Page 502 - Atlas of Small Animal CT and MRI

492 Atlas of Small Animal CT and MRI

extend into the retroperitoneal fat to cause increased soft‐tissue septations present (Figure 5.1.10). Their often
attenuation and streaks of soft‐tissue attenuation large size may displace neighboring organs or extend into
(Figure 5.1.6). the peritoneal or retroperitoneal space. Large lipomas may
Foreign bodies that penetrate through the skin may become hemorrhagic, with increased soft‐tissue attenua-
cause multilobular, peripherally contrast‐enhancing tion interspersed with fat on CT images (Figure 5.1.11).
masses with fluid‐attenuating centers in the subcutane- Infiltrative lipomas are characterized by dissection within
ous tissues (Figure 5.1.7). They may also penetrate into surrounding musculature (Figure 5.1.10). Lipomas may
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the retroperitoneal space, causing an abscess in this also infiltrate fat planes within the pelvis, causing addi-
region with a connecting draining tract to the skin tional mass effects (Figure 5.1.12).
(Figure 5.1.8). Other types of penetrating foreign bodies, Feline injection‐site sarcomas occur in the abdomi-
such as projectiles, can introduce infection into the nal wall or pelvic soft tissues because of extension of a
peritoneal or retroperitoneal space (Figure 5.1.9). distant tumor or misdirected injection meant for the
Abscesses due to fungal disease occasionally occur in distal extremity. Involvement of the musculature of
the retroperitoneal space. On MR images, they appear as the body wall appears as contrast enhancement of
T2 hyperintense and T1 contrast‐enhancing masses that the tumor with extension to the musculature and sur-
may surround the vasculature or cause vascular rounding tissues. Differentiating postsurgical inflam-
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thrombosis. 4 mation from tumor extension in animals scanned for
Increased attenuation of peritoneal fat can be seen as radiation therapy can be challenging and is not always
“fat stranding” on CT images. The proposed mechanism possible. Fibrosarcoma in dogs is an invasive neoplasm
of fat stranding is edema due to increased vascular perme- that may involve the peritoneal cavity or pelvic canal.
ability and engorgement of the lymphatics. This can Contrast‐enhanced imaging of the lesions is necessary
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appear as a ground‐glass (Figure 5.1.4) or linear, reticu- to accurately define the tumor margins and evaluate for
lated pattern and has been associated with inflammatory local lymph node enlargement. Fibrosarcomas and
lesions, such as abscesses and gastrointestinal perforation, other sarcomas are moderately to intensely contrast
in dogs. Localization of the fat stranding to a particular enhancing with poorly defined margins (Figures 5.1.13,
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organ can help to pinpoint the source of the abnormality. 5.1.14, 5.1.15), and tendrils of neoplastic tissue may
CT is also capable of detecting free gas within the abdo- extend to regional tissues.
men without the superimposition of organs encountered Hemangiosarcoma occasionally occurs as a mass in
with ultrasonography. the pelvis or retroperitoneal space without apparent
primary abdominal involvement. These masses are
irregular and lobular in shape, with heterogeneous
Neoplasia
contents on both CT and MR images. Hemorrhagic
CT of the abdomen has become a mainstay in imaging components of the mass are hypoattenuating on CT
lesions involving complex fascial planes of the abdo- (Figure 5.1.16) and T1 hypointense and T2 hyperintense
men, especially for surgical planning and surgical on MR images (Figure 5.1.17). In both modalities, the
oncology. Surgical margins surrounding inflamma- masses tend to be peripherally contrast enhancing.
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tory or neoplastic lesions can be assessed to determine Various tumor types may occur on the abdominal wall
the depth and width of resection, as well as removal of and pelvis, causing mass effects and imaging characteris-
neighboring structures, such as ribs. Multiplanar tics ranging from solid to cystic (Figure 5.1.18). The
reformatting is especially helpful in visualizing and medial iliac lymph nodes, internal iliac lymph nodes, and
measuring for surgical margins. lumbar lymph nodes should be evaluated for enlargement
Lipomas are frequently encountered in the subcutane- and regions of poor contrast enhancement, indicative of
ous tissues and within the intermuscular fat depositions. metastatic disease from the body wall, peritoneum,
These are rounded, fat‐attenuating masses that may have pelvis, or pelvic limbs (Figures 5.1.18, 5.1.19).

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