Small Animal Radiology and Ultrasound: A Diagnostic Atlas and Text

metal. Soft tissues or organs surrounded by fluid cannot be defined, because their tissue
density is the same as that of the fluid surrounding them. The internal architecture of
organs is similarly obscured due to the aggregation of similarly dense tissues inside a given
organ. Patient manipulation, as well as the addition of appropriately administered positive
(more opaque than soft tissue) or negative contrast (less opaque than soft tissue) media
may facilitate the radiographic assessment. Be sure appropriate indications and contraindications
for these media are understood. 16-19
Ultrasonography frequently is performed in addition to radiography (e.g., for evaluation
of the internal structure of an abdominal mass) but may be performed independent
of the radiograph (e.g., for confirmation of a pregnancy). Ultrasonography is superior to
radiography in some circumstances but has limitations in other areas. Ultrasonography
provides information about size, shape, and location of structures; however, it also provides
information about the soft-tissue architecture of the structure or organ being examined.
Ultrasonography is best for distinguishing solid from cavitating (fluid-filled)
structures and provides internal detail not demonstrated radiographically. When identified,
an abdominal mass should be evaluated to determine (1) its internal architecture, particularly
whether the mass is solid or cavitating, (2) the organ from which the mass arises,
(3) the extent of that organ infiltrated, (4) the degree of local spread (e.g., to peritoneal surfaces
or adjacent organs), and (5) the presence of abnormalities compatible with metastases
in distant organs (e.g., liver metastases from a splenic mass). Ultrasonography is
ideally suited for evaluation of animals with pleural or peritoneal fluid. In those patients
with pleural fluid, mediastinal masses, or cardiac disease, ultrasonography provides information
that may not be evident on the radiograph. Pulmonary lesions usually are not
accessible for ultrasonographic examination unless the area of lung involved is against the
thoracic wall or surrounded by soft tissue–equivalent lung infiltrate that permits sound
penetration (remember, air blocks ultrasonographic transmission into tissue). Although
ultrasonography is not as useful for broad examination of the axial, appendicular skeleton
or the skull as are survey radiographs, some information may be obtained from ultrasonographic
evaluation of muscles, tendons, and the joints, as well as examination of the orbit
and brain (in animals with open fontanels).
Ultrasonography is also extremely valuable for guiding fine-needle aspirates or biopsies.
20-26 The needle can be observed as it passes through or into a lesion, and samples can
be obtained from specific sites within an organ or mass (Fig. 1-3). When the needle cannot
be identified, movement of the organ or lesion as the needle is moved can be used as indirect
evidence of the needle’s position. The individual performing the biopsy must be careful
to ensure that the tip of the needle is identified. Short-stroke manipulation of the needle
parallel to its path can facilitate identification of the shaft and the tip. Remember that most
real-time images are two-dimensional representations of a three-dimensional slice of tissue
that is from 4 to 15 mm thick, depending on transducer frequency and focusing.
Therefore, if the tip is out of the scan plane (slice thickness), it cannot be seen. Some biopsy
needles have been altered to increase the intensity of the echoes reflecting from them.
Biopsy guides are helpful because they keep the needle in the scanned plane and often project
the path of the needle within the video image. Biopsies may also be performed without
these guides but these require greater operator skills. Because most current ultrasonographic
transducers are focused for optimal tissue visualization within a specific depth
range, surface stand-off and off-set materials may be useful in improving tissue and needle
visualization in “long-focused” transducers. 27 Surface stand-off and off-set materials may
also aid in the assessment of structures at or near the skin surface when sector scanners are
used (i.e., allow the sector sound beam to diverge within the off-set, giving a broader view
of surface and superficial structures). Aspiration of fluid from cysts, abscesses, or
hematomas can be performed to obtain samples for cytology or to reduce the size of the
fluid-filled cavity. Ultrasonographically guided biopsy is safe when performed on patients
with appropriate coagulation parameters. 24,25 Major complications may include hemorrhage,
bile peritonitis, spread of infection, and possibly even spread of previously localized
neoplasia. Sonographically guided fine-needle aspirations are relatively safe even on a
patient with compromised coagulation status, but their use requires a risk-benefit assessment.
In addition to sonographically guided biopsies and needle aspirations, guided
Chapter One Introduction 5