Head and Neck Imaging

Thyroid gland

1. Anatomy

The thyroid gland has the overall form of a shield; on each side of the proximal trachea a thyroid lobe is present, usually connected by the isthmus overlying the third tracheal cartilage. In more than half of the population, a pyramidal lobe is seen extending superiorly from the isthmus, sometimes reaching the hyoid bone; this pyramidal lobe is on the tract of the (embryological) thyroglossal duct. The thyroid gland appears on all imaging modalities as a homogeneous parenchymatous structure. On CT images it exhibits a higher density than the surrounding structures, owing to its high iodine content. Also, see thyroid gland.

2. Pathology

Congenital abnormalities

These anomalies are associated with failure of the thyroid gland to migrate from its anlage to the definitive gland position low in the neck, or with failure of the thyroglossal duct to obliterate completely after descent of the gland:

  • ectopic thyroid

  • thyroglossal duct cyst

    Inflammation

    Apart from demonstrating liquefaction in suppurative thyroiditis, the role of radiological imaging is rather limited in inflammatory thyroid gland diseases.

  • suppurative thyroiditis: rare bacterial infection of the thyroid gland, caused by direct inoculation (trauma), spread from a nearby infectious focus, or by haematogenic spread. Suppurative thyroiditis may also be due to a fourth branchial anomaly (see branchial apparatus).

  • Hashimotos thyroiditis: chronic autoimmune thyroiditis

  • Riedel's thyroiditis: chronic thyroiditis, characterized by the formation of fibrous tissue in the thyroid gland and surrounding tissues.

  • de Quervains thyroiditis: subacute thyroiditis, presumably of viral origin

    Benign thyroid mass lesions

  • thyroid adenoma: an adenoma may be functioning ('hot' on scintigraphy) or nonfunctioning ('cold' nodule). Hyperthyroidism due to an autonomous hot nodule is known as Plummer's disease. A solitary cold nodule, if not appearing completely cystic on sonography, needs to be aspirated or biopsied to exclude malignancy.

  • colloid cyst thyroid: usually corresponding to cystic degenerated adenomas.

  • goitre: enlarged thyroid gland, commonly caused by nodular hyperplasia.

    Malignant tumours

    Imaging is important in determining the regional spread of thyroid neoplasms. Extracapsular spread of the primary tumour can be shown by CT and MRI, and tracheal invasion, oesophageal invasion and/or laryngeal framework invasion can be demonstrated (Fig.1). The relationship of the tumour to the large neck vessels can be determined. Imaging can show the presence of adenopathies, and is particularly useful for identifying such adenopathies in the tracheoesophageal groove and superior mediastinum.

  • papillary thyroid carcinoma: most common of the malignant thyroid neoplasms, commonly showing lymphatic spread. The overall prognosis is good if it is adequately treated.

  • follicular carcinoma: less frequent neoplasm, spreading less commonly to the lymph nodes and with the potential of haematogeneous metastasis. The prognosis is less good than for papillary carcinoma.

  • medullary carcinoma thyroid: tumour arising from calcitonin-secreting cells.

  • anaplastic carcinoma: a very aggressive tumour, with poor prognosis.

  • non-Hodgkin lymphoma, head and neck manifestation  : patients with Hashimoto's thyroiditis  are predisposed to thyroid gland lymphoma.

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    Fig.1

    Axial gadolinium-enhanced T1-weighted spin-echo images of thyroid gland. a. Diffuse tumoral enlargement of the thyroid gland, growing behind the pharynx and in the larynx (right paraglottic space; arrowhead); the soft tissue mass grows along the thyroid cartilage (arrows). Parajugular lymphadenopathies. b. Lower in the neck, the tumoral mass is seen to invade the trachea through its membranous part (arrowhead; note interruption by the tumour of the enhancing mucosal rim). The proximal oesophagus is displaced (arrow). Non-Hodgkin lymphoma.
    Thyroid gland, Fig.1 (a)
    Thyroid gland, Fig.1 (b)