Page 151 - Atlas of Small Animal CT and MRI

1.11

Thyroid and parathyroid

Normal thyroid and parathyroid to adjacent cervical muscle (Figure 1.11.2). Normal
thyroid glands are markedly and uniformly contrast
The normal thyroid gland is composed of two flattened enhancing. 4
ellipsoid lobes that lie adjacent and dorsolateral to the Normal parathyroid glands are not routinely recog-
cranial segment of the cervical trachea. In dogs the right nized on either CT or MR images, although larger‐
lobe is located slightly cranial to the left lobe. Canine diameter glands may appear as focal hyperintensities in
thyroid gland size is variable and may be conjoined by T2‐weighted MR images.
1
an isthmus. Feline thyroid lobes are approximately 2 cm Because high‐resolution ultrasound and scintigraphy
in length by 0.5 cm in maximum width. The thyroid studies are highly effective for evaluation of primary
2
gland is supplied by the cranial and caudal thyroid arter- thyroid and parathyroid disorders, CT and MRI may be
ies and is highly perfused. Parathyroid glands average best employed as adjunct imaging techniques to assess
four in number, although there is individual variability. extent and operability of aggressive thyroid neoplasia or
These glands are ovoid and less than 5 mm in longest for detection of ectopic thyroid and parathyroid masses. 5
diameter. Parathyroid glands are located toward the cra-
nial and caudal poles of each thyroid lobe, with cranial Hypothyroidism
glands tending to be located superficially and caudal
glands embedded in thyroid parenchyma. Approximately half of dogs with functional hypothy-
On transverse CT images, thyroid lobes are usually roidism have lymphocytic thyroiditis, while the majority
easily detected as small oval or triangular paratracheal of the remainder suffer from idiopathic thyroid atrophy.
structures that are denser than surrounding tissues as Although descriptions of CT and MR features of
a result of the presence of iodine within the gland hypothyroidism are lacking in the veterinary literature,
parenchyma (Figure 1.11.1a). On long‐axis reformatted those patients with thyroiditis would be expected to
CT images, thyroid lobes have a characteristic elongated have thyroid enlargement, whereas idiopathic thyroid
ovoid shape (Figure 1.11.1b). Unenhanced attenuation atrophy would result in reduced thyroid size
of canine and feline thyroid tissue is approximately (Figure 1.11.3). Thyroid glands of people with thyroiditis
110 HU and 125 HU, respectively, and glands uniformly have reduced density on CT images as a result of
contrast enhance. 1,3 diminished iodine concentration thought to be the result
On MR images, thyroid glands may have a homoge- of follicular cell destruction. Thyroiditis in people is
6
neous or heterogeneous appearance and are T1 isointense also associated with increased T2 hyperintensity on MR
or mildly hyperintense and T2 hyperintense compared imaging. 6

Atlas of Small Animal CT and MRI, First Edition. Erik R. Wisner and Allison L. Zwingenberger.
© 2015 John Wiley & Sons, Inc. Published 2015 by John Wiley & Sons, Inc.
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