Page 419 - Atlas of Small Animal CT and MRI

Mediastinum and esophagus 409

most commonly involved. Generalized lymph node which can create pseudo‐filling defects. Intraluminal
enlargement occurs in response to an inflammatory tumors appear as relatively hypoattenuating masses
insult and may progress to abscessation (bacterial or surrounded by hyperattenuating blood (Figures 4.3.10,
fungal) or granuloma formation (fungal) (Figures 4.3.5, 4.3.11). Filling defects can also result from tumor‐asso-
4.3.6, 4.3.7). Lymph nodes are soft‐tissue attenuating on ciated thrombi, which usually cannot be distinguished
unenhanced CT images and are moderately and uni- from tumor invasion.
formly contrast enhancing. Abscessed or granulomatous
nodes may peripherally contrast enhance with variable Lymphoma
or no central enhancement. Lymphoma in the mediastinum may involve the thy-
mus or the mediastinal lymph nodes, the latter often
Mediastinitis resulting in marked nodal enlargement with affected
Mediastinitis may result from contamination through lymph nodes retaining their normal shape. Lymph
6,7
direct penetrating injury, such as occurs with penetrat- nodes are normally soft‐tissue attenuating on unen-
ing esophageal foreign bodies, or from systemic infec- hanced CT images and may have a uniform or mildly
tion. The mediastinum is often widened by the presence heterogeneous pattern of moderate contrast enhance-
of inflammatory fluid and associated lymphadenopathy ment (Figure 4.3.13).
(Figure 4.3.8).
esophagus
Mediastinal neoplasia The entire length of the normal esophagus can be seen

Thymoma and other solid mediastinal on CT images, and detection is made easier with the
neoplasms presence of luminal gas or fluid that defines the esopha-
Thymomas are variable in size but can be quite large, geal lumen and outlines the characteristic rosette pat-
occupying a significant volume in the cranial thorax and tern of the esophageal mucosal folds when viewed in
causing cranial lung lobe displacement and atelectasis as cross‐section.
well as displacement of the heart, mediastinal blood ves- Megasophagus
sels, and the cranial thoracic esophagus and trachea.
Because of the orientation of the ventral recess of the Generalized or regional esophageal distension is easily
cranial mediastinum, which is often positioned to the recognized on CT images, and the appearance depends
left of midline, large thymomas often extend caudally on the extent of dilation and the presence of luminal gas
primarily along the left hemithorax. 1,4,5 Thymomas can or fluid (Figure 4.3.14).
have a cystic center with a thick and internally irregular Esophageal stricture and entrapment
parenchymal margin on CT images, and solid compo-
nents have a moderate to intense heterogeneous pattern Although esophageal strictures can be detected using
of contrast enhancement (Figures 4.3.9, 4.3.10, 4.3.11). CT or MR, these are typically not the preferred imaging
Thymomas can also be associated with development of modalities for diagnosis, unless an associated mass is
megaesophagus in some patients. present. A stricture may not be detected directly on CT
A number of other mediastinal neoplasms have been images, but its presence may be implied by gas or fluid
reported, including thyroid carcinoma, carcinomas of distension cranial to the stricture (Figure 4.3.15).
other origin, sarcomas, and round cell tumors. Imaging
6
features of these neoplasms may be similar to those Esophagitis
described for thymomas but vary depending on cell type Esophagitis would typically not be detected with CT
(Figure 4.3.12). imaging but may occasionally appear as a focal or
An important reason for imaging cranial mediasti- regional thickening of the esophageal wall, perhaps
nal neoplasms is to determine the presence and extent associated with an irregular mucosal margin when gas is
of vascular invasion, which can determine operability present in the lumen (Figure 4.3.17).
and prognosis. With CT imaging, vascular luminal
defects representing local tumor extension are best Paraesophageal abscess
seen on images acquired shortly after contrast admin- Paraesophageal abscesses preferentially involve the cau-
istration while intravascular contrast medium concen- dal thoracic esophagus, intimately involve the esopha-
tration is high. However, if images are acquired too geal wall, and are presumably caused by penetrating
4
quickly, intravascular contrast concentration may be foreign bodies. Paraesophageal abscesses are generally
nonuniform because of inadequate recirculation, well‐delineated, fluid‐filled, spheroid to ellipsoid masses

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