Ultrasonography (US) is generally accepted in veterinary medicine as a very valuable non-invasive diagnostic imaging modality. Ultrasonography provides information on the size and shape of organs and has the unique ability to image the internal structure of organs and soft tissues. In addition, motion pattern of the heart and gastrointestinal tract can be evaluated, and Doppler techniques allow mapping of velocity and flow patterns of blood within the heart and vessels.

In order to gain as much information as possible from an ultrasonographic examination, high-quality images and knowledge of the normal anatomy and appearances of the various body tissues are necessary. An important factor in the image quality is the frequency of the sound beam. For large patients usually a 5.0-MHz transducer is used, but, when examining more superficial structures, higher-frequency transducers are recommended, such as 7.5-MHz and 10-MHz transducers. For small- to medium-sized dogs and cats, a 7.5-MHz or 10.0-MHz transducer is preferred. Abdominal US is usually performed in the conscious animal, through the ventral abdominal wall with the patient in dorsal recumbency. The hair is clipped and contact gel is applied for good transducer-skin contact. During the systematic examination of the abdomen, the size, shape, delineation, and interrelations of the abdominal contents are evaluated and the textures of the different parenchymatic organs are compared with each other in both longitudinal and transverse planes. Additional oblique scan planes can be used to obtain further information. If image quality remains poor after optimal adjustment of the ultrasound machine, it is important not to give up too soon. Ultrasound examinations often require a lot of patience and practice. Image quality usually improves during the examination, possibly through better transducer-skin contact, through relaxation of the abdominal muscles of the patient, or through displacement of gas-filled intestinal loops. Changing the position of the patient in left or right lateral recumbency or in upright position may improve the image quality and provide additional information. Furthermore, through different approaches some organs can be imaged more easily; for instance, imaging the right liver lobe, the portal vein, and the right kidney can be more rewarding by scanning via the right abdominal wall in left lateral recumbency. The ability to image internal parenchymal texture enables the detection of focal lesions that involve or displace normal architecture. Diffuse parenchymal abnormalities that may only alter the relative echogenicity of an organ's parenchyma may be subtle and more difficult to detect than focal lesions. Through comparing the echogenicity of the different parenchymal organs with each other, diffuse changes may be recognized. Focal parenchymal lesions involving an organ may be solitary or multiple, the changes in the structure may be solid and homogenous, heterogeneous, and/or cavitated. The more numerous the lesions and the greater their heterogeneity, the easier they can be detected by US. Lesion detection is one in a series of steps employed in obtaining a list of ultrasonographic differential diagnosis. The main disadvantage of US is the lack of tissue specificity, so that US rarely provides enough characteristic information for a specific histopathologic-type tissue diagnosis. Ultrasonography is used to find lesions and to determine the organ or organs involved and to characterize the lesion's size, shape, echogenicity, outer border definition, and interrelations with other organs and soft-tissue structures. Finally, the ultrasonographic findings must be correlated with the patient's anamnesis, clinical signs, findings from physical examination, and clinical laboratory results. It is also very important to realize that a normal ultrasonographic appearance does not preclude disease. So for a specific diagnosis, tissue or fluid samples are often required and US is preeminently suitable to guide biopsies.

An understanding of the physical laws of sound beam propagation in tissue is necessary to interpret ultrasound images correctly and to understand and recognize occurring artifacts. There are two categories of artifacts: (1) Artifacts that affect the image quality and therefore the interpretation through improper use of equipment, machine settings, scanning procedures or patient preparation and (2) artifacts that may be useful, being the result of interactions between ultrasound and matter. These are produced under proper technical conditions and may enhance accurate interpretation.

In the following some frequently occurring artifacts will be explained.

