poorer blood flow than normal tissue and do not enhance as much as the surrounding tissue, whereas tumors may enhance more and retain the contrast for a longer time than the surrounding tissue. Contrast agents hold great promise for improving both the sensitivity and specificity of ultrasonographic examinations. However, they are extremely expensive, which precludes their use in all but special instances or funded research.