The initial imaging modality for the thyroid gland is ultrasound (US), performed using a high-resolution linear probe (7–13 MHz). US is useful for evaluating thyroid nodules and cervical nodes, guiding fine-needle aspirate (FNA) biopsies, and performing surveillance for disease progression or recurrence. US can also be used to assess for fetal goiters and to screen for the presence of thyroid nodules in high-risk groups (e.g., history of childhood radiation exposure).

On US, the thyroid parenchyma appears homogeneous with echogenicity greater than the adjacent muscles. The longus colli muscle and trachea can be seen posteriorly to the thyroid gland on sagittal views. On axial views, the common carotid artery and internal jugular vein are present laterally; sternohyoid, sternothyroid, and sternocleidomastoid muscles are present anteriorly (Fig. 12.4).

Ultrasonography findings suspicious for thyroid cancer include a taller-than-wide shape, spiculated or microlobulated margins, marked hypoechogenicity, and micro- and macrocalcifications. A combination of size and suspicious US features is more predictive of malignancy than size alone. When FNA biopsy of a nodule is clinically appropriate, US guidance is recommended for nodules that are nonpalpable, pre- dominantly solid, and/or located posteriorly in the thyroid lobe. US-guided FNA is done using narrow-gauge needles (22G or 25G), with or without local anesthesia, and adequate samples can be obtained in 90–97 % of solid nodules (Fig. 12.5). The American Thyroid Association and Society of Radiologists in Ultrasound recommendations are summarized in Box 12.1.

In the past, radionuclide imaging was used to assess the functionality of all thyroid nodules as a predictor of malignancy (hot nodules are rarely malignant). Given the effectiveness of thyroid US and FNA to clearly delineate malignant nodules, nuclear thyroid scans are currently used to distinguish between various causes of hyperfunctioning thyroid (Graves’ disease, toxic multinodular goiter, autonomous adenoma) and in the workup of ectopic thyroid tissue. Whole body imaging using iodine-123 (¹²³I) or iodine-131 (¹³¹I) is used to monitor post-thyroidectomy patients for recurrence or metastasis and to assess the response to therapy. ¹²³I is the imaging agent of choice for benign thyroid processes due to its short half-life (13 h) and lower energy (159 keV). Because of its longer half-life (8 days) and higher energy (364 keV), ¹³¹I is the radionuclide of choice for treatment of Graves’ disease, toxic multinodular goiter, and well differentiated thyroid cancers (papillary and follicular thyroid cancers). ¹³¹I is also used for posttreatment surveillance of well-differentiated thyroid cancer. Medullary and anaplastic thyroid carcinomas do not take up radioiodine, but can be evaluated by either technetium-99 m (^99mTc) sestamibi imaging or fluorine-18 deoxyglucose (FDG) PET-CT, which are discussed below. ^99mTc pertechnetate is the preferred thyroid imaging agent in children due to its low radiation dose, short half-life (6 h), and high count rate.

A normal thyroid gland shows homogeneous radiotracer uptake with areas of increased uptake in the region of greater gland thickness. The pyramidal lobe can be seen as linear uptake extending superiorly from the isthmus in cases of hyperthyroidism (Fig. 12.6). Most cold nodules represent benign lesions such as cyst, adenomas, or colloid nodules; the incidence of cancer in a cold nodule is 15–20 %; thus, they warrant further assessment with US. If a cold nodule is solid on US, biopsy is recommended. Almost all hot nodules represent benign hyperfunctioning adenomas. Given the limited diagnostic information and the frequent need for follow-up US, iodine uptake imaging is no longer considered the preferred initial evaluation for a thyroid nodule.

PET-CT is performed using ¹⁸F-FDG and is useful in detecting recurrent thyroid cancers in the setting of negative radioiodine scans. The sensitivity of PET-CT is more than 90 % in the setting of elevated thyroglobulin and negative ¹³¹I scan after thyroidectomy. Due to their low ¹⁸F-FDG uptake, PET-CT is limited in its ability to evaluate poorly differentiated and aggressive thyroid tumors, except to identify distant metastasis.

The primary role of cross-sectional imaging (CT or MRI) is for presurgical evaluation for extra thyroidal extension and regional metastasis of thyroid cancers. This includes evaluation for invasion of the trachea, esophagus, larynx, recurrent laryngeal nerve, infrahyoid strap muscles, subcutaneous soft tissue, mediastinal and great vessels, and prevertebral fascia. While US can evaluate for cervical lymphadenopathy, retropharyngeal and mediastinal nodes are difficult to access with US and are better evaluated by CT or MRI. Contrast-enhanced CT is discouraged prior to surgery as it can impact the uptake of ¹³¹I used for postoperative thyroid ablation. When iodinated contrast agents have been used, ¹³¹I treatment is typically withheld for 6 weeks and urine iodine measurements may be necessary to ensure uptake. Thus, non-contrast CT or contrast-enhanced MRI is preferred preoperatively; of the two, MR provides better soft tissue resolution.

Thyroid incidentalomas are nonpalpable thyroid nodules that are detected during imaging procedures ordered for other reasons. Nonpalpable nodules have approximately the same risk of malignancy as palpable nodules. However, ¹⁸F-FDG-avid nodules discovered on PET scans have a higher risk of malignancy relative to those discovered on CT or MRI. The American Thyroid Association (ATA) and Society of Radiologists in Ultrasound (SRUS) guidelines, summarized in Box 12.1, are based on clinical risk factors and ultrasound evaluation. Although there are no uniform guidelines for non-US (CT, MRI, or PET)-detected incidentalomas, a practical approach includes further work based on imaging findings and clinical history. This has been summarized in Box 12.2.