Page 66 - Atlas of Small Animal CT and MRI

56 Atlas of Small Animal CT and MRI

T2* GRE images accentuated the signal from bone and A contrast‐enhancing tract may help to localize any
provided good image quality for evaluating hyperosto- foreign material or to trace the origin of the wound.
sis. Contrast‐enhanced T1 images with fat saturation
were recommended to reveal tissue enhancement. CT Osteomyelitis
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imaging also demonstrates the increased bone attenua- Penetrating wounds to the head can result in intramuscular
tion in this syndrome (Figure 1.4.2). abscesses in the muscles of the head (Figure 1.4.9,
Figure 2.7.6). Bite wounds and direct trauma can cause bac-
Trauma terial osteomyelitis of the skull to develop. CT features of
osteomyelitis in the skull include soft tissue swelling, multi-
Skull fractures focal bone lysis with poorly defined cortical margins,
Skull fractures due to trauma, such as vehicular trauma regions of sclerosis, and irregular periosteal reaction.
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or fall from a height, are best appreciated on CT images. Sequestra, identified as separate bone fragments in the
Radiographs cause superimposition of complex anat- affected region, can develop in chronic infections (Figure
omy, and the asymmetry of the fractures and skull make 1.4.9). The infection can extend to the meninges or brain if
interpretation difficult. Significantly more maxillofacial the full thickness of the skull is involved, appearing as
injuries were identified on CT images as compared to contrast enhancement on CT and MR images (Figure 2.7.6).
radiographs in cats and dogs. Common regions of
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trauma to the calvarium include the sphenoid and ptery-
goid bones, the frontal bone, and the temporal bone Neoplasia
(Figures 1.4.3, 1.4.4). Fractures of the temporomandibular Osteomas are benign tumors of unknown etiology, com-
joint and maxilla/mandible are discussed in Chapters prised of compact or cancellous bone, that occasionally
1.3 and 1.9. Gas may enter the calvarium as a result of occur in the skull. Periosteal osteomas arise from the
open trauma to the skull and is identified as signal void surface of the bone, while endosteal osteomas develop in
on MR images and hypoattenuating regions on CT the center of the bone. These tumors have been
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images. Associated hemorrhage may be seen in the dural reported in cats and dogs in the region of the skull. These
tissues or brain (see Chapter 2.4). 3D reformations of CT may appear on CT images as primarily compact periph-
images of the skull may be helpful in depicting the spatial eral types, with uniform, hyperattenuating centers and
location of fragments. However, small fractures are often smooth margins; or central cancellous types, with
best seen in the two‐dimensional images. slightly lower attenuation and more irregular margins
with invasion into adjacent bone (Figure 1.4.10). These
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Inflammatory disorders masses may affect the skull, oral cavity, or orbit. 11,12
Osteosarcoma occurs most commonly in the maxilla
Masticatory myositis and mandible in the axial skeleton and also occurs in the
Masticatory myositis is an autoimmune inflammatory bones of the calvarium (Figure 1.4.11). Chondro-
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condition of the masseter, temporal, and pterygoid sarcoma occurs in the flat bones of the skull, most com-
muscles in which autoantibodies are directed against monly in the nasal cavity. Imaging characteristics of
myosin. Affected dogs have pain opening the mouth and primary bone tumors are similar on CT and MR images,
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atrophy of the muscles of mastication. The atrophy can be with expansile irregular new bone production, cortical
seen on both CT and MR images. The affected muscles lysis, and asso ciated soft‐tissue masses with heterogeneous
are hypoattenuating on CT on unenhanced images and contrast enhancement (Figure 1.4.12). Other primary
have diffuse or peripheral enhancement on contrast- bone tumors, such as fibrosarcoma, hemangiosarcoma, as
enhanced images (Figure 1.4.5). Regions of myositis well as metastatic neoplasia, are infrequently encountered.
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appear hyperintense on MR T2 sequences and, similar to Multilobular osteochondrosarcoma occurs in the
CT, are contrast enhancing (Figure 1.4.6). Nonenhancing flat bones of the skull of dogs and occasionally in cats.
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regions represent areas of necrosis (Figure 1.4.7). It is comprised of multiple lobules of bone or cartilage
separated by fibrous septae, which give it a characteristic
Abscess stippled appearance on CT images (Figure 1.4.13). These
Abscesses can occur in the musculature of the head tumors tend to be round and well circumscribed to
secondary to penetrating wounds from the skin, oral cav- irregular in shape. They often expand into the calvarium
ity, and pharynx or secondary to otitis media. Areas of or orbit, causing a significant mass effect. Brain edema
abscessation appear hypoattenuating on CT and hyper- can be seen as T2 hyperintensity, and obstructive
intense on T2 MR images. On both modalities, contrast hydrocephalus may result (Figure 1.4.14). On CT
enhancement tends to be peripheral (Figure 1.4.8). images, the masses are mildly contrast enhancing. MR
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