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Fig. 29. Medullary carcinoma appearing as a heterogeneous and enlarged left thyroid lobe with ipsilateral lymph node metastasis lateral to the jugular vein. The trachea is displaced slightly to the right.

The complete extent of larger lesions is - in some cases - better evaluated with CT (or MRI).

Thyroid nodules often appear as low density lesions but CT cannot differentiate benign nodules from papillary and follicular carcinomas. As with US, calcifications are easily detected and invasive growth into surrounding structures, as well as lymph node metastases (neck and mediastinum), can be revealed by CT (fig. 29) [138].

Magnetic Resonance Imaging

MRI offers excellent anatomic resolution and generation of images in multiple planes. Conventional T1- and T2-weighted imaging is highly sensitive but just as nonspecific as US and CT in differentiating benign thyroid nodules from carcinomas. Sensitivity does not increase with additional gadolinium-enhancement but primary thyroid lymphoma enhances less than other solid thyroid tumors [147]. MRI can distinguish solid from cystic nodules (fig. 30) [132]. Like CT, it provides highly accurate estimates of thyroid volume with a low observer variability and is useful, especially in irregularly enlarged goiters [148]. As CT, and in contrast to US, it can identify thyroid tissue in the retrotracheal and intrathoracic regions (fig. 31). The obvious limitations of MRI are its cost, limited

Fig. 30. Axial MR examination with T2- (on the left side) and T1-weighted (on the right side) scans of a cystic-solid thyroid nodule in the right thyroid lobe. In the picture on the left side a hypointense solid component (arrow) can be seen in comparison with the relatively hyperintense fluid. In the T1-weighted picture on the right side, the lesion can not be recognized in the hypointense fluid.

Fig. 30. Axial MR examination with T2- (on the left side) and T1-weighted (on the right side) scans of a cystic-solid thyroid nodule in the right thyroid lobe. In the picture on the left side a hypointense solid component (arrow) can be seen in comparison with the relatively hyperintense fluid. In the T1-weighted picture on the right side, the lesion can not be recognized in the hypointense fluid.

Left Substernal Thyroid Goiter

Fig. 31. Coronal T1-weighted MRI of a large multinodular goiter shows compression of trachea (white arrow) and left-sided substernal extension (black arrows).

availability, length of the procedure, need for preparation and patient cooperation - the examination cannot be carried through in 5-10% of adult patients due to claustrophobia - and anesthesia is required in early childhood [149]. Tissue movement decreases image quality, and calcifications are better seen with CT [150].

Indications for Thyroid MRI

MRI is rarely required to define anatomy and parenchyma of the thyroid gland itself, but is more useful in defining the exact extension of very large thyroid glands and large masses caused by lymphadenopathy, which may be difficult to achieve with US alone. Metastatic lymph nodes in the neck as well as invasion of the aerodigestive tract are also in the realm of MR imaging [142]. In this context, the extent of thyroid carcinoma can be determined preoperatively, which may be useful in planning surgery. Another potential implication of MRI is for the detection of the site of recurrent carcinoma in thyroglobulin-positive patients with normal clinical examinations. Features such as asymmetry, increased signal intensity in the thyroid bed, and invasion or displacement of adjacent tissue, as well as enlarged lymph nodes with increased signal intensity suggest recurrent carcinoma [151]. Additional gadolinium injection may be useful because enhancement is seen in recurrent carcinoma and also in meta-static nodes [152].

Acute suppurative thyroiditis and thyroid abscess are rare disorders and congenital pyriform fistula should be suspected, especially in case of recurrent infections on the left side. MRI or CT is valuable in addition to barium esopha-gography in the workup of such patients [153].

Normal Thyroid

On T1-weighted images the normal thyroid gland has a nearly homogeneous signal with an intensity similar to that of the adjacent neck muscles (fig. 32) [154]. Air, blood, and vessels usually appear black. On T2-weighted images, the normal thyroid gland has a greater signal intensity than the adjacent muscles. Blood vessels, lymph nodes, fat, and muscle are clearly identified and distinguished from the thyroid.

Developmental Defects

Ectopic thyroid tissue may be encountered in the tongue (foramen cecum), along the midline between the posterior tongue and the isthmus of the thyroid gland, but also in the oral cavity, lateral neck and mediastinum. Scintigraphy is the first-line imaging modality. MRI, however, is also useful as demonstrated in a small series of 21 patients with submucosal lesions in the base of the tongue [155]. MRI depicted lingual thyroid and additional ectopic thyroid tissue in the floor of

Lingual Thyroid Vallecula Mri
Fig. 32. Axial MRI of the neck showing a T1-weighted image with a normal thyroid gland appearing homogeneous and with signal intensity similar to that of the adjacent neck muscles.

the mouth and lateral neck in concordance with the scintigraphic findings. Ectopic thyroid glands appear isointense or hyperintense relative to muscle tissue on Trweighted images and show slight to moderate contrast enhancement, and the T2 signal appears low to intermediate. In the same study all ectopic thyroid tissue had well-defined margins on MRI and in case of ill-defined margins malignancy with invasion of adjacent structures was confirmed surgically [155]. Although rare, goiter and malignant tumors may develop in ectopic thyroid tissue [142].

Diffuse Thyroid Disease

In Graves' disease both T1- and T2-weighted images show a diffusely increased but slightly heterogeneous signal [156]. Dilated vessels within the thyroid can often be identified [157]. In autoimmune thyroiditis the thyroid appears heterogeneous on T1-weighted images and often with diffusely increased signal on T2-weighted images [157]. A morphological overlap on T1- and T2-weighted images is seen between patients with Graves' disease, subacute thyroiditis and Hashimoto's thyroiditis, but additional calculation of the diffusion coefficient can distinguish Graves' (highest values) from the other two [158]. In subacute thyroiditis T1-weighted images demonstrate regions of abnormality with irregular margins and slightly high intensity while on T2-weighted images, markedly increased intensity can be seen in the same sites [159].

Infiltration of adjacent neck structures and hypointensity on T1- and T2-weighted images are suggestive of Riedel's thyroiditis [160].

Nodular Thyroid Disease

Multinodular goiters have various degrees of heterogeneity and low to increased signal intensity on T1-weighted images [154]. Focal hemorrhage and areas of cystic degeneration, often seen in multinodular goiters, are characterized by high signal intensity [157]. Nodules are better visualized on T2-weighted images [157] and simple cysts show a homogeneous high-intensity signal (increases with increasing protein and lipid content) on both T1- and T2-weighted images. The MR characteristics of hyper- or hypofunctioning nodules do not differ. Hyperplastic-colloid nodules and benign adenomas appear round or oval with a heterogeneous signal equal to or greater than that of normal thyroid tissue [156].

No MRI characteristics accurately distinguish between benign nodules and carcinomas, although a nodule with a smoother, more uniform, and thicker capsule is more likely to be benign [161]. Thyroid carcinomas appear as focal or multifocal lesions of variable size, and iso- or slightly hyperintense on T1-weighted images and hyperintense on T2-weighted images. MRI is valuable to assess extracapsular spread, especially into the trachea, larynx, esophagus, vessels, and muscles [162].

The complete extent of larger lesions is - most often - better evaluated with MRI or CT than with US.

On MRI, metastatic lymph nodes can have low to intermediate T1- and high T2-weighted signal intensities or high T1- and T2-weighted signal intensities, the latter reflecting primarily a high thyroglobulin content. The metastatic nodes in papillary carcinoma may enhance markedly (hypervascular) [142].

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