Ultrasonography

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Because of the superficial location of the thyroid gland, high-resolution real-time gray-scale and color Doppler sonography can demonstrate the normal thyroid anatomy and pathologic conditions with remarkable clarity [58]. With increasing availability, this technique has come to play an ever more important role in the diagnostic evaluation of thyroid diseases. High-frequency transducers (7.5-15.0 MHz) provide both deep ultrasound penetration (up to 5 cm) and a high-definition image, with a resolution of 0.7-1.0 mm. It can distinguish solid nodules from cysts and allows accurate estimation of size, shows vascular flow (Doppler), and aids in the accurate placing of needles for diagnostic or therapeutic purposes [59]. It is also an excellent tool for use in the follow-up for estimation of changes in size of a lesion or the entire thyroid gland over time. Finally, it allows in utero investigation of the fetal thyroid [60] and can be helpful in fetal diagnosis of thyroid dysfunction [61]. The major limitations of sonography are the high degree of observer variability [62] and the inability to identify retrotracheal, retroclavicular, or intrathoracic extension of the thyroid [2, 59].

Examination is performed with the patient in the supine position and the neck hyperextended. A small pillow may be placed under the shoulders to provide better exposure of the neck. The thyroid gland must be examined in both transverse and longitudinal planes. The examination should be extended laterally to include the region of the carotid artery and jugular vein to identify enlarged jugular chain lymph nodes, superiorly to visualize submandibular adenopathy, and inferiorly to define any pathologic supraclavicular lymph nodes.

Indications for Thyroid Sonography

It is important to remember that thyroid scintigraphy (imaging providing information on functionality and to some degree anatomy) and sonography (providing information on morphology and anatomy) are complementary imaging modalities. Based on the lack of prospective comparative studies in childhood thyroid disease, indications for each will often be based on local traditions and nuclear medicine and radiology facilities and expertise. The evasion of ionizing radiation and sedation, in addition to a short examination time and wide availability, makes ultrasound an ideal initial examination in children [36]. Sonography will provide valuable diagnostic information in a number of

Table 3. Indications for thyroid US

Aid in the diagnosis of congenital hypothyroidism Differentiate different types of thyrotoxicosis Differentiate thyroid masses Guide biopsy of nodules Aspirate thyroid cysts

Guide interventional procedures (e.g. laser ablation) Identify ectopic thyroid Identify thyroid metastases

Identify recurrence in the follow-up of patients treated for thyroid cancer clinical situations (table 3). In one series, one third of pediatric neck masses were located in the thyroid gland [36]. Sonography, and sonography-guided fine-needle aspiration biopsy, often has substantial impact on the final diagnosis of a thyroid mass (table 4). Sonographic tissue characteristics aid in classifying the lesion as inflammatory, neoplastic, congenital, traumatic, or vascular, and are diagnostic in the majority of cases [63]. In some genetic disorders attention must be drawn to the frequent involvement of the thyroid. For instance, Cowden syndrome, a rare autosomal-dominant disease, is characterized by multiple hamartomas of the skin and often (two-thirds of the patients) coexisting benign thyroid nodules, but also increased risk of nonmedullary thyroid carcinoma [64]. Genetic confirmation of Cowden syndrome warrants regular thyroid US because of the increased risk of thyroid malignancy.

Normal Thyroid Sonography

The thyroid gland is made up of two lobes located along either side of the trachea (seen in the midline of the lower neck as a markedly echogenic area with shadowing), and connected across the midline by the isthmus (fig. 15). The pyramidal lobe can often be visualized in younger patients, but it undergoes progressive atrophy in adulthood and eventually becomes invisible. Generally, the parathyroid glands are not identified.

The size and shape of the thyroid lobes vary widely. In the newborn, the gland is 18-20 mm long, with an anteroposterior diameter of 8-9 mm. By 1 year of age, the mean length is 25 mm and the anteroposterior diameter is 12-15 mm [58].

Sonography is an accurate method for calculating thyroid volume. The most common mathematical method is based on the ellipsoid formula (length X width X thickness X for each lobe) (fig. 16). This method has an estimated mean error of 15% [58] but the accuracy decreases with increasing size, irregularity of the thyroid, and with retroclavicular extension [1]. The most

Table 4. Potential causes of thyroid masses in childhood and adolescence

Acute suppurative thyroiditis Subacute thyroiditis (DeQuervain) Congenital goiter (often diffuse) Diffuse goiter

Nodular goiter (uni- or multinodular): Benign thyroid nodules

• Colloid/hyperplastic nodule

• Follicular adenoma

• Thyroid teratomas

• Lymphocytic thyroiditis

• Thyroglossal duct cyst Malignant thyroid nodules

• Papillary carcinoma

• Follicular carcinoma

• Hürthle-cell carcinoma

• Anaplastic carcinoma (extremely rare in childhood)

• Medullary carcinoma

• Cancer metastatic to the thyroid

Nonthyroid lesions (clinically mistaken for being of thyroid origin)

• Branchial cleft cyst and other epithelial cysts

• Parathyroid adenoma or cyst (rarely palpable)

• Lymph node precise mathematical method is the integration of partial volume estimates obtained at cross-sectional scans of the thyroid gland through evenly spaced sonographic scans [65]. This method has an estimated error of 5-10%. Modern three-dimensional ultrasound technology permits the simultaneous measurement of the three orthogonal planes of each thyroid lobe [66]. Planimetric three-dimensional sonography seems less observer-dependent and is more accurate than conventional sonography with an intraobserver variability of 5% [67].

Goiter prevalence in school-age children is an important indicator of iodine deficiency disorders in a population. The 1994 WHO criteria provides an acceptable estimate of goiter prevalence in areas of severe iodine deficiency, but in areas of mild iodine deficiency sonography-determined thyroid volume is the method of choice [68]. Thyroid volume is correlated with iodine status, age, weight, height, sex and body surface area in non-iodine-deficient areas [69]. Thyroid volumes increase with advancing age with a relative sudden increase between the age of 11 and 12 in girls and between 13 and 14 in boys

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