Mirror Artifact

These are produced by rounded, strongly reflective interfaces (diaphragm-lung artifact). Part of the ultrasound beam is reflected back into the liver and the echoes from the liver return to the transducer along the same path, via the diaphragm-lung interface. Because the ultrasound machine assumes that the sound pulse and the reflected echoes travel to and from the transducer in a straight line, the echo time is delayed through multiple reflections and the machine places echoes of more superficial structures at deeper locations along the beam's axis. Mirror artifact of the liver may locate this organ in the thoracic cavity immediately cranial to the diaphragm, simulating lung consolidation or a diaphragmatic hernia.

Acoustic Shadowing Artifact

An acoustic shadowing appears as an anechoic band deep to structures of high attenuation (bone or gas-containing structures). It occurs as a result of nearly complete reflection (gas) or absorption (bone or other mineralized structures) of the sound. With soft tissue-bone interface, approximately two thirds of the sound are reflected and a significant portion absorbed, therefore a complete absence of reverberations and a 'clean,' sharp-edged shadow (uniformly black) is created (for instance urinary calculus, gallstones). With soft tissue-gas interfaces, 99% of the sound are reflected, and multiple reflections or reverberations develop, so the appearing shadow is 'dirty' (inhomogeneous) with hazy indistinct borders. The differentiation in 'clean' and 'dirty' shadowing is not absolute but also depends on size, location relative to the focal zone, transducer frequency, and composition of a calculus.

Edge Shadowing Artifact

Edge shadowing can be seen deep to the edge of curved surfaces of round or oval structures (such as bladder, gallbladder, kidney) as a hypoechoic or anechoic shadow. The sound beam strikes the curved surface and is refracted as its velocity changes with passage from normal through abnormal tissue. The refracted portion of the beam broadens or becomes more diffuse, and beam intensity decreases.

Acoustic Enhancement

A hyperechoic zone, or a relative increase in the reflected sound beam amplitude distal to a structure with low attenuation, for instance occurring distal to the gallbladder, characterizes acoustic enhancement. It may also be seen deep to other weakly attenuating structures such as inflamed or neoplastic lymph nodes. Acoustic enhancement is best seen when imaging fluid-filled structures in the focal zone. This artifact may help differentiate cystic structures from solid, hypoechoic masses.

Section Thickness Artifact

This occurs when part of the ultrasound beam's width (thickness) is outside a cystic structure and can mimic the presence of sediment in the gallbladder of bladder. Echoes displayed in the image result from both the center and edges of the beam. When scanning the curved wall of a round or oval structure, echoes from the wall are displayed in the anechoic fluid. Therefore anechoic fluid may appear to contain particulate material or a cyst may appear to be solid. True sediment shows a horizontal line, whereas the surface of pseudosediment is curved.

Reverberation Artifact

Reverberation of the ultrasound beam is characterized by multiple, evenly spaced, hyperechoic foci deep to a highly reflective interface. Usually it results from gas because high-amplitude reflections striking the transducer reflect back into the body and then create evenly spaced, deeper, hyperechoic foci similar to that of the original reflector.

Comet Tail Artifact

This is a form of reverberation in which a series of tightly spaced and regular arranged bright echoes appear deep to a highly reflective interface. This occurs for instance by metal objects or discrete gas bubbles.

So, when performing abdominal US, not only diffuse or focal alterations in the architecture of organs and soft tissues have to be recognized, but also artifact presence. Both have to be interpreted correctly. Interpretation of the lesion type should be combined with the patient history, clinical signs, physical examination findings, clinical laboratory results, and possibly cytological or histological examination of samples of biopsy tissues or fluids.

References

1.  Penninck D, d'Anjou M-A. Atlas of Small Animal Ultrasonography. Ames, IA: Blackwell Publishing; 2008.

2.  Nyland TG, Mattoon JS. Veterinary Diagnostic Ultrasound. Philadelphia, PA: Saunders; 1995.

3.  Saunders HM. Ultrasonography of abdominal cavitary parenchymal lesions. Vet Clin North Am Small Anim Pract. 1998;28(4):755–775.