Thyroid imaging.

Abstract

Four modalities are being used to image the thyroid gland: (1) scintigraphy ("scanning"), employing one of several currently available radiopharmaceuticals, (2) ultrasound (US), (3) computed tomography (CT, "CAT" scan), and (4) magnetic resonance imaging (MRI). The first method, scintigraphy, provides an image of the spatial distribution of thyroid functional attributes, the nature of which are dependent on the interaction between the particular radiopharmaceutical employed and the tissue in question, whereas the latter three modalities provide an image of the spatial distribution of structural attributes such as the varying degrees of echogenicity of the tissues examined or the differential tissue attenuation of an x-ray beam, which permits visualization of the structures. A fifth modality, fluorescent thyroid scanning, in which fluorescence of the iodide within the thyroid gland is induced by low-dose external radiation and which gives an image of iodine distribution, is generally unavailable and only rarely used. For most patients, the combination of careful history, skilled physical examination, tests of thyroid function (and serum thyroglobulin and calcitonin for cancer evaluation), fine needle aspiration biopsy, and scintigraphy provide the most cost-effective means of evaluating the thyroid gland and its diseases. Of the four modalities currently used to image the thyroid gland--scintigraphy, ultrasound, computerized tomography, and magnetic resonance imaging--only scintigraphy has the widest application. It is employed to determine gland size, locate thyroid tissue, evaluate nodules and masses, determine the cause of a painful tender gland, differentiate various forms of goiter, detect differentiated thyroid carcinoma and gland remnants, assess suppressibility or stimulatability of the gland, and identify nonfunctioning cancers. Ultrasonography, computed tomography, and magnetic resonance imaging are not useful in differentiating between benign and malignant nodules, and their sensitivity in detecting impalpable nodules is not clinically useful, because nodules less than 1 to 1.5 cm in diameter are only rarely clinically significant. These modalities have limited utility in the evaluation of the thyroid gland: they are useful in sizing known lesions and for the detection of cervical lymphadenopathy in thyroid cancer cases.