In CT scan of the thyroid, focal and diffuse thyroid abnormalities are commonly encountered. These findings can often lead to a diagnostic dilemma, as the CT reflects nonspecific appearances. Ultrasound (US) examination has a superior spatial resolution and is considered the modality of choice for thyroid evaluation. Nevertheless, CT detects incidental thyroid nodules (ITNs) and plays an important role in the evaluation of thyroid cancer.

This pictorial review covers a wide spectrum of common and uncommon, incidental and non-incidental thyroid findings from CT scans. It will also include the most common incidental thyroid findings. In addition, the role of imaging in the assessment of thyroid carcinoma (before and after treatment) and preoperative thyroid goiter is explored, as well as localization of ectopic and congenital thyroid tissue.

Thyroid ultrasonography is the modality of choice for thyroid evaluation.Originally copied from: Creative Commons Attribution 4.0 International License Yet, focal and diffuse thyroid abnormalities are commonly encountered during the interpretation of computed tomography (CT) exams performed for various clinical purposes. For example, CT often detects incidental thyroid nodules (ITNs). It plays an important role in the evaluation of abnormal structures including thyroid cancer.

Thyroid disorder Thyroid disorders are common and include many entities. They can be symptomatic, asymptomatic, diffuse, focal, neoplastic, or non-neoplastic processes. Neck ultrasound (US), with the prospect of proceeding to fine needle aspiration (FNA), is the first line of investigation; however, other options are available. Thyroid Uptake Scans using Tc-99 m or I-123 are typically reserved for specific clinical scenarios. Cross-sectional imaging including computed tomography (CT) and magnetic resonance imaging (MRI) detect incidental thyroid nodules (ITNs) and can be used in the evaluation of thyroid cancers and goiter. The aim of this article is to provide a pictorial review of a broad spectrum of incidental and non-incidental thyroid findings on CT scans.

CT scans of the thyroid: normal anatomy and imaging techniques The thyroid gland is a vascular, encapsulated structure made up of right and left lobes, which are connected at the midline by the isthmus. Each lobe is about 2 cm thick, 3 cm wide, and 5 cm long. The thyroid apex is located superiorly at the level of the mid-thyroid cartilage. The inferior margin of the gland is at the level of the fifth or sixth tracheal ring. The thyroid gland is encapsulated by the middle layer of deep cervical fascia and is part of the visceral space in the infrahyoid neck. It wraps around the trachea and is separated from the esophagus by the tracheoesophageal groove on each side, which houses the recurrent laryngeal nerves. The thyroid has variable lymphatic drainage to the internal jugular chain, para-tracheal region, mediastinum, and retropharyngeal area. It has homogeneous high attenuation values on a CT scan, as compared to adjacent muscles, due to its high iodine concentration. It shows avid iodine contrast enhancement due to its hypervascularity.

Multi-detector volumetric acquisition from the skull base to the tracheal bifurcation is usually obtained. Multiplanar 2-mm axial, coronal, and sagittal images are typically available. Examination can be acquired with or without administration of intravenous (IV) iodinated contrast.

Incidental thyroid findings on CT scan The thyroid gland can have variable CT scan findings, such as calcifications, single or multiple nodules, cysts, or diffuse enlargement.

Thyroid calcifications on a CT scan can be seen in both benign and malignant thyroid lesions. Sonographic examination of the thyroid can differentiate between micro-calcifications, which are highly associated with papillary thyroid carcinoma, and eggshell calcifications, which favour a benign process such as colloid cysts (Figs. 1 and 2). In a retrospective review of preoperative CT scan, 35% (135 of 383) of the patients had detectable intrathyroidal calcifications. Among them, 48% had a histopathologically proven thyroid cancer. Calcified nodules had a significantly higher incidence of thyroid cancer and lymph node metastases. The incidence of thyroid cancer among nodules with different calcifications patterns was 79% of nodules with multiple punctate calcifications, 58% of nodules with a single punctate calcification, 21% of nodules with coarse calcification, and 22% of nodules with peripheral calcification. Most of the single calcified nodules were malignant. However, this did not include patients with ITNs and the sample is skewed towards malignancy. Another study evaluated the presence of ITNs on CT scans and found that 12% of thyroid nodules were calcified, with no significant correlation between malignant or potentially malignant histology and punctate calcifications. As a result, some researchers believe that calcification per se is not a suspicious CT sign, and have suggested that calcified thyroid nodules on CT scans should be treated the same as non-calcified nodules.

Thyroid cystic changes are variable, ranging from simple cysts with a thin wall to complex cysts with septations and solid components. An adenoma may undergo cystic degeneration. Papillary carcinoma may mimic a benign-looking cyst. Simple serous cysts appear with fluid density on a CT scan, whereas a cyst with haemorrhage or high thyroglobulin content is iso-dense to muscle.

