Page 152 - Atlas of Small Animal CT and MRI

142 Atlas of Small Animal CT and MRI

Thyroid neoplasia contrast enhancing, and tumor thrombi may be evident in
neoplasms with vascular invasion. 5
Feline functional thyroid nodular hyperplasia On MR images, tumors are generally T1 hyperintense
and adenoma on unenhanced images and of mixed hyperintensity
Functional benign adenomatous neoplasms and hyper- on T2 images. Contrast enhancement on MR images
plastic masses are common in the older cat. Because parallels that seen on CT, with tumor parenchyma
these are usually adequately characterized using other markedly and heterogeneously contrast enhancing. 5
methods, descriptions of the CT and MR appearance of Ectopic thyroid tumors may occasionally be encoun-
these lesions may have little clinical utility. tered in the ventral cervical region (Figure 1.11.10) or
On both imaging modalities, thyroid glands are cranial mediastinum (Figure 1.11.11). Ectopic thyroid
unilaterally or bilaterally enlarged and may include tumors involving the hyoid apparatus are discussed in
discrete mass lesions or diffuse lobar enlargement. Chapter 1.10. CT and MR appearances of ectopic thyroid
Affected thyroid glands may have irregular margins and carcinomas are the same as described for in situ masses.
cystic components that appear hypoattenuating on CT Other ventral cervical masses that can mimic the
images and hypointense and hyperintense on T1 and T2 imaging appearance of thyroid carcinomas include
MR images, respectively (Figure 1.11.4). The thyroid carotid body tumors (Figure 1.11.12), hemangiosarcoma,
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glands of affected cats are moderately to markedly con- undifferentiated carcinoma, granulomatous lymphade-
trast enhancing and may be nonuniform in appearance. nitis, and paraesophageal abscess.

Thyroid adenocarcinoma Parathyroid nodules
Canine thyroid carcinomas are most commonly unilat- Disorders of the parathyroid glands resulting in
eral, are usually poorly encapsulated, and aggressively hypercalcemia are categorized as primary or secondary
invade adjacent tissues and vessels. A tentative diagnosis hyperparathyroidism. Primary hyperparathyroidism is
is often made before CT or MR imaging is performed, usually due to the presence of a solitary autonomously
and these studies are most useful for confirming thyroid functional parathyroid adenoma or carcinoma, while
origin, determining operability, and for specific surgical secondary hyperparathyroidism is caused by hypocalce-
planning. 5 mia that leads to parathyroid hyperplasia of multiple
On both CT and MR imaging, thyroid carcinomas are glands. Both entities lead to parathyroid gland
often large and may displace or invade adjacent cervical enlargement; however, neoplastic glands tend to be
musculature, blood vessels, trachea, larynx, and esopha- solitary and, on average, are larger than hyperplastic
gus (Figures 1.11.5, 1.11.6, 1.11.7, 1.11.8, 1.11.9). Some glands, although there is considerable overlap.
tumors appear to be well encapsulated, but others are Little has been reported on the CT and MR imaging
unconstrained and highly invasive to adjacent tissues. appearance of functional parathyroid nodules and masses
Malignant tumors are typically highly vascular, and the in veterinary or human patients, presumably because
parenchyma often appears heterogeneous and may have they are easily detected and adequately characterized
cystic and mineralized components. Regional lymph using diagnostic ultrasound. CT would be expected to be
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node metastasis is common. an insensitive modality for detecting parathyroid lesions
On CT images, thyroid adenocarcinomas are generally because of the relatively small size of parathyroid nodules
isoattenuating to adjacent ventral cervical musculature on and similarity in density to surrounding thyroid tissue.
unenhanced images, with hypoattenuating and hyperat- On MR images parathyroid nodules may appear as
tenuating regions within the parenchyma corresponding to well delineated T1 hypointense and T2 hyperintense
cavitary lesions or mineralization, respectively, when pre- lesions within or adjacent to thyroid parenchyma
sent. Malignant neoplasms are markedly and nonuniformly (Figure 1.11.13). 8

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