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ANATOMY FOR DIAGNOSTIC IMAGING
axial plane, but with MRI sagittal and coronal imaging is also possible. The cartilages are of low density on CT unless calcified, which occurs increasingly with 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, with 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 width 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 with 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.
