Neck Ultrasonography

1

What is the basic normal anatomy and sonographic imaging appearance of the thyroid gland?

The thyroid gland is a butterfly-shaped organ located in the anterior neck, just below the thyroid cartilage and anterior to the trachea. It extends posteriorly to the esophagus and laterally to the carotid sheath, and consists of two lateral conically shaped lobes and the isthmus, a narrow band of midline tissue that connects the lateral lobes. The thyroid gland also has a pyramidal lobe, a superior extension of thyroidal tissue of variable length which is a vestigial remnant of the thyroglossal tract that arises from the isthmus and ascends toward the hyoid bone ( Figure 61-1 ). The normal thyroid gland has relatively homogeneous, bright parenchymal echogenicity with a smooth overlying capsule ( Figure 61-2 ). The dimensions of the thyroid gland vary with body habitus, but the general values for an adult are 15 to 20 mm in anteroposterior diameter, 20 to 30 mm in lateral width, and 40 to 60 mm in craniocaudal length, and the anteroposterior diameter of the isthmus is usually 6 mm. On ultrasonography (US) examination, the pyramidal lobe is quite small, but when hypertrophied, for example in the setting of Graves' disease, it can be imaged and may be discontinuous from the isthmus.

2

What is the basic normal anatomy and sonographic imaging appearance of the parathyroid glands?

The parathyroid glands are typically four in number and occur as a superior pair and an inferior pair. Supernumerary glands are seen approximately in 3% to 13% of patients, and fewer than four glands can occur. The superior glands are derived from the fourth branchial pouch along with the lateral lobes of the thyroid. The superior glands tend to be more consistent in location, with most (>90%) located on the posterior surface of the thyroid, toward the upper pole ( Figure 61-3 ). Infrequently, they may be located above the lateral lobe of the thyroid or rarely in the retropharyngeal (1%) or retroesophageal (1%) space or in the thyroid gland itself (0.2%). The inferior glands arise from the third branchial pouch along with the thymus gland and their location is more variable. Sixty percent can be found posterior to the thyroid gland at the lower pole, and 25% are located inferior to the thyroid gland along a migratory path that extends into the superior mediastinum including within the thymus. The inferior glands may fail to descend and may be located above the superior glands. Ectopic parathyroid glands are not unusual and may be found within the carotid sheath, within the aorticopulmonary window, posterior to the carina, within the pericardium, or within the posterior triangle of the neck. Most parathyroid glands individually measure 3 mm in anteroposterior diameter, 1 mm in lateral width, and 5 mm in craniocaudal length, and due to their small size are infrequently identified on neck US.

3

What is the basic normal anatomy and US imaging appearance of cervical lymph nodes?

The American Joint Committee on Cancer (AJCC) classification system of nodal location is recommended when reporting nodal location on imaging exams because it more accurately corresponds with surgical and pathology reporting ( Figure 61-4 ). This classification system divides the neck into three compartments, the central compartment and the paired lateral compartments, each having multiple nodal levels. The distinction of the lateral and central compartments is very important for surgical planning, because abnormal nodes in the lateral neck cannot be accessed from the central compartment and vice versa. The lateral compartment nodes (levels I through V) are found in the submental region (level I), along the jugulocarotid vascular bundle (levels II through IV) and posterior to the sternocleidomastoid muscle (V). The jugulocarotid or anterior cervical nodes are further classified as follows: level II lymph nodes are located above the level of the hyoid bone to the base of the skull; level III nodes are between the levels of the hyoid bone and the cricoid cartilage; and level IV nodes are below the level of the cricoid cartilage extending to the clavicle. Level V nodes are in the posterior neck, posterior to the lateral border of the sternocleidomastoid muscle. The central compartment contains the thyroid and parathyroid glands, is bordered laterally by the carotid sheaths, and extends superiorly to the hyoid bone and inferiorly into the upper mediastinum. The central compartment level VI nodes are located anterior to the thyroid gland (prelaryngeal nodes), posterior and inferior to the thyroid gland, and adjacent to the trachea (paratracheal nodes). Central compartment nodes that are below the level of the suprasternal notch are level VII nodes.

Normal lymph node morphology is characterized by a hypoechoic outer cortex with densely packed lymphocytes and a central hyperechoic hilum containing lymphatic sinuses and vessels. The size of normal lymph nodes may vary depending on the neck region, with submandibular or level II lymph nodes tending to be larger, perhaps due to reactive hyperplasia from repeated oral cavity inflammation, with a long axis up to 2 cm. The short axis diameter varies less and is typically less than 8 mm in level II and less than 5 mm in the other cervical regions ( Figure 61-5 ).

4

What are the indications for US of the thyroid gland, parathyroid glands, and neck lymph nodes?

US of the neck is commonly used to both characterize and localize palpable neck lesions, and to guide fine needle aspiration (FNA). Specifically, US of the thyroid gland can be used to (1) determine the echotexture of the gland and assess for the presence of a nodule, (2) assess nodule characteristics to determine the need for FNA, (3) follow up thyroid nodule size, (4) screen for occult thyroid cancer in high risk patients, and (5) guide FNA of thyroid nodules. Sonographic evaluation of cervical lymph nodes is commonly performed to detect metastatic disease in patients with a history of malignancy, and to examine palpable nodes for malignant features and guide FNA of abnormal-appearing nodes.

Primary hyperparathyroidism is the most frequent indication for performing US of the parathyroid glands. This condition leads to accelerated bone destruction, renal stone formation, and elevated serum calcium levels and is most commonly diagnosed in the fifth through seventh decades of life. Most cases of primary hyperparathyroidism are caused by a single parathyroid adenoma (89%). Other causes include hyperplasia of all four glands (6%), double adenomas (4%), and, rarely, parathyroid carcinoma. In most instances, parathyroid adenomas are sporadic. There is an increased incidence of parathyroid hyperplasia in patients with multiple endocrine neoplasia (MEN) syndromes.

5

What is a thyroid goiter?

Thyroid goiter refers to an enlarged thyroid gland of any cause and may occur in the setting of autoimmune thyroid disease or as a result of one or more focal lesions in the gland. The thyroid gland may also be enlarged due to nonthyroid conditions such as during pregnancy or in the setting of chronic renal disease.