Small Animal Radiology and Ultrasound: A Diagnostic Atlas and Text

20 Small Animal Radiolo g y and Ultrasono graphy
Fig. 1-14 Longitudinal sonogram of
the cranial abdomen of a cat. The
diaphragm is evident as an
echogenic (bright) curved line
(arrows). The normal liver can be
seen adjacent and caudal to the
diaphragm and a mirror image of
the liver can also be identified deep
and cranial to the diaphragm. There
is a small amount of anechoic peritoneal
fluid (curved arrows) in the
dorsal abdomen between the liver
and stomach.
A mirror image artifact is also a form of reverberation artifact (Fig. 1-14). This is
observed most often when examining the liver and produces an image of the liver on the
thoracic side of the diaphragm. The image is a mirror of that seen on the abdominal side
of the diaphragm and is due to the sound traversing a reflected rather than a direct path.
The resulting increased path length is displayed by the machine as if it traveled in a straight
line, placing it farther away from the transducer.
The ultrasound beam has a finite thickness in the third dimension, and the echogenicities
of structures encountered across that thickness are averaged when displayed.
Consequently, echoes that originate from structures within the center as well as from the
edges of the beam are included in the image. These produce beam-width or slice-thickness
artifacts and can be responsible for adding echoes to an anechoic structure or subtracting
echoes from a hyperechoic structure (Figs. 1-15 and 1-16). This can result in the false presence
of echogenic material within the urinary or gall bladder or the illusion of a mass in
the liver because of catching the edge of the stomach. Electronic noise also may add echoes
to an anechoic structure.
Because of the number of artifacts that occur during an ultrasonographic examination,
when an abnormality is observed it must be evaluated carefully to determine that it
is not an artifact. The ultrasonographic findings must fit the patient’s clinical signs and
should be visible in multiple planes. Lesions detected that are not consistent with those
signs should be suspect. The ultrasonographer must be aware of the lesions that may
accompany the presumptive diagnosis. If an area in which a lesion would be anticipated
(based on the presumptive diagnosis) cannot be evaluated thoroughly because of poor
patient cooperation or interference from overlying gas or bone, the ultrasonographer
should be prepared to repeat the ultrasonographic examination or to suggest alternative
diagnostic techniques or additional patient preparation, anesthesia, or sedation. As with
radiographs, rarely is the ultrasonographic diagnosis specific, and additional studies,
such as aspiration or biopsy, frequently are required for a specific anatomical diagnosis.
Difficulty in examining certain patients adds to the problem in determining a specific
ultrasonographic diagnosis.