الفهرس 
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Abstract
Thyroid imaging approach is evaluation. Lesions that are smaller than 2 cm should be assessed with US, which is capable of discriminating masses as small as 2 mm and distinguishing solid from cystic nodules. US-guided FNAB pro¬vides tissue for cytological examination of thy¬roid nodules. CT and MR imaging are indi¬cated for larger tumors (greater than 3 cm diameter) that extend outside the gland to adjoining structures, including the mediasti¬num, and retropharyngeal region.
based on the preliminary clinical
Metastatic lymph nodes in the neck and invasion of the aero digestive tract are also in the realm of CT and MR imaging. Thyroid nodules are categorized on scintigraphy as hot or cold nodules. Hot nodules are rarely malignant, whereas cold nodules have an in¬cidence of 10% to 20% of malignancy.
Calcifications (amorphous, globular, nodu¬lar, and linear) occur in adenomas and carci¬nomas and have no differential diagnostic features except for psammomatous calcifica¬tions, which are a path gnomonic finding in papillary carcinomas and a small percentage of medullary carcinomas.
Papillary carcinoma is the most common malignant tumor (80%) followed by follicular (20% to 25%); medullary (5%); undifferentiated; Ana plastic carcinomas (<5%); lymphoma (5%); and metastases. Lymph node metastases are common in papillary carci¬noma, 50% at presentation, and less common in follicular carcinomas. The metastatic nodes in papillary carcinoma may enhance mark¬edly (hyper vascular); show increased signal intensity on Tl-weighted images (increased thyroglobulin content or hemorrhage); and reveal punctate calcifications.
Localized invasion of the larynx, trachea, and esophagus occurs predominantly in pap¬illary and follicular carcinomas; the incidence is less than 5%. Ectopic thyroid tissue may be encountered in the tongue (foramen cecum); along the midline between posterior tongue and isthmus of thyroid gland; lateral neck; mediastinum; and oral cavity. Goiter and ma¬lignant tumors, notably papillary carcinoma, may develop in ectopic thyroid tissue. Carci¬nomas may also arise in thyroglossal duct cysts, which develop from duct remnants be¬tween the foramen cecum and thyroid isth¬mus. Infectious disease of the thyroid gland is not common and the CT and MR imaging findings are similar as described under neck infection. Other types of inflammatory disor¬ders including Hashimoto’s thyroiditis, gran-ulomatous thyroiditis, and Riedel’s struma display no specific imaging features. Imaging studies may, however, be indicated to confirm a suspected clinical diagnosis and assess com¬promise of the airway (Riedel’s struma).
HPT is a clinical diagnosis in which hyper-calcemia is the most important finding. Para-thyroid hyperplasia, adenoma, and carcinoma represent underlying lesions. To relieve the patient’s symptoms surgical extirpation is in¬dicated. The surgical success rate without im¬aging is 95%. The indications for imaging studies vary but it is generally agreed that reoperation after a previous failed surgical attempt and suspicion of an ectopic parathy¬roid adenoma should be investigated by im¬aging. These consist of US, nuclear medicine studies, CT and MR imaging. US and technetium sestamibi scanning have the highest ac¬curacy rate for localizing an adenomatous gland at and near the thyroid gland. Ectopic adenomas, particularly if they are located in the mediastinum, are preferably investigated with CT and MR imaging with gadolinium and fat suppression. Carcinomas and para¬thyroid cysts are optimally evaluated by CT and MR imaging. On MR imaging adenomas are low in signal intensity on Tl-weighted images, high in signal intensity on T2-weighted images, and enhance post introduc¬tion of gadolinium.