The thyroid and parathyroid glands

axial plane, but w i th MRI sagittal and coronal imaging is also possible.

The cartilages are of low density on CT unless calcified, which occurs increasingly w i th age. They are of high signal intensity on MRI as they contain fatty marrow. The mucosa of the subglottic larynx and the anterior commissure should not be thicker than 1 mm on MR images. The true cords (ligament) are of low signal intensity and the false cords (fat containing) are of high signal intensity.

THE THYROID AND PARATHYROID GLANDS

The thyroid gland (Figs 1. 42-1. 44) The thyroid gland is derived from the first and second pharyngeal pouches. It consists of two lateral lobes joined by a midline isthmus, and lies anterior and lateral to the trachea. The lobes are approximately 4 cm in height and extend from the thyroid cartilage of the trachea superiorly to the sixth tracheal ring inferiorly They are described as having upper and lower poles. The lobes are often asymmetrical, w i th the right being larger and more vascular than the left. The isthmus is draped across the second to fourth tracheal rings at the level of C6. The gland is invested in the pretracheal fascia. This fascial layer also invests the larynx and trachea, and the pharynx and oesophagus. The deep surface of the gland lies on these structures. Posterolaterally are the neck vessels, invested in their own fascia, the carotid sheath. Behind these, on either side, are the prevertebral muscles and their fasciae.

Anterior to the gland are the strap muscles of the neck and the sternomastoid muscles invested in an outer layer of fascia. Superficially, the anterior jugular vein runs in the midline, and the external jugular vein runs inferiorly on either side. The parathyroid glands lie close to the deep surface of the gland and may be intracapsular.

Cross-sectional anatomy (Fig. 1. 43b) At the level of C6 the gland appears as two triangles of tissue draped across the trachea by the connecting isthmus. Each triangular lobe measures approximately 3 cm in depth by 2 cm in w i d th and has a convex anterior surface. The strap muscles of the neck, sternomastoid and the jugular veins are anterior. The posterolateral surface is related to the carotid sheath. The posteromedial surface lies on the trachea and oesophagus and may be interposed between them.

Blood supply and lymph drainage Two constant pairs of arteries supply the thyroid gland. The superior thyroid artery is the first branch of the external carotid and supplies the upper pole. The inferior thyroid artery arises from the thyrocervical trunk, which is a branch of the subclavian artery. This passes behind the carotid sheath to gain access to the deep part of the gland. Both arteries anastomose freely w i th each other. A variable (3%) third artery, the thyroidea ima, may arise from the brachiocephalic artery or the aortic arch and ascends anterior to the trachea to join in the anastomotic plexus.

(a) 1. Base of tongue 2. Median glossoepiglottic fold 3. Lateral glossoepiglottic fold 4. Vallecula 5. Epiglottis 6. Laryngopharynx 7. Pharyngeal constrictor muscle 8. Greater cornu of hyoid bone 9. Tip of piriform sinus (the sinus is separated from the laryngopharynx on lower cuts by the aryepiglottic fold) 10. Internal carotid artery 11. Internal jugular vein 12. Sternomastoid muscle 13. External jugular vein 14. Submandibular gland 15. Platysma muscle 16. Subcutaneous fat 17. Prevertebral muscle 18. Foramen transversarium

(b) (There is a pathological abscess in the left sternomastoid muscle).

1. Thyroid cartilage 2. Cricoid cartilage 3. Vocal process of arytenoid cartilage 4. Vocal cord 5. Anterior commissure 6. Laryngopharynx 7. Upper pole of thyroid gland 8. Anterior jugular vein 9. Strap muscles 10. Sternomastoid muscle 11. Internal jugular vein 12. Common carotid artery 13. External jugular vein 14. Prevertebral muscle 15. Abscess in sternomastoid muscle

(c) 1. Cricoid cartilage 2. Cricothyroid membrane 3. Right lobe of thyroid gland 4. Left lobe of thyroid gland 5. Oesophagus (collapsed) 6. Prevertebral muscles 7. Common carotid artery 8. Internal jugular vein 9. Sternomastoid muscle 10. External jugular vein 11. Anterior jugular veins

T h r ee p a i rs of v e i ns arise f r o m a v e n o us p l e x us on t he surface of the gland. The superior and middle thyroid veins drain into the internal jugular vein. The inferior thyroid veins (often multiple) end in the left brachiocephalic vein.

Lymph drainage is directly into the thoracic duct and the right lymphatic duct.

Ectopic thyroid tissue

The thyroid gland develops from an outpouching of the pharynx and descends into the neck, passing anterior to the hyoid bone and trachea.

Maldevelopment may cause thyroid tissue to be found anywhere along a line from the base of the tongue to its

normal position. Less commonly, thyroid tissue may migrate inferiorly to the mediastinum or even to the pericardium or myocardium. The thyroglossal duct may persist as a midline structure extending superiorly from the isthmus of the gland and a thyroglossal cyst may be found at any site related to this. More commonly (40%), part of the duct persists as the pyramidal lobe of the gland, extending superiorly from the isthmus or the medial part of either lobe. Radiology of the thyroid gland (Fig. 1. 44)

Plain films The normal thyroid is not seen on plain radiographs. If enlarged, it may be seen to displace the trachea or bariumfilled oesophagus.

Ultrasound Ultrasound of the gland w i th a high-frequency transducer provides excellent detail. The normal thyroid gland has a homogeneous echotexture of medium echogenicity. The carotid vessels may be seen as anechoic structures on either side of the gland. The strap muscles are seen as structures of low echogenicity. Linear echogenic lines may be seen separating the muscles. The prevertebral muscles may be identified posteriorly. Numerous vascular structures may be seen surrounding the gland, and its extreme vascularity is readily appreciated w i th colour flow imaging.

Nuclear medicine studies Isotope scanning provides functional rather than anatomical detail. Both lobes and the isthmus can be identified. It is useful for identifying ectopic thyroid tissue, which is most likely to be in the base of the tongue, reflecting its site of development. 99mTc- or iodine-labelled agents are used.

CT (see Fig. 1. 42c) CT may be used to assess the gland in the axial plane. It shows as soft-tissue areas of high attenuation because of the