Thyroid nodules that are detected by an imaging study but have not been previously detected or suspected clinically are considered to be ITNs. ITNs are one of the most common incidental findings on neck imaging. ITNs are reported in up to 25% of chest CT scans, and in 16–18 % of cervical region cross-sectional imaging, including CT and MRI scans. The rate of malignancy in the detected ITNs on CT and MRI scans varies from 0% to 11%. Incidentally detected thyroid carcinomas are more likely to be papillary thyroid carcinomas (PTCs) (Fig. 3). Incidentally detected cancers tend to be smaller in size and less likely to have distant metastasis, as compared to clinically suspected thyroid cancers.

On CT scans, a malignant lesion is suspected when the margins are ill-defined and there is extra-thyroid extension, lymph node involvement, or invasion of the surrounding structures. The absence of these features does not exclude malignant tumours, especially papillary, follicular, and medullary thyroid carcinomas (Fig. 3). Therefore, ultrasound is the modality of choice for thyroid lesion evaluation, due to its superior spatial resolution compared to CT examinations. Sonographic features of malignancy are micro-calcifications, acoustic shadowing, anti-parallel orientation, marked hypoechogenicity, irregular or microlobulated margins, and increased vascularity. CT scans lack the ability to detect these reliable sonographic signs of malignancy. Therefore, further management of ITNs, if required, usually begins with thyroid ultrasound and FNA should be considered according to the ultrasound findings.

The American College of Radiology (ACR) flowchart and recommendations for ITNs detected by CT or MRI offer general guidance and are not applicable to all patients. The recommendations are primarily based on the presence or absence of suspicious features, nodule size, patient's age, patient's life expectancy, and patient's comorbidities. Suspicious features that can be detected on CT scans include signs of local invasion and abnormal lymph nodes. Abnormal lymph nodes may show cystic components, calcifications, and/or increased enhancement. Mere nodal enlargement is less specific for thyroid cancer metastasis; however, further evaluation should be considered if the ITN has ipsilateral jugulodigastric lymph nodes > 1.5 cm on the short axis or > 1 cm for other groups. Cervical Level IV and VI lymphadenopathies raise a higher suspicion of thyroid carcinoma metastasis. Almost all patients with ITNs and suspicious imaging features should be evaluated with a neck ultrasound. Patients with comorbidities or limited life expectancy without suspicious features should not undergo further evaluation. Nevertheless, further workup might be necessary for such individuals if it is clinically warranted, or specifically requested by the referring physician or the patient.

In patients with ITNs, it is important to inquire about pertinent historical factors predicting malignancy. These factors include a history of childhood or adolescent head and neck or total body radiation exposure, and familial thyroid carcinoma or thyroid cancer syndrome. Syndromes associated with thyroid cancer include multiple endocrine neoplasia 2, familial adenomatous polyposis, Carney complex, Cowden's disease, and Werner syndrome/progeria. If a patient has a first-degree relative with such a syndrome, screening based on the various components of that syndrome is advised. Nevertheless, there are no guidelines specifically addressing ITNs detected on CT scans in patients at risk of thyroid cancer. Therefore, in the absence of suspicious features on the CT scan, other criteria such as nodule size on the CT scan, patient age, and levels of thyroid stimulating hormone (TSH) are important in guiding management in such a patient population.

Although the correlation between thyroid nodule size and malignancy risk is limited, nodule size affects prognosis in malignant nodules. Small thyroid cancers (less than 2 cm) tend to have an indolent course, with favourable prognosis even if not treated. Less than 7% of the imaging-detected ITNs are seen in younger populations. However, Shetty et al. found a higher rate of malignancy in the ITNs detected on CT scans among patients younger than 35 years. Ito et al. found a higher tumour progression risk among young patients (<40 years) with subclinical, low-risk PTCs who undergo observation rather than surgery. Therefore, nodule size and patient age should determine the need for workup in the general population without suspicious imaging features and with normal life expectancy. Further evaluation with ultrasound is required for patients less than 35 years old with nodules measuring more than 1 cm in the axial plane. The cutoff size for further evaluation is raised to 1.5 cm for patients more than 35 years old. This recommendation should be applied to the largest thyroid nodule in cases of multiple thyroid nodules. Incidentally discovered heterogeneous and enlarged thyroid glands should undergo dedicated ultrasound if the patient has no limited life expectancy or serious comorbidities.

Thyroid cancers Epidemiology Primary thyroid carcinomas include papillary, follicular, medullary, and anaplastic carcinomas. Lymphoma and metastasis of other primary malignancies to the thyroid gland represent a minority of thyroid carcinomas. Differentiated thyroid carcinomas (DTCs) originate from follicular epithelial cells and encompass PTCs and follicular thyroid carcinomas, including the Hurthle cell variant of follicular carcinoma. DTCs have an excellent prognosis and fortunately represent the majority of thyroid carcinomas. PTCs and follicular thyroid carcinomas represent 88% and 8%, respectively, of all thyroid malignancies. Medullary thyroid carcinoma arises from neuroendocrine C-cells and has a good prognosis. Anaplastic carcinoma is an aggressive undifferentiated tumour that usually affects the elderly and tends to have a worse prognosis.

Role of imaging