14 Structures of the Neck Amenable to Ultrasound Evaluation

Ultrasound (US) is based on the reflection of generated sound waves. There have been tremendous advances in algorithms that transform the captured sound waves into a much more refined image. The depth that the sound waves reach is dependent upon frequency. Lower-frequency sound waves penetrate deeper but typically have poorer resolution. Higher-frequency sound waves do not penetrate as well but have superior resolution. Sound waves are reflected at the boundaries of tissues with different impedance, which is dependent on the density of the tissues and the velocity of the sound within a particular tissue.¹ Transducers both generate the sound waves and capture reflected sound. Transducers today commonly have variable frequency capabilities. A common frequency used to evaluate the thyroid lobes and superficial structures is 14 MHz. Slightly lower frequencies up to 12 MHz are often employed to localize parathyroid adenomas. Lower frequencies, from 7 to 10 MHz, are used to look at deep structures in the neck, up to 4 cm from the surface.¹

To perform a US of the neck, the patient is typically positioned either supine or reclined up to an angle of 30 to 45 degrees. For some patients in a wheelchair or others who cannot lie supine, US can be performed in the near sitting position with the chin tilted back slightly. One common method to perform a US of the neck is to start in the thyroid gland with a frequency of 12 to 14 MHz. The transducer is placed transversely on the neck and the frequency is optimized for thyroid tissue. The transducer is slowly moved vertically to evaluate the thyroid lobe and nodules in transverse and anteroposterior dimensions. The transducer is then rotated 90 degrees to obtain the axial dimensions of structures of interest. Once the thyroid has been completely evaluated, the transducer is again placed in the transverse orientation over the thyroid and moved caudally, paying particular attention to the area around the hyoid to pick up a possible thyroglossal duct cyst. With the transducer still in the transverse position, it is moved one-half to three-fourths of the width of the transducer laterally and then slowly moved inferior down to the clavicle. The transducer is tilted caudally for the sound beam to characterize lymph nodes or other structures beneath or inferior to the clavicle. This process is repeated moving into the contralateral neck to evaluate the presence of lymph nodes, the size and shape of the submandibular and parotid glands, and for the presence of any abnormal masses.

14.2 Thyroid/Parathyroid Gland

The thyroid gland is often a focus in the planning of a neck dissection. US of a normal thyroid gland is shown in Fig. 14.1. A normal thyroid gland is homogeneous with a density slightly brighter than that of the muscle. Thyroid tissue that arises from the isthmus and extends superiorly is known as a pyramidal lobe and may be present up to 30% of the time.

Fig. 14.1 (a) Normal thyroid lobe longitudinal view. (b) Normal thyroid lobe transverse view

There are many pathologic states that conceivably could influence the extent of neck dissection. Hashimoto’s thyroiditis is a benign autoimmune disorder that can trigger an extensive inflammatory response potentially causing adhesions and difficulty during a dissection. The most common clinical presentation is a moderately enlarged gland, often one and a half times normal, that is very firm on physical examination. There are several US appearances of a Hashimoto gland.² The most common has been termed “giraffe skin” because of its mottled appearance with innumerable hypoechoic areas alternating with hyperechoic areas ( Fig. 14.2). These nodularities are typically small ranging from 3 to 5 mm. The hyperechoic areas are sometimes confluent, forming benign, well-defined nodules termed “white knights” ( Fig. 14.3). Hashimoto’s thyroiditis can also manifest with large areas that are diffusely hypoechoic with some fibrous banding ( Fig. 14.4). Often, there is very little normal thyroid tissue present. The fibrous banding often forms pseudonodules, where in the transverse view the hypoechoic area appears exactly as a thyroid nodule; however, when the transducer is rotated to the longitudinal view, the nodule disappears. This form of Hashimoto’s thyroiditis can appear almost identical to thyroid lymphoma ( Fig. 14.5) and any biopsy should always include a sample sent for flow cytometry in order to differentiate between the two possible diagnoses. A Hashimoto gland also may present as a hybrid of fibrous banding and hypoechogenicity, with hypoechoic areas often encompassing one-fourth or more of the thyroid gland with no normal tissue present and then the remainder of the thyroid lobe appearing relatively normal. Finally, the thyroid gland in early Hashimoto’s thyroiditis may have a near-normal appearance. It is not unusual to see benign lymph nodes around the periphery of a Hashimoto gland. These can be mildly enlarged from 1 to 1.5 mm but are benign appearing with an elongated appearance and a good hilum.

Fig. 14.2 Hashimoto’s giraffe skin (moth-eaten) appearance.

There appears to be a correlation between the appearance of a Hashimoto gland and antithyroid peroxidase (thyroperoxidase [TPOAb]) and thyroglobulin antibody levels. Antibody levels are higher in homogeneously, hypoechoic glands and correlate with the degree of lymphocytic infiltration.³ The prevalence of thyroid cancer in a Hashimoto gland is controversial. A large meta-analysis of over 10,000 papillary thyroid carcinoma (PTC) cases demonstrated that Hashimoto’s gland was far more likely associated with PTC than normal thyroid tissue.⁴ A second meta-analysis of over 64,000 patients showed a modest increase in PTC.⁵ However, a recent, well-conducted meta-analysis compared studies categorized into a fine-needle aspiration biopsy (FNAB) group and an archival thyroidectomy group. The prevalence of PTC in the FNAB group was 1.2%, whereas in the archival thyroidectomy group it was 27%, strongly suggesting no real association.⁶

It is important to ascertain whether or not thyroid cancer is present. The characteristics of malignant or suspicious nodules that would trigger an FNAB will be discussed here, but the details of the workup of indeterminate nodules, etc., are beyond the scope of this publication. Four patterns of thyroid nodules are commonly associated with benign results⁷: (1) simple cysts with or without colloid clot that do not contain substantial solid element; (2) hyperechoic nodules or white knights ( Fig. 14.3) in a Hashimoto gland; (3) giraffe pattern ( Fig. 14.2) also in a Hashimoto gland; and (4) nodules resembling a wet sponge or “spongiform” nodules ( Fig. 14.6). However, the morphology of these benign lesions should be carefully standardized. As shown in Fig. 14.7, a nodule that closely resembled a spongiform nodule was found to be PTC on biopsy.

There are several US characteristics that suggest malignancy, with microcalcifications less than 1 mm having the highest specificity ( Fig. 14.8).⁸ Intact eggshell calcifications are most often benign, but can be malignant depending upon the continuity of the calcifications. Incomplete eggshell calcifications and those with protruding elements are often malignant.⁹ However, inspissated colloid can form bright objects that strongly resemble microcalcifications. The two can often be discriminated by shadowing often seen with microcalcifications that is rarely present with inspissated colloid.

Fig. 14.3 Hashimoto’s benign hyperechoic nodules (white knights).

Fig. 14.4 Hashimoto’s large hypoechoic areas.

Fig. 14.5 Thyroid lymphoma.

Fig. 14.6 Spongiform nodule.

Fig. 14.7 Spongiform-appearing nodule positive for papillary thyroid carcinoma.

Fig. 14.8 Primary papillary thyroid carcinoma with microcalcifications.